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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