Groundwater Lowering and Drainage Techniques, Hydrogeology, Civil Engineering

7/29/2019 Groundwater Lowering and Drainage Techniques, Hydrogeology, Civil Engineering

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CHAPTER 1

GROUNDWATER LOWERING ANDDRAINAGE TECHNIQUES

1-1 COMMON REASONS TO LOWER GROUNDWATER LEVELSCom mon reasons to low er groundw ater levels are for construction excavations andfor permanent structures that are below the water table and are not waterproof or arewaterproof but are not designed to resist the hydrostatic pressure. Permanent de-watering systems are far less commonly used than temporary or construction dew-atering systems. When construction below the water table is planned, choices fordealing with this problem include construction in the wet (Le., with water orsome other type of fluid remaining in the excavation during construction, see Figure1 - 1 ) , use of cutoff walls, which limit inflow into the excavation (Figure 1-2), orlowering of the groundwater levels to reduce the hydraulic head and hence theinflow into the excavation (Figure 1-3). Even when cutoff walls are used, dewater-ing within the confines of the cutoffs may still be required to improve the stability ofworking areas, but probably to a lesser extent.This chapter deals primarily with techniques used for lowering groundwaterlevels and related issues, such as effects on adjacent structures and the use ofimpermeable barriers in combination w ith groundwater lowering techniques. In thischapter, as well as in others in the book, the reader will find overlap betweensubjects. For instance, grouting can be used in conjunction with dewatering toreduce the quantity of water inflow into the excavation. Grouting is addressed inChapter 7. Drainage trenches, cutoff walls, and ground freezing are often used inconjunction with or in place of dewatering systems and are addressed in Chapters 3,10, and 11 , respectively. The art of ground improvement is now beyond the stage ofunique application to special individual problems and has m oved into the realm of

1

Copyright 1994 John Wiley & Sons Retrieved from: www.knovel.com


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2 GROUNDWATER LOWERING AND DRAINA GE TECHNIQUES

A ir l i f t

Figure 1-1Hill .) Construction in the wet. (From Xanthakos, 1979, by permission of McGraw-

Ex is t ing g round l eve l7

w i t h i n e x c a v a t i o n

Fsgure 1-2 Qpical water cutoff wall. (From Xanthakos, 1991 .)

Or;g;no/ Wofer 7%/e----- L --

Figure 1-3 Typical excavation dewatering system. (From Peck et al . 19 74 .)



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1-1 COMMON RE ASONS TO LOWER GROUNDWATER LEVE LS 3multiple uses and purposes on a wide variety of construction problems. The follow-ing discussions specifically address issues related to groundwater lowering or dew-atering; pertinent information related to other subjects is referenced as appropriate.Construction DewateringConstruction dewatering is most often used by contractors to decrease water inflowinto excavations, thereby improving working conditions in the excavation and in-creasing the stability of soils in the sides and base of the excavations. The height ofgroundwater above the base of the excavation (Le., the anticipated head and thus theamount of inflow) will dictate the methods used. A contractors experience andavailable equipment will also affect the methods chosen. Dewatering is a necessaryevil. It is avoided to the extent possible because of cost, disruption to other construc-tion tasks, schedule constraints, discharge and disposal requirements, sensitivity todischarge water quality, potential effects on water supply, and potential effects onadjacent structures. Often construction below the water table cannot be avoided andwill cost less than other alternatives such as impermeable excavation supports (e.g.,slurry walls).The most common dewatering methods chosen by contractors are: sumps,trenches, and pumps; well points; and deep wells with submersible pumps.

Briefly, the first method involves handling minor amounts of water inflow into anexcavation by channeling the water to trenches and sumps and then pumping thesumps out with a submersible pump as necessary to keep the excavation bottom dryand stable. This method is usually used where the height of groundwater above the

er

,Sand andgrave/flJfer

Head

Figure 1-4 Sp ic a1 well point pumping system . (From Peck et a l . , 1974.)



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4 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUES

Figure 1-5 Typical well point system. (From Johnson, 1975.)

excavation bottom is relatively small (5 f t or less) and the surrounding soil mass isrelatively impermeable (clayey soil for instance).The well point method involves multiple closely spaced wells connected by pipesto a strong pump that can suck the water out of the ground through the well pointsvia the header manifold pipe, through the pump, and out of the discharge end ofthe pump (Figure 1-4). Multiple lines or stages of well points are required forexcavations that extend more than about 15 to 20 ft below the groundwater table(Figure 1-5). Ejectors or eductors can be used to enhance the capabilities of a wellpoint system but require careful design for maximum efficiency at the anticipatedhead and discharge conditions.The most common alternative to using well points is use of deep wells withsubmersible pumps. In this method, the pumps are placed at the bottom of the wellsand the water is discharged through a pipe connected to the pump and run upthrough the well hole to a suitable discharge point. These wells are usually morepowerful than well points, require a wider spacing and therefore fewer well holes,and can be installed farther outside of the excavation limits. Deep wells are usedalone or in combination with well points.Permanent DewateringAnyone who has a sump pump in the basement has, in a crude way, a permanentdewatering system. When groundwater or rainwater rises to a predetermined level inthe basement sump, a pump automatically switches on and pumps the water levelback down to another predetermined level that is more tolerable. This is more or lesshow a permanent dewatering system works. Most structures built below the ground-water level leak. There is a need, therefore, to dispose of the leakage with sumpsand pumps. Also, permanent dewatering systems can be used where the structure is



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1-2 DESIGN INPUT PARAMETERS 5

Figure 1-6 Buoyancy effects on underground structure.

far below the prevailing ground wate r level outside the structure a nd w here it is m oreecono mic al to dewater than to design the struc ture walls and sla bs to resist the w aterpressure outside and also to counteract the buoyancy effects (Figure 1-6). In thiscas e, the hig her initial capital cost of building a stron ger and heavier struc ture mu stbe traded off against the future operating and maintenance costs.

1-2 DESIGN INPUT PARAMETERSThe most important input parameters for selecting and designing a dewateringsystem are the height of the groundwater above the base of the excavation and thepermeability of the ground surrounding the excavation. To know the depth ofgroun dwa ter lowe ring, one must know what the prevailing grou ndw ater levels are atthe site and the depth of excav ation. Th e groundw ater level is usually lowered to atleast 2 ft below the bottom of the excavation. The field permeability of the groundmust be known to estimate the amount of pumping, or flow rate, that will berequired to attain the required groun dw ater level. A lso of importance is the shape ofthe dewatered zone, often referred to as the cone of depression. The cone ofdepre ssion must encom pass the excavation limits or the excavation bottom will beonly partially dewatered (Figure 1-7). Water quality must be known too. Theamount of discharge from a dewatering system is usually significant. If treatmentwill be required, this must be known in advance of construction.



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6 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUESHeader

Figure 1-7 Partially dewatered excavation.

Narrow utility trench

Large building excavationFigure 1-8 Comparison of dewatering requirements.



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1-2 DESIGN INPUT PARAMETERS 7Existing Groundwater Levels and FluctuationsA good starting point in assembling the information necessary to select and design adewatering system is to determine where the prevailing groundwater level is at thesite. This is usually accomplished with observation wells or with piezometers,instruments installed in boreholes to sense piezometric surfaces in the ground. If thesite is near a large body of water such as a river, lake, or ocean, chances are that thegroundwater level is at or near the river, lake, or sea level. Also, variations in waterlevels due to floods, storms, control structures, and tides must also be considered.Another important consideration is the presence of artesian (high pressure) andperched water, which can exhibit piezometric levels different from that expected.Any offsite pumping for water supply, hazardous waste remediation, or other con-struction projects should be known and evaluated for possible effects on the de-watering system being considered.The most conservative approach would anticipate the highest water level possibleduring construction. One might also want to weigh the effects of assuming a lowerwater level with the potential economic impacts of that lower level being exceededduring the construction duration. That analysis usually results in the decision to beon the safe side by assuming the high water level. The potential costs of excavationflooding, construction delays, or the emergency mobilization of additional equip-ment usually far exceed the initial costs of mobilizing larger equipment or a fewextra wells.Depth of Required Groundwater LoweringThe required depth of groundwater lowering is usually related to the bottom of theexcavation. The water level should be lowered to about 2 to 5 f t below the base ofthe excavation. If the absolute bottom of the excavation is not known, a conserva-tive (i.e., lowest possible) estimate should be made. This should include any over-excavation required for footings, slabs, and shafts. The maximum depth of ground-water lowering is then the difference between the prevailing groundwater level andthe required level during construction.Zone of Groundwater LoweringThe zone of groundwater lowering involves not only the depth but the three-dimensional shape required. For instance, the requirements for a thin, linear, utilitytrench will be different from those for a large, square parking structure (Figure 1-8).The limits of the excavation must be known or estimated. For a long linear excava-tion, the entire site may not need to be dewatered at the same time.PermeabilityPermeability or hydraulic conductivity is the rate of water movement through theground at a hydraulic gradient of one. Common values for a variety of soil and rocktypes are shown in Table 1-1. Of the parameters needed for dewatering system



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8 GROUNDWATER LOWER ING AND DRAINAGE TECH NIQUESTABLE 1-1Formation Value of k(cmlsec)

Permeability Values for Common Soils and Rocks

River DepositsRhone at GenissiatSmall streams, eastern AlpsMissouriMississippi

Outwash plainsEsker, Westfield, Mass.Delta, Chicopee, Mass.Till

Dune sandLoessLoess loam

Glacial Deposits

Wind D eposits

Lacustrine and Marine Offshore DepositsVery fine uniform sand, C, = 5 to 2Bull's liver, Sixth Ave., N. Y. , C, = 5 to 2Bull's liver, Brooklyn, C, = 5Clay

Up to 0.400.02-0.160.02-0.200.02-0 * 12

0.05-2.000.01-0.130.0001-0.015Less than 0.0001

0.1-0.30.001 2o.oO01 k

o.Ooo1-0.00640.Ooo1-0.00500.m1-0.Ooo1Less than 0.0000001

Porosityk(cm lsec) Intact Rock n (% ) Fractured Rock~~Prac tically im- Massive low-porosity

rocks10-8permeable 10-7Low discharge, Ioor drainage 10-5 Weathered granite'g3 schist10-3High discharge, lo-*free draining lo -'

1.o10'102

0.1-0.50.5-5.05.0-30.0 Clay-filled joints

Jointed rockOpen-jointed rockHeavily fracturedrock

Source: Tenaghi and Peck, 1967 (Wiley).



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1-2 DESIGN INPUT P ARAMETERS 9selection and design, permeability is probably the most elusive and hardest topredict for the field case. Common methods for estimating permeability includeempirical formulas, laboratory permeability tests, borehole packer tests, and fieldpump tests. The reliability and cost of these methods increases more or less in theorder given. Field pum p tests are the most reliable method but also the most costly.It is easy to say that permeability may vary by one or two orders of magnitude.Pump discharge rates are proportional to the coefficient of permeability. Pumpdischarge rates, however, must be predicted within more refined limits than plus orminus two orders of magnitude. So one of the most important parameters needed fordewatering analysis is one of the hardest to predict.TransmissibilityThe coefficient of transmissibility indicates how much water will move through theformation. It is defined as the rate at which water will flow through a vertical stripof the formation 1 ft wide and extending through the full saturated thickness under ahydraulic gradient of 1 or 100 percent. It can be calculated by multiplying thecoefficient of permeability by the thickness of the formation. Comm on values rangebetween 1000 and 1 million gallons per day per foot. The higher the value is, themore water will flow through the formation. Also, formations with higher trans-missibility values will exhibit shallower cones of depression (less drawdown) thatextend farther from the well (larger radius of influence) than form ations with lowervalues at the same pumping rate (Johnson , 1975). The coefficient of transmissibilitycan be determined at a site by conducting a field pump test and recording therelationship between pumping rate and drawdow n, as discussed later.Storage CapacityThe coefficient of storage indicates how much can be removed from the formationby pumping. It is defined as the volume of water released from storage per unit ofsurface area of the formation per unit change in head. For a nonartesian groundwatertable, the storage coefficient is the same as the specific yield of a formation. This isa dimensionless parameter often ranging between 0.1 and 0.35. he coefficient ofstorage of a site is a function of transmissibility and can be determined from a fieldpump test if the coefficient of transmissibility and the time-drawdown relationship isrecorded.Groundwater QualityRequirements on the disposal of water from construction sites have become restric-tive during recent years due to heightened awareness of water quality. It is morecostly now to dispose of construction water and water quality requirements on theeffluent are more stringent than ever. Therefore groundwater quality must be deter-mined before a dewatering effort is undertaken . The quan tity of discharge as well as



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10 GROUNDWATER LOWERING AND DRAINAGE TEC HNIQUESthe quality must be known ahead of time so that discharge facility requirements areknown and any needed treatment can be planned far ahead of time.The actual water quality requirements governing a construction project vary fromlocation to location as well as from agency to agency. The city sewer company mayhave different requirements for discharge into a sanitary or storm sewer than theArmy Corps of Engineers or Environmental Protection Agency may have for Oceanor river discharge. The concept of putting the same kind of water back to where itcame from no longer passes as a justification for minimal processing and handling.Once the water is removed from the ground, it is subject to requirements that aremore stringent than the ones governing its condition in situ. It may have to bedisposed of in a cleaner condition than it left the ground!Another dilemma is determining who has jurisdiction over the groundwater anddischarge and what quality requirements must be met. Governing agencies havingsimilar charges have a variety of names from place to place. For each dewateringproject, there is no alternative to contacting the city, state, and federal agencies eachtime to ascertain the requirements for a specific project. Also, the regulations arechanging almost continuously. The requirements now may be different than theywere last time. On a more positive note, the agencies tend to have knowledgeablepeople who are anxious and willing to help, not only with their own regulations butalso in pointing us in the right direction for determining other agencies regulations.

1-3 INVESTIGATION METHODSThere are certain geotechnical investigation methods that should be used whenplanning or designing a dewatering program. Firstly, conventional borings shouldbe conducted to characterize the subsurface profile in terms of soil and rock types(i.e., gravel, sand, silt, clay, fractured basalt, massive limestone, etc.), lateral andvertical extent and variability of zones and layers, and the location and variability ofthe groundwater table including artesian and perched groundwater conditions. Onecommon characteristic of soils that relates to dewatering is the grain size distribu-tion. For some soils this is a reliable predictor of permeability. Sometimes labora-tory and field tests are run to estimate the permeability of the soil and rock. Finally,the most reliable, and unfortunately the most costly, test method for dewateringprograms is a full-scale field pump test. On large projects, field pump tests areusually cost effective.

BoringsMany geotechnical references (Hvorslev, 1949; Peck et al., 1974; Sowers, 1979;Hunt, 1984) discuss the wide variety of geotechnical investigative methods avail-able. The staple of these methods is conventional borings, usually drilled usinghollow-stem augers or casing and a roller bit. The most common methods of soil



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1-3 INVESTIGATION METHODS 11

Figure 1-9 Examples of geologic profiles.

sampling are the standard penetration test (SPT) with a split-spoon sampler fordisturbed samples and thin-walled Shelby tubes for relatively undisturbed samples.The latter sampling method is more expensive than the former method and is mostcommonly used for strength testing when required. Continuous rock coring isusually the method of sampling used in bedrock.

The three most important pieces of information that must be obtained from aboring program for dewatering are: (1) the lateral and vertical extent and variation ofthe soil and rock deposits at the site; (2) the hydraulic characteristics of each soil androck deposit; and (3) the level and characteristics of the groundwater table. Exam-ples of geologic site profiles are given in Figure 1-9. Dewatering requirements forthese example sites would be quite different from site to site.Important considerations in conducting a boring program include:

Understanding the geologic origins of the siteCareful field classification of the samplesRetention of representative samples for testingInstallation of observation wells or piezometers



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12 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUE S

Entire Borchole Specific Zone

F S T ECTIONS MAY BE PERR)RATEDWITH S I M S OR DRILLED HOLESFigure 1-10 Typical observation well detail. (From NAVFAC, 1982.)

Observation wells (Figure 1-10) and piezometers can be used for sensing where thegroundwater table is and what pressure it is under, as well as for performingborehole field permeability tests. Borehole field permeability tests (Figure 1-1 1) areconducted by adding water to the well (falling head test) or baling w ater out of thewell (rising head test) and timing how long it takes for the well to reestablish anequilibrium condition. The foregoing test methods are used in soil zones. In rock,permeability tests can be run by using inflatable packers (Figure 1- 12) to seal a zoneof rock off and by pumping water into the sealed-off zone.

Grain Size DistributionGrain size distribution of soil deposits affects their permeability and therefore is ofprimary concern to predicting water inflow into an excavation. Relative per-meabilities of a variety of soil and rock types are given in Table 1-1. Th e amount offines in the soil has a significant effect on permeability. A sand with 10 percent fines(10 percent passing a No. 100 sieve) could have 100 to 1000 times lower per-meability than a cleaner sand with no fines (Bush, 1971).



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1-3 INVESTIGATION METHODS 13The most comm on way of running a grain size distribution test in the laboratoryis according to ASTM Test Method D-422 (ASTM, 1990). These are done withdifferent sizes of sieves and a vibratory shaker for coarse-grained soils and a hydro-meter for fine-grained soils. Typical grain size distribution curves are shown in

Figure 1 13. Since there is usually variability from sample to sample, the grain sizedistribution limits or bounds will often be shown for different zones. Suitabledew atering and other treatment m ethods are shown according to grain size distribu-tion in Figure 1-14.

F =SHAPE FACTOR OF INTAKE POINTIN GENERAL: A =STANDPIPE AREA

' OBSERVATION WELL PIEZOMETER

PLOT OF OBSERVATIONS1 o0.90 .89' .73 0 .68 I 0.5

d 0.4I"k 0.30a

v)

cE 1 0.2aIw

0.10 2 4 6 8 1 0TIME, 1 . (ARITHMETIC SCALE)

F igu re 1-11 Typical borehole permeability test. (From NAVFAC, 1982.)



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14 GROUNDWATER LOWERING AND DRAINA GE TECHNIQUE S

packer test in g only)

Perfo rated outer pipeto t es t be tween(Inn er pipe provides

Lower tes t

Figure 1-12 Typical packer test detail. (From Hunt, 1984.)

From grain size distribution testing, the grain size of the sample where only10 percent of the sample passes the sieves, or the D lo size, can be determined.Hazen (1893) found that, for uniformly graded filter sands, permeability is roughlyequal to (Dlo)z in centimeters per second when D ,o is in millimeters. q i s elation-ship agrees with other studies (USACE, 1956) but should be used with caution,especially in fine-grained soils and in soil deposits with a high degree of vari-ability.



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U.S. STANDaRD SIEVE SIZE3" 1.6" 3/4' W E ' 84 811 028 8 41 868 8188 8288

PARTICLE SIZE IN MILLIMETERS

B A R T - Pi t t sbu rg -An t iochGRAIN SIZE ANALYSIS

Figure 1-13 Example of grain s ize distribution curves.



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Clean Gravels Very Fine Sands ,! 4

lean SandsCoarse Fine

Silts, organic & inorganic

lc - -and.gravel Mixtures, T il l Varied Clays. etc.Horizontal vertici'Sand.Silt.Clay M ixtures, T illk 4

Excessive water Largs dia. wells Educator Vacuum systems. Vacuum plus Dewatering usuallyyields. wide wide spacing wells. low yields electrwsmosis not requiredspacingLoss of compressed air /Sand.cement Freezing possible throughout

B ituminws Grouts-

narrow narrowspacing spacing E ectroosmosis. electrochemical stabilizationPOSSIELE OEWATERING METHODS

Cement- Colloidal Grouts- Polymers.Betonite- C hromeL ignin- not required

Silicates. J uasten-uspensions Colloidal Solutions

>J & V?t- 0 i& < s jz z ; :

Figure 1-14 Treatment methods according to grain size. (From McCusker, 1982.)

Example Problem 1-1Given: Grain size distribution curves below:

Calculating Permeability from D,, Size

3" 1.8"314"318" 04 010 020 04006001000200100908070

2 60," 5022 40

3020100

c0

0.1 0.01 0.16



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1-3 INVESTIGATIONMETHODS 17Required: Permeability values for the five soil samples.Solution: Permeability, k(cmlsec) =D fo (mm)Curve Number Soil Type D,, (mm) D,, Particle Size k(cm/sec)

1 Sand 0.0 6 Silt2 Sand 0.1 Fine sand3 Gravel 0. 3 Fine sand4 Gravel 1.5 Medium sand5 Gravel 9.0 Fine sand

3.6 x 10-31.0 x 10-29.0 X 10-22.3 X 1008.1 X 10'

D,,SIZE (mm)

Permeability TestsA more accurate method of determining soil permeability is by conducting labora-tory permeability tests on representative samples obtained from the boring program.A device called a permeameter is used for testing (ASTM , 1990). A constant-headpermeameter (Figure 1-15) is used for sands and gravels. A falling-head permeame-ter (Figure 1-16) is used for silts and clays. Because of changes imposed on the so ilsduring the sampling, transportation, and preparation processes, samples are nevercompletely undisturbed. Therefore, laboratory test results can be unreliable andmisleading (Carson, 1961).



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18 GROUNDWATER LOWERING AND DRA INAGE TECH NIQUES

Figure 1-15 Typical constant-head permeameter. (From Hunt, 1984.)

Figure 1-16 'Qpical falling-head permeameter. (From Hunt, 1984.)



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Rmpinpwater kvelor dmwdown curve

I , l t l I I * L0 20 40 60 80Dirtonce In feet from pumped wellTypical pump test setup. (From Johnson, 1975.)igure 1-17

Pump TestsThe best method for predicting field permeability rates at a site is by conducting afull-scale field pump test where a test well, similar to the anticipated dewateringwells, is installed and pumped fo r a duration of time to predict flow rates and coneof depression geometry (Figure 1-17). The test well should penetrate the aquifer ifpractical and should be located near the center of the project site. A constant pumprate should be used and con tinued until equilibrium or static levels are reached in theobservation w ells. Two other pum p rates can be tried to verify the results of the firstpump test. The results should be analyzed using both equilibrium and non-equilibrium formulas.The equilibrium w ell formula according to Johnson (1973 , the one that appliesto m ost groundwater conditions (Figure 1-18), is

k (H* h2)= 1055 log R / r

where Q = pumping rate, gallmink = permeability, gal/day/ft*H = aquifer thickness, fth = depth of drawdown in the well, f tR = cone of depression radius, ftr =well radius, f t



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20 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUE S

Diameterof well-F T i u s of influence -

Cone of

R

\\\5 Drawdown_/)\PBz Pumping level-a

\CUNec

v)

c._5P Well screen-P--rnn

Ground surfaceL -"7Depth towater table

'ilepression,/n well,H-h

h

Figure 1-18 Equilibrium well formula. (From Johnson, 1975.)

Figure 1-19 Pumping from an artesian aquifer. (From Johnson, 1975 . )



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1-3 INVESTIGATIONMETHODS 21Recharge at the periphery of the cone of depression is assumed. If the aquifer isconfined, or in other words, if it is an artesian aquifer (Figure 1 -19), the equilibriumwell formula becomes

km (H - h )= 528 log R / rwhere the terms are as defined above except

m = aquifer thickness, ftH = static head at the bottom of the aquifer, ftThese equations are frequently used to determinepump test.

(1-2)

the field permeability from a

Example Problem 1-2 Calculating Permeability from Pump TestGiven

~ Q =300 g a l h i n8' $deep well Observation well

R =1 7 0 O ' l i - 4 1 HIh I 1 1 r n I I I I I I II I I 1I I I 1 m

Shale bedrock

Required: Permeability of sandy alluvium based on pump test results.Solutionk(gal/day)(H2 - h i )1055 log R / r(gal/min) =

or1055 Q og R / r - 1055 X 300 X log 1700/0.33-k = H 2 - hi (70' - 252)

= 275 gal/day = 1.3 X 10-2 cm/secThe nonequilibrium well formula, developed by Theis (1 9 33 , takes into accountthe effect of time on pumping. By use of this formula, the drawdown can bepredicted at any time after pumping begins. Using this method can eliminate theneed to reach a static condition in the observation wells during a pump test, thereby



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22 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUESreducing the time and cost of the test. Also, only one observation well is required todevelop the site hydraulic characteristics from a pump test, instead of the twoneeded for the equilibrium well formulas given above. While the Theis non-equilibrium formula is useful in running field pump tests, it is not often used inconjunction with dewatering calculations.

1-4 THEORETICAL BACKGROUNDDewatering involves theory dealing with fluid flow through soil and rock media,aquifer properties, and hydraulic flow through pumps and pipes. To understand therequirements of a dewatering system and response characteristics to pumping, onemust understand the concepts of permeability, transmissibility, storage, specificcapacity, pump hydraulics, and flow through pipes. These concepts are discussedbelow.Soil and Rock PermeabilityThe capacity of soil and rock to transmit water is called permeability. Flow throughsoil and rock is quantified by a characteristic termed the coefficient of permeabilityor k. Permeability is expressed in terms of Darcys law and is valid for laminar flowin a saturated, homogeneous material as follows:

where q = quantity of flow per unit of timei = hydraulic gradient (head loss/length of flow)A = cross-sectional area of flow streamHydrostatic conditions refer to pressures in fluids when there is no flow. The

pressure at a given depth in water equals the unit weight of water multiplied by thedepth and is equal in all directions. Groundwater flow occurs when there is animbalance of pressure from gravitational forces acting on the water, and the ground-water seeks to balance the pressure. Hydraulic gradient and permeability are the twofactors upon which groundwater movement is dependent. The hydraulic gradientbetween two points on the water table is the ratio between the difference in elevationof the two points and the distance between them. It reflects the friction loss as thewater flows between the two points. Flow condition nomenclature is illustrated inFigure 1-20.Flow in soil is affected by the grain size distribution and the dependent volume ofvoids through which water can pass. Since soil formations are often stratified andconsist of alternating layers of coarse-grained and fine-grained soil, horizontalpermeability may be greater than vertical permeability.Flow in rock generally follows the path of joints, partings, shear zones, andfaults of the formation. The intact rock is generally much less permeable than thejointing except in the case of highly porous rock such as coral. Joint conditions that



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1-4 THEORETICAL BACKGROUND 23

n n Horizontal l ine,

ri r notum elevat ionNo Flow Condition

Tailu;atumFlowing Condition

Figure 1-20 Hydraulic flow nomenclature. (From Hunt, 1984.)

affect rock mass permeability include spacing, orientation, continuity, interconnec-tivity, aperture width, and filling characteristics. Sedimentary rock formations oftenexhibit stratification of water flow, especially when coarse-grained rock such assandstone is interlayered with fine-grained rock such as claystone and shale. Rockjoints often stop at fine-grained rock boundaries, causing water to flow alongbedding until another geologic feature permits flow across bedding.

Flow NetsFlow through a soil medium may be represented by a flow net: a two-dimensionalgraphical presentation of flow consisting of a net of flow lines and equipotentiallines, the latter connecting all points of equal piezometric level along the flow lines(Figure 1-21). Flow-net construction is accomplished by trial and error. The flowzone, bounded by the phreatic surface and an impermeable stratum, is subdividedon a scaled drawing of the problem area as nearly as possible into equidimensionalquadrilaterals formed by the flow lines and equipotential lines crossing at rightangles. The basic assumptions in flow-net construction are that Darcys law is valid



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24 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUESFree water surface/ uppermost line of seepage)

Figure 1-21 Flow net concepts. (From Cedergren, 1967.)and that the soil formation is homogeneous and isotropic. Seepage quantity can becalculated, using the flow net, from the following equation:

where N, = number of flow channelsN e =number of equipotential drops along each flow channelk =coefficient of permeabilityh = total head loss

Aquifer CharacteristicsOther terms of interest when contemplating a dewatering program include trans-missibility, storage, specific capacity, and radius of influence. The transmissibility,T, of an aquifer can be described as the ease with which water moves through a unitwidth of aquifer (Figure 1-22) and is defined as follows:

T = k B (1-5)where k = coefficient of permeability

B = thickness of aquiferIf T is being determined from a pump test, then

T = -dwhere Q = pumping rate

d = change in drawdown per log cycleThe storage coefficient, C ,, is defined as the volume of water released from



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Transmissib i l i ty T:f l ow th rough a un i tw i d t h o f aqu i fe rt-B Permeabil i ty K:F low through a u n i tarea of aquifer

figure 1-22 Aquifer characteristics. (From Powers, 1992.)storage, per unit area, per unit reduction in head. In the average water table aquifer,C , approaches 0.2 as water drains by gravity from the pores (Powers, 1992). In aconfined aquifer, the pores remain saturated, but there is nevertheless a smallrelease from storage when the head is reduced, due to the elasticity of the aquifer,and the compressibility of water. For confined aquifers, C , is on the order of 0.0005to 0.001. In rock aquifers, C , can be lower than the above values by several ordersof magnitude because of low effective porosity and rigid aquifer structure. If C, isbeing determined from a pump test,

TtOc, =-r2where T = transmissibility

to = zero drawdown intercept (Figure 1-23)r =distance of measurement from pumping wellThe specific capacity of a well at time t , q,, is defined asQq s =-d

(1-7)

where Q = pump rate at time td =drawdown at time tThe variables defined above are interrelated as shown in Figure 1-24.An ideal aquifer has no recharge within the zone of influence of pumping. But,as illustrated in Figure 1-25, most natural aquifers are constantly discharging andbeing recharged. When dewatering begins, natural discharge from the aquifer di-minishes. Recharge usually increases. For mathematical convenience, we say thatthe sum of the recharge from all the sources acts as an equivalent single source,large in capacity, acting on a vertical cylindrical surface at distance Ro from the



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26 GROUNDWATER LOWERING AND DRAINAGE TECHNIQUES

0.1 1.0 10 100 lo00Time since pumpingstarted I (min)

Figure 1-23 Zero drawdown intercept from pump test. (From Powers, 1992.)

center of pumping. R , is called the equivalent radius of influence. Dewateringvolume varies inversely as the log of RoeThe only reliable indication of R, is from aproperly conducted pump test (Powers, 1992). Lacking that, a rough guide to totalrecharge, and to the probable equivalent R,, can be inferred from soil borings,permeability estimates, areal geology, and surface hydrology. Other ways to esti-mate R , includeR , = r , +(Tt/C,) lQ (1-9)

where r, = equivalent radius of the pump arrayT = transmissibilityr =pumping timeC, = storage coefficientand

R, = 3 ( H - h) k* (1-10)where H = initial headh = final head

k =coefficient of permeability, microns/secH - h = amount of drawdown, ftWater table aquifers can be analyzed using two-dimensional computer modelssuch as FLOW-PATH and MODFLOW developed by the U .S. Geological Survey. Apump test is still recommended to define the characteristics of the aquifer and to



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1-4 T H E O R E TI C A L B A C K G R O U N D 27

Specif ic capacity (gprn/ft lAt One Hour

Speci f ic capaci ty (gprn l f t lA t Eight Hours

Figure 1-24 Specific capacity of wells. (From Powers, 1992.)

calibrate the model. Even with a computer model, several iterations are required tomodel the aquifer correctly. Figure 1-26 illustrates the type of output one cangenerate with such a program.Pump TheoryCompared to the complexities of soils and groundwater, the pump is a ratherstraightforward mechanical device, whose performance should be predictable and



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Figure 1-25 Natural aqu ifer characteristics. (From Powers,1992.)Copyright 1994 John Wiley & Sons Retrieved from: www.knovel.com


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1-4 THEORETICAL BACK GROUND 29Hydraullic Head Distribution

7

SteadystateflowMin:5.70 E+01Max:9.00 +01Inc:3.00 E+OOUnits:I f t l

0.0 150.0 300.0 450.0 600.0 750.0Figure 1-26 Computer modeling of well drawdown. (From Powers, 1992.)

reliable. T he work a pump must accomplish, termed the water horsepower (WH P),is the product of the volume pumped times the total dynamic head (TDH) on theunit. TDH is the sum of all energy increase, dynamic and potential, that the waterreceives. Figure 1-27 illustrates the calculation of TDH in various pumping applica-tions.The well pump in Figure 1-27 faces a static discharge head h, from the operatinglevel in the well to the elevation of final disposal from the discharge manifold. Inaddition, the pump must provide the kinetic energy represented by the velocity headh, And it must overcome the frictionf, in the discharge column and fittings andf,in the discharge manifold.

TDH = h, +h, + i +fi (1-11)The velocity head, h , is calculated at the point of maxim um velocity by

(1-12)where v =water velocityThe sump pump in Figure 1-27 faces a discharge head h,, plus a suction head h,,plus the velocity and friction heads. For the well point pump in Figure 1-27, it is not

g = acceleration of gravity



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30 GROUNDWATER LO WERING AND DRA INAGE TECHNIQUE SValve A

Well Pump Sump Pump

Wellpoint PumpFigure 1-27 Variables in pump performance curves. (From Powers, 1992.)

possible to measure the suction head h,. An approximate value can be estimated forh, as equal to the maximum operating vacuum of the well point pump, usually 28 ft(Powers, 1992).Figure 1-28 shows the basic performance curve o f a centrifugal well point pump.The head-capacity curve sh ow s the capacity o f the pum p at various values of T D H .



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120-I 1

Model 240 SFSpeed 140 0 rpmImpeller diam. 12.09 in.-P 2Head capacity /

The water horsepower (WHP) the pump is producing is the product of head andcapacity with appropriate conversion factors.

0-2 0I?, - 1 s -g a r -10-E- - 5 --0

(1-13)DH(ft) x Q(gal/min)3960HP =

20- - 0I I

The brake horsepower (BHP) is the amount of power that must be applied to thepump. It is greater than the W HP by the amount of hydraulic and mechanical lossesin the pump. The efficiency, e , of the pump isW HPBHPe = -

The BHP required by the pump is thereforeTDH x Q3960 X eHP =

(1-14)

(1-15)Figure 1-29 shows a performance curve for a w ell point pump operating at a speedof 1600 revolutions per minute (rpm) for various suction lifts. Figure 1-30 show s afamily of curves indicating the performance of one diameter impeller at variousspeeds. Figure 1-31 shows the performance at 1150 revolutions per m inute (rpm)with various diameters.



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32 GROUNDWATER LOWERING AND DRAINAGE TECH NIQUES140120100

-0- 803d 60r-0

ec-

4020

0

Discharge, gp mFigure 1-29 Performance curve of a well point pump. (From Mansur and Kaufman, 1 96 2. )

US. gallons per minu teFigure 1-30 Pump performance versus speed. (From Powers, 1992.)



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wwU.S. gallan per minute

Figure 1-31 Pump performance versus impeller size. (From Powers, 1992.)



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34 GROUNDWATER LOWERINGAND DRAINAGE TECHNIQUES1-5 ESTIMATIONOF FLOW RATESAt best, flow rates into a dew atered excavation are hard to predict, especially in theabsence of pump test data and previous site experience. T he variability in geologicformations and the difficulties involved with making reliable estimates of per-meability complicate the task significantly. Three methods are presented herein.These methods have proven to provide reasonably accurate estimates of inflow(Cedergren, 1967). When using these methods, it is better to err on the high side.Having excess pumping capacity at the site is preferable to not having enoughcapacity, which may cause costly delays on the project. Most contractors do notmind oversizing their dewatering systems by a modest amount.Darcys L awDarcys law relates flow rate to permeability, hydraulic head, and area of flow asfollows:

Q = qL = kiAL (1-16)where Q = total flow

q = flow through a unit areaL = length of areak = permeabilityi = hydraulic headA = area of flowTo use this formula in computing a flow rate into a dewatered excavation (Figure1-32), the following assumptions can be made:

whereH

and

Therefore

H = height from impermeable zone to water tableh, = height to lowered water tableho= amount of drawdownR = the radius of influencer , = average radius of the bottom of excavation

AL = 1.571 (H +h,) ( R +ro)

(1-17)

(1-18)

(1-19)



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1-5 ESTIMATION OF FLOW RATES 35R

c-

Figure 1-32 Darcys aw. (FromCedergren, 1967.)

Well FormulasAnother way to compute inflow into a dewatered excavation is by using the wellform ulas given ab ove unde r the discussions of pum p tests. Simplified, the nonarte-sian equation would appear as follows for this case:

(1-20)

Tkro-Dlmenslonal Flow NetsFlow nets are a graphical representation of water flow (F igure 1-33) whereby, in thedirection of flow, lines are drawn to designate individual flow channels (nf) andperpendicular to the direction of flow, cross-flow lines, called equipotential lines,are drawn to designate head drops (nd) . A classic publication discussing the con-struction of flow nets is Casagrande (1940). After a flow net is constructed for thedewatered excavation, the amount of inflow can be calculated with the followingequation:

Q = 3.14 k (H- h,) (R +1,) n f l n d (1-21)where nr = number of flow channels in flow net

nd = number of head drops in flow netR

Figure 1-33 Sample flow net. (From Cedergren, 1967.)



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36 GROUNDWATER L OWERING AND DRAINAGE TECHNIQUESThis is the most complex of the three methods given for predicting inflow into adew atered excavation and in m ost cases is not warranted in view of the reliability ofthe input data.

Example Problem 1-3 Excavation Dewatering Flow VolumeGiven: A cut-and-cover transit station is to be constructed in a glacial outwashdeposit below the groundwater table.

Ilan

Glacial outw ash sands and gravelk =4.7 x 10-3 crn/sec

1bo1

Very st i f f lacustr ine clayk =8.1 x lo6 cmlsec

ElevationRequired: Total dewatering flow rate to lower the groundwater table to 5 ftbelow the bottom of the excavation.Solution: Equivalent radius of excavation

r, =Aqu ifer thickness, H = 140 ft +20 ft = 160 ft. Required draw down , h, =70 ft+5 ft - 20 ft =55 ft. Permeability, k =4.7 X 10-3 cm/sec =0.00925 ft /min.Radius of dewatering, R , is unknown. Assume 1500 ft. Using Darcys law,



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1 -5 ESTIMATION OF FLOW RATES 37

Method Darcy's Law

(160 - 55)(160 +55)(1500 + 357)(1500 - 357)1.57 X 0.00925= 533 ft3/min or 3998 gal/min

Well Formula Flow Net

Using the simple well formu la,

- 1.37 X 0.00925(1602- 552)log (1500/357)=459 ft3/min or 3442 gal/min

Using a flow net,

R =1500'r0I2 =178.5'

Number of flow drops, nd = 43. Number of flow channels, nf = 3.Q = 3.14 k ( H - ho)(R + r o ) 13fnd

3= 3.14 X 0.00925 X (160 - 55)(1500 +357)3=395 ft3/min or 2963 gal/min

Based on the three alternative methods, the flow can be expected to be between3000 and 4000 gal/min.

cm/secand the radius of influence R =2200 ft. The new solutions based on the pumptest results are:A pump test indicates that the field permeability rate k =9.2 x

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