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  4 Water Removal and Wellpointing 4.1 SUMPS Whenever excavation is taken into the water table, groundwater will enterthe excavation. Water can also enter the excavation from precipitation andsurface runoff.Water removal is much simpler when the bottom of the excavation isabove the surface of the ground water (the phreatic line). Surface runoff water which accumulates in a pool within the excavation can be removed bypumping from a sump. Sumps are made at a low area in the excavation byburying a container (such as a 55 gallon drum) with its upper rim level withor just below ground surface. As water accumulates in the drum, it ispumped away from the site.Sumps may not be adequate to handle both precipitation and surfacerunoff. Precipitation, of course, cannot be prevented from falling into anexcavation, so even if the surface runoff is diverted, one or more smallsumps may still be needed. 4.2 DRAINAGE DITCHES Surface runoff may be kept out of an excavated area by a system of ditches.The shape of the cross-section of the ditch is not important as long as the Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  sides are stable. Ditches should be lined when placed in soils which canslough off as water flows through. Geotextiles may be used for this purpose.In lieu of a lining, the ditch may be filled with a narrowly grade gravel orcrushed stone. In addition, porous pipe may be placed in the ditch toincrease its flow capacity.Ditches must be designed and constructed with sufficient carryingcapacity for the anticipated runoff flow. Local contractors’ experience maybe the best source of design data. If needed, porous pipe capacities may beobtained from the manufacturer, and the amount of water a gravel-filledditch can carry may be estimated as illustrated in the example which follows: A trapezoidal ditch averaging 18 inches wide and 18 inches deep has been placed on a 2 % slope, and filled with narrowly graded gravel. How much watercan the ditch carry without overflowing?  Darcy’s Law can be used:Q  ¼  kiAThe value of k must be assumed. Many empirical charts list values for gravelranging from 1 to 10cm/sec. Use an intermediate value of 5. ThenQ  ¼  5 ð 0 : 02 Þ½ 18 ð 2 : 54 Þ 2 ¼  209 cm 2 = sec  ¼  3 : 3gpm : The discharge from a ditch is small, as the example indicates. It may beincreased by increasing its cross section and/or slope. The increase gained byeither alternative is linear, but the cost increase is more than linear.Increasing the slope results in progressively deeper ditches, so a system of ditches will generally consist of relatively short segments, each emptying intoa sump. This avoids excess digging as well as the undesirable possibility of the ditch going into the water table.In the preceding example, the capacity will obviously vary directlywith the assumed value of k. The difference between choosing values at thelower or upper range of gravel permeability could well be the differencebetween an acceptable or unacceptable design.Sumps and ditches placed to divert or remove surface water aregenerally referred to as  drainage  facilities. The various processes used toremove water from below the water table (which may sometimes includesumps and ditches), are referred to as  dewatering . Knowing the actual valueof k is of major importance in all dewatering applications (and also ingrouting applications, as covered in later chapters). Field tests to determinek are generally done after a job has been contracted, and prior to the start of field work. For preliminary estimates, many empirical relationships can befound in texts and technical publications. Some of these are shown inFigure 4.1 and Table 4.1. Permeability values from laboratory tests are also Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  F IGURE  4.1  Empirical relationships among grain size, uniformity coefficient,and permeability. T ABLE  4.1  Degree of PermeabilityDescriptive term k,cm/sec SoilsHigh 10  1 and over Gravel and coarse sandMedium 10  1 to 10  3 Medium and fine sandLow 10  3 to 10  5 Very fine sandVery low 10  5 to 10  7 SiltsImpermeable 10  7 and less Clays Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  used, but represent for granular materials a value somewhere between thevertical and horizontal permeabilities. For most dewatering methods, it isthe horizontal value that is critical. Permeability values most appropriate forfield work can be obtained from pumping tests. 4.3 WELLPOINTS When water is withdrawn from a point below the groundwater surface, aconcavity is formed in that surface above the withdrawal point. Generally,this concavity will reach down to the withdrawal point. The initial and finalelevations of groundwater in the vicinity of the withdrawal point are shownin Figure 4.2. The radius R is related to the pumping and recharging ratesand represents an equilibrium between these two factors. If the pumpingrate is increased, the distance R will increase by an amount ‘‘m’’, and anadditional volume of soil (represented by the shaded area) will become dry.Because the drawdown is linear and the soil volume is a cubic function,continued increase in pumping rate from one withdrawal point soonbecomes an inefficient process. Because the rate of groundwater slopechange is greatest near the well, efficiency is obtained for field work by usinga number of closely spaced points of withdrawal. Pumping tests done on siteuse only one withdrawal point, and may show somewhat different dischargevolumes than will be achieved with the individual points of a fieldinstallation. Such tests do, however, give reliable values of permeability. F IGURE  4.2  Drawdown from a well or well point. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
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