CHAPTER 4.
DISTRIBUTION OF STRESSES
Section 1.
INTRODUCTION
1. SCOPE. This chapter covers the analysis of stress conditions at a
point, stresses beneath structures and embankments, and empirical methods
for estimating loads on buried pipes, conduits, shafts, and tunnels.
2. RELATED CRITERIA. For certain criteria not covered in this
publication, but concerning the design of buried pipes and conduits and
other underground structures, see the following sources:
Subject
Source
Airfield Pavements ..................................... NAVFAC DM-21 Series
Drainage Systems ....................................... NAVFAC DM-5.03
3. STATE OF STRESS. Stresses in earth masses are analyzed using two
basic and different assumptions. One assumes elastic conditions, and the
other assumes full mobilization of shear strength (plastic equilibrium).
Elastic solutions apply to problems for which shear failure is unlikely. If
the safety factor against shear failure exceeds about 3, stresses are
roughly equal to values computed from elastic theory. Plastic equilibrium
applies in problems of foundation or slope stability (see Chapter 7) and
wall pressures where shear strength may be completely mobilized (see
DM-7.02, Chapter 3).
Section 2.
STRESS CONDITIONS AT A POINT
1. MOHR'S CIRCLE OF STRESS. If normal and shear stresses at one
orientation on an element in an earth mass are known, stresses at all other
orientations may be determined from Mohr's circle. Examples of stress
transformation are given in Figure 1.
a. Plastic Equilibrium. The use of Mohr's circle for plastic
equilibrium is illustrated by analysis of triaxial shear test results (see
Figure 5 of Chapter 3).
2. STRESSES IN SOILS. The normal stress at any orientation in a
saturated soil mass equals the sum of two elements: (a) pore water pressure
carried by fluid in soil spaces, and (b) effective stress carried by the
grain skeleton of the soil.
a. Total Stress. The total stress at any point is produced by the
overburden pressure plus any applied loads.
b. Pore Water Pressure. Pore water pressure may consist of (a)
hydrostatic pressure, (b) capillary pressure, (c) seepage or (d) pressure
resulting from applied loads to soils which drain slowly.
7.1-161
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