design of dewatering systems has been discussed in

chapter 3. Mathematical, graphical, and electroanalo-

gous methods of analyzing seepage flow through gen-

groundwater control system first requires determina-

eralized soil conditions and boundaries to various

tion of the type of groundwater flow (artesian, gravity,,

types of dewatering or pressure relief systems are pre-

or combined) to be expected and of the type of system

sented in paragraphs 4-2, 4-3, and 4-4.

that will be required. Also, a complete picture of the

groundwater and the subsurface condition is neces-

sary. Then the number, size, spacing, and penetration

design of dewatering, permanent drainage, and sur-

of wellpoints or wells and the rate at which the water

face-water control systems are considered in this chap-

must be removed to achieve the required groundwater

ter.

lowering or pressure relief must be determined.

in paragraphs 4-2, 4-3, and 4-4 and figures 4-1

source of seepage must be determined and the bounda-

through 4-23 are for a steady state of groundwater

ries and seepage flow characteristics of geologic and

flow. During initial stages of dewatering an excava-

soil formations at and adjacent to the site must be gen-

tion, water is removed from storage and the rate of

eralized into a form that can be analyzed. In some

flow is larger than required to maintain the specified

cases, the dewatering system and soil and groundwa-

drawdown. Therefore, initial pumping rates will prob-

ter flow conditions can be generalized into rather sim-

ably be about 30 percent larger than computed values.

ple configurations. For example, the source of seepage

can be reduced to a line or circle; the aquifer to a homo-

relief systems are given in appendix D.

geneous, isotropic formation of uniform thickness; and

the dewatering system to one or two parallel lines or

circle of wells or wellpoints. Analysis of these condi-

tions can generally be made by means of mathematical

formulas for flow of groundwater. Complicated con-

quires the determination of the number, size, spacing,

figurations of wells, sources of seepage, and soil forma-

and penetration of wells or wellpoints and the rate at

tions can, in most cases, be solved or at least approxi-

which water must be removed from the pervious strata

mated by means of flow nets, electrical analogy mod-

to achieve the required groundwater lowering or pres-

els, mathematical formulas, numerical techniques, or a

sure relief. The size and capacity of pumps and collec-

combination of these methods.

tors also depend on the required discharge and draw-

down. The fundamental relations between well and

electrical analogy, is not better than the validity of the

wellpoint discharge and corresponding drawdown are

formation boundaries and characteristics used in the

presented in paragraphs 4-2, 4-3, and 4-4. The equa-

analysis. The solution obtained, regardless of the rigor

tions presented assume that the flow is laminar, the

or precision of the analysis, will be representative of

pervious stratum is homogeneous and isotropic, the

actual behavior only if the problem situation and

water draining into the system is pumped out at a con-

boundary conditions are adequately represented. An

stant rate, and flow conditions have stabilized. Proce-

approximate solution to the right problem is far more

dures for transferring an anisotropic aquifer, with re-

desirable than a precise solution to the wrong problem.

spect to permeability, to an isotropic section are pre-

The importance of formulating correct groundwater

sented in appendix E.

flow and boundary conditions, as presented in chapter

3, cannot be emphasized too strongly.

graphs 4-2, 4-3, and 4-4 are in two groups: flow and

drawdown to slots *(b *below and fig. 4-1 through 4-9)

their suitability for various types of excavations and

and flow and drawdown to wells (c below and fig. 4-10

soil conditions were described in chapter 2. The inves-

through 4-22). Equations for slots are applicable to

tigation of factors relating to groundwater flow and to

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