CHAPTER 7 Movement of Water Through Soils · Movement of Water Through Soils The movement of water into or through a soil mass is a phenomenon of great practical importance in engineering

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CHAPTER 7

M o v e m e n t o f W a t e r

T h r o u g h S o i l s

The movement of water into or through asoil mass is a phenomenon of great practicalimportance in engineering design and con-struction. It is probably the largest singlefactor causing soil failures. For example,water may be drawn by capillarity from a freewater surface or infiltrate through surfacecracks into the subgrade beneath a road orrunway. Water then accumulated may greatlyreduce the bearing capacity of subgrade soil,allowing the pavement to fail under wheelloads if precautions have not been taken indesign. Seepage flow may be responsible forthe erosion or failure of an open cut slope orthe failure of an earth embankment. Thischapter concerns the movement of water intoand through soils (and, to some extent, aboutthe practical measures undertaken to controlthis movement) and the problems associatedwith frost action.

Section I. WaterSOURCES OF WATER

Water in the soil maybe from a surface or asubsurface source.

Surface WaterWater enters through soil surfaces unless

measures are taken to seal the surface toprevent entry. Even sealed surfaces mayhave cracks, joints, or fissures that admitwater. In concrete pavement, for instance,

the contraction and expansion joints arepoints of entry for water. After the pavementhas been subjected to many cycies of expan-sion and contraction, the joint filler may beimperfect, thereby allowing water to seep intothe base course or subgrade.

Surface water may also enter from the sidesof the road or airfield, even though the surfaceitself is entirely adequate. To reduce this ef-fect, shoulders and ditches are sloped to carrythe water away at an acceptable rate. If ade-quate maintenance is not providedthroughout the life of the facility, vegetationor sedimentation may reduce the efficiency ofthe drainage so that water will frequentlystand in the drainage ditch. This water maythen be absorbed by the base and subgrade.

Subsurface WaterSubsurface moisture in soils may come

from one of the following sources:Free, or gravitional, water (controlledby the force of gravity).Hydroscopic moisture (controlled byforces of absorption).Capillary moisture (controlled by for-ces of capillarity).

Free, or Gravitational, Water. Water thatpercolates down from the surface eventuallyreaches a depth at which there is somemedium that restricts (to varying degrees)

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the further percolation of the moisture. Thismedium may be bedrock or a layer of soil, notwholly solid but with such small void spacesthat the water which leaves this zone is not asgreat as the volume or supply of water added.In time, the accumulating water completelysaturates the soil above the restrictingmedium and fills all voids with water. Whenthis zone of saturation is under no pressureexcept from the atmosphere, the water it con-tains is called free, or gravitational, water. Itwill flow through the soil and be resisted onlyby the friction between the soil grains and thefree water. This movement of free waterthrough a soil mass frequently is termedseepage.

The upper limit of the saturated zone of freewater is called the groundwater table, whichvaries with climatic conditions. During a wetwinter, the groundwater table rises. How-ever, a dry summer might remove the sourceof further accumulation of water. Thisresults in a decreased height of the saturatedzone, for the free water then flows downward,through, or along its restricting layer. Thepresence of impervious soil layers may resultin an area of saturated soil above the normalgroundwater table. This is called a “perched”water table.

Hydroscopic Moisture. When wet soil isair-dried, moisture is removed by evaporationuntil the hydroscopic moisture in the soil is inequilibrium with the moisture vapor in theair. The amount of moisture in air-dried soil,expressed as a percentage of the weight of thedry soil, is called the hydroscopic moisturecontent. Hydroscopic moisture films may bedriven off from air-dried soil by heating thematerial in an oven at 100 to 110 degrees Cen-tigrade (C) (210 to 230 degrees Fahrenheit(F)) for 24 hours or until constant weight is at-tained.

Capillary Moisture. Another source ofmoisture in soils results from what might betermed the capillary potential of a soil. Drysoil grains attract moisture in a mannersimilar to the way clean glass does. Outward

evidence of this attraction of water and glassis seen by observing the meniscus (curvedupper surface of a water column). Where themeniscus is more confined (for example, as ina small glass tube), it will support a column ofwater to a considerable height. The diagramin Figure 7-1 shows that the more the menis-cus is confined, the greater the height of thecapillary rise.

Capillary action in a soil results in the“capillary fringe” immediately above thegroundwater table. The height of the capil-lary rise depends on numerous factors. Onefactor worth mentioning is the type of soil.Since the pore openings in a soil vary with thegrain size, a fine-grained soil develops ahigher capillary fringe area than a coarse-grained soil. This is because the fine-grainedsoil can act as many very small glass tubes,each having a greatly confined meniscus. Inclays, capillary water rises sometimes as highas 30 feet, and in silts the rise is often as highas 10 feet. Capillary rise may vary from prac-tically zero to a few inches in coarse sands andgravels.

When the capillary fringe extends to thenatural ground surface, winds and hightemperatures help carry this moisture awayand reduce its effects on the soil. Once a pave-ment of watertight surface is applied,however, the effect of the wind and tempera-ture is reduced. This explains theaccumulation of moisture often found directlybeneath an impervious pavement.


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Capillary moisture in soils located abovethe water table may be visualized as occur-ring in the following three zones:

Capillary saturation.Partial capillary saturation.Contact moisture.

In the zone of capillary saturation, the soilis essentially saturated. The height of thiszone depends not only on the soil but also onthe history of the water table, since the heightwill be greater if the soil mass has beensaturated previously.

The height of the zone of partial capillarysaturation is likely to be considered greaterthan that of the zone of capillary saturation; italso depends on the water-table history. Itsexistence is the result of a few large voidsserving effectively to stop capillary rise insome parts of the soil mass. Capillary waterin this zone is still interconnected or “con-tinuous,” while the air voids may not be.

Above the zones of capillary and partialcapillary saturation, water that percolatesdownward from the surface may be held inthe soil by surface tension. It may fill thesmaller voids or be present in the form ofwater films between the points of contact ofthe soil grains. Water may also be broughtinto this zone from the water table byevaporation and condensation. This mois-ture is termed “contact moisture.”

One effect of contact moisture is apparentcohesion. An example of this is the behaviorof sand on certain beaches. On these beaches,the dry sand located back from the edge of thewater and above the height of capillary rise isgenerally dry and very loose and has littlesupporting power when unconfined. Closerto the water’s edge, and particularly duringperiods of low tide, the sand is very firm andcapable of supporting stationary or movingautomobiles and other vehicles. This ap-parent strength is due primarily to theexistence of contact moisture left in the voidsof the soil when the tide went out. The sur-face soil may be within the zone of partial orcomplete capillary saturation very close to

the edge of the water. Somewhat similarly,capillary forces may be used to consolidateloose cohesionless deposits of very fine sandsor silts in which the water table is at or nearthe ground surface. This consolidation is ac-complished by lowering the water table bymeans of drains or well points. If the opera-tion is properly carried out within the limitsof the height of capillary rise, the soil abovethe lowered water table remains saturated bycapillary moisture. The effect is to place thesoil structure under capillary forces (such astension in the water) that compress it. Thesoil may be compressed as effectively asthough an equivalent external load had beenplaced on the surface of the soil mass.

Methods commonly used to control thedetrimental effects of capillarity, particularlyconcerning roads and airport pavements, arementioned briefly here. Additional attentionis given to this subject in Section II, which isdevoted to the closely allied subject of frost ac-tion.

As has been noted, if the water table iscloser to the surface than the height of capil-lary rise, water will be brought up to thesurface to replace water removed by evapora-tion. If evaporation is wholly or partiallyprevented, as by the construction of imper-vious pavement, water accumulates and maycause a reduction in shearing strength orcause swelling of the soil. This is true par-ticularly when a fine-grained soil or a coarsesoil that contains a detrimental amount ofplastic fines is involved.

One obvious solution is to excavate thematerial that is subject to capillary action andreplace it with a granular material. This isfrequently quite expensive and usually maybe justified only in areas where frost action isa factor.

Another approach is to include in the pave-ment structure a layer that is unaffected bycapillary action. This is one of the functionsof the base that is invariably used in flexiblepavements. The base serves to interrupt theflow of capillary moisture, in addition to its


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other functions. Under certain circumstan-ces, the base itself may have to be drained toensure the removal of capillary water (seeFigure 7-2). This also is usually not justifiedunless other circumstances, such as frost ac-tion, are of importance.

Still another approach is to lower the watertable, which may sometimes be accomplishedby the use of side ditches. Subdrains maybeinstalled for the same purpose (see Figure 7-3,page 7-6). This approach is particularly effec-tive in relatively pervious or free-drainingsoils. Some difficulty may be experienced inlowering the water table by this method inflat country because finding outlets for thedrains is difficult. An alternative, used inmany areas where the permanent water tableis at or near the ground surface, is simply tobuild the highway or runway on a fill.Material that is not subject to detrimentalcapillarity is used to form a shallow fill. Thebottom of the base is normally kept a mini-mum of 3 or 4 feet above the natural groundsurface, depending on the soil used in the filland other factors. A layer of sand, known as asand blanket, or a geotextile fabric may beused to intercept capillary moisture, prevent-ing its intrusion into the base course.

PERMEABILITYPermeability is the property of soil that per-

mits water to flow through it. Water maymove through the continuous voids of a soil inmuch the same way as it moves through pipesand other conduits. As has been indicated,this movement of water through soils is fre-quently termed seepage and may also becalled percolation. Soils vary greatly in theirresistance to the flow of water through them.Relatively coarse soils, such as clean sandsand gravels, offer comparatively little resis-tance to the flow of water; these are said to bepermeable or pervious soils. Fine-grainedsoils, particularly clays, offer great resistanceto the movement of water through them andare said to be relatively impermeable or im-pervious. Some water does move throughthese soils, however. The permeability of asoil reflects the ease with which it can bedrained; therefore, soils are sometimesclassed as well-drained, poorly drained, or


impervious. Permeability is closely related tofrost action and to the settlement of soilsunder load.

The term k is called the coefficient of per-meability. It has units of velocity and mayberegarded as the discharge velocity under aunit hydraulic gradient. The coefficient ofpermeability depends on the properties of thefluid involved and on the soil. Since water isthe fluid normally involved in soil problems,and since its properties do not vary enough toaffect most practical problems, the coefficientof permeability is regarded as a property ofthe soil. Principal factors that determine thecoefficient of permeability for a given soil in-clude—

Grain size.Void ratio.Structure.

The relationships among these differentvariables for typical soils are quite complexand preclude the development of formulas forthe coefficient of permeability, except for thesimplest cases. For the usual soil, k is deter-mined experimentally, either in thelaboratory or in the field. These methods arediscussed briefly in the next paragraph.Typical values of the coefficient of per-meability for the soil groups of the USCS aregiven in column 8 of Table 5-4, page 5-15.

DRAINAGE CHARACTERISTICSThe general drainage characteristics of

soils classified under the USCS are given incolumn 12 of Table 5-3, page 5-11. Soils maybe divided into three general groups on thebasis of their drainage characteristics. Theyare—

Well-drained.Poorly drained.Impervious.

Well-Drained SoilsClean sands and gravels, such as those

included in the (GW), (GP), (SW), or (SP)groups, fail into the classification of well-drained soils. These soils may be drainedreadily by gravity systems. In road and

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airfield construction, for example, openditches may be used in these soils to interceptand carry away water that comes in from sur-rounding areas. This approach is veryeffective when used in combination with thesealing of the surface to reduce infiltrationinto the base or subgrade. In general, if thegroundwater table around the site of a con-struction project is controlled in these soils,then it will be controlled under the site also.

Poorly Drained SoilsPoorly drained soils include inorganic and

organic fine sands and silts, organic clays oflow compressibility, and coarse-grained soilsthat contain an excess of nonplastic fines.Soils in the (ML), (OL), (MH), (GM), (GC),(SC), and (SM) groups, and many from the(Pt) group, generally fall into this category.Drainage by gravity alone is likely to be quitedifficult for these soils.

Impervious SoilsFine-grained, homogeneous, plastic soils

and coarse-grained soils that contain plastic

fines are considered impervious soils. Thisnormally includes (CL) and (CH) soils andsome in the (OH) groups. Subsurfacedrainage is so slow on these items that it is oflittle value in improving their condition. Anydrainage process is apt to be difficult and ex-pensive.

FILTER DESIGNThe selection of the proper filter material is

of great importance since it largely deter-mines the success or failure of the drainagesystem. A layer of filter material ap-proximately 6 inches deep should be placedaround all subsurface piping systems. Theimproper selection of a filter material cancause the drainage system to become inopera-tive in one of three ways:

The pipe may become clogged by theinfiltration of small soil particles.Particles in the protected soil maymove into or through the filters, caus-ing instability of the surface.Free groundwater may not be able toreach the pipe.


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> 1.2

> 1

> 5

< 5

< 25

> 5

To prevent these failures from occurring,criteria have been developed based on thesoil’s gradation curve (see Chapter 5).

To prevent the clogging of a pipe by filtermaterial moving through the perforations oropenings, the following limiting require-ments must be satisfied (see EngineerManual (EM) 1110-2-1901):

For slotted openings:50 percent size of filter material

slot width

For circular holes:50 percent size of filter material

hole diameter

For porous concrete pipes:D85 filter (mm)

D15 aggregate (mm)

For woven filter cloths:D85 surrounding soil

Equivalent opening size (EOS) of cloth>1

To prevent the movement of particles fromthe protected soil into or through the filters,the following conditions must be satisfied:

15 percent of filter material85 percent size of protected soil

and50 percent size of filter material

50 percent size of protected soil

To permit free water to reach the pipe, thefilter material must be many times more per-vious than the protected soil. This conditionis fulfilled when the following requirement ismet:

15 percent size of filter material15 percent size of protected soil

If it is not possible to obtain a mechanicalanalysis of available filter materials and

protected soils, concrete sand with mechani-cal analysis limits as shown in Figure 7-4,page 7-8, may be used. Experience indicatesthat a well-graded concrete sand is satisfac-tory as a filter material in most sandy, siltysoils.

Section II. Frost ActionPROBLEMS

Frost action refers to any process that af-fects the ability of the soil to support astructure as a result of—

Freezing.Thawing.

A difficult problem resulting from frost actionis that pavements are frequently broken up orseverely damaged as subgrades freeze duringwinter and thaw in the spring. In addition tothe physical damage to pavements duringfreezing and thawing and the high cost oftime, equipment, and personnel required inmaintenance, the damage to communicationsroutes or airfields may be great and, in someinstances, intolerable strategically. In thespring or at other warm periods, thawing sub-grades may become extremely unstable. Insome severely affected areas, facilities havebeen closed to traffic until the subgraderecovered its stability.

The freezing index is a measure of thecombined duration and magnitude of below-freezing temperature occurring during anygiven freezing season. Figure 7-5, page 7-9,shows the freezing index for a specific winter.

FreezingEarly theories attributed frost heaves to

the expansion of water contained in soil voidsupon freezing. However, this expansionwould only be about 9 percent of the thicknessof a frozen layer if caused by the water in thesoil changing from the liquid to the solidstate. It is not uncommon to note heaves asgreat as 60 percent; under laboratory condi-tions, heaves of as much as 300 percent havebeen recorded. These facts clearly indicate


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that heaving is due to the freezing of addi-tional water that is attracted from the non-frozen soil layers. Later studies have shownthat frost heaves are primarily due to thegrowth of ice lenses in the soil at the plane offreezing temperatures.

The process of ice segregation may be pic-tured as follows: the thin layers of wateradhering to soil grains become supercooled,meaning that this water remains liquid below32 degrees Fahrenheit. A strong attractionexists between this water and the ice crystalsthat form in larger void spaces. This super-cooled water flows by capillary action towardthe already-formed crystals and freezes oncontact. Continued crystal growth leads tothe formation of an ice lens, which continues

to grow in thickness and width until thesource of water is cut off or the temperaturerises above the normal freezing point (see Fig-ure 7-6, page 7-10).

ThawingThe second phase of frost damage occurs

toward the end of winter or in early springwhen thawing begins. The frozen subgradethaws both from the top and the bottom. Forthe latter case, if the air temperature remainsbarely below the freezing point for a sufficientlength of time, deeply frozen soils graduallythaw from the bottom upward because of theoutward conduction of heat from the earth’sinterior. An insulating blanket of snowtends to encourage this type of thawing.From an engineering standpoint, this


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thawing condition is desirable, because it per- leaves a frozen layer beneath the thawed sub-mits melted water from thawed ice lenses toseep back through the lower soil layers to thewater table from which it was drawn duringthe freezing process. Such dissipation of themelted water places no load on the surfacedrainage system, and no tendency exists toreduce subgrade stability by reason of satura-tion. Therefore, there is little difficulty inmaintaining unpaved roads in a passable con-dition.

Thawing occurs from the top downward ifthe surface temperature rises from below thefreezing point to well above that point andremains there for an appreciable time. This

grade. The thawed soil between thepavement and this frozen layer contains anexcessive amount of moisture resulting fromthe melting of the ice it contained. Since thefrozen soil layer is impervious to the water,adequate drainage is almost impossible. Thepoor stability of the resulting supersaturatedroad or airfield subgrade accounts for manypavement failures. Unsurfaced earthenroads may become impassable when super-saturated.

Thawing from both the top and bottom oc-curs when the air temperature remainsbarely above the freezing point for a sufficient


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time. Such thawing results in reduced soilstability, the duration of which would be lessthan for soil where the thaw is only from thetop downward.

CONDITIONSTemperatures below 32 degrees Fahren-

hiet must penetrate the soil to cause freezing.In general, the thickness of ice layers (and theamount of consequent heaving) is inverselyproportional to the rate of penetration offreezing temperature into the soil. Thus,winters with fluctuating air temperatures atthe beginning of the freezing season produce

more damaging heaves than extremely cold,harsh winters where the water is more likelyto be frozen in place before ice segregation cantake place.

A source of water must be available topromote the accumulation of ice lenses.Water may come from—

A high groundwater table.A capillary supply from an adjoiningwater table.Infiltration at the surface.A water-bearing system (aquifer).Voids of fine-grained soils.


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Ice segregation usually occurs in soils whena favorable source of water and freezingtern peratures are present. The potential in-tensity of ice segregation in a soil dependslargely on the size of the void space and maybe expressed as an empirical fuction of grainsize.

Inorganic soils containing 3 percent ormore by weight of grains finer than 0.02 mmin diameter are generally considered frostsusceptible. Although soils may have as highas 10 percent by weight of grains finer than0.02 mm without being frost susceptible, thetendency of these soils to occur interbeddedwith other soils makes it impractical to con-sider them separately.

Frost-susceptible soils are classified in thefollowing groups:

F-1.F-2.F-3.F-4.

They are listed approximately in the orderof increasing susceptibility to frost heaving or

weakening as a result of frost melting (seeTable 7-1 ). The order of listing of subgroupsunder groups F-3 and F-4 does not necessarilyindicate the order of susceptibility to frostheaving or weakening of these subgroups.There is some overlapping of frost suscep-tibility between groups. The soils in groupF-4 are of especially high frost susceptibility.Soil names are defined in the USCS.

Varved clays consist of alternating layers ofmedium-gray inorganic silt and darker siltyclay. The thickness of the layers rarely ex-ceeds 1/2 inch, but occasionally much thickervarves are encountered. They are likely tocombine the undesirable properties of bothsilts and soft clays. Varved clays are likely tosoften more readily than homogeneous clayswith equal water content. However, local ex-perience and conditions should be taken intoaccount since, under favorable conditions (aswhen insufficient moisture is available forsignificant ice segregation), little or nodetrimental frost action may occur. Someevidence exists that pavements in theseasonal frost zone, constructed on varvedclay subgrades in which the deposit and


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depth to groundwater are relatively uniform,have performed satisfactorily. Where sub-grade conditions are uniform and localevidence indicates that the degree ofheave is not exceptional, the varved clay sub-grade soil should be assigned a group F-4frost-susceptibility classification.

EFFECTSFrost action can cause severe damage to

roads and airfields. The problems includeheaving and the resultant loss of pavementstrength.

HeavingFrost heave, indicated by the raising of the

pavement, is directly associated with icesegregation and is visible evidence on the sur-face that ice lenses have formed in thesubgrade material. Heave may be uniform ornonuniform, depending on variations in thecharacter of the soils and the groundwaterconditions underlying the pavement.

The tendency of the ice layers to developand grow increases rapidly with decreasinggrain size. On the other hand, the rate atwhich the water flows in an open systemtoward the zone of freezing decreases withdecreasing grain size. Therefore, it isreasonable to expect that the worst frostheave conditions would be encountered insoils having an intermediate grain size. Siltsoils, silty sands, and silty gravels tend to ex-hibit the greatest frost heave.

Uniform heave is the raising of adjacentareas of pavement surface by approximatelyequal amounts. In this type of heave, the in-itial shape and smoothness of the surfaceremains substantially unchanged. Whennonuniform heave occurs, the heave of ad-jacent areas is appreciably different,resulting in objectionable unevenness orabrupt changes in the grade at the pavementsurface.

Conditions conducive to uniform heavemay exist, for example, in a section of pave-ment constructed with a fairly uniform


stripping or fill depth, uniform depth togroundwater table, and uniform soil charac-teristics. Conditions conducive to irregularheave occur typically at locations where sub-grades vary between clean sand and silty soilsor at abrupt transitions from cut to fill sec-tions with groundwater close to the surface.

Lateral drains, culverts, or utility linesplaced under pavements on frost-susceptiblesubgrades frequently cause abrupt differen-tial heaving. Wherever possible, suchfacilities should not be placed beneath thesepavements, or transitions should be providedso as to moderate the roughening of the pave-ment during the period of heave.

Loss of Pavement StrengthWhen ice segregation occurs in a frost-sus-

ceptible soil, the soil’s strength is reduced asis the load-supporting capacity of the pave-ment during prolonged frost-melting periods.This often occurs during winter and springthawing periods, because near-surface icemelts and water from melting snow or rainmay infiltrate through the surface causing anexcess of water. This water cannot drainthrough the still-frozen soil below, or throughthe shoulders, or redistribute itself readily.The soil is thus softened.

Supporting capacity may be reduced in claysubgrades even through significant heavehas not occurred. This may occur becausewater for ice segregation is extracted from theclay lattice below, and the resultingshrinkage of the lattice largely balances thevolume of the ice lenses formed.

Further, traffic may cause remolding ordevelop hydrostatic pressure within the poresof the soil during the period of weakening,thus resulting in further-reduced subgradestrength. The degree to which a soil losesstrength during a frost-melting period andthe length of the period during which thestrength of the soil is reduced depend on—

The type of soil.Temperature conditions during freez-ing and thawing periods,

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The amount and type of traffic duringthe frost-melting periods.The availability of water during thefreezing and thawing periods.Drainage conditions.

Rigid Pavements (Concrete). Concretealone has only a little tensile strength, and aslab is designed to resist loads from abovewhile receiving uniform support from thesubgrade and base course. Therefore, slabshave a tendency to break up as a result of theupthrust from nonuniform heaving soilscausing a point bearing. As a rule, if rigidpavements survive the ill effects of upheaval,they will generally not fail during thawing.Reinforced concrete will carry a load by beamaction over a sub grade having either frozen orsupersaturated areas. Rigid pavements willcarry a load over subgrades that are bothfrozen and supersaturated. The capacity tobear the design load is reduced, however,when the rigid slab is supported entirely bysupersaturated, semiliquid subgrades.

Flexible Bituminous Pavements. Theductility of flexible pavements helps them todeflect with heaving and later resume theiroriginal positions. While heaving mayproduce severe bumps and cracks, usually itis not too serious for flexible pavements. Bycontrast, a load applied to poorly supportedpavements during the thawing period nor-mally results in rapid failure.

Slopes. Exposed back slopes and side slopesof cuts and fills in fine-g-rained soil have a ten-dency to slough off during the thawingprocess. The additional weight of water plusthe soil exceeds the shearing strength of thesoil, and the hydrostatic head of water exertsthe greatest pressure at the foot of the slope.This causes sloughing at the toe of the slope,which multiplies the failure by consecutiveshear failures due to inadequate stability ofthe altered slopes. Flatter slopes reduce thisproblem. Sustained traffic over severelyweakened areas afflicted with frost boils in-itiates a pumping action that results incomplete pavement failure in the immediatevicinity.

INVESTIGATIONAL PROCEDURESThe field and laboratory investigations con-

ducted according to Chapter 5 of this manualusually provide sufficient information todetermine whether a given combination ofsoil and water conditions beneath the pave-ment are conducive to frost action. Thisprocedure for determining whether the condi-tions necessary for ice segregation arepresent at a proposed site are discussed in thefollowing paragraphs. As stated earlier inthis chapter, inorganic soils containing 3 per-cent or more by weight of grains finer than0.02 mm are generally considered susceptibleto ice segregation. Thus, examination of thefine portion of the gradation curve obtainedfrom hydrometer analysis or the recantationprocess for these materials indicates whetherthey should be assumed frost susceptible. Inborderline cases, or where unusual materialsare involved, slow laboratory freezing testsmay be performed to measure the relativefrost susceptibility.

The freezing index value should be com-puted from actual daily air temperatures, ifpossible. Obtain the air temperatures from aweather station located as close as possible tothe construction site. Differences in eleva-tions, topographical positions, and nearnessof cities, bodies of water, or other sources ofheat may cause considerable variations infreezing indexes over short distances. Thesevariations are of greater importance to thedesign in areas of a mean design freezingindex of less than 100 (that is, a design freez-ing index of less than 500) than they arefarther north.

The depth to which freezing temperaturespenetrate below the surface of a pavementdepends principally on the magnitude andduration of below-freezing air temperaturesand on the amount of water present in thebase, subbase, and subgrade.

A potentially troublesome water supply forice segregation is present if the highestgroundwater at any time of the year is within5 feet of the proposed subgrade surface or thetop of any frost-susceptible base materials.When the depth to the uppermost water table


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is in excess of 10 feet throughout the year, asource of water for substantial ice segregationis usually not present unless the soil containsa significant percentage of silt. Inhomogeneous clay soils, the water contentthat the clay subgrade will attain under apavement is usually sufficient to providewater for some ice segregation even with aremote water table. Water may also enter afrost-susceptible subgrade by surface infiltra-tion through pavement areas. Figure 7-7illustrates sources of water that feed growingice lenses, causing frost action.

CONTROLAn engineer cannot prevent the temperat-

ures that cause frost action. If a road orrunway is constructed in a climate wherefreezing temperatures occur in winter, in allprobability the soil beneath the pavementwill freeze unless the period of loweredtemperatures is very short. However, severalconstruction techniques may be applied tocounteract the presence of water and frost-susceptible soils.

Every effort should be made to lower thegroundwater table in relation to the grade ofthe road or runway. This may be ac-complished by installing subsurface drains oropen side ditches, provided suitable outletsare available and that the subgrade soil isdrainable. The same result maybe achievedby raising the grade line in relation to thewater table. Whatever means are employedfor producing the condition, the distance fromthe top of the proposed subgrade surface (orany frost-susceptible base material used) tothe highest probably elevation of the watertable should not be less than 5 feet. Distancesgreater than this are very desirable if theycan be obtained at a reasonable cost.

Where it is possible, upward water move-ment should be prevented, In many cases,lowering the water table may not be practical.An example is in swampy areas where an out-lets for subsurface drains might not bepresent. One method of preventing the rise ofwater would be to place a 6-inch layer of per-vious, coarse-grained soil 2 or 3 feet beneaththe surface. This layer would be designed as

a filter to prevent clogging the pores withfiner material, which would defeat theoriginal purpose. If the depth of frostpenetration is not too great, it maybe less ex-pensive to backfill completely with granu-lar material. Another successful method,though expensive, is to excavate to thefrost line and backfill with granular material.In some cases, soil cement and asphalt-stabilized mixtures 6 inches thick have beenused effectively to cut off the upward move-ment of water.

Even though the site selected may be onideal soil, invariably on long stretches ofroads or on wide expanses of runways, local-ized areas will be subject to frost action.These areas should be removed and replacedwith select granular material. Unless this ismeticulously carried out, differential heavingduring freezing and severe strength loss uponthawing, may result.

The most generally accepted method ofpreventing subgrade failure due to frost ac-tion is to provide a suitable insulating cover tokeep freezing temperatures from penetratingthe subgrade to a significant depth. This in-sulating cover consists of a suitable thickpavement and a thick nonfrost-susceptiblebase course.

If the wearing surface is cleared of snowduring freezing weather, the shouldersshould also be kept free of snow. Where this isnot the case, freezing will set in first beneaththe wearing surface. This permits water to bedrawn into and accumulate in the subgradefrom the unfrozen shoulder area, which isprotected by the insulating snow. If bothareas are free of snow, then freezing willbegin in the shoulder area because it is notprotected by a pavement. Under this condi-tion, water is drawn from the subgrade to theshoulder area. As freezing progresses to in-clude the subgrade, there will be little frostaction unless more water is available fromgroundwater or seepage.

Base Composition RequirementsAll base and subbase course materials lying

within design depth of frost penetration


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should be nonfrost-susceptible. Where thecombined thickness of pavement and base or-subbase over a frost-susceptible subgrade isless than the design depth of frost penetra-tion, the following additional designrequirements apply.

For both flexible and rigid pavements, thebottom 4 inches of base or subbase in contactwith the subgrade, as a minimum, will consistof any nonfrost-susceptible gravel, sand,screening, or similar material. This bottomof the base or subbase will be designed as a fil-ter between the subgrade soil and theoverlying material to prevent mixing of thefrost-susceptible subgrade with the nonfrost-susceptible base during and immediatelyafter the frost-melting period. The gradationof this filter material shall be determinedusing these guidelines:

To prevent the movement of particlesfrom the frost-susceptible subgradesoil into or through the filter blanket,all of these must be satisfied:15 percent size of filter blanket85 percent size of subgrade soil 5

50 percent size of filter blanket50 percent size of subgrade soil 25

The filter blanket in the above caseprevents the frost-susceptible soilfrom penetrating; however, the filtermaterial itself must also notpenetrate the nonfrost-susceptiblebase course material. Therefore, thefilter material must also meet the fol-lowing requirements:

15 percent size of base course85 percent size of filter blanket

5

50 percent size of base course50 percent size of filter blanket

25

In addition to the above require-ments, the filter material will, in nocase, have 3 percent or more byweight of grains finer than 0.02 mm.

A major difficulty in the construction of thefilter material is the tendency of the grain-size particles to segregate during placing;therefore, a CU > 20 is usually not desirable,For the same reason, filter materials shouldnot be skip- or gap-graded. Segregation ofcoarse particles results in the formation ofvoids through which fine particles may washaway from the subgrade soil. Segregation canbest be prevented during placement by plac-ing the material in the moist state. Usingwater while installing the filter blanket alsoaids in compaction and helps form satisfac-tory transition zones between the variousmaterials, Experience indicates that non-frost-susceptible sand is particularly suitablefor use as filter course material. Also fine-grained subgraded soil may workup into animproperly graded overlying gravel orcrushed stone base course. This will occurunder the kneading action of traffic duringthe frost-melting period if a filter course is notprovided between the subgrade and basecourse.

For rigid pavements, the 85-percent size offilter or regular base course material placeddirectly beneath pavement should be 2.00mm in diameter (Number 10 US standardsieve size) for a minimum thickness of 4 in-ches. The purpose of this requirement is toprevent loss of support by pumping soilthrough the joints.

Pavement DesignPavement may be designed according to

either of two basic concepts. The design maybe based primarily on—

Control of surface deformation causedby frost action.Provision of adequate bearing capac-ity during the most critical climaticperiod.

Control of Surface Deformation. In thismethod of pavement design, a sufficientcombined thickness of pavement and non-frost-susceptible base is provided to reduce


FM 5-410

subgrade frost penetration. Consequently, subgrade strength during the frost-meltingthis reduces pavement heave and subgrade period.weakening to a low, acceptable level.

Detailed design methods used in determin-Provision of Adequate Bearing Capacity. ing the required thickness of pavement, base,In this method, the amount of heave that will and subbase for given traffic and soil condi-result is neglected and the pavement is tions where frost action is a factor aredesigned solely on the anticipated reduced described in FM 5-430.


Documents

CHAPTER 7 Movement of Water Through Soils · Movement of Water Through Soils The movement of water into or through a soil mass is a phenomenon of great practical importance in engineering