paragonpdh.com · 2019. 10. 24. · spaced. Consequently, installation may be considered too expensive for use of subsurface drains. Soils must have sufficient depth and permeability

14–45(210-VI-NEH, April 2001)

650.1420 General

Subsurface drainage removes or controls free waterfrom the soil surface and below the surface of theground. The principal function of subsurface drainageis to prevent, eliminate, or control a high water table.Lowering the water table can improve growing condi-tions for crops, the condition of the soil surface, andtrafficability on the field as well as around the farm-stead. It also facilitates tillage practices.

Subsurface drainage also provides a period to applyagricultural wastes during high and prolonged rainfallseasons, and it maintains water intake and soil profilestorage capacity, thereby reducing runoff during highrainfall periods. This type of drainage functions inirrigated areas to control saline and sodic soil condi-tions by removing excess salt accumulations, to pro-vide for subirrigation, to help control seepage fromcanals and laterals, and to remove excess irrigationwater from sources upslope as well as onsite. Subsur-face drainage is accomplished by various kinds ofburied or open drains.

(a) Plans

A plan should be made of every subsurface drainagelayout. The size and detail of the plan vary in differentlocations; however, the plan should have the basicinformation required for the construction of the sub-surface drainage system. Figures 14–24 and 14–25 arean example of a subsurface drain plan and layout.

(b) Soils

Subsurface drainage is applicable to saturated soilconditions where it is physically and economicallyfeasible to use buried conduits to remove or controlfree water from the root zone.

The need for and the design of subsurface drainagesystems are related to the amount of excess waterentering the soil from rainfall, irrigation, or canalseepage; the permeability of the soil and underlyingsubsoil material; and the crop requirements. In soilswith slow permeability that causes water to flow

slowly into the drain, the drains must be closelyspaced. Consequently, installation may be consideredtoo expensive for use of subsurface drains.

Soils must have sufficient depth and permeability topermit installation of an effective and economicalsubsurface drainage system. Some sandy soils andpeat and muck have large pore spaces that allow rapidmovement of water. Wetness occurs in these soilsbecause of a high water table, particularly in the springin nonirrigated areas, late in summer, or during theirrigation period. For maximum crop yields, the wet-ness problem must be corrected by drainage. Thesesoils can be successfully drained.

Some fine sand soils have insufficient colloidal mate-rial to hold the sand particles together. This can causeexcessive movement of the particles into the drains.Special precautions, such as filters or envelopes, areoften required.

In highly permeable, coarse sands and some peat soils,excessive lowering of the water table causes a mois-ture deficiency during periods of drought. Such soilshave limited capillary rise and are unable to deliverwater up into the plant root zone of certain crops if thewater table falls much below the root zone. Watertable control systems should be used for these condi-tions.

Other soil conditions make construction of drainshazardous or impractical. In some soils, boulders orstones make drainage costs prohibitive. In others, thetopsoil is satisfactory, but it is underlain by unstablesand at the depth where drains should be installed,thus making installation more difficult. A chemicalaction, which takes place in soils that have glauconite,iron oxide, or magnesium oxide, can cause drain jointsor perforations to seal over.

In soils where iron is present in soluble ferrous form,ochre deposits in drain lines can be a serious problem.If the problem is recognized, it can be solved by mak-ing adjustments in design and maintenance of thedrainage system.

Ochre is formed as a combination of bacterial slimes,organic material, and oxidized iron. It is a highlyvisible, red, gelatinous, iron sludge that often occurs inthe valleys of the corrugations of drain tubing as wellas at the drain outlets.

Chapter 14 Water Management (Drainage)Subchapter C Subsurface Drainage

Part 650Engineering Field Handbook

Chapter 14

(210-VI-NEH, April 2001)

Water Management (Drainage)

14–46

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14–47

Water Management (Drainage)Chapter 14


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Chapter 14



14–48

Soil conditions that contribute to ochre formationhave been identified throughout the United States.Studies have shown a relationship among soil types,iron ochre, and related sludge deposits in subsurfacedrain lines. Four known sludge deposits are associatedwith bacterial activity in subsurface drains—ochre,manganese deposits, sulfur slime, and iron sulfide.Iron deposits, collectively named ochre, are the mostserious and widespread of the sludge deposits. Theochre and associated slimes are a sticky mass that isgenerally red, yellow, or tan. This sticky mass can clogdrain entry slots, drain envelopes, and the valleys ofthe corrugations between envelope and inlet slots.Such elements as aluminum, magnesium, sulfur, andsilicon are often present.

The soils that tend to show the most hazard for ochreformation are fine sand, silty sand, muck or peat thathas organic pans (spodic horizons), and mineral soilsthat have mixed organic matter. Those that have theleast potential hazard of ochre are silty clay and clayloam. Sites used for spray irrigation of sewage effluentand cannery plant wastes generally furnish sufficientiron and energy for reduction reactions; therefore, thepotential hazard of ochre deposits is serious.

In certain areas of the western United States, manga-nese, when present under suitable conditions in theground water, can form a drain-clogging, gelatinousblack deposit. Manganese has not been a seriousproblem in the Eastern United States.

Sulfur slime is a yellow to white stringy depositformed by the oxidation of the hydrogen sulfide inground water. This slime has not been a serious prob-lem in most agricultural drains. It is most frequent inmuck soils. It may be on sites designed for subirriga-tion if the well water used for irrigation containshydrogen sulfide.

(c) Economics

Some soils can be drained satisfactorily, but the instal-lation cost of drainage structures is so great that thebenefits derived do not justify the expense. In mostinstances, drain spacing of less than 40 feet for relief(field) drainage can be justified where high valuecrops or substantial indirect benefits are involved. Forexample, indirect benefits should be considered wherethe drying of soil in orchards makes it possible forspray rigs and harvesting equipment to be used with-out bogging down and where agricultural wastes canbe applied during high and prolonged rainfall periods.

Some soil can be drained satisfactorily, but inherentproductivity is so low that yields do not justify theexpense. Suitable outlets and disposal for drainageeffluent may not be available at an acceptable cost.Even if returns from increased crop yields and reduc-tion in the cost of production should pay for drainsystem installation within 5 to 10 years, the financialability of the land user may not allow such an invest-ment. Final economic decisions should be made by thelanduser based on best estimate of cost and benefits.

(d) Use of local guides

Drainage recommendations for the varied soil types,soil conditions, crops, and economic factors can bemade only in general terms. Because investigationalprocedures, planning, and methods of improvingdrainage conditions differ in many sections of thecountry, local technical guides and State drainageguides should be consulted for recommendations andprocedures for agricultural drainage. Local and Stateregulations for disposal of drainage water must becomplied with.


14–49



650.1421 Applications ofsubsurface drainage

(a) Field drainage

Relief drains are those installed to remove excessground water percolating through the soil or to controla high water table. They should systematically lowerthe water table for an area. The drains may be alignedparallel or perpendicular to the direction of groundwater flow.

Relief drains will develop similar drawdown condi-tions on either side of the drain, and, if the soil ishomogeneous, the water table on either side will bethe same at equal distances from the drain.

(b) Interception drainage

Interception drains are installed at right angles to theflow of ground water to intercept subsurface flows.The drainage is applicable to broad, flat areas that arewet because of seepage from canals or adjoining high-lands. Drains for interception of seep planes must belocated properly to dry wet areas caused by upslopewater. Seep planes are first located by soil borings,and then the drain is located so that continuous inter-ception of such seep planes, adequate soil cover overthe drain, and uniform grade to an available outlet areestablished. In steeply graded depressions or draws, alayout may include a main or submain drain in thedraw or to one side of the draw, with the interceptorlines across the slope on grades slightly off contour.

(c) Drainage by pumping

The objective of all subsurface drainage is to removeor control excess water from the root zone of the crop.This generally is accomplished by installing subsurfacedrains or open ditches. Water table levels may also becontrolled by pumping from the ground water reser-voir to maintain the upper limit of the water tablelevel. In some irrigated areas where irrigation water isobtained from wells, irrigation and drainage may both

be affected by the pumping of wells. This combinationof practices is limited to areas where the soil has lowsalinity and a proper salt balance can be maintained.In salty areas where drainage is accomplished bypumping, the drain water is generally discharged into anatural outlet or planned disposal system and notdirectly reused for irrigation. In some cases the draineffluent can be used conjunctively with a water sourceof higher quality, making the water suitable for irriga-tion use.

Planning pump drainage can be quite complex. De-tailed information on the geologic conditions and thepermeability of soil and subsoil material is important.Design involves anticipating what the shape andconfiguration of the cone of depression will be afterpumping. This, in turn, involves spacing of wells withrespect to their areas of influence to obtain the desireddrawdown over the area to be drained. Experiencewith this type of drainage indicates that it is costly andconsideration of its use may be limited to high-produc-ing lands that have a high net return per acre.

(d) Mole drainage

Mole drains are cylindrical channels artificially pro-duced in the subsoil without trenching from the sur-face. Various kinds of plastic liners and methods ofinstallation of the liners have been tried with varyingdegrees of success. The object of the lining is to ex-tend the life of the mole channel.

Mole drainage is used in organic soils and soils with adense, impervious, fine-textured subsoil and normallyin more undulating areas. The problem is not thecontrol of a ground water table (which may be verydeep), but the removal of excess water from the fieldsurface or from the topsoil. The water reaches themole channel mainly through fissures and cracksformed during installation of the mole opening. Theoutflow from mole drainage systems differs consider-ably from that of subsurface drainage systems control-ling the ground water table. Normally, the outflowfrom mole drainage systems show a quick response torainfall or irrigation water; thus, as the water applica-tion or rainfall event ceases, the outflow ends quickly.The time lag between water intake at the surface anddrain outflow is a few hours at most.


Chapter 14



14–50

Experience with use of mole drainage in the UnitedStates is so variable that it is impractical to describeits use here. Refer to the National Engineering Hand-book, section 16 (NEH-16), for general informationand to local field office technical guides for specificinformation.

(e) Water table management

Water table management is the operation and manage-ment of a ground water table to maintain proper soilmoisture for optimum plant growth, to sustain orimprove water quality, and to conserve water. It is theoperation of a subsurface drainage system for thepurpose of lowering the water table below the rootzone during wet periods (drainage), raising the watertable during dry periods (subirrigation), and maintain-ing the water table during transition (controlled drain-age). For more information, refer to Subchapter E,Controlled and reversible drainage.

(f) Salinity control

Salinity control is practiced for both irrigated croplandand dryland farming operations. The same salt balanceprinciples apply, but the source of the salts and thewater source and management must be understoodand treated appropriately.

Drainage for salinity control on dry land commonlyintercepts and removes excess ground water upslopeof areas in which the subsoil has a high salt content.The excess water may come during high precipitationperiods, noncrop growing periods, or during heavysnow accumulation. Removal of the excess waterabove the seep prevents the accumulation of salts leftby the surfacing of the fluctuating water table. Drain-age facilities may be used alone or in combinationwith special agronomic practices to manage the ex-cess water.

(1) Dryland salinity management

Management of dryland seeps is described in detail inchapter 17 of the Agricultural Salinity Assessment andManagement Manual Number 71 (ASCE 1990). Be-cause interception drainage may play an importantrole in management of ground water, refer also to thesubchapter on interception drainage. Open interceptordrains may be effective and better suit a particular sitecondition than a subsurface drain.

Saline or sodic conditions can also occur on non-irrigated land at or near sea level in coastal areas.Controlled drainage structures or pumping systemsmay be needed to make a drainage system functionsatisfactorily.

(2) Salinity management for irrigated

conditions

The management of saline conditions in irrigatedagriculture is practiced more extensively and perhapsbetter understood. The source of the salts is generallyfrom the irrigation water. As the water is used byevapotranspiration, the salts are either leached belowor out of the root zone, or they are left to accumulatein the root zone. If the salts are left to accumulate inthe soil profile, a noticeable reduction in yield occurs,depending on the specific crop tolerance. Monitoringsalinity level of soil-moisture is shown in figure 14–26.To correct this situation, a natural or supplied source

Figure 14–26 Monitoring salinity level of soil-moisture


14–51



650.1422 Investigationsand planning

(a) Topography

The amount of surveying needed to obtain topographicinformation depends on the lay of the land. Whereland slopes are uniform, only limited survey data areneeded to locate drains. On flat or slightly undulatingland, proper location of drains is not obvious, and atopographic survey is necessary. Sufficient topo-graphic information should be obtained for planning ofthe complete job. Insufficient data often result in apiecemeal system, which can eventually be morecostly to the landowner. Methods for making topo-graphic surveys and maps are covered in chapter 1 ofthis handbook.

(b) Soils

Unless sufficient soil borings were taken during thepreliminary investigation, more should be taken aspart of the design survey. The number of boringsnecessary depends upon variations in the subsoilmaterial. Borings should be in sufficient number anddepth to determine the extent of soil texture variationsand to locate any lines of seepage, water movement, orwater table elevations.

Permeability (hydraulic conductivity) of the soilshould be determined at each change in soil layerswhen making the borings. Soil permeability should bedetermined by the auger hole or other field methods asdescribed in 650.1423(e).

If the hydraulic conductivity tests cannot be per-formed because a water table is not present, the sys-tem could be planned using estimated soil hydraulicconductivity. Hydraulic conductivity can vary signifi-cantly within the same soil in any given field; thusestimates should be used carefully. Estimates can bemade for each soil using the Mapping Unit Interpreta-tions Record (MUIR) as shown in figures 14–27 and14–28. The MUIR lists permeability values that can beused as hydraulic conductivity in lieu of measuredvalues.

of water is used to leach the salts beyond or out of theroot zone. If natural drainage is not adequate to dis-charge the saline water, then a drainage system needsto be installed. The depth and spacing of subsurfacedrain tubes are generally much greater in irrigatedareas of the Western United States compared to sub-surface drainage systems in the humid eastern region.

The proper operation of a productive and sustainableirrigated agriculture, especially when using a salinewater source, requires periodic information on thelevels and distribution of soil salinity within the croproot zones. Direct monitoring in the field is the mosteffective way of determining where and when soilsalinity conditions need corrective action. Figure 14-26shows a team of specialists measuring the soil salinitylevel in the root zone of a cotton crop that is irrigatedwith moderately saline water. In addition to hand-heldequipment in recent years mobile equipment has beenperfected to spatially monitor infield soil salinityconditions. Refer to Appendix 14C Salinity MonitoringEquipment.

Reuse or disposal of saline drainage water has re-ceived a lot of attention and special studies in recentyears (ASCE 1990). Saline water may be used for moretolerant crops, or the timing of saline water use oncrops may be adjusted till the crop growth stage willtolerate the increased saline level without hamperingquality or yield. Because reuse of different qualitywater for sustainability of production is a complexactivity, this should only be undertaken after consulta-tion with skilled specialists. Refer to NEH-16 (NRCS1971); NEH, Part 652, Irrigation Guide, chapters 2 and13; and ASCE Manual 71 (ASCE 1990) for additionalguidance.

(g) Residential and non-agricul-tural sites

A major application of subsurface drainage in non-agricultural settings is for foundation, basement, andlawn areas of residential homes. Refer to the appendix14F. A recent publication on this subject matter is theUrban Subsurface Drainage Manual, referred to asManual of Engineering Practice 95 (ASCE, 1998).


Chapter 14



14–52

Figure 14–28 Soil profile with calculations of hydraulic conductivity using soil interpretation record

Figure 14–27 Estimating soil hydraulic conductivity using soil interpretation record

0

19

35

72

120"

Loam ------------------------------------- 0.6 - 6.0 in/hr ------------------------------------- Use 3.0 in/hr --------- K1 = 3.0 --------- D1 = 19"

Sandy clay loam ---------------------- 0.6 - 2.0 in/hr ------------------------------------ Use 1.5 in/hr --------- K2 = 1.5 --------- D2 = 16"

Coarse sand ---------------------------- 6.0 - 20 in/hr -------------------------------------- Use 18 in/hr

K3 = 18.0" --------- D3 = 85"

6.0 - 20 in/hr -------------------------------------- Use 18 in/hr

Greatest depth classifiedon soil sheet

"Estimated to be the same as coarse sand basedon boring logs from 35 to 120 inches."

"Portsmouth soil"

Estimated permeabilities "Educated guess"

K values(educated guess)

D values(Thickness of horizon)


14–53



Saline and sodic problems are most common in aridand semi-arid climates. Where salt crusts are visible orsalinity is suspected, the soil should be sampled atvarious depths. A record of the species and conditionof plant cover is important. More details about salineand sodic problems are in ASCE Manual 71 (ASCE1990) and local drainage guides.

Soil investigations should be conducted to estimatethe maximum potential for ochre development in theplanned drainage system.

(c) Biological and mineralclogging

The ferrous iron content of the ground water flowinginto a drain is a reliable indicator of the potential forochre development. Soluble ferrous iron flowing inground water enters a different environment as itapproaches the drain and passes through the drainenvelope. If the level of oxygen is low, certain filamen-tous and rod-shaped bacteria can precipitate insolubleferric iron and cause its incorporation into the com-plex called ochre. The amounts of iron in groundwater that can stimulate bacteria to produce ochre canbe as low as 0.2 ppm.

Laboratory and field methods are available to estimatethe ochre potential for a given site. Of particularimportance is whether ochre may be permanent ortemporary. Temporary ochre occurs rapidly, usuallyduring the first few months after drain installation. Ifthe drains can be cleaned or maintained in functionalorder, the ochre problem may gradually disappear asthe content of iron flowing to the drains is reduced.Such soil environments must be low in residual or-ganic energy sources to prevent the continual releaseof iron during short-term flooding.

Permanent ochre problems have been found in pro-files with extensive residual iron, such as cementediron subhorizons or rocks, and from iron flowing infrom surrounding areas. Many factors influence ochredeposition, including the pH, type, temperature, andreducing conditions of the soil.

Certain onsite observations may give clues to potentialochre formation before a drainage system is installed.Surface water in canals may contain an oil-like filmthat is iron and may contain Leptothrix bacterial

filaments. Gelatinous ochre may form on the ditch-banks or bottom of canals. Ochre may also formwithin layers of the soil. Iron concretions, sometimescalled iron rocks, are in some areas. The presence ofspodic horizons (organic layers) suggests ochre poten-tial; and most organic soils, such as mucks, have somepotential for ochre problems.

If a site has potential for ochre deposits, certain plan-ning and design practices should be followed to mini-mize this hazard to the system. No economical, long-term method for effectively controlling this problem isknown. For sandy soils where a filter is necessary, agraded gravel envelope is best, although it can becomeclogged under conditions of severe ochre potential.When synthetic fabrics were evaluated for ochreclogging, the knitted polyester material showed theleast clogging.

A submerged outlet may be successfully used to mini-mize ochre development with the entire drain perma-nently under water. The line should be completelyunder water over its entire length throughout the year.This may require that the drains be on flat grade. Thedepth of ground water over the drain should be at least1 foot.

Herringbone or similar drain designs should haveentry ports for jet cleaning.

Use drain pipe that has the largest slots or holes al-lowed within the limits of drain pipe and envelopestandards. Slots or holes should be cleanly cut andshould not have fragments of plastic to which ochrecan adhere. Both smooth bore and corrugated pipescan accumulate ochre.

(d) Ground water

Water table contour maps and depth to water tablemaps are very useful in planning subsurface drainage.The elevation of the water table at selected points, asobtained from borings, is plotted on a topographicmap of the area. By interpolation, lines of equal watertable are drawn. The result is a contour map of thewater table. Where a ground surface contour crosses awater table contour, the depth to the ground water isthe difference in elevation of the two contours. Areasthat have a range of depths can be delineated on themap. The direction of ground water flow and extent of


Chapter 14



14–54

high water table areas are also shown on the map. Foradditional information on the preparation and use ofthe water table contour maps, refer to National Engi-neering Handbook, part 623 (section 16), chapter 10.

(e) Outlets

(1) System outlets

The starting point in planning a subsurface drainagesystem is normally the location of the outlet. Drainsmay discharge by gravity into natural or constructeddrains. Any of these outlets are suitable if they aredeep enough and of sufficient capacity to carry all thedrainage water from the entire drainage system. Theadequacy of the outlet should be determined beforeproceeding with the design of the system.

(2) Capacity and depth of open ditch outlets

The outlet ditch must have the capacity to remove thedrainage runoff from its watershed quickly enough toprevent crop damage. It should be deep enough toallow at least 1 foot of clearance between the flow lineof the drain and the normal low water stage in theditch when drains are installed at the specified depth.

(3) Capacity and depth of subsurface outlets

If existing subsurface drains are used for the outlet,they should be in good condition and working prop-erly. The main drain should have sufficient capacity tohandle the proposed drainage system in addition toother systems it serves, and it should be deep enoughto permit the new system to be installed at the depthspecified.

(4) Pump outlet

An outlet by pumping should be considered for drain-age sites where a gravity outlet is not available. Referto subchapter F for information on pumped outlets.

(5) Vertical drain outlet

A vertical drain is a well, pipe, pit, or bore, drilled intoporous underlying strata, into which drainage watercan be discharged. It is sometimes called a drainagewell.

Wells tap permanent sources of ground water forlivestock and domestic use. In some areas, shallowwells are the only reasonable and available watersource for domestic use and must be protected fromcontaminants in agricultural drainage water. Publichealth laws in some States regulate the use of wells fordrainage, and in many cases, prohibit this use becauseof the potential contamination of ground water and thedanger to public health. However, in some parts of thecountry vertical drainage is a satisfactory solution todrainage water disposal and deep ground water re-charge.

Where the possibility of using wells for a drainageoutlet is considered, an engineer or geologist, or both,should assist in the investigation and planning.


14–55



650.1423 Field drainagesystem design

To plan a field drainage system, a pattern should beselected that fits the topography, sources of excesswater, and other field conditions. The following basicsystems may be considered (fig. 14–29).

(a) Random system

A random field drainage system is used where thetopography is undulating or rolling and has isolatedwet areas. The main drain is generally placed in thelowest natural depression, and smaller drains branchoff to tap the wet areas. Because such drains oftenbecome outlets for a more complete system estab-lished in the higher areas of the field, the depth, loca-tion, and capacity of the random lines should be con-sidered as part of a complete drainage system. Gener-ally, the logical location of these drains obviously fitthe topography.

(b) Parallel system

The parallel field drainage system consists of lateralsthat are perpendicular to the main drain. Variations ofthis system are often used with other patterns. Inmany cases, the parallel system is desirable because itprovides intensive drainage of a given field or area. Itcan also be used in depressional or low areas that canbe graded before installation of the system.

(c) Herringbone system

The herringbone field drainage system consists oflaterals that enter the main drain at an angle, generallyfrom both sides. If site conditions permit, this systemcan be used in place of the parallel system. It can alsobe used where the main is located on the major slopeand the lateral grade is obtained by angling the lateralsupslope. This pattern may be used with other patternsin laying out a composite system in small or irregularareas.

(d) Drainage coefficient

(1) Humid areas

Drains should have sufficient capacity to removeexcess water from minor surface depressions and themajor part of the root zone within 24 to 48 hours afterrainfall ceases. The required amount of water to beremoved in some specified time is the drainage coeffi-cient. For field drainage, it is expressed as inches ofwater depth to be removed over a safe period of time,generally 24 hours, or as an inflow rate per unit lengthof drain. Because of the differences in soil permeabil-ity, climate, and crops, as well as the manner in whichwater may enter the drain (i.e., all from subsurfaceflow or part from subsurface flow and part fromsurface inlets), the coefficient must be modified to fitsite conditions in accordance with local drainageguides or within the approximate limits as follows:

• Where drainage is uniform over an area througha systematic pattern of drains and surface wateris removed by field ditches or watercourses, thecoefficient should be within the range as shownin table 14–5. Figure only the area to be drainedas the drainage area.

• Where surface water, including roof runoff, mustbe admitted through surface inlets to the drain,an adjustment in the capacity of the drain isrequired. Runoff from an area served by a sur-face water inlet takes place soon after the rain-fall and enters the drain ahead of the groundwater. In short drainage lines or small systemsthat have only one or two inlets, the size of thedrain may not need to be increased. As systemsbecome larger or the inlets more numerous, anadjustment to the drainage coefficient should bemade. The timing of the surface water flow inrelation to the entrance of ground water into thedrain should be the basis for increasing thecoefficient over those shown in table 14–5.

• A higher coefficient than those given in table14–5 is sometimes necessary to hold crop dam-age to a minimum. Refer to local drainage guides.

Table 14–5 Drainage coefficients

Soil Field crops Truck crops

(inches to be removed in 24 hours)

Mineral 3/8 – 1/2 1/2 – 3/4

Organic 1/2 – 3/4 3/4 – 1 1/2


Chapter 14



14–56

Figure 14–29 Field drainage systems

Outlet

Drain

Parallel system

Herringbone system

Random system

Drain

Outlet

Outlet

Drain


14–57



(2) Arid areas

In areas where rainfall is light, drainage coefficientsapplicable to local areas are variable and depend uponthe quality and amount of irrigation water applied,methods of irrigation, crops to be grown, and charac-teristics of the soil.

Local drainage guides and other information should beused to determine drainage coefficients. However,where experience is lacking, the following formula canbe used to estimate drainage coefficients. The deeppercolation volume must be accounted for, includingleach water that must be added to maintain a favor-able salt balance in the soil and the water that is aresult of inefficient or excess irrigation. This formuladoes not account for upslope water sources or upwardflux from ground water.

q

P Ci

=

+ ( )100

24Fwhere:

q = drainage coefficient, inches/hourP = deep percolation from irrigation including

leaching requirement, percent (based on con-sumptive use studies)

C = field canal losses, percenti = irrigation application, inchesF = frequency of application, days

A graphical solution to the formula is provided in thechart in figure 14–30. An example problem solved bygraphical method follows.

Example:

Assume:Total loss (P+C) = 30 percentIrrigation application (i) = 6 inchesFrequency of application (F) = 8 days

Using figure 14–30, find 30 on the left vertical scale;follow horizontally from this point to intersect withline (i) = 6 in. Translate this point vertically to inter-sect line (F) = 8. Follow horizontally from this point tothe right scale and read 0.0092 inches per hour as thedrainage coefficient.

(e) Hydraulic conductivity

Any drainage survey should involve a hydraulic con-ductivity (K) investigation in the field. Twice the depthof drains or the top 7 feet of the soil profile, whicheveris less, can be surveyed using the auger hole method.When using this method, the observation points shouldbe selected according to available soil maps andinformation on soil morphology and its lateral varia-tion pattern in relation to physiography. Typically, theintensity of a hydraulic conductivity survey for thepurpose of project design varies from one hole per 10acres to one hole per 20 acres, depending on thedegree of homogeneity in the obtainable values. Mapsshould be prepared at a suitable scale showing the Kvalues differentiated, if possible, for hydraulic conduc-tivity above and below drain level and also showingdepths to an impermeable stratum if one exists. Allauger holes should be drilled at least to the depth thedrains will be installed. It is advisable to extend onehole in 10 to the relatively impermeable layer (barrier),three holes in 10 to a depth below the drain level, andsix holes in 10 to the depth of the drain level. Appen-dix 14D describes the auger hole procedure.

A farm or large field on which the values of hydraulicconductivity are quite uniform may be given an aver-age value by simple calculation of the arithmeticmean. Sometimes the farm or field must be dividedinto sections, each having values that are uniform butwhose averages are different.

Areas with an average hydraulic conductivity of 0.3foot per day or less normally are not provided withsubsurface drainage. The chosen figure, 0.3 foot perday, is based on economic considerations. Hydraulicconductivity is subject to change with time as a resultof alternate drying and wetting of the soil or bychanges in the soil structure through chemical ormechanical means, as well as the farming practices.

Isolated spots of very low hydraulic conductivity maybe encountered on farms that are otherwise reason-ably permeable. These spots should not be included inthe average. The drainage of such localized imperme-able mounds is best achieved by draining the sur-rounding, more permeable soil into which thesemounds can then dewater. Installation of subsurfacedrainage systems in such impermeable soil should beavoided. In some situations shallow, open field ditches


Chapter 14



14–58

Figure 14–30 Curves to determine drainage coefficient, q, for arid areas

80

0.005

0.006

0.007

0.008

0.009

0.010

0.011

0.012

0.013

0.014

0.015

0.016

0.017

0.018

0.019

0.020

70

60

50

40

30

20

102 4 6 8 10

i/h

(P

+C

),

percen

t

i=6

F=10

12

14

18

21

28

5

1

2

3

4

F, days

PC

q

PC F

=

+ 100

24in

hr/


14–59



may be feasible. The landowner decides on the feasi-bility of installing a drainage system with an appropri-ate drain spacing. Some over or under drainage isinevitable, thus the decisionmaker should be informedof these potential irregularities as decisions are madeon spacings.

(f) Depth of impermeable layer

The impermeable layer (barrier) is not necessarily amore fine textured subsoil. It may be a product ofother factors, such as consolidation by the weight ofglaciers or a stratification of alluvial sediment.

The impermeable layer can be defined as a layer with ahydraulic conductivity value of one-tenth or less thanthat of the soil stratum containing the water tablethrough which the drainage water moves towards thefield drains. In that regard, the effects of possible up ordownward seepage through the impermeable layerhave been neglected, and any lateral seepage (result-ing from canal leakage or hillside seepage) is notconsidered.

The influence of an impermeable layer on the behaviorof a ground water table depends on its depth belowthe level of field drains and on the drain spacing. Theflow pattern of the water moving toward the drain isaltered drastically by the impermeable layer if thedrain level above the impermeable layer is less than afourth the spacing between drains. The drains need tobe placed closer together to achieve the effect theywould have in a deep permeable soil. However, if thedepth of the impermeable layer below drain levelexceeds a fourth of the drain spacing, the flow systemcan be treated as if such a layer was absent.

(g) Depth and spacing

The two basic formulas to determine spacing are thesteady state and the nonsteady state, also called tran-sient state. A basic form of the steady state equation,referred to as the ellipse equation, is presented in thissection. The importance of a good drainage formulafor depth and spacing is often overestimated. The

transient state formula may better relate actual condi-tions than the ellipse equation, but the accuracy ofresults from any of the accepted equations is con-trolled primarily by the accuracy of the assumed ormeasured parameters used in the calculations. Theaccuracy of the input parameters, especially hydraulicconductivity, may be in error by as much as 20 per-cent; therefore, a minor difference in the result of oneequation over another is not that important. Sensitivityanalysis of input parameters has been done and docu-mented in conjunction with evaluations ofDRAINMOD. For these reasons and to simplify use, itis justifiable to use the presented steady state equa-tion.

The DRAINMOD computer program can satisfy theneed for a more precise analysis of the water tablefluctuations and resulting crop impact analysis. SeeDRAINMOD User’s Guide (USDA NRCS 1994). If atransient state analysis is desired, refer to appendix14E. Many field offices have locally developed drain-age guides with recommended depth and spacingcriteria related to the individual soil series as mappedand published in cooperative soil survey reports.

(1) Humid areas

Generally, the greater the depth of a field drain, thewider the spacing can be between drains. Other fac-tors that help determine the depth and spacing are thesoil, climate, subsurface barriers, frost depth, and croprequirements. Where the experience with drain instal-lations is extensive, the depth and spacing of drainagesystems in various soils are already well established.

Drains should be deep enough to provide protectionagainst tillage operations, equipment loading, andfrost. Initial settlement in organic soils should beconsidered in depth selection. Main and submaindrains must be deep enough to provide the specifieddepth for outlets of lateral drains. Also, the maximumdepth at which drains can be laid to withstand trenchloading varies with the width of the trench and thecrushing strength of the drain to be used. Allowablemaximum depths are given later in this section.


Chapter 14



14–60

In areas where drainage installations and knowledgeof effective spacings are limited, the ellipse equationcan be used to determine drain spacing.

S

K b a

q=

−

4 2 20 5.

where (see fig. 14–31):S = spacing of drains, feet,K = hydraulic conductivity, inches/hourb = distance from the drawdown curve to barrier

stratum at midpoint between the drains, feeta = distance from drains to the barrier, feetq = drainage coefficient, inches/hour

Note: The units of K and q may also be in inchesremoval in 24 hours or gallons per square foot per day,but both must be in the same units in this formula. Inusing this formula for conditions where no knownbarrier is present, it is assumed that a barrier ispresent at a depth equal to twice the drain depth.

Example:

1. Parallel drains are installed at a depth of 6.0 feet(d = 6).

2. Subsoil borings indicate an impervious barrier at 11feet below the ground surface (a = 11.0 - 6.0 = 5.0feet).

3. Distance from ground surface to drawdown curvedesired is 3 feet. Then (b = 11 - 3 = 8 feet)

4. The hydraulic conductivity of subsurface materials(K), is 1.2 inches/hour.

5. The applicable drainage coefficient (q) is 3/8 inchremoved in 24 hours, or 0.0156 inch/hour.

Then:

S ft=( ) −( )

=4 1 2 8 5

0 0156109 5

2 20 5

.

..

.

(use 110 feet)

Charts for the graphical solution of the ellipse equa-tion are shown in figure 14–32, sheets 1 and 2. Refer tothe figure and solve the above example as follows:

Step 1. On sheet 1, find [a = 5] on the bottom scale.Project this point vertically to intersect the curve line[b = 8]. From this point, follow horizontally to inter-sect radial line [K = 1.2]. From this point go verticallyto intersect the top scale. Read the index number of380.

Step 2. On sheet 2, find index number (380) on thebottom scale. Project this point vertically to intersectthe curve line [q = 0.0156]. From this point, followhorizontally to the right vertical scale and read thespacing of [S =109 feet]. [Use 110 feet.]

Figure 14–31 Nomenclature used in ellipse equation

d

a

b

S2

Ground surface

Drawdown curve

Drain

Barrier


14–61



Figure 14–32 Graphical solution of ellipse equation (sheet 1 of 2)

9.0

20.0

10.0

8.0

6.0

5.0

4.0

K-values

3.0

2.0

1.51.4

1.31.2

1.11.0

0.50.3 0.1

8.0 7.0 6.0 5.0

a-values

Index numbers

4.0 3.0 2.0 1.0 0

1000

11.0

10.0

9.0

b-va

lues

8.0

7.0

6.0

5.0

4.0

900 800 700 600 500 400 300 200


Chapter 14



14–62

Figure 14–32 Graphical solution of ellipse equation (sheet 2 of 2)

1000 900 800 700 600Index numbers

S-v

alu

es-s

pacin

g i

n f

eet

500 400 300 200

200

250

300

150

100

50

(380

)

0.040

0.030

0.025

0.020

0.019

0.018

0.017

0.0160.0156

0.015

q-values

0.010

0.008

0.006

0.004

0.002

(109)


14–63



(2) Arid areas

For field drainage systems, depth and spacing ofdrains are given in state drainage guides where experi-ence has been extensive enough to determine thecorrect figures. Where this information is not avail-able, the ellipse equation, as previously discussed, isused to determine the spacing of field drains.

To control salinity, drains generally are placed deeperin arid and semiarid irrigated areas than in humidareas. The minimum effective depth is 6 feet for me-dium textured soils and up to 7 feet for fine texturedsoils, assuming there is no barrier above this depth.This is necessary to maintain the ground water at adepth of 4 to 5 feet midway between the drains. Inmost cases, the depth of the laterals depends upon theoutlet depth, slope of the ground surface, and depth toany barrier.

The depth capacity of the drain-laying machine maylimit the depth of the drain unless provisions are madefor ramping part of the line. In ramping, a depressedroadway is excavated for the machine to obtain theneeded depth in the deeper reaches of the drain line.

If the outlet for the drain is not deep enough to permitits installation to the required depth, then outletting

into a sump with an automatic pump cycling may beconsidered. Refer to subchapter D for information onpumped outlets.

(3) Allowable depths

In most cases subsurface drains should not be placedunder an impermeable layer, and they should belocated within the most permeable layer. Economicdepth should also receive consideration. Economicdepth figures can easily be developed locally by evalu-ating changes in the cost for different depth and spac-ing ratios for each major soil type. The maximumdepth at which subsurface drains can be safely laidvaries with the crushing strength of the drain, type ofbedding or foundation on which the drain will be laid,the width of the trench, and the weight of the backfill.

Temperature is also a factor to consider for installa-tion and loading of thermoplastic pipe. The maximumdepth of cover recommended for subsurface drains isgiven in the American Society of Agricultural Engi-neers (ASAE) Engineering Practice 260.4.

Table 14–6 is provided for guidance on maximumdepth. The type of bedding depends upon constructionmethods. If a drain is installed by backhoe or draglineand the bottom of the trench is not shaped to conform

Table 14–6 Maximum trench depths for corrugated plastic pipe buried in loose, fine-textured soils (feet) *

Nominal tubing Tubing quality - - - - - - - - - - - - Trench width at top of corrugated plastic pipe (feet) - - - - - - - - - - - -diameter (ASTM)(inches) (1) (1.3) (2) (2.6 or greater)

4 Standard 12.8 6.9 5.6 5.2Heavy-duty § 9.8 6.9 6.2



10 — 9.2 6.6 6.212 — 8.9 6.6 6.215 — — 6.9 6.218

Note: Depths are based on limited research and should be used with caution. Differences in commercial tubing from several manufacters,including corrugation design and pipe stiffness and soil conditions, may change the assumptions; and, therefore, maximum depths maybe more or less than stated above.

* Based on 20 percent maximum deflection.§ Any depth is permissible for this or less width and for (0.67 ft) trench width for all sizes.Source: ASAE-EP260.4


Chapter 14



14–64

to the drain, special care should be exercised to fill allthe spaces under and around the tile or tubing withgranular material.

In the ordinary bedding method, the trench bottom isshaped by the shoe of the trenching machine to pro-vide a reasonably close fit for a width of about 50percent of the conduit diameter. The remainder of theconduit is covered with granular material to a heightof at least 0.5 feet above its top or topsoil is placed tofill all spaces under and around the drain. The presentmethod of installing drains using trenching machinesis effective, and for practical purposes ordinary bed-ding may be assumed. The bedding (ordinary) allowsthe drain to be installed in a deeper or wider trenchthan if no attention had been paid to the bedding ofthe drain.

For computation of maximum allowable loads onsubsurface drains, use the trench and bedding condi-tions specified and the crushing strength of the kindand class of drain.

The design load on the conduit should be based on acombination of equipment loads and trench loads.Equipment loads are based on the maximum expectedwheel loads for the equipment to be used, the mini-mum height of cover over the conduit, and the trenchwidth. Equipment loads on the conduit may be ne-glected if the depth of cover exceeds 6 feet. Trenchloads are based on the type of backfill over the con-duit, the width of the trench, and the unit weight of thebackfill material.

(h) Size of drains

The size of drains depends upon the required flow andthe grade on which they are laid. The required flow isdetermined from the drainage coefficient and the areaor length of drains contributing flow, plus any allow-ances for concentrated flow entering from the surface,springs, or other sources. The contributing drainagearea for a complete drainage system is about the sameas the total length of all contributing lines multipliedby the spacing between such lines.

Random drains in poorly drained depressions areoften used later as main drains for a more complete

drainage system. Where such expansion is likely, theadditional area that such drains would serve should beincluded in determining the size of the initial randomline. Where surface water is admitted directly into adrain by surface inlets, the entire watershed contribut-ing to the inlet should be included. Flow from suchwatersheds often can be reduced by diversion ditches.

(1) Main drain

The required discharge can be determined using figure14–33 for a given drainage coefficient and area (acres).The required size of the corrugated plastic drainagetubing can be determined directly from figure 14–34.After grade, coefficient, and drainage area have beendetermined, the size of clay or concrete drain tilerequired can be determined directly from the tiledrainage chart in figure 14–35. The same charts may beused if the required flow and the grade of the drainsare known. The size required for all types of drains canbe calculated using Manning's equation with the ap-propriate roughness coefficients (table 14–7). Theexample on page 14–68 illustates the use of thesecharts for the subsurface drainage system shown infigure 14–36.

Table 14–7 Values of Manning's n for subsurface drainsand conduits

Description of pipe Values of n

Corrugated plastic tubing3 to 8 inch diameter 0.01510 to 12 inch diameter 0.017>12 inch diameter 0.020

Smooth plastic, unperforated 0.010 – 0.012

Smooth plastic, perforated 0.010 – 0.012

Annular corrugated metal 0.021 – 0.025

Helical corrugated 0.015 – 0.020

Concrete 0.012 – 0.017

Vitrified sewer pipe 0.013 – 0.015

Clay drainage tile 0.012 – 0.014


14–65



Figure 14–33 Subsurface drain discharge

10090008000

7000

6000

500045004000

3500

3000

2500

2000

1500

1200

1000900800

700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10

1/4 in 3/8 in 1/2 in 3/4 in 1 in 1-1/2 in 2 in

6.4 9.5 12.7 19.1

Depth removed in 24 hours

Drainage coefficient

25.4 38.1 50.8

9080706050

40

30

20

15

10

5

4

3

2

1.5

1.0

0.8

0.6

0.4

0.3

0.2

0.15

0.10

0.06

in.

mm

Dra

in d

isch

arg

e in

cu

bic

feet

per

sec

on

d

0.006

0.01

0.015

0.02

0.03

0.04

0.050.06

0.10

0.15

0.2

0.4

0.6

0.8

1.0

1.5

2

3

4

56

Dis

char

ge

in c

ub

ic m

eter

s p

er s

eco

nd

6000

500045004000

3500

3000

2500

2000

1500

1200

1000900800

700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10987

6

5

4

45004000

3500

3000

2500

2000

1500

1200

1000900800

700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10

5

4

3

3000

Acres or hectaresArea Drained

2500

2000

1500

1200

1000900800

700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10

5

4

3

2

2000

1500

1200

1000900800700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10

5

4

3

2

1500

1200

1000

900800700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10

5

6

7

89

4

3

2

1

1000900800700

600

500450400350

300

250

200180160140

120

100908070

60

504540

35

30

25

20

15

10

5

6

7

89

4

3

2

1

Note: Use acres with ft3/s and hectares with m3/s (Source-ASAE Standard EP260.4)


Chapter 14



14–66

Figure 14–34 Determining size of corrugated plastic pipe

(Source-ASAE Standard EP260.4)

0.010.01

0.02

0.03

0.040.05

0.1

0.2

0.3

0.40.5

1.0

2.0

3.0

4.0

5.0

10.0

0.02 0.03 0.1 0.2 0.3 0.5 1.0 2.0 3.0 5.0 10.0

0.2831

0.1416

0.1132

0.0849

0.0566

0.0283

0.0142

0.0113

0.0085

0.0057

0.0028

0.00140.0011

0.0008

0.0006

0.0003

Drain

dis

ch

arge i

n c

ub

ic m

ete

rs p

er s

eco

nd

Drain

dis

ch

arge i

n c

ub

ic f

eet

per s

eco

nd

Drain grade in percent

Drain grade = hydraulic gradient 3 to 8 in (76 to 203 mm) : n = 0.015Percent = ft per 100 ft, or m per 100 m 10, 12 in (254, 305 mm) : n = 0.017V = velocity in ft/s (m/s) 15, 18 in (381, 457 mm) : n = 0.02

Space between the solid lines is the rangeof pipe capacity for the size shown betweenlines.

V=0.5 ft/s (0.155 m/s) n/sec)

V=1.0 ft/s (0.31 m/s)

V=1.4 ft/s (0.434 m/s)

V=2.0 ft/s (0.61 m/s)

V=3.0 ft/s (0.91 m/s)

V=4.0 ft/s (1.22 m/s)

V=5.0 ft/s (1.52 m/s)

V=7.0 ft/s (2.13 m/s)

18 in (457 mm)

15 in (381 mm)

12 in (305 mm)

10 in (254 mm)

8 in (203 mm)

6 in (152 mm)

5 in (127 mm)

4 in (102 mm)

3 in (76mm)


14–67



Figure 14–35 Determining size of clay or concrete drain tile (n = 0.013)

(Source-ASAE Standard EP260.4)

0.01

0.1

0.2

0.3

0.4

0.5

0.01

0.02

0.03

0.040.05

1.0

2.0

3.0

4.0

5.0

10

20

30

40

50

100

0.02 0.05 0.1 0.5 1.0 5.0 10

2.831

1.4155

1.1324

0.8493

0.5662

0.2831

0.1416

0.1132

0.0849

0.0566

0.0283

0.01420.0113

0.0085

0.0057

0.0028

0.00140.0011

0.0008

0.0006

0.0003

Drain

dis

ch

arge i

n c

ub

ic m

ete

rs p

er s

eco

nd

Drain

dis

ch

arge i

n c

ub

ic f

eet

per s

eco

nd

Drain grade in percent

Drain grade = hydraulic gradient(Percent = ft per 100 ft, or m per 100 m)V = velocity in ft/s (m/s)

Space between the solid linesis the range of tile capacity forthe size shown between lines.

V=12 ft/s (3.55 m/s)

V=10 ft/s (3.05 m/s)

V=9 ft/s (2.74 m/s)

V=8 ft/s (2.44 m/s)

V=7 ft/s (2.13 m/s)

V=6 ft/s (1.83 m/s)

V=5 ft/s (1.52 m/s)

V=4 ft/s (1.22 m/s)

V=3 ft/s (0.91 m/s)

V=2 ft/s (0.61 m/s)

V=1 ft/s (0.31 m/s)

V=0.8 ft/s (0.24 m/s)

(1219 mm)

48 in

(1066 mm)

(914 mm)

42 in

36 in

(834 mm)

33 in

(762 mm)

30 in

(686 mm)

27 in

(610 mm)

24 in

(533 mm)

21 in

(457 mm)

18 in

(406 mm)

16 in

(381 mm)

15 in

(355 mm)

14 in

(305 mm)

12 in

(254 mm)

10 in

(203 mm)8 in

(152 mm)

6 in

(127 mm)

5 in

(102 mm)4 in

(76 mm)

3 in


Chapter 14



14–68

Example for main drain (see fig. 14–36):

Given:

A tract of land about 640 by 725 feet is to be drainedfor general crops. The drainage area is 10.65 acres. Thedrainage coefficient is 3/8 inch in 24 hours (0.0156inch/hour). A parallel system that has laterals spaced66 feet apart, requires 4 lines, 660 feet long; 11 lines,370 feet long; and 1 line, 200 feet long; making a totalof 6,910 feet of drain. The main, as shown from aplotted profile, is on a grade of 0.08 percent and is tobe corrugated plastic tubing.

Required:

Size of the corrugated plastic main at the outlet and itscapacity.

Using figure 14–33, find 10.65 acres in the 3/8 inchcoefficient column under Area Drained to determinethe discharge of 0.17 cubic feet per second. Using thisdischarge, enter figure 14–34 and a slope of 0.08 verti-cal gradeline. The point of intersection lies within therange for an 8-inch drain. The top line of the spacemarked 8 represents the 8-inch drain flowing full whenthe hydraulic grade is the grade of the drain. From theintersection of the top of 8-inch range and grade of0.08 percent, produce a line horizontally to intersectthe vertical on the left. The drain flowing full willdischarge 0.3 cubic feet per second, which shows thatthe drain selected will not be flowing full.

Figure 14–36 Subsurface drainage system

66 ft

Main

Mai

n

725 ft

660 ft

640

ft

370

ft

370

ft

Subsurface pipe drain


14–69



(2) Field or drain lateral

To compute the size of a lateral, first determine therequired discharge for the lateral. The following for-mula or figure 14–33 can be used. When the dischargeis determined, use figure 14–34 to determine the drainsize for plastic pipe or figure 14–35 for clay or con-crete tile.

In the case of parallel drains, the area served by thedrain is equal to the spacing times the length of thedrain plus one-half the spacing. The discharge can beexpressed by the following formula:

Q

q S LS

r =+

2

43 200,

where:Q

r= Relief drain discharge, ft3/s

q = Drainage coefficient, in/hrS = Drain spacing, ftL = Drain length, ft

Example:

Drain spacing – 200 feet (S)Drain length – 3,000 feet (L)Drain coefficient – 0.04 inches/hour (q) (1 inch/day)Drain grade – 0.30 percent

Using figure 14–37, find spacing of 200 feet on thevertical scale on the left; follow horizontally to theright to intersect with the drainage coefficient curve0.04. From that point follow vertically to intersect thelength curve of 3,000 feet, then go horizontally to theright to read the discharge of 0.575 cubic feet persecond. Using figure 14–35 for plastic tubing, find theabove discharge on the vertical scale on the left andlook horizontally to intersect the grade of 0.30 percent.An 8-inch drain will be required.

The drain chart has velocity lines. In the example, thevelocity in the drain is between 1.4 and 2.0 feet persecond, thus, minimizing sediment accumulation. In adrainage system, different sizes of drains may beneeded. Required drain size may change at breaks ingrade and changes in tributary area.


Chapter 14



14–70

Figure 14–37 Curves to determine discharge, Qr, for main drain

2000

1000

3000

4000

5000

600070

00

0.06

0.05

0.04

0.03

0.02

0.01

Length in feet (L)

Drainagecoefficient (q) (in/hr)

400

350

300

250

200

150

100

50

0

qS (L+ )

43,200

0.00

0.167±

0.50

0.575

0.75

1.00

1.25

1.50

1.75

2.00

Sp

acin

g (

feet)

(S

)

Desig

n d

isch

arge (

cu

bic

feet

per s

eco

nd

) (

Qr)

Qr =

S2


14–71



650.1424 Drain envelopes

Drain envelope is used here as a generic term thatincludes any type of material placed on or around asubsurface drain for one or more of the followingreasons:

• To stabilize the soil structure of the surroundingsoil material, more specifically a filter envelope.

• To improve flow conditions in the immediatevicinity of the drain, more specifically a hydrau-lic envelope.

• To provide a structural bedding for the drain,also referred to as bedding.

Refer to the glossary for more complete definitions ofenvelopes (hydraulic envelope, filter envelope, andbedding).

Soils in which drains are prone to mineral clogging arecommonly referred to as problem soils because thesoil particles tend to migrate into the drain. In prac-tice, all very fine sandy or silty soils with low claycontent are probable problem soils. Finer texturedsoils, even with high clay content if the soil is consid-ered dispersed, may present clogging problems inaddition to being difficult to drain. Envelope materialsplaced around subsurface drains (drain envelopes)have both hydraulic and mechanical functions(Dierickx 1992). The protection and stabilizing of thesurrounding soil material should be the planned objec-tive as it is not the filter envelope that fails, but thestructure of the surrounding soil (Stuyt 1992b). Morecomplete information on drain envelopes is in theUrban Subsurface Drainage Manual (ASCE 1998).

(a) Drain envelope materials

Drain envelope materials used to protect subsurfacedrains include almost all permeable porous materialsthat are economically available in large quantities.Based on the composition of the substances used, theycan be divided into three general categories: mineral,organic, and geotextile envelope materials. Mineralenvelopes consist of coarse sand, fine gravel, and glassfiber membranes that are applied while installing thedrain pipe. Organic envelopes include prewrappedloose plant materials, fibers, chips, or granules. Syn-thetic materials are geotextile fabrics specifically

manufactured for use in drainage and soil stabilization.Drain envelope materials are most effective whenplaced completely around the pipe. General drainenvelope recommendations are summarized in figure14–38.

The practice of blinding or covering subsurface drainswith a layer of topsoil before backfilling the trenchactually provides many humid area drains with perme-able envelope material. Humid area surface soils tendto have a well developed, stable, and permeable struc-ture that functions well as a drain envelope. In strati-fied soils, drains are blinded by shaving the coarsesttextured materials in the soil profile down over thepipe.

(1) Sand-gravel

Traditionally, the most common and widely used drainenvelope that also satisfies the definition of filterenvelope material is graded coarse sand and finegravel. The envelope material may be pit run coarsesand and fine gravel containing a minimum of fines.Properly designed or selected sand-gravel drain enve-lopes can fulfill all the mechanical, soil stabilizing, andhydraulic functions of a filter envelope. Figure14–39 shows typical bedding or sand-gravel envelopeinstallations. One example uses an impermeable sheetor geotextile filter. This is used to reduce costs wheresand and gravel envelope materials are expensive.

(2) Organic material

The service life and suitability of organic materials asdrain envelopes for subsurface drains cannot bepredicted with certainty. Organic matter placed as adrain envelope may also affect chemical reactions inthe soil that result in biochemical clogging problems.Where ochre clogging of drains is expected, organicmatter should be used with caution.

(3) Synthetic fiber materials

In the United States during the 1970's, several dozeninstallations of thin filter envelopes of fiberglass andspun bonded nylon were monitored with the assis-tance of the NRCS (SCS at that time). The drain sys-tems typically used 4-inch (100 mm) corrugated poly-ethylene tubing (CPE). The fiberglass membrane usedin these installations did not successfully span thecorrugations of the tubing while the spun bondednylon did. As a result the fabricator of fiberglassadopted the use of spun bonded nylon. Figure 14–38was issued to the field offices at that time to provide


Chapter 14



14–72

Figure 14–38 Filter envelope recommendation relative to soil texture (source USDA)

Perce

nt siltP

erc

ent

clay

Percent sand

Silty clayloam

Sandy loam

Sandy clay loam

Sandyclay

Sand

Clay loam

Loam

Loam sand

Silt loam

Silt

Silty clay

Clay

50 10 5 2 1 0.5 0.42

Grain size in millimeters

0.25 0.1 0.05 0.02 0.01 0.005 0.002 0.001100

3 2 1 1/2 3/4 1/2 3/81 4 10 20U.S. standards sieve numbers

Comparison of particle size scales

Sieve openings in inches

USDA Gravel Silt

Silt

V. fineFineMed.CoarseV. crs

Clay

ClaySand

UNIFIED(USCS)

AASHO

Gravel

Gravel or stone Sand Silt-clay

Coarse Fine Coarse Medium Fine

Coarse FineFineCoarse Medium

Silt or claySand

40 60 200

Filter envelopesare typically notneeded

Sand-gravel filterenvelope recommended

Filter envelope needed(Sand and gravel or geotextile)

100

10

20

30

40

50

60

70

80

90

100

10

20

30

40

50

60

70

80

90

100

90

80

70

60

50

40

30

20

10


14–73



guidance on the application of the two major types offilter envelopes, which were sand and gravel or thinspun bonded nylon. The figure has been updated toreflect recommendations for geotextiles rather thanjust nylon.

(4) Prewrapped loose materials

Prewrapped loose materials (PLM) used for drainenvelopes have a permeable structure consisting ofloose, randomly oriented yarns, fibers, filaments,grains, granules, or beads surrounding a corrugatedplastic drain pipe. These materials are assembled withthe pipe at the time of manufacture as a permeablehydraulic envelope of uniform thickness held in placeby twines or netting. The voluminous materials in-volved are either organic or geotextile.

(5) Prewrapped geotextiles

A geotextile is a permeable, polymeric material thatmay be woven, nonwoven, or knitted. Materials knownas geotextiles are widely used as pre-wrapped syn-thetic drain envelopes. Geotextiles are made of polyes-ter, polypropylene, polyamide, polystyrene, and nylon.

Geotextiles differ widely in fiber size or weight,smoothness, and weave density. No single geotextile issuitable as a drain envelope for all problem soils. Thematerials vary in weight, opening size, fiber diameter,thickness, and uniformity. The geotextiles are com-monly wrapped on the corrugated plastic drain pipe inthe production plant. The finished product must besufficiently strong to withstand normal handling that ispart of the construction and installation process.

Figure 14–39 Typical bedding or envelope installations

Ground surface

Trench width Trench widthTrench width

Backfill Backfill Backfill

Minimum

Minimum

Granular soilMin.

6"

Sand and gravelBedding

Bottom of trench

Min.4"

Min.3"

Sand and gravelFilter envelope

Bottom of trenchSand and gravelfilter envelope

3" 3"

Granularsoil

Impermeablesheet or geotextilefilter

Min.6"

Min.3 "

3" 3"

3" 3"


Chapter 14



14–74

(b) Principles of drain envelopedesign

(1) Exit gradients in soil near drains

As water approaches a subsurface drain, the flowvelocity increases as a result of flow convergence. Theincreased velocity is related to an increase in hydraulicgradient. The hydraulic gradient close to the drain mayexceed unity resulting in soil instability. Using a graveldrain envelope increases the apparent diameter of thedrain and, therefore, substantially decreases the exitgradient at the soil and drain envelope interface. Amajor reason for using a filter envelope is to reducethe hydraulic gradient at the soil and envelope inter-face, which acts to stabilize the soil in the proximity ofthe drain system.

(2) Hydraulic failure gradient

The hydraulic failure gradient is the change in hydrau-lic head per unit distance that results in soil instability,generally a gradient exceeding unity. As long as theflow rate (and the associated hydraulic gradient) in asoil is low, no soil particle movement occurs. If thevelocity of waterflow through the soil toward drains iskept below the hydraulic failure gradient, no failure ofthe drain and drain envelope system should occur.

To reduce the hydraulic gradients in the soil near thedrain:

1. Increase the effective diameter of the drain byusing a hydraulic envelope (i.e., gravel).

2. Increase the perforation area of the drain.3. Reduce the drain depth and spacing to decrease

the possible magnitude of the gradient.4. Use a geotextile having innerflow characteristics

to make the full surface of the corrugated drainpipe permeable. If the geotextile does not haveinnerflow characteristics, perforations in everycorrugation should be required (Willardson andWalker 1979, Salem and Willardson 1992).

If a soil has a high hydraulic failure gradient, a drainenvelope may not be necessary. Many humid area soilsdo not require use of a drain envelope. If the draintubes have an opening or perforation area from 1 to2.5 square inches per foot, the drain functions wellwithout sedimentation problems in structurally stablesoils. In some areas, criteria based on the soil claycontent and type is used to determine whether a filterenvelope is required. Such criteria are based on localexperience and field observations.

(c) Design of drain envelopes

(1) Sand-gravel filter envelope design

The general procedure for designing a sand-gravelfilter envelope for a given soil is:

• Make a mechanical analyses of both the soil andthe proposed filter envelope material.

• Compare the two particle size distributioncurves.

• Use criteria to determine whether the filterenvelope material is satisfactory.

The criteria include:• The D15 (defined below) size of the filter material

should be at least 4 times the diameter of the d15of the base material. (This would make the filtermaterial roughly more than 10 times more per-meable as the base material.)

• The D15 of filter material should not be morethan 4 times larger than the d85 of the base mate-rial. (This prevents the fine particles of the basematerial from washing through the filter mate-rial.)

The following gradation limits are recommended:• Upper limit of D100 is 38 mm (1.5 inches).• Upper limit of D15 is the larger of 7 times d85 or

0.6 mm.• Lower limit of D15 is the larger of 4 times d15 or

0.2 mm.• Lower limit of D5 is 0.075 mm (number 200

sieve).

D100 represents the particle size in the filter materialfor which 100 percent, by weight, of the soil particlesare finer (similarly for D15 and D5). The d85 and d15represent the particle size in the surrounding basematerial for which 85 percent and 15 percent, byweight, of the soil particles are finer. In the case ofdrainage, the base material is the soil.

Procedures for determining filter gradation designlimits are found in NEH, Part 633, Chapter 26, Grada-tion Design of Sand and Gravel Filters.

Research on filter envelopes show that:• If a filter envelope does not fail with the initial

flow of water, it is probably permanently safe.• The size ratios are critical.• Materials with a D15/d85 ratio greater than nine

always fail.


14–75



• Well graded materials are more successful thanuniform sized materials.

• A well-graded gravelly sand is an excellent filteror filter envelope for very uniform silt or fineuniform sand.

• It is not necessary for the grading curve of thefilter envelope to be roughly the same shape asthe grading curve of the soil.

(2) Sand-gravel hydraulic envelope design

The criteria for a sand-gravel hydraulic envelope isless restrictive than for a sand-gravel filter envelope asfollows:

• Upper limit of D100 is 38 mm (1.5 inches).• Upper limit of D30 is 0.25 mm (number 60 sieve).• Lower limit of D5 is 0.075 mm (number 200

sieve).

Pit run coarse sand and fine gravel containing a mini-mum of fines often meet this criteria.

Sand gradations used for concrete as specified byASTM C-33 (fine aggregate) or AASHTO M 6-65 willsatisfy these hydraulic envelope criteria and will meetthe filter envelope requirements for most soils.

(3) Geotextile filter envelope design

In filter envelope applications, the geotextile mustphysically survive installation, allow adequate flow ofwater, and basically retain the soil on its hydraulicallyupstream side. Both adequate flow capacity (requiringan open geotextile structure) and soil retention (re-quiring a tight geotextile structure) are required simul-taneously. Therefore, critical geotextile parameters forfilter envelope applications are permittivity, survivabil-ity, and soil retention.

Permittivity—Unrestricted flow of water through thegeotextile is essential. Therefore, the flow capacity(permittivity) of the geotextile should be much greaterthan the flow capacity of the soil, typically 10 timesgreater or more. Permittivity values in excess of 1 unitper second (ft3/ft x ft2 x sec) are typically required andare determined according to ASTM D 4491 (1992).Permittivity, not permeability, should be specifiedbecause permeability measures the rate at whichwater will pass through the geotextile under a givenhead without regard to geotextile thickness.

Survivability—The geotextile must survive installa-tion without being damaged. AASHTO DesignationM288-90 (1990) includes recommendations on mini-mum physical strength properties for geotextile surviv-ability.

Soil retention—The geotextile must prevent exces-sive loss of fines (soil piping) from the upstream side.This is accomplished by checking the coarser soilparticles, which in turn retain the finer soil particles.Numerous approaches can accomplish soil retention,all of which use the soil particle grading characteris-tics and compare them to the apparent opening size(AOS) of the geotextile. AOS is the approximate larg-est particle that will effectively pass through a geo-textile and is determined by glass ball dry sieving(ASTM D 4751). Both AOS and O95, effective openingsize of the envelope pore, represent the apparentopening size in millimeters (mm) or sieve size.

The simplest method uses the percentage of fines (soilpassing the No. 200 sieve). AASHTO Designation M288-90 recommends the following retention criteria:

• Soil ≤ 50% passing the No. 200 sieveAOS of the geotextile No. 30 sieve(O95 < 0.59 mm)

• Soil > 50% passing the No. 200 sieveAOS of the geotextile No. 50 sieve(O95 < 0.297 mm)

These criteria should meet most drainage require-ments.

For more critical applications, table 14–8 recommendsO95 values based on relative density (DR), coefficientof uniformity (CU), and average particle size (d50). Theterms are defined as:

d50 = soil particle size corresponding to 50% finerCU = coefficient of uniformity = d60/d10d60 = soil particle size corresponding to 60% finerd10 = soil particle size corresponding to 10% finerAOS = O95 apparent opening size of geotextile ex-

pressed in millimeters or sieve size

Because the three approaches are restrictive in differ-ent degrees, choose one of the three approaches intable 14–8 (Koener 1986) based on the critical natureof the application.


Chapter 14



14–76

Clogging—Once the geotextile is designed, the nextquestion is “Will it clog?” Obviously, some soil par-ticles will embed themselves within the geotextilestructure; therefore, the question really is if thegeotextile will completely clog such that the liquidflow through it will be shut off before the soil matrixstabilizes. Laboratory tests, such as the Gradient RatioTest given in ASTM D 5101, are available to answerthis question.

Another approach is to simply avoid situations knownto lead to severe clogging problems. Three conditionsare necessary for a high likelihood of completegeotextile clogging (Koerner 1986):

• cohesionless sands and silts• gap graded particle size distribution• high hydraulic gradients

If these three conditions are present, use of geotextilesshould be avoided. A gravel or sand gravel filter enve-lope can be used.

(4) Prewrapped loose material filter envelope

design

Subsurface drain filter envelopes using prewrappedloose materials may be characterized by pore sizedistribution, filter thickness, and hydraulic conductiv-ity. Filter thickness and pore size distribution aredetermined for a natural, compressed condition, butfor prewrapped loose materials, the hydraulic conduc-tivity is generally so high that it has no bearing onselection of a filter envelope.

Retention criterion defines the capability of a filterenvelope to retain soil particles and is expressed as aratio of a characteristic pore opening size of the filterenvelope to a particle size of the soil granular material

in contact with the envelope. The characteristic poreopening size of the envelope material is the O90 value.

Depending on the pore size index O90, prewrappedloose materials are classed into three groups, withrecommendations as follows:

Label Class Pore size index range

PLM-XF: XF extra fine 0.1 mm < O90

PLM-F: F fine 0.3 mm < O90 < 0.6 mm

PLM-S: S standard 0.6 mm < O90 < 1.1 mm

Coil ends are labeled with tape imprinted with identifi-cation PLM-XF, PLM-F, or PLM-S.

Minimum thickness is required to guarantee a homoge-neous filter envelope. In addition to these O90 ranges,the following minimum filter envelope thicknesses arerequired regardless of the O90 range involved.

Material Mimimum filter envelope thickness

synthetic, fibrous 3 mm (e.g., poly-propylenefibers)

synthetic, granular 8 mm (e.g., polystyrene beads)

organic, fibrous 4 mm (e.g., coconut fibers )

organic granular not yet fixed (e.g., wood chips,sawdust)

(5) Combination gravel and geotextile filter

envelope design

Properly graded gravel or sand gravel material neededfor a satisfactory filter envelope may not be readilyavailable, or the cost of handling, including transporta-tion, may be prohibitive. Also the soil material in theproximity of the subsurface drain may be either diffi-cult or impossible to stabilize with economicallyavailable geotextile materials alone. The opportunity

Table 14–8 Relationships used to obtain fabric opening size to protect against excessive loss of fines during filtration(source: Giuard 1982)

Relative density of base material 1 < CU < 3 CU > 3

Loose (DR < 50%) O95 < (CU)(d50) O95 < (9d50)/CU

Intermediate (50% < DR > 80%) O95 < 1.5(CU)(d50) O95 < (13.5d50)/CU

Dense (DR > 80%) O95 < 2(CU)(d50) O95 < (18d50)/CU


14–77



to use gravel and geotextile material together for apractical and economic filter envelope should beconsidered. On many sites the most feasible filterenvelope can be designed and constructed from areadily available pit run sand or gravel that would notbe satisfactory alone, but can be used along with aneconomical geotextile to satisfy the filter envelopedesign requirements.

A common application incorporates a thin geotextilematerial adjacent to the pipe with the pit run sand orgap graded gravel surrounding the geotextile. Thecombination system should be designed using theappropriate criteria given above for each of the filterenvelope materials acting independently, resulting intwo filter envelopes working in unison. The geotextileis designed to retain the sand or gravel envelopematerial. The thickness of the sand or gravel envelopeshould be designed to increase the effective radius ofthe combination drain envelope to the point that theresulting hydraulic gradient in the soil adjacent to theenvelope is reduced satisfactorily.

The configuration may be reversed with the geotextileoutside the gravel envelope and adjacent to the soilbeing protected. For this combination the gravelshould be coarse enough that migration to or into thepipe is not a concern. The key factor is to increase thearea of the geotextile in contact with the soil to satis-factorily reduce the flow velocity associated with theexit gradient. This configuration uses more geotextileper linear length of drain than the combination havingthe geotextile adjacent to the pipe, but in confinedareas it may be the most cost effective.

650.1425 Materials

Common subsurface drainpipe materials includeplastic, concrete, metal, and clay. Standards are con-tinually updated by standards organizations, such asASTM and AASHTO, so pipe materials meeting recog-nized standards adopted by these types of organiza-tions should always be used. Current standards thatcan be considered follow.

(a) Concrete pipe

Reinforced and nonreinforced concrete pipes are usedfor gravity flow systems. Concrete fittings and appur-tenances, such as wyes, tees, and manhole sections,are generally available. A number of jointing methodsare available depending on the tightness required.Concrete pipe is specified by diameter, type of joint,and D-load strength or reinforcement requirements.

The product should be manufactured in accordancewith one or more of the following standard specifica-tions:

ASTM C14/AASHTO M86 (ASTM C14M/AASHTO

M86M)—Concrete Sewer, Storm Drain and CulvertPipe. These specifications cover nonreinforced con-crete pipe from 4- through 36-inch (100 through 900mm) diameters in Class 1, 2, and 3 strengths.

ASTM C76/AASHTO M170 (ASTM C76M/

AASHTO M170M)—Reinforced Concrete Culvert,Storm Drain, and Sewer Pipe. These specificationscover reinforced concrete pipe in five standardstrengths: Class I in 60- through 144-inch diameters,and Class II, III, IV, and V in 12- through 144-inch (300through 3,600 mm) diameters.

ASTM C118 (ASTM C118M)—Concrete Pipe forIrrigation or Drainage. These specifications coverconcrete pipe to be used for the conveyance of waterunder low hydrostatic heads, generally not exceeding25 feet (75 kPa), and for drainage in sizes from 4-through 24-inch (100 through 600 mm) diameters instandard and heavy-duty strengths.


Chapter 14



14–78

ASTM C361 (ASTM C361M)—Reinforced ConcreteLow-Head Pressure Pipe. These specifications coverreinforced concrete pipe with low internal hydrostaticheads generally not exceeding 125 feet (375 kPa) insizes from 12- through 108-inch (100 through 2700 mm)diameters.


AASHTO M178M)—Concrete Drain Tile. Thesespecifications cover nonreinforced concrete drain tilewith internal diameters from 4 through 24inches (100through 600 mm) for standard quality and 4 through36 inches (100 through 900 mm) for extra-quality,heavy-duty extra-quality, and special quality concretedrain tile.


AASHTO M175M)—Perforated Concrete Pipe. Thesespecifications cover perforated concrete pipe intendedto be used for underdrainage in 4-inch (100 mm) andlarger diameters.

ASTM C505 (ASTM C505M)—NonreinforcedConcrete Irrigation Pipe with Rubber Gasket Joints.These specifications cover pipe to be used for theconveyance of water with working pressures up to 30feet (90 kPa) of head.


AASHTO M206M)—Reinforced Concrete ArchCulvert, Storm Drain, and Sewer Pipe. These specifica-tions cover reinforced concrete arch pipe in sizes from15- through 132-inch (375 through 3,300 mm) equiva-lent circular diameters.


AASHTO M207M)—Reinforced Concrete EllipticalCulvert, Storm Drain, and Sewer Pipe. These specifica-tions cover reinforced elliptical concrete pipe in fivestandard classes of horizontal elliptical. 18- through144-inch (450 through 3,600 mm) in equivalent circulardiameter, and five standard classes of vertical ellipti-cal, 36- through 144-inch (900 through 3,600 mm) inequivalent circular diameter.


AASHTO M176M—Porous Concrete Pipe. Thesespecifications cover porous nonreinforced concretepipe in sizes from 4- through 24-inch (100 through 600mm) diameters and in two strength classes.


AASHTO M242M)—Reinforced Concrete D-LoadCulvert, Storm Drain, and Sewer Pipe. These specifica-tions cover acceptance of pipe design and productionbased on the D-load concept and statistical samplingtechniques.


AASHTO M259M)—Precast Reinforced ConcreteBox Sections for Culverts, Storm Drains, and Sewers.These specifications cover precast reinforced concretebox sections from 3-foot (900 mm) span by 2-foot (600mm) rise to 12-foot (3,600 mm) span by 12-foot (3,600mm) rise.


AASHTO M273M)—Precast Reinforced ConcreteBox Sections for Culverts, Storm Drains, and Sewerswith less than 2 feet (0.6 m) of Cover Subject to High-way Loading. These specifications cover box sectionswith less than 2 feet (0.6 m) of earth cover in sizesfrom 3-foot (900 mm) span by 2-foot (600 mm) rise to12-foot span (3600 mm) by 12-foot (3600 mm) rise.

ASTM C985 (ASTM C985M)—NonreinforcedConcrete Specified Strength Culvert, Storm Drain, andSewer Pipe. These specifications cover acceptance ofnonreinforced concrete pipe design and productionbased on specified strengths and statistical samplingtechniques.

(b) Thermoplastic pipe

Thermoplastic pipe materials include high densitypolyethylene (HDPE), poly (vinyl) chloride (PVC), andacrylonitrile-butadiene-styrene (ABS). Thermoplasticpipes are produced in a variety of shapes and dimen-sions.

(1) High density polyethylene (HDPE) pipe

HDPE pipe is available for gravity and low-pressureflow systems. The application will dictate the qualityof the joining system used. Fittings are widely avail-able and can be adapted to many other products.HDPE pipe should be manufactured according to oneor more of the following standard specifications:

AASHTO M252—Corrugated Polyethylene DrainageTubing. This specification covers corrugated polyeth-ylene tubing from 3- through 10-inch diameter (75


14–79



through 250 mm), couplings, and fittings for use insurface and subsurface drainage applications. Provi-sions are included for corrugated and smooth interiorpipe.

AASHTO M294—Corrugated Polyethylene Pipe, 12-to 48-inch Diameter. This specification covers therequirements of corrugated polyethylene pipe, cou-plings, and fittings for use in storm sewers and subsur-face drainage systems. Provisions are included forboth corrugated and smooth interior pipe.

AASHTO MP7-95—Corrugated Polyethylene Pipe 54and 60-inch Diameter. This specification covers therequirements of corrugated polyethylene pipe, cou-plings, and fittings for use in storm sewers and subsur-face drainage systems. Provisions are included forsmooth interior pipe.

ASTM F405—Corrugated Polyethylene Pipe andFittings. This specification covers pipe with 3- through6-inch (75 through 150 mm) diameter. This product iscommonly used for subsurface and surface drainageinstallations.

ASTM F667—Large Diameter Corrugated Polyethyl-ene Pipe and Fittings. This specification covers pipesfrom 8- through 24-inch (200 through 600 mm) diam-eters commonly used for surface and subsurfacedrainage.

ASTM F810—Smoothwall Polyethylene (PE) Pipe forUse in Drainage and Waste Disposal AbsorptionFields. This specification covers smoothwall HDPEpipe, including co-extruded, perforated andnonperforated, from 3- through 6-inch (75 through 150mm) diameter.

ASTM F892—Polyethylene (PE) Corrugated PipeWith a Smooth Interior and Fittings. This specificationcovers corrugated PE pipe 4 inches (100 mm) in diam-eter.

ASTM F894—Polyethylene (PE) Large DiameterProfile Wall Sewer and Drain Pipe. The specificationcovers profile wall PE pipe from 18- to 120-inch (450 to3,000 mm) diameter for low pressure and gravity flowapplications.

(2) Polyvinyl chloride (PVC) pipe

PVC pipe is used for gravity and low pressure flowsystems. PVC composite pipe is a combination of aPVC pipe with a series of truss annuli. It is filled withlightweight portland cement concrete or other suchmaterial. PVC fittings are widely available. PVC pipeshould be manufactured in accordance with one ormore of the following standard specifications:

AASHTO M304—Poly (Vinyl Chloride) (PVC) RibbedDrain Pipe and Fittings Based on Controlled InsideDiameter. This specification covers 18- to 48-inchdiameter ribbed PVC pipe.

ASTM D2680/AASHTO M264—Acrylonitrile-Butadi-ene-Styrene (ABS) and Poly (Vinyl Chloride) (PVC)Composite Sewer Piping. These specifications coverABS or PVC composite pipe, fittings, and a joiningsystem for storm drain systems in 6- through 15-inch(150 through 375 mm) diameter.

ASTM D2729—Poly (Vinyl Chloride) (PVC) SewerPipe and Fittings. This specification covers PVC pipeand fittings for sewer and drain pipe from 2-inch (50mm) to 6-inch (150 mm) diameters.

ASTM D3034—Type PSM Poly (Vinyl Chloride)(PVC) Sewer Pipe and Fittings. This specificationcovers PVC pipe and fittings from 4- through 15-inch(100 to 375 mm) diameters.

ASTM F679—Poly (Vinyl Chloride) (PVC) Large-Diameter Plastic Gravity Sewer Pipe and Fittings. Thisspecification covers PVC gravity sewer pipe andfittings from 18- through 36-inch (450 through 900 mm)diameters with integral bell elastomeric seal joints andsmooth inner walls.

ASTM F758—Smooth-Wall Poly (Vinyl Chloride)(PVC) Plastic Underdrain Systems for Highways,Airports, and Similar Drainage. This specificationcovers PVC pipe and fittings for underdrains from 4-through 8-inch (100 through 200 mm) diameters withperforated or nonperforated walls for use in subsur-face drainage systems.

ASTM F789—Type PS-46 Poly (Vinyl Chloride) (PVC)Plastic Gravity Flow Sewer Pipe and Fittings. Thisspecification covers requirements for PVC gravitysewer pipe and fittings from 4- through 18-inch (100through 450 mm) diameters.


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ASTM F794—Poly (Vinyl Chloride) (PVC) ProfileGravity Sewer Pipe and Fittings Based on ControlledInside Diameter. This specification covers PVC pipeand fittings from 4 through 48 inches (200 through1200 mm) with integral bell and elastomeric sealjoints.

ASTM F949—Poly (Vinyl Chloride) (PVC) CorrugatedSewer Pipe With a Smooth Interior and Fittings. This speci-fication gives requirements for PVC pipe and fittings from 4-through 36-inch (100 through 900 mm) diameters withcorrugated outer wall and smooth inner wall.

(3) Acrylonitrile-butadiene-styrene (ABS)

pipe and ABS composite pipe

ABS and ABS composite pipe should be manufacturedin accordance with one of the following standard specifi-cations:

ASTM D2680/AASHTO M264—Acrylonitrile-Butadi-ene-Styrene (ABS) and Poly (Vinyl Chloride) (PVC)Composite Sewer Piping. These specifications coverABS or PVC composite pipe, fittings, and a joiningsystem for 4- to 15-inch (100 to 375 mm) diameter.

ASTM D2751—Acrylonitrile-Butadiene-Styrene(ABS) Sewer Pipe and Fittings. This specificationcovers ABS pipe and fittings from 3- through 12-inch(75 through 300 mm) diameter.

(c) Metal pipe

Corrugated metal pipe is fabricated from corrugatedsteel or aluminum sheets or coils. Corrugated metalpipe is specified by size, shape, wall profile, gauge orwall thickness, and coating or lining. Appurtenancesincluding tees, wyes, elbows, and manholes are avail-able. Corrugated metal pipe should be manufacturedin accordance with one or more of the followingstandard specifications:

AASHTO M190—Bituminous Coated Corrugated MetalCulvert Pipe. This specification covers characteristics ofbituminous coated corrugated metal and pipe archesmeeting AASHTO M36.

ASTM A760/AASHTO M36—Corrugated Steel Pipe,Metallic-Coated for Sewers and Drains. These specifi-cations cover metallic-coated corrugated steel pipefrom 4- through 144-inch (100 to 3600 mm) diameter.

ASTM A762/AASHTO M245—Corrugated SteelPipe, Polymer Precoated for Sewers and Drains. Thesespecifications cover polymer precoated corrugatedsteel pipe from 4- through 144-inch (100 through 3600mm) diameter.

ASTM B745/AASHTO M196—Corrugated AluminumPipe for Sewers and Drains. These specifications covercorrugated aluminum pipe from 4- through 144-inch(100 through 3600 mm) diameter.

(d) Vitrified clay pipe (VCP)

VCP is manufactured from clays and shales and vitri-fied at high temperatures. VCP is available in severalstrength classifications, and is specified by nominalpipe diameter, strength and type of joint. The productshould be manufactured in accordance with one ormore of the following standard specifications:

ASTM C4/AASHTO M179—Clay Drain Tile. Thesespecifications cover drain tile from 4- through 30-inch(100 through 750 mm) diameter in standard, extraquality, and heavy duty strengths.

ASTM C498—Clay Drain Tile, Perforated. This speci-fication covers perforated drain tile from 4- through18-inch (100 through 450 mm) diameters in standard,extra quality, heavy duty, and extra strength.

ASTM C700/M65—Vitrified Clay Pipe, Extra-Strength, Standard Strength, and Perforated. Thesespecifications cover perforated and nonperforatedpipe from 3- through 42-inch (75 through 1,050 mm)diameters in extra strength and standard strength.

(e) Other materials and products

Geocomposites, geomembranes, geotextiles, aggre-gates, wick drains, and pump and lift stations may notbe covered by conservation practice standards. Therequirements for such materials and products must bespecified in construction contract documents by anengineer. Contact individual manufacturers for moredetail on specific products.


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650.1426 Appurtenances

(a) Surface inlets

Surface inlets should be used in low areas wheresurface drainage otherwise cannot be provided. Theymust be properly constructed to prevent washouts andsilting of the line. Surface inlets should be avoidedwherever possible. If silt is a hazard, place a silt trap(fig. 14–40) at a convenient location immediatelydownstream from the inlet or use a blind inlet (fig.14–41). Blind inlets allow entry of surface water fromsmall ponded areas into the drain without an openriser. The sand-gravel material for the porous mediummust be appropriately designed to keep out sedimentand prevent piping of base soil material, yet providefree water movement into the drain.

(b) Junction boxes

Junction boxes should be used where two or moremain or submain drains join or where several lateralsjoin at different elevations. If the junction is in a culti-vated field, the box should be constructed so that thetop is at least 18 inches below the surface of theground. It can be capped and covered and its positionreferenced for future relocation (fig. 14–40).

(c) Vents and relief wells

Vents, or breathers, are used to alleviate vacuum ornegative pressure in the line. Breathers should be usedwhere the line changes abruptly from a flat section toa steep section. Permanent fence crossings are goodlocations for installation. Relief wells relieve pressurein the line. They should be installed where steepsections change to flat sections unless the flattersection has about 25 percent greater capacity than thesteeper section. They should be used on lines thathave surface inlets, particularly when such inlets arelarge (fig. 14–42).

Figure 14–40 Junction box and silt trap

Out

Vari

able

but

not

less

tha

n 5'

-0"

Min

imum

-2'

-0"

2'-0"Minimum-

7"Va

riab

le

Use solid cover of iron or precastreinforced concrete

Vari

able

In

Con

stru

ct w

alls

of

conc

rete

, bri

ck,

bloc

k or

cur

ved

segm

ent,

con

cret

e or

sew

er t

ile

Always use a concrete base

Silt trap

��Ground surface

Precast reinforcedconcrete cover

Con

stru

ct w

alls

of

conc

rete

, bri

ck,

bloc

k or

cur

ved

segm

ent

Incoming drain

Out

Always use a concrete base

7"Va

riab

le4"

12"

Minimum 2'-0" square

Junction box


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Figure 14–41 Blind surface inlet

Minimum of 6'-0"Width gap between tiles

Use 1/4" to 1/2" gap

Slope 1:1

Batter side2" per 1" of depth

Elevation

Cross-Section

Ground surface

Ground surface

Top of drain

Minimum width

5" tile — 16"6" tile — 20"8" tile — 25"10" tile — 30"12" tile — 36"

Bottom of drain trench

Drain

Min.12"

Cobblestone, old brick,and tile bats graded upwardfrom coarse to fine

Broken tile, coarsegravel, or crushed rock

Porous topsoil for cultivated fieldsPea gravel, small stone or coarse sandmay be used in other locations

6"


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(d) Outlet protection

Where drains outlet into an open ditch, the end of thedrainage line should be protected. If surface waterenters the outlet at the same location as the drain,some type of structure, such as a headwall or earthberm, is needed over the outlet. Where there is nosurface water, the most practical and economicaloutlet is a section of rigid pipe. The pipe should con-form to the requirements shown in figure 14–43.

Figure 14–42 Vents or relief wells

1 foot(typical)

Tile or CPP

Figure 14–44 Outlet pipe protection

Figure 14–43 Pipe outlet

Where burning to control weeds may occur, the pipeshould be fireproof. A swing gate or some type ofgrating or coarse screen should be used on all outletsto exclude rodents and other small animals (fig. 14–44). The screen mesh should not be less than 1 inch.Swing gates, rather than fixed screens or grates,should be used where surface water enters a systemdirectly.

Drain

Corrugated metal pipeor other rigid, nonperforated pipe(fireproof)

1' min.

Low water flow

Provide swinginggate or iron gratingto exclude rodents


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650.1427 Drain installation

(a) Inspection of materials

All materials of a subsurface drainage system shouldbe inspected before the system is installed. Materialsshould be satisfactory for the intended use and shouldmeet standards and specifications. Any defective ordamaged clay or concrete drain tile should be rejected,and defective or damaged sections of plastic tubingshould be removed. The perforations in the plastictubing must be the proper size. Check pipe for thespecification to which it is manufactured (ASTM,AASHTO) as well as NRCS Practice Standard.

Figure 14–45 Laser grade control

(b) Storage of materials

Drainage materials should be protected from damageduring handling and storage. More precautions shouldbe taken to protect plastic tubing. End caps can beused if rodents are a problem. Tubing that has filterwrap should be covered. Because tubing can beharmed by excessive exposure to ultraviolet rays, itmust be protected from long exposures to sunlight.Coils of tubing should be stacked no more than fourhigh, and reels should not be stacked.

(c) Staking

Presently, field staking is at a minimum because mostinstallations are done with laser controlled equipment(fig. 14–45).


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Figure 14–46 Drain crossings and outlets

Road right-of-way

Roadway

RoadditchRoad

ditch

Use rigid pipe, extra strength sewer tile with joints cemented, or drain tile encased in 4 inches of concrete.

Waterway or drain

Use rigid pipe, extra strength sewer tile with joints cemented, or drain tile encased in 4 inches of concrete.

Drain crossing under waterway or drain

Drain crossing under road

Pipe drain

Pipe drain

(d) Utilities

Special caution must be taken when trench or trench-less work is performed because of the danger if utili-ties are too near. Many jurisdictions have systems inplace that require notification and location of utilitylines before any excavations. Most require advancenotification when excavation is to take place and havespecial telephone numbers for notification. Somestates and metropolitan areas use a single telephonecontact to alert local utility companies of pendingconstruction activities (ASCE 1993).

Utilities should be located when preparing plans, andprocedures are needed to assure contractors havenoted the utilities and have taken the necessary pre-cautions. The location of all underground utilities andstructures should be indicated on construction plans

or drawings. Safety is the primary concern, but inter-ruption of services can create tremendous economicproblems. Whether underground utilities are shown onthe plans or not, the contractor is required by OSHAand possibly local or state law to contact local utilitycompanies to ascertain if there is a potential for in-volvement.

(e) Crossing waterways and roads

Special precautions should be taken where drains areplaced under waterways or roads. Figure 14–46 pro-vides some guidance for these crossings, but, if excep-tionally heavy trucks and equipment are expected, anengineer should be consulted.


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Figure 14–46 Drain crossings and outlets (Continued)

Pipe drainRigid pipe outlet

1

2

3

Outletdrain Less than 2 feet of cover

To provide minimum coverplace fill over pipe drain

4' min

Pipe drain

Use rigid pipe through section wherecover over drain is less than 2 feet

Less than 2 feet of cover

Pipe drain

Metal pipe outlet

Excavate an open ditch back to where cover over the pipe drain is more than 2 feet

Pipe drain

Profile Cross Section of Fill

Methods for handling shallow depths at drain outlet

81

81


14–87



(f) Shaping the trench bottom

The bottom of the drain trench should be shaped sothat a fourth or more of the drain's circumference ison solid ground. Trenching machines shape the trenchproperly as a part of the trenching operation. Backhoebuckets can also be modified to provide a propershape. Where drains are laid through unstable pocketsof soil, one of the following materials should be placedin the bottom of the trench to support the drain:

• stable soil• crushed rock• sand/gravel bedding

For corrugated plastic pipe, a specially-shaped groovemust be made in the trench bottom if the design doesnot call for a gravel envelope. The groove shape can bea semicircle, trapezoid, or a 90-degree V. A 90-degreeV-groove of sufficient depth is recommended for 3- to6-inch pipe; however, if the pipe is installed on a steepgrade, the bottom of the trench should be shaped to fitthe pipe closely (fig. 14–47).

(g) Laying corrugated plastic pipe(CPP)

Trenching machines or drainage plows are used toinstall most CPP. Any stretch that occurs duringinstallation decreases the pipe strength somewhat andmay pull perforations open wider than is desirable.

The amount of stretch that occurs during installationdepends on the temperature of the CPP at the time it isinstalled, the amount and duration of drag that occurswhen the CPP is fed through the installation equip-ment, and the stretch resistance of the pipe. The use ofa power feeder is recommended for all sizes of CPP.Stretch, which is expressed as a percentage of length,should not exceed 5 percent.

(h) Drain envelope installation

(1) Drain filter envelopes

The best quality filter envelope material cannot com-pensate for improper installation, especially in fine,weakly structured soils that are saturated. Reliabledrain envelope material will only be successful ifinstalled under favorable physical soil conditions.General excess wetness of a soil may adversely affectstructural stability, hence the soil manipulation causedby the trenching operation while installing drains maydestroy the soil structure. This leads to soil slaking,enhanced risk of mineral clogging of filter envelopesand pipes, and a low hydraulic conductivity of the soilitself. Gravel filter envelopes tend to be less suscep-tible to poor installation conditions, but they can alsofail because of adverse conditions at the time of instal-lation. Geotextile filter envelopes are normallyprewrapped and have sufficient mechanical strengthto withstand the mechanical stresses of installation.Because of this, attention should be primarily onpreserving the hydraulic function.

Figure 14–47 Dimensions for a 90 degree V groove for corrugated plastic pipe

Insidediameter

Outsidediameter

Undisturbedtrench bottom

r r

X

Z

Y

Diameter(D)

34568

r(D/2)

1.52.02.53.04.0

X(0.707r)

1.0601.4141.7682.1212.828

Y(o.293r)

0.4390.5860.7320.8791.171

Z(0.414r)

0.6210.8281.0361.2421.657

Values are based on typical outside diameter, which is assumed to be 20 percent greater than inside diameter.

a


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14–88

The ideal condition for installation of subsurfacedrains is to place the drains in an unsaturated soil. Ifthe soil has a high water table that cannot be loweredbefore installation, every effort should be made topreserve the existing soil structure and to protect thedrain from trench wall failure. Adjusting the forwardspeed of the installation machine may help to limit thedestruction of soil structure. If the condition of theexcavated material is observed, it can be a guide to theproper machine speed. The machine should move fastenough to preserve the structure of the soil and notturn the excavated soil into a slurry. Simultaneous andinstantaneous backfilling can prevent trench wallfailure.

Drain plows have been developed that install drainswith synthetic and gravel drain envelopes. Plowed in(trenchless) drains avoid many of the problems oftrenched or backhoe excavated drain installation.Unfortunately, they present their own unique set ofproblems. They are limited to shallow depths andsmall pipe sizes, and may produce compaction aroundthe drain under certain soil texture and moistureconditions. Moreover rocky soils can be a problem forthis equipment.

(2) Sand-gravel drain envelopes

Most of the water entering a subsurface drain movesthrough openings in the sides and bottom of the drain,below the hydraulic gradeline inside the drain. Thehydraulic gradients that develop at the drain openingsare often high enough to cause an unstable conditionat the opening, and consequently piping of the soilmaterial may occur. The noncohesiveness of manysoils makes them particularly susceptible to move-ment when saturated. For these reasons, an adequateamount of filter envelope material is needed aroundthe drain pipe.

Where drains are laid by hand, a layer of drain enve-lope material is placed in the bottom of the trench andis leveled to the design grade before the drain is laid.The drain pipe is then put into place and covered withenvelope material to the required depth. The trench isthen backfilled with soil. Some trenching machines arefitted with two hoppers for placing drain envelopematerial under and over a drain on a continuous basis.One hopper near the digging device covers the trenchbottom with the required thickness of drain envelopematerial. The pipe is placed and the second hopper at

the rear of the trenching machine covers the pipe withdrain envelope material.

In a common variation of the two hopper design, thepipe is guided through an enclosed single gravel hop-per chute and emerges at the rear of the shield alongwith the gravel. In either case, the shield design iscritical. If gravel segregation occurs within the shield,an improper gradation results and often leads to drainfailure. Also, the design must be such that the pipe isnot subject to tension caused by friction between thegravel and the shield walls. Such tension results instretching the pipe beyond acceptable limits.

Drainage contractors have recently developed proce-dures for placing drain envelope material completelyaround a drain pipe in one operation using a singlehopper. Single hopper placement is used for both rigidand flexible pipes. The pipe within the machine issuspended above the bottom of the trench so thegranular envelope material can flow around the pipe.This single stage placement has resulted in materialeconomies since it is possible to make an approxi-mately concentric drain envelope by preshaping thetrench bottom. Drain plows that install flexible corru-gated plastic drain pipes with drain envelopes haveuniformly concentric envelope placement.

In unstable soil the drain pipe and drain envelope aresometimes displaced by soil movement before andduring backfilling. The sides of the open trench mayfall or slough causing lateral misalignment of thepipes. If the soil around or in the bottom of the trenchis saturated and unstable, it may move upward as afluid displacing the envelope material and pushing thepipe out of line. Simultaneous backfilling is particu-larly desirable in unstable soil conditions. Movementof saturated unstable soil may also cause puddling ofthe backfill material and plugging of the filter enve-lope, or any drain envelope material, during construc-tion. A slurry in the bottom of a trench generallycauses immediate and complete failure of syntheticdrain envelope material.

Protection of the drain envelope and drain systemimmediately following installation is important. Noheavy loads, mechanical or hydraulic, should be im-posed until the soil in the trench is consolidated. Theloose backfill material will settle naturally with time.Passage of a light weight vehicle wheel in the trench


14–89



speeds up the process, but care must be taken to avoidcrushing the drain pipe.

Application of irrigation water to unconsolidatedmaterial in the trenches to settle the backfill is apractice that should be done carefully. Muddy watermoving through the porous backfill material directlyinto the filter envelope under high hydraulic heads cancause plugging of the filter envelope material at thedrain openings. Such plugging reduces the effective-ness of the drain envelope. It may also result in sedi-mentation in the drain or even complete plugging ofthe filter envelope.

(3) Envelope thickness

One of the benefits of drain envelope placement is theincrease in permeability along the pipe that enableswater to flow more freely to the open joints or perfora-tions. The effect is similar to converting the pipe fromone with limited openings to one that is completelypermeable. This increased permeability can probablybe obtained with an envelope 0.5 inches thick. Theo-retically, corrugated pipes should be perforated inevery corrugation to reduce secondary convergence atthe openings.

Increasing the diameter of the drain envelope effec-tively reduces the waterflow velocity and exit gradientat the soil and envelope interface (Willardson andWalker 1979), thereby decreasing the probability ofsoil particle movement. If the permeable hydraulicenvelope material is considered to be an extension ofthe pipe diameter, then the thicker the envelope thebetter. Some practical limitations to increasing drainenvelope thickness include:

• The perimeter of the envelope through whichflow occurs increases as the first power of thediameter of the envelope, while the amount ofenvelope material required increases as thesquare of the diameter.

• Doubling the diameter of the envelope andconsequently decreasing the inflow velocity atthe soil and envelope interface by half wouldrequire four times the volume of envelope mate-rial.

Corrugated plastic drain pipes with close perforationspacing reduce the requirement for a hydraulic enve-lope material to transport water to widely spacedopenings that were common where 1- to 3-foot lengths

of rigid pipe were used for drainage. The practicalproblems of placement probably dictates a designminimum sand-gravel drain envelope thickness ofapproximately 3 inches. The principal reason for athicker envelope in a problem soil would be to reducethe exit gradient to a value below the hydraulic failuregradient of the soil and to nullify the effects of con-struction inconsistencies. Figure 14–39 illustratessand-gravel envelope placement recommendations.

(i) Alignment and joints

(1) Plastic pipe

Manufactured couplers should be used at all joints andfittings of corrugated plastic pipe, at all changes indirection where the centerline radius is less than threetimes the pipe diameter, at changes in diameter, and atthe end of the line. All connections must be compat-ible with the pipe. Where certain fittings are not avail-able, hand-cut connections are acceptable if they arereinforced with a cement mortar or other material thatmakes a strong, tight joint. The connection should notcreate a means of obstructing flow, catching debrisinside the conduit, or allowing soil to enter the line.

(2) Tile

Alignment in main and lateral drains should generallybe straight, and junction boxes should be used toaffect changes in direction. Y and T joints can be used.Manufactured connections are preferred, but chippedor fitted connections that are sealed may be installed ifmanufactured connections are not available.

Laterals should be connected to mains so that theircenterlines meet. Any curves in mains and lateralsshould have a radius of more that 50 feet. If gaps inexcess of 1/4 inch in clay soils or 1/8 inch in sandysoils occur in the outer side of a curved line, theyshould be covered with an impermeable material.

Joints between tile laid in straight or nearly straightlines should be about 1/8 inch wide unless the soil issandy. Tile laid in sandy soil should be butted to-gether. If gaps exceed 1/4 inch in clay soils or 1/8 inchin sand, they should be covered with broken tile battsor wrapped with impermeable material. In certain soilswhere experience shows that tile lines fill with sedi-ment within a few years, joints should be protected bywrapping or covering.


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14–90

(j) Safety and protection duringconstruction

At the end of each day's work, the end of the drainbeing placed should be completely closed to preventsmall animals or, in the event of rain, silt and debrisfrom entering the line. A wooden or metal plate orsome other device can be used. Upon completion ofthe line, the upper end of the drain should be closedtightly using a plate, end cap, or some other perma-nent material.

Contractors are responsible for construction sitesafety. Federal regulations covering safety for all typesof construction are published in the Safety and HealthRegulations for Construction under the Department ofLabor, Occupational Safety and Health Administration(OSHA). Many states, municipalities, and other localagencies have established codes and safety practicesregarding construction. These regulations apply tosubsurface drainage installation as well as all types ofconstruction, including alteration and repair work.Personnel and contractors associated with drainageinstallation should be thoroughly familiar with thesafety requirements and follow the required practices,procedures, and standards.

(k) Blinding and backfill

As soon as the drains are placed, they should beblinded by covering them with soil to a depth of 6 to 12inches. They should not be left exposed overnightbecause damage can occur from rain and trenchcaving. Loose topsoil, either taken from the sides ofthe trench or excavated during trenching operations,provides good blinding material.

Backfilling of the trench should be done as soon afterblinding as possible to prevent damage from surfacewater. This generally is done by mechanical means.Some trenchers have backfilling attachments thatplace the excavated material in the trench as the drainis laid.

(l) Protection for biological andmineral clogging

The following installation procedures may minimizeochre problems for shallow drains in humid areas.

In ochreous areas, drains should not be installed

below the water table. If possible, drains should beinstalled during the dry season when the water table islow because the iron in the soil will be in the insolubleform and stabilization of the drain and surroundingsoil will help to minimize the possibility of ochrebecoming a serious problem.

Drains should open into ditches, rather than

through collector systems. If a small area in a fieldis ochreous, the trouble could be confined to a singledrain. Cleaning is also easier for single drains.

Clogging is more severe shortly after drain

installation. The best cleaning method is to jet thedrains during the first year after installation ratherthan wait until the drains are clogged. One method ofcleaning has vents at the upper end of the lines thatare used as ports to pour large quantities of water intothe drains for flushing action. This method will notclean the valleys of the corrugations.

Shallow drains and closely spaced drains that

flow infrequently are not as troublesome, even

though the site may be rated serious for ochre

potential.

Drains in marl soils generally have fewer prob-

lems, unless the drains are installed deep in the

soil profile.

Avoid blinding the drain with topsoil or organic

materials.

(m) Checking

The most practical way to check the drain installationis after the drain has been laid and before the trench isbackfilled; however, checking can be done using aprobe after backfilling.

(n) As-built plans

As-built or record drawings are recommended forfuture reference. They can be done by GPS mappingprocess, aerial photography, or traditional surveymethods.


14–91



650.1428 Maintenance

Maintenance of subsurface drains is needed through-out the drain’s expected useful life. Outlets should beinspected regularly. If they are not fire-resistant orfire-proof, they need to be protected from weed burn-ing operations. Corrugated plastic tubing is not suit-able for the outlet section. Maintenance problems arereduced if the outlet is a short section of solid pipe.The gates or screens of outlets must be checked toassure that entry of rodents and other small animals isrestricted and that they are free of sediment build-up,weeds, debris, and seasonal ice blockage.

General observation of the entire subsurface drainagesystem will reveal areas of possible failure. Sinkholesor cave-ins over the drains indicate that soil pipingproblems have occurred. The problem may be a bro-ken or collapsed drainage conduit or an opening in thefilter or envelope material that allows soil material toenter the drain. Following the spring drying period,puddles or wet areas can indicate a plugged line orfilter fabric or areas where additional drains areneeded.

(a) Jet cleaning

High pressure jet cleaning has been successfully usedfor removal of ochre, silt, and roots from subsurfacedrains (fig. 14–48). This practice has been used exten-sively in Northern Europe and in the U.S.

Timely maintenance of subsurface drainage systems inareas of ochre development is critical. Subsurfacedrains should not be installed in sites having perma-nent ochre potential unless some provision is made forfrequent jet cleaning.

Temporary ochre as a clogging factor may diminish ordisappear over a period of 3 to 8 years if drains aremaintained in a free-flowing condition. It generallyoccurs rapidly and often can be detected at drain out-lets within the first few months after drain installation.If drains can be maintained in working order, ferrousiron reaching them may diminish over a period of time.

Permanent ochre is the most serious problem becauseit continues to be a clogging agent for the life of thedrainage system, regardless of treatment. The use ofhigh and low pressure water jetting has been success-ful in cleaning many drains clogged with ochre. Nozzle

Figure 14–48 High pressure jet cleaning


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14–92

pressure should not exceed 400 psi in sandy soils;otherwise sand around the drains may become un-stable and flow into the drain. Jetting nozzles designedfor agricultural drains should be used rather thanthose designed for cleaning municipal sewer lines. Jetcleaning should not be delayed until the ochre hasaged and become crystalline.

(b) Acid solutions

A second method for cleaning drains involves an acidsolution to dissolve the iron. This method cannot beused with synthetic envelopes, and the outflow aftertreatment may need to be neutralized to preventpollution downstream. Some acids, especially sulfuric,may damage concrete lines.

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paragonpdh.com · 2019. 10. 24. · spaced. Consequently, installation may be considered too expensive for use of subsurface drains. Soils must have sufficient depth and permeability