DESIGN OF DEWATERING, PRESSURE feELIEF, AND GROUNDWATER CONTROL SYSTEMS
2- 1. Analysis of groundwater flow.
a. Design of a dewatering and pressure relief or groundwater control system first requires determination of the type of groundwater flow (artesian, gravity, or combined) to be expected and of the type of system that will be required. Also, a complete picture of the groundwater and the subsurface condition is necessary. Then the number, size, spacing, and penetration of wellpoints or wells and the rate at which the water must be removed to achieve the required groundwater lowering or pressure relief must be determined.
b. In the analysis of any dewatering system, the source of seepage must be determined and the boundaries and seepage flow characteristics of geologic and soil formations at and adjacent to the site must be generalized into a form that can be analyzed. In some cases, the dewatering system and soil and groundwater flow conditions can be generalized into rather simple configurations. For example, the source of seepage can be reduced to a line or circle; the aquifer to a homogeneous, isotropic formation of uniform thickness; and the dewatering system to one or two parallel lines or circle of wells or wellpoints. Analysis of these conditions can generally be made by means of mathematical formulas for flow of groundwater. Complicated configurations of wells, sources of seepage, and soil formations can, in most cases, be solved or at least approximated by means of flow nets, electrical analogy models, mathematical formulas, numerical techniques, or a combination of these methods.
c. Any analysis, either mathematical, flow net, or electrical analogy, is not better than the validity of the formation boundaries and characteristics used in the analysis. The solution obtained, regardless of the rigor or precision of the analysis, will be representative of actual behavior only if the problem situation and boundary conditions are adequately represented. An approximate solution to the right problem is far more desirable than a precise solution to the wrong problem. The importance of formulating correct groundwater flow and boundary conditions, as presented in chapter 3, cannot be emphasized too strongly.
d. Methods for dewatering and pressure relief and their suitability for various types of excavations and soil conditions were described in chapter 2. The investigation of factors relating to groundwater flow and to
design of dewatering systems has been discussed in chapter 3. Mathematical, graphical, and electroanalogous methods of analyzing seepage flow through generalized soil conditions and boundaries to various types of dewatering or pressure relief systems are presented in paragraphs 4-2,4-3, and 4-4.
e. Other factors that have a bearing on the actual design of dewatering, permanent drainage, and surface-water control systems are considered in this chapter.
f. The formulas and flow net procedures presented in paragraphs 4-2, 4-3, and 4-4 and figures 4-1 through 4-23 are for a steady state of groundwater flow. During initial stages of dewatering an excavation, water is removed from storage and the rate of flow is larger than required to maintain the specified drawdown. Therefore, initial pumping rates will probably be about 30 percent larger than computed values.
g. Examples of design for dewatering and pressure relief systems are given in appendix D.