CHAPTER 6.
SEEPAGE AND DRAINAGE
Section 1.
INTRODUCTION
1. SCOPE. This chapter covers surface erosion, and analysis of flow
quantity and groundwater pressures associated with underseepage.
Requirements are given for methods of drainage and pressure relief.
2. RELATED CRITERIA. Other criteria, relating to groundwater utilization
or control, can be found in the following sources:
Subject
Source
Drainage Systems ........................................ NAVFAC DM-5.03
Drainage for Airfield Pavements ......................... NAVFAC DM-21.06
Dewatering and Groundwater Control for Deep Excavations.. NAVFAC P-418
Additional criteria for permanent pressure relief and seepage control
beneath structures are given in DM-7.02, Chapter 4.
3. APPLICATIONS. Control of soil erosion must be considered in all new
construction projects. Seepage pressures are of primary importance in
stability analysis and in foundation design and construction. Frequently,
drawdown of groundwater is necessary for construction. In other
situations, pressure relief must be incorporated in temporary and permanent
structures.
4. INVESTIGATIONS REQUIRED. For erosion analysis, the surface water flow
characteristics, soil type, and slope are needed. For analysis of major
seepage problems, determine permeability and piezometric levels by field
observations. See Chapter 2 for techniques.
Section 2.
SEEPAGE ANALYSIS
1. FLOW NET. Figure 1 shows an example of flow net construction. Use this
procedure to estimate seepage quantity and distribution of pore water
pressures in two-dimensional flow. Flow nets are applicable for the study
of cutoff walls and wellpoints, or shallow drainage installations placed in
a rectangular layout whose length in plan is several times its width. Flow
nets can also be used to evaluate concentration of flow lines.
a. Groundwater Pressures. For steady state flow, water pressures
depend on the ratio of mean permeability of separate strata and the
anisotropy of layers. A carefully drawn flow net is necessary to determine
piezometric levels within the flow field or position of the drawdown curve.
7.1-259
Change 1, September 1986
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