TM 5-815-5/AFM 88-5, Chap 6/NAVFAC P-418
CHAPTER 4
DESIGN OF DEWATERING, PRESSURE RELIEF, AND
GROUNDWATER CONTROL SYSTEMS
4-1. Analysis of groundwater flow.
design of dewatering systems has been discussed in
chapter 3. Mathematical, graphical, and electroanalo-
a. Design of a dewatering and pressure relief or
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-
e. Other factors that have a bearing on the actual
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.
f. The formulas and flow net procedures presented
b. In the analysis of any dewatering system, the
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
g. Examples of design for dewatering and pressure
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
4-2. Mathematical and model analyses.
circle of wells or wellpoints. Analysis of these condi-
a. General.
tions can generally be made by means of mathematical
(1) Design. Design of a dewatering system re-
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-
c. Any analysis, either mathematical, flow net, or
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
(2) Equations for flow and drawdown to drainage
3, cannot be emphasized too strongly.
slots and wells. The equations referenced in para-
graphs 4-2, 4-3, and 4-4 are in two groups: flow and
d. Methods for dewatering and pressure relief 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
4-1
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