What’s in Your Piezometer? Evaluating and …€™s in Your Piezometer? Evaluating and Maintaining Piezometers, Relief Wells, and Drains Charles N. Easton, P.E., Senior Geotechnical

Copyright © 2016 Association of State Dam Safety Officials, Inc. All Rights Reserved Page 1 of 28

What’s in Your Piezometer? Evaluating and Maintaining Piezometers, Relief Wells,

and Drains

Charles N. Easton, P.E., Senior Geotechnical Engineer, Freese and Nichols, Inc., Fort Worth, TX; and Russell G. Springer, P.E., Geotechnical Engineer, Freese and Nichols, Inc., Fort Worth, TX

Abstract—Many dams have seepage control and monitoring systems. These features are subject to damage, deterioration and reduced function with time. This paper is intended to help dam managers and maintenance staff understand the purpose of these systems, how to evaluate their condition, and how to perform necessary maintenance. Procedures and equipment are described for evaluating and cleaning standpipe piezometers, relief wells, toe drains and underdrains. Some technical guidance is necessary, but much of the work can be done by maintenance staff using equipment that is on-site, rented, or shop-built. Examples are illustrated.

I. INTRODUCTION Many medium-sized dams have seepage-control systems such as relief wells, toe drains, and spillway underdrains. Many

dams also have piezometers to monitor the performance of these systems. The owners, managers, and maintenance supervisors for these dams often have limited knowledge about the intent, function and condition of the seepage-control systems and piezometers. They often have such questions as:

What is the purpose of these systems? How can we know whether the systems are working as intended? What regular maintenance do they need to keep them working properly? Can my staff do the necessary maintenance and repairs?

This paper is intended to help the people responsible for operating and maintaining dams understand the purpose, function, and condition of these assets and address the repair needs that are identified.

A. What is the Purpose of Relief Wells and Drains? The purposes of seepage control systems are: To limit the pressure of the pore water within and under the dam to magnitudes that do not induce landslides in the

embankments, sliding or flotation of the structures, or internal erosion of the soils, and To provide filtered outlets for water that seeps through or under the dam so soil is not carried away with the water.

B. How Can We Know if They Are Working Properly? Whether the systems are performing as intended can be addressed by measuring the pore water pressure at selected

locations with piezometers and comparing the measured pressures with those assumed for the design analyses or using them in new analyses. Piezometers, however, are subject to deterioration, damage and clogging and need evaluation, maintenance, and repairs to provide accurate information. We will spend some time discussing how to evaluate the condition of the piezometers and how to fix some of the common problems that develop. Many existing piezometers are 1” or 1-1/4” diameter, and tools to evaluate and treat them are not widely available commercially. Examples of adapted or home-built tools for this purpose are presented.

C. What Regular Maintenance do the Wells and Drains Need? Problems include mechanical damage, corrosion, and clogging. We will discuss some common problems with relief wells

and drains and how they can be detected and treated.

D. Can the Local Staff Perform the Maintenance? Some guidance by a geotechnical or dam engineer is needed, but much of the actual work can be accomplished by the

responsible and resourceful persons that are already on staff and familiar with the dam. Some special tools and equipment are needed, but they need not be expensive. Examples will be shown.


II. GENERAL SEEPAGE PATTERNS IN EARTH DAMS Water in the spaces between soil grains is called pore water. Seepage through soil usually occurs under saturated

conditions, where the pore spaces are essentially full of water. Water in saturated soil tends to move from areas of high energy to areas of lower energy under the influence of gravity and pressure. The energy level of water is the sum of the elevation head, the pressure head, and the velocity head. Seepage velocities are usually so small that the velocity head can be ignored. The relative energy level of seeping pore water at any point is equal to the elevation of the point in feet above a given datum plus the pressure head, calculated as the pressure divided by the unit weight of water and expressed in feet. If a standpipe is inserted into the soil with a screen and filter to let the water in but keep the soil out, the water will rise in the pipe to an elevation equal to the total energy head at that point. That elevation is called the “piezometric elevation”, and the screened standpipe used to measure it is a piezometer.

Water in a lake has a piezometric elevation equal to the elevation of the lake surface, or “headwater”. Water in the channel or pool downstream from the dam has a piezometric elevation equal to the surface of the water in the channel or pool, called the “tailwater”. Pore water within and beneath the embankment will generally move from the headwater toward the tailwater, and its piezometric elevation will decline as it progresses.

A. Embankments Lake water enters the embankment with a piezometric elevation equal to the headwater. The soil at the upstream face will

be essentially saturated below the lake surface elevation. As the water seeps through the dam, it loses energy, so the line between saturated and unsaturated soil, called the “phreatic surface” slopes downward through the embankment until it reaches an exit point, either on the downstream slope or in some outlet such as a toe drain or blanket drain. (Strictly speaking, the soil may be saturated to some distance above the phreatic surface, thanks to capillary attraction. The phreatic surface is more precisely defined as the elevation where the pore water is at atmospheric pressure. The water in the saturated capillary zone is at a pressure less than atmospheric pressure.)

In a homogeneous dam, the phreatic surface may intercept the downstream slope, creating a perennially wet surface condition. A wedge of sand and gravel or a perforated pipe in a shallow trench at the bottom of the slope, called a “toe drain”, can move the phreatic surface down and cure the wet slope problem. A layer of sand under the downstream part of the embankment, called a “blanket drain”, can lower the phreatic surface further so that most of the embankment beneath the downstream slope is unsaturated. This reduces the risk of landslides in the downstream slope and filters the seeping water to prevent erosion of the soil, called “piping”. A vertical or steeply-sloped column of sand, called a “chimney drain” can lower the phreatic surface even further and also help halt internal erosion if cracks develop in the embankment. See Figure 1.

Piezometers in the downstream zones can show whether the drains are working properly.

Figure 1. Seepage through a dam embankment with and without drains.

B. Foundations Some dam foundations, such as a thick bed of clay or unfractured bedrock, may be essentially impervious. Stratified

alluvial foundations, found in most meandered river valleys, often have relatively permeable layers of sand that can conduct significant seepage flows from the lake to the tailwater. A layer of clay above the sand can separate the seepage through the foundation from the seepage through the embankment. If the thickness of the sand layer is reasonably uniform, the piezometric elevation in the sand may slope downward nearly linearly from some point upstream of the dam to some point downstream from the toe. The “piezometric profile” in the permeable layers of the foundation may be quite different from the phreatic surface in the embankment (Figure 2). A deeper sand layer may have a higher piezometric elevation below the toe than an


overlying shallow sand layer because water from the deeper layer has to pass through an additional layer of clay or shale layer before reaching the atmosphere.

Figure 2. Piezometric profiles in two permeable foundation layers.

If the piezometric profile is near or above the ground surface, slope stability may be reduced, or seepage may appear at the

surface. A seepage cutoff such as a clay-filled trench may be constructed through a sand layer to slow the seepage and lower the piezometric elevation under the downstream slope as shown in Figure 3.

Figure 3. Piezometric profile through a dam with a clay-filled cutoff trench.

If a sand layer is too deep to be economically cut off, relief wells can be installed to provide an outlet for the seepage and

lower the piezometric elevation as shown in Figure 4.


Figure 4. Piezometric profiles in deep sand layer with and without relief wells.

III. PIEZOMETERS

A. Description Open-standpipe piezometers are the most common type used on medium-sized dams, and they are the most amenable to

evaluation and repair. As shown in Figure 5, an open-standpipe piezometer is essentially a small well. It is installed in a hole in the ground, has a screen, a riser, usually a sand pack or other filter, and has a seal at the surface to keep out surface water. A proper piezometer also has a seal just above the top of the filter to be sure that the piezometer measures only the pressure of the water in the soil within a specific range of depth.

Figure 5. Schematic of an open-riser piezometer.

Water in the soil pore spaces moves into the piezometer and fills up the riser until the pressure generated by the column of

water in the riser is equal to the pressure in the pore water. We measure down from the top of the piezometer to the water


surface using a water level indicator (Figure 6). The indicator has a battery, a buzzer, and an electric circuit that feeds a voltage to a cable leading to a probe with two contacts separated by an insulator. When the contacts enter the water, the water completes the circuit, and the buzzer sounds. The cable is marked off in feet or meters so the distance from the top of the riser to the water surface can be measured. This depth is subtracted from the surveyed elevation of the top of the riser to determine the elevation of the water surface, which is equal to the piezometric elevation of the pore water in the ground around the screen.

If you have only a few piezometers and don’t want to invest $500 for a water level indicator, use a fiberglass tape with a popper at the tip (Figure 6). You will hear a pop when it strikes the water surface.

Figure 6. Water level indicator, tape with popper, and tape with sounding rod.

B. Problems Many piezometers installed before about 1980 had galvanized steel pipe risers and wellpoints for screens. The wellpoints

were commonly bronze or brass screen wire wrapped around a 2-to 3-foot length of steel pipe with holes. The risers are subject to corrosion, especially at the joints and where the concrete meets the soil if the piezometer was cast into a concrete spillway structure. Water can enter through holes created by corrosion and cause erroneous readings. See Figure 7. Corrosion products can block the inside of the pipe, stopping the water level indicator. Most modern piezometers are PVC pipe, and the screens are made by cutting narrow saw slots in the side of the pipe. The corrosion problem is greatly reduced.

Figure 7. Extracted galvanized steel piezometer riser damaged by corrosion.

Piezometers can collect sediment washed in from the ground surface or brought in by groundwater when the pore pressure

rises. Rust flakes and dead bugs also tend to collect in the bottom. The effect on performance is uncertain, but with time the sediment may clog the screen and prevent the piezometer from responding to changes in pore pressure.


Screens can be clogged by deposition of minerals such as calcium carbonate. At one dam which had a limestone downstream shell and a good deal of limestone gravel in the borrow soil, 14 of 21 piezometers were found to be unresponsive. Flushing produced white powder. Treatment with acid as described later in this paper restored 10 of them to usefulness.

Embankment or structure settlement can compress and rupture piezometers, especially if the tip rests on or in bedrock. The broken riser may receive water from a layer above the one the piezometer was intended to monitor or lose water to it.

C. Evaluation Begin by reading the water level with the water level indicator as described above. Erroneous readings can occur because

the wall of the piezometer is wet and can cause a false reading. False readings can be reduced by setting the sensitivity control as low as possible. Lower the water level indicator until you are sure it is in the water and the buzzer is sounding. Turn down the sensitivity control until the buzzer goes off; then turn it back up until the buzzer sounds, then just a little farther. Then find the water level several times by raising and lowering the probe. When the buzzer sounds several times at the same depth, read the depth.

Some piezometers will flow because the head is higher than the piezometer top. Pressure gauges are often installed to measure the excess pressure. Read the pressure in psi, multiply it by 2.31, and add it to the elevation of the gage to get the piezometric elevation in feet. Gages must be insulated to prevent freezing, and they tend to deteriorate quickly. The groundwater may carry air or other gas which collects in the riser. The air must be bled off through a tee and valve and the pressure allowed to stabilize before reading to eliminate the error. Another way to read a flowing piezometer is to add a temporary riser extension. Measure from the top of the riser and add the length of the riser to the calculated water elevation. If the water level rises higher than you can reach, fasten a length of transparent tubing and a scale to a long rod with a scale attached. At one dam, a thin layer of sand about 70 feet deep had a piezometric elevation about 20 feet above the top of the downstream berm. Clear plastic tubing was attached to “telephone poles” fitted with scales from survey rods to allow the piezometers to be read with binoculars (Figure 8). The accuracy was probably better than most pressure gages, and freezing did no damage.

Figure 8. Piezometer riser extended with clear tubing on pole.

Next lower a sounding rod on a second fiberglass tape (Figure 6). The rod should be steel, about ½-or 5/8-inches in diameter and one or two feet long overall. Feel for the first reduction in weight; that is the top of the sediment. Lower the rod until you feel a solid stop at the bottom. If the rod stops in sediment, you may be able to get it down to the bottom by working it up and down. Record both depths. The difference is the amount of sediment. Compare the bottom depth to the expected depth based on the installation records in the dam plans or the maintenance manual. If the measured distance is too short, there may be several feet of dense or stiff sediment, or the riser may be broken. Sometimes the rod will stop on an obstruction such as a pebble or a knob of rust. You may be able to judge what the obstruction is by the way it feels and sounds.

Run a time lag test to evaluate how well the piezometer communicates with the soil stratum around it. Pour in enough water to raise the water level about 10 feet. Quickly insert the water level indicator and measure the depth to the water surface. Note the time, and record both the depth and the time it was measured (see Table I). Take another reading 30 seconds after the first one, then one minute, then two minutes, four minutes, etc. Continue until the water level has declined at least two-thirds of the initial change. If the intervals become long, move on to another piezometer and return for the later readings. The exact


time intervals are not critical; just be sure to record the time each reading is taken. (Note: Do not use distilled water; it is non-conductive and won’t activate the water level indicator.)

TABLE I

Sample of Data and Data Reduction for Standard Time Lag Test

Note: H=Initial depth to to water-current depth

Head ratio =Current H/initial H If the normal water level in the piezometer is near the top, you can remove water with a bailer or displace it with a rod

instead of pouring water in. The principle is the same. Reduce the readings using a spreadsheet as shown in Table I and plot them as shown in Figure 9. The points should form

a straight line, but don’t be concerned if it curves a little; many factors affect the recovery rate. Determine the Standard Time Lag [1] as the time when 37% of the initial change remains. For a piezometer installed in sand, this is usually a few minutes or a few tens of minutes. For a piezometer installed in clay It could be hundreds of minutes (you may have to extrapolate). If the water level does not recover, the piezometer has a clogged screen, and the readings probably don’t accurately reflect the pore water pressure.

Note: Head Ratio is plotted on a logarithmic scale.

Figure 9. Graphed results of standard time lag test.


D. Rehabilitation The sediment can be flushed out and the screen washed by inserting a piece of ½-inch PEX tubing to the bottom and

pumping in water. Use potable water or water sterilized with one gallon of chlorine bleach per 500 gallons of water to avoid introducing bacteria into the piezometer. A 55-gallon barrel and a small domestic well pump and pressure tank can be used for flushing (Figure 10). A 2-inch gas-engine centrifugal pump will also work. Cut the end of the PEX tubing at an angle so you can work the sediment loose by twisting the tubing (Figure 11). You may be able to judge the nature of the sediment or blockage by the feel of the tubing.

Figure 10. Electric pump and pressure tank used for flushing piezometers.

Figure 11. Flushing soil from piezometer with 3/4-inch PEX pipe.

Watch the soil being washed out. White sand with all the grains the same size is probably from the filter pack placed

around the screen and may indicate that there is a hole in the screen. Let the piezometer set overnight or longer to stabilize, then re-run the time lag test. Record the results of both tests and

other data as shown in (Table II). This data forms a base line to compare with when you test the piezometer again in a few years.


TABLE II Example of a Summary Table of Piezometer Data

If the time lag seems too long, you can try to clear the screen or develop the piezometer by jetting using a home-made jet

nozzle on the PEX pipe (Figure 12). Better results may be obtained using a jetter on a pressure washer, but take care not to subject the piezometer to high pressure. You can also try surging; raising and lowering a surge block up and down in the riser.

Figure 12. Jetter made by plugging and drilling PEX pipe to clean piezometer screens.

The surge block is a piston that fits loosely inside the riser (Figures 13 and 14). These are best for PVC piezometers; as

they may tend to snag on a joint in steel pipe. The surge block can be inserted on threaded PVC pipe or on ½-inch or ¾-inch PEX pipe. Do not apply too much force; you can generate a lot of pressure in a small-diameter riser.


Figure 13. Surge block for 2-inch piezometers made from 1-1/4-inch PVC pipe and fittings.

Figure 14. Surge block for 1-1/4-inch piezometers made with ½-inch all-thread rod, washers and nuts, threaded into ½-inch PEX Pipe.

You can also try developing the piezometer by pumping the water out and letting it recover repeatedly. There are 12-volt

submersible pumps that will fit inside 2-inch risers, or you can use an airlift. An airlift consists of a pipe and a smaller tube connected to an air compressor to feed air into the bottom of the pipe. The bubbles reduce the weight of the water column in the pipe, and water enters the pipe at the bottom, pushing water out the top. The greater the submergence of the tip of the pipe, the higher the water can be raised. For pumping a relief well, the pipe can be 1-inch to 4-inch diameter PVC (Figure 15). For a piezometer, it can be ¾-inch diameter PVC or even a garden hose.

Figure 15. 2-inch air lift with 3/8-inch air hose. Fittings end in a 1/8-inch nipple directing air up into the pipe.

As a last resort, you can blow the water out with compressed air and the PEX tube. This, however, may push air into the

formation and make the piezometer behave oddly for a while. If flushing brings up white powder, the screen may be clogged with calcium carbonate. Muriatic acid, available in the tile

or concrete department at building supply stores, can be used to dissolve the precipitate. The fixture shown in Figures 16 and 17 can be used to blow nearly all the water out. Air is forced into the piezometer riser to push the water up through the PEX pipe inserted to the bottom. Pour a 1:1 mixture of water and acid into the screen and agitate the acid with a slow flow of compressed air for about 30 minutes. Then inject fresh water through the PEX pipe to flush out the acid. Catch the strong acid and store it in a drum for proper disposal. Diluted acid with a pH greater than 6.5 can usually be disposed on the ground.


Figure 16. Fixture made to treat clogged piezometers with acid.

Figure 17. Acid-treatment fixture connected to piezometer riser with hose and clamps.

If the screen is clogged and can’t be cleaned, or if the screen is crushed or the riser is broken off, or if water is entering

through a hole in the side of the riser, the piezometer may have to be abandoned and, if it is essential, replaced. Try to set the screen of the replacement at the same elevation so the readings can be expected to be similar to those obtained when the existing piezometer was good.

E. Using the Data Set up a spreadsheet similar to Table III to manage and summarize the data. Determine the elevation of the top of each

riser from the plans, records, or a survey. Subtract the depth to the water surface from the top elevation to get the water surface elevation, which is the piezometric elevation of the water in the soil pores at the location and elevation of the screen. Piezometric elevations are more meaningful than the depth to water because they can be compared to the lake level (headwater), the water level at the dam toe (tailwater), the piezometric elevations at other piezometers, and the values predicted or assumed in the design analyses.


TABLE III. Spreadsheet for Managing Piezometer Readings.

Using the spreadsheet, plot the piezometric elevation of each piezometer against time to watch for changes (Figure 18). It

is also useful to plot piezometric elevation vs. headwater elevation and vs. tailwater elevation (Figure 19). Usually, the piezometers located closest to the lake are most strongly affected by the lake elevation and those closer to the downstream toe are most strongly affected by the tailwater level.

Figure 18. Chart of piezometric elevations vs time.


Figure 19. Plot of piezometric elevations vs headwater elevation.

Compare the piezometric levels associated with a normal lake level over time. Gradual reduction in piezometric elevations

over several years may indicate that new sediment deposited on the lake bottom is retarding seepage. This is normal. A gradual rise may indicate that relief wells or drains are becoming clogged, which can become serious. Sudden changes not related to changes in the headwater or tailwater elevation may indicate failure of a well or drain or development of cracks in the dam.

Draw a cross section through the dam and plot the piezometer locations and the piezometric elevations on it as illustrated in Figure 20. Try to compare it to one of the sections in Figures 1 through 4.

Figure 20. Piezometer locations and piezometric elevations shown on a cross section of the dam.

Piezometric elevations typically decrease as the piezometers are farther from the lake and closer to the tailwater because

the water loses energy as it seeps through the soil. Typically, all piezometric elevations fall between the headwater and tailwater elevations. Exceptions are possible, but piezometers that read higher than the lake surface or lower than the free water surface near the toe of the dam should be viewed with suspicion and investigated further.


Data that indicates a possible problem with the dam or cannot be explained should be promptly reported to a qualified dam engineer or geotechnical engineer.

IV. RELIEF WELLS

A. Description and Purpose Relief wells are installed near the toe of the dam to provide a filtered outlet for water seeping under the dam and to relieve

excess pressure that could cause slides of the downstream slope, heaving of shallow clay layers, or sand boils that can cause subsurface erosion called piping. A relief well schematic looks like a big piezometer, including a screen, riser, filter pack, and seal. Most flow like artesian wells, which they are. Some discharge right out the top or through a tee. Some are connected to a header pipe that collects the flow from a line of wells and discharges it into a stream or toe ditch. The lower the elevation of the outlet, the more effective the well can be in reducing the uplift pressure acting against the base of the dam. See Figures 21 through 25.

Figure 21. Relief well schematic.

Figure 22. Relief well manhole on right and collector pipe manhole on left.


Figure 23. Manhole with 12-inch PVC collector pipe and 6-inch feeder from one well.

Figure 24. Relief well with vertical check valve on the ground to the right.

Figure 25. Relief well in manhole with discharge pipe in toe ditch.

In some cases, pumps are installed in relief wells to increase the drawdown of the aquifer. Pumps require some type of

controller to keep the pump from running dry and ruining the motor. Both pumps and controls need maintenance. The water levels should be checked frequently to verify that the pumps are keeping the water level within the intended range.


B. Evaluation Start by measuring the discharge rate if the well is flowing, or the water level if it is not. Sound the well to determine the

total depth and the amount of sediment. Run a caliper down it to check for screen damage. The caliper in Figure 26 was made with a 4-inch cleanout adapter, pipe nipple, and wire.

Figure 26. Well caliper used to check for screen damage.

The water discharge from each well can be monitored over time to detect loss of performance. Flow can be measured with

a bucket and stopwatch if there is room at the outlet. If the water discharges through a tee, a bailer (Figure 27) can be used to keep the water level just below the tee for a given period of time. Measure the volume removed with the bailer during that period.

Figure 27. Old fashioned well bailer useful for measuring discharge rate of relief well.

The effectiveness of the relief well system is best determined by installing piezometers beneath the area of interest, often

midway between wells. Look at piezometric elevations obtained at times when the head water and tailwater conditions are similar, for example, with a full lake and normal tailwater. A gradual rise in piezometric elevation can indicate gradual clogging of the well screens. A decline in piezometric elevation under similar conditions may indicate that the water has found another exit, possibly from piping.

The piezometric elevations can be compared with the expected conditions used in the design analyses or used in new stability analyses.

Run a pump test with a submersible pump, a trash pump with a suction hose, or an air lift (Figure 28).


Figure 28. Pump testing a well with submersible pump, generator, and calibrated barrel.

Pump the well at a more or less constant rate for a given period of time such as 30 minutes. Measure the drawdown (the change from the static water level) and the discharge rate every 10 minutes. A calibrated plastic barrel and stop watch are useful for measuring the discharge rate. You may need to hang a garden hose or a string of 3/4” PVC pipe in the well and run the water level indicator down inside it if the water is turbulent. The discharge divided by the drawdown is the “specific capacity”, expressed in gallons per minute per foot of drawdown. It is a useful measure of the productivity of the well. More importantly, it is a repeatable index that can be compared from year to year to detect changes in the well’s performance.

More sophisticated pumping tests can be run to produce information about the aquifer and the well’s resistance to flow. See [2] or standard groundwater textbooks if you are interested.

Check the discharge pipe from the well to the outlet, if there is one. They can become clogged with sediment or bacterial sludge or converted into homes by critters. Check the outlet grates and check valves. Check valves protect the well from sediment and pressure surges during floods or high releases. Corrosion is tough on them. A diver was asked to examine a submerged check valve. He tried to open it, and it broke off in his hand (Figures 29 and 30).

Figure 29. 6-inch check valve in spillway wall.


Figure. 30. Corroded check valve broken off by diver.

Sometimes relief wells are installed beside the spillway and pipes are run through the spillway wall to discharge into the

chute or stilling basin. Rigid pipes such as cast iron can be broken off when the backfill settles (Figure31). Repairs can be difficult if the pipe is buried deep, especially if it is below the groundwater (Figure 32).

Figure 31. 6-inch cast iron well discharge pipe cast in spillway wall, broken by backfill settlement.


Figure 32. Replacing 6-inch C.I. discharge pipes broken at spillway wall with 4-inch PVC pipes

C. Treatment and Repairs Pipe cleaning companies such as some Roto-Rooter franchises can readily clean the pipes with high-pressure jets that

propel themselves and their hoses several hundred feet into the pipe (Figure 33). Begin at the discharge end and work upstream. If such services are too expensive for your budget or your dam is just too far out of town, you can build a low-pressure jetter by drilling a few small holes in PVC pipe, feed it with a 2-inch or larger centrifugal pump, and push it in on 1-1/2-inch PVC pipe. Stiff clay, however, requires high-pressure jets. After cleaning, have the pipe cleaning crew run their video camera up the pipe to check for breaks, open joints, sags, and corrosion.

Figure 33. Cleaning well discharge pipe with high-pressure jetter.

Well screens and filter packs can become clogged with time by collecting fine sand, silt or clay grains; by deposition of

minerals such as calcium carbonate or iron oxides; or by development of bacterial colonies and the slime they form for protection. Clogging by soil grains can be addressed by renewed development activities such as surging with a gorge block, brushing, jetting, air flushing, or hard pumping. A drill rig or well service rig is useful for this work, but if the locations are


inaccessible or funds are short, considerable cleaning can be done by hand using purchased or home-made surge blocks and jetting nozzles with trash pumps or pressure washers. Ropes and pulleys hung from a tripod, small crane, or manure bucket can help reduce the strain on backs (Figures 34 and 35).

Figure 34. Brushes for 6-inch well screens.

Figure 35. Tripod, motor, and cathead used for brushing, surging, and air-lifting.

Air flushing consists of lowering an air hose weighted with a length of pipe into the well and turning the air on suddenly,

blowing much of the water out of the well. The pressure pushes water out into the aquifer; then the drop in water level causes a strong inflow. The process is repeated, or the air is allowed to continue flowing, causing turbulence and periodic discharge. Air flushing is a cheap and popular development method. But be careful! The pressure can rupture screens and risers of PVC, light construction, or corroded carbon steel. This method should not be used on such materials.


While developing, pause periodically to remove the sediment. If the static water level is shallow, a trash pump with a polyethylene suction pipe inserted to the bottom of the well can be used. Air lifts work well if the water column is long enough and the well produces a good flow (Figures 36 and 37). If flow is slow, a vacuum truck can be used.

Figure 36. 3-inch airlift pipe with two air injection fittings.

PVC pipe in 5- or 10-foot lengths fitted with male and female threaded adaptors can be run into and out of wells with a small tripod, rope and pulley, or even by hand. Plywood forks (Figure 38) can be used to hold the pipe while connecting or removing sections.

Figure 37. Removing sediment from a relief well with a 3-inch air lift.


Figure 38. PVC pipe with threaded adaptors, plywood forks, and tools for coupling pipe.

If the discharge from the well leaves red, orange or black deposits (Figure 39); or if the well is not improved by mechanical

cleaning and development; it may be clogged by bacterial growth. Bacterial infestation is serious; it can severely reduce the flow or block it entirely. The colony can grow exponentially, so it can be there for years without noticeable effect, then block the well over a period of months. The U.S. Army Corps of Engineers treats wells subject to bacterial clogging on a rotating 5-year cycle. Treatment requires acid and dispersant to break up the slime and dissolve the mineral deposits the bacteria create, mechanical re-development to remove the solids, and sterilization to reduce the colony and make the well inhospitable for a while. Reference [2] has a good chapter on the subject, and [3] is a complete book on the subject. The issue is complex. Treatment chemicals and the gas they produce are dangerous. Specialized assistance is recommended. Find a well driller or licensed pump installer that does this work regularly. Well material suppliers can help you find one.

Figure 39. Relief well discharge pipe with bacterial deposits.


Some wells were built with carbon steel screens or with stainless steel screens and carbon steel risers that can corrode and rupture. A professional well driller may be needed to repair or replace such a well. If such problems develop and there are a number of such wells, pre-emptive action can be taken by lining the wells with screens and risers of smaller diameter (Figure 40). The well should be cleaned and re-developed first, then the new screen and riser can be installed, and the annular space can be filled with filter sand. The sand will provide support to the outer screen and minimize the amount of sand and soil that can enter the well if the weak part ruptures.

Figure 40. Lining a 6-inch well with a 4-inch screen and riser.

If a well fails and can’t be repaired, a replacement well can be drilled nearby. If possible, leave enough room between the

two wells to accommodate flaring the trench needed to install the new discharge pipe and connect it to the existing pipe. In most states, new well installation and old well abandonment are required to be done by a licensed water well driller and registered with the state.

V. DRAINS

A. Description Embankment dams are often provided with toe drains and/or blanket drains to lower the phreatic surface and create a

filtered outlet to safely dispose of water that seeps through the embankment. Chimney drains serve a similar purpose and also halt internal erosion caused by transverse cracks in the embankment. Perforated drain pipes are often included to collect the seepage and discharge it into ditches or into a header pipe. The pipes also create an opportunity to measure the seepage rate and monitor it for changes.

Underdrains consisting of perforated pipes in layers of sand and gravel are constructed under the downstream portions of spillway structures to provide a filtered outlet for water seeping through the dam or the foundation and to reduce hydrostatic uplift that can heave the floor, float the structure, or cause sliding of the structure or associated soil slopes. (Figure 41) The drains may discharge downstream or alongside the spillway or into the structure itself through openings in the floor or walls (Figure 41). Such openings can be subjected to strongly fluctuating back pressures during flood releases, so they may be protected by check valves or turbulence suppressors.


Figure 41. Spillway cross section showing underdrain pipes.

B. Problems Sediment can collect in the embankment drain pipes when tailwater or storm water backs up into the pipes. Check valves

intended to keep storm water out may corrode and fail. Soil may be eroded internally from the embankment into the pipes if the filter is not properly constructed or becomes damaged. Corrugated steel pipes can become damaged by corrosion (Figure 42).

Figure 42. Corrugated metal pipe toe drain outlet damaged by corrosion.

There may be unseen damage to the drain system. Rigid pipes entering a rigid structure cannot tolerate much differential

settlement and may break near the face of the structure (Figures 31 and 32). At one dam, cleanouts were provided through the floor of the stilling basin to allow cleaning the underdrains. The recesses housing the cleanouts were covered with heavy cast-


iron covers bolted down to the frames (Figure 43). Time, corrosion, and turbulent water loosened and tore the covers off, but the damage went unnoticed because the basin was always full of water. Diving revealed that eight of ten covers were missing and the cleanout pipes had never been fitted with plugs.

Figure 43. Underdrain cleanout and cast iron cover.

Underdrain pipes and/or filters can become clogged with sediment or bacterial slime. Pipes can become damaged and

allow soil to enter. Gravel toe drains not properly designed as filters can become clogged with the soil they are intended to drain.

The effectiveness of the drain systems is best evaluated by monitoring the pore water pressure with piezometers screened in the zones of soil being drained. If piezometers were not installed in the downstream slope or blanket drain during construction, they can be added. Piezometers of galvanized steel pipe and brass wellpoints were often installed beneath spillways and then cast into the concrete structure. These piezometers are especially subject to corrosion just below the concrete/soil contact, and many have failed.

Depending on the configuration of the drain, it may be possible to install new or replacement piezometers adjacent to the structure and measure the pressure in the underdrain. If necessary, holes can be cored through the floor of the structure to install piezometers in or below the underdrain.

C. Treatment and Repairs Embankment drains often have discharge pipes extending to the toe of the dam every few hundred feet. These can be jet

cleaned as discussed above for relief well discharge pipes. They can also be inspected using video cameras. Push cameras have stiff cables that can be used to push the camera in, perhaps as much as 200 feet. Camera vehicles, both wheeled and tracked, are available for longer runs up to about 500 feet (Figure 44). Good cameras can obtain excellent pictures of pipe interiors as long as the camera is above water (Figure 45). Photography in dirty water can be disappointing. Many of the vehicle-mounted cameras can pan and tilt, allowing a look around a bend too tight to be passed. Sometimes it is best to use the camera before jetting to be sure there is not damage that might be made worse by jetting. Better pictures of the drain conditions are obtained, however, after cleaning.


Figure 44. Pipe inspection vehicle with pan/tilt camera.

Figure 45. Interior of 12-inch PVC collector pipe photographed with underwater camera and vehicle.

If a toe drain has manholes or cleanouts to provide access to the longitudinal pipe, the pipe can be cleaned with high

pressure jet nozzles that propel themselves by the impulse of the jets and pull the hose behind them. More care is needed when cleaning perforated pipes than unperforated discharge pipes. Reducing the pressure is prudent to avoid disturbing the gravel filter surrounding the pipe. If a significant amount of non-native sand or gravel is being removed, stop cleaning!

Spillway underdrains can be harder to access. Jetters on hoses may be able to negotiate pipe bends but unable to lift coarse sediment up to the outlets. It may be feasible to bring the sediment to the end of a horizontal section with the jetter and then remove it with the suction hose on a trash pump or with a vacuum truck. Silt and bacterial sludge can be flushed out by pumping a high flow of water through the pipe. If an underdrain has two outlets, one can be blocked and water pumped or air-flushed from the other to cause a strong inflow of groundwater into the drain (Figure 46). If groundwater is the only possible source, and a strong flow can be generated, the drain system is probably working well.


Figure 46. Air flushing water from an underdrain to remove sediment.

VI. CAUTIONS Drains beneath structures can be very difficult and expensive to repair, so they should be handled gently to avoid damage.

The adage, “First, do no harm.” applies. If a drain flows freely and the piezometric levels are acceptable, it doesn’t need to be perfectly clean.

Air flushing done gently can cause a steady flow of ground water up a well and out the discharge pipe, accomplishing modest cleaning of both. A larger flow of air can cause turbulence to help re-develop the well. Lowering an air hose to the bottom of the well and abruptly turning the air on fully, however, can damage a fragile well beyond repair.

Flushing with water and PEX tubing is a convenient way to remove sediment from a piezometer. If the water stops returning, however, it may be fracturing the formation, creating seepage paths, or destabilizing the slope. Stop flushing if no water is returning.

Dam maintenance staff members often have numerous skills and can perform much of the necessary evaluation and maintenance tasks carefully and economically. However, the potential undesirable results of some actions are not anticipated without considerable experience. It is best to have a geotechnical engineer or dam designer help develop an action plan and demonstrate it to the crew. The discussion should include clear constraints on what should and should not be done and red flags that require that the work be suspended until the consultant returns.

VII. REFERENCES 1. Time Lag and Soil Permeability in Ground-water Observations, by M. Juul Hvorslev, Bulletin No. 36, Waterways

Experiment Station COE Vicksburg MS.

2. Sterrett, Robert J. Groundwater and Wells. Third ed. New Brighton, MN: Johnson Screens, 2007.

3. Schnieders, John H. Chemical Cleaning Disinfection and Decontamination of Water Wells. First ed. St. Paul, MN: Johnson Screens, 2003.


VIII. AUTHOR BIOGRAPHIES Charles N. Easton, P.E. Senior Geotechnical Engineer Freese and Nichols, Inc. 4055 International Plaza, Suite 200 Fort Worth, TX 76109 (817) 735-7335 cne @freese.com

Chuck Easton was raised on a small farm in southern Iowa and earned a B.S. in Civil Engineering and an M.S.in Geotechnical Engineering from Iowa State University. He practiced geotechnical engineering with Woodward-Clyde Consultants in Omaha for 25 years on projects ranging from schools and grain storage structures to hospitals, office towers, water and wastewater plants, and bridges. He provided geotechnical services to Freese and Nichols, Inc. in Texas through Woodward-Clyde for 4 years before joining Freese and Nichols in 1999, where he remains. Projects include geotechnical engineering for new and old dams, pump stations and pipelines, and water and wastewater treatment and storage facilities. He has performed comprehensive hands-on evaluation and maintenance of dam instruments, relief wells, and drains at more than a dozen major Texas dams, often working with local dam staff and occasional subcontractors. Life Member ASCE.

Russell Glenn Springer Project Geotechnical Engineer Freese and Nichols, Inc. 4055 International Plaza, Ste 200 Fort Worth, TX 76123 [email protected]

Russ Springer is a registered professional engineer with over 8 years of experience in design, construction, and rehabilitation of dams and hydraulic structures. Mr. Springer is an engineering graduate from the University of Missouri-Rolla with a degree in Geological Engineering. Subsequent to completing his bachelor’s degree, Mr. Springer worked for Terracon as a geotechnical engineer and construction materials testing project manager in both St. Louis, Missouri, and Tulsa, Oklahoma. For 3-1/2 years, he worked for SGT, LLC as a field engineer at the Callaway Nuclear Plant in Fulton, Missouri, as part of a steam generator replacement project. Mr. Springer joined Freese and Nichols, Inc. in 2007 and has since been involved in several dam projects, primarily in Texas. In addition to performing periodic dam inspections, he has been involved in the design, implementation, and documentation for rehabilitation projects involving dam instrumentation and underdrain systems.

Documents

What’s in Your Piezometer? Evaluating and …€™s in Your Piezometer? Evaluating and Maintaining Piezometers, Relief Wells, and Drains Charles N. Easton, P.E., Senior Geotechnical