c. Effective Stress. Effective stress equals the total stress minus

the pore water pressure, or the total force in the soil grains divided by

the gross cross-sectional area over which the force acts.

d. Overburden Pressure. Division of weight of overlying soil and water

into effective stress and pore water pressure depends on the position of the

groundwater table or the flow field induced by seepage. For static water

condition, effective stresses at any point below the groundwater level may

be computed using the total unit weight of soil above the water level and

buoyant unit weight below the water level. Pore water pressure is equal to

the static head times the unit weight of water. If there is steady seepage,

pore pressure is equal to the piezometric head times the unit weight of

water, and the effective stress is obtained by subtracting the pore water

pressure from the total stress.

e. Applied Load. Division of applied load between pore pressure and

effective stress is a function of the boundary conditions, the stress-strain

properties, and the permeability of the stressed and surrounding soils.

When drainage of pore water is inhibited, load is compensated for by

increased pore water pressures. These pressures may decrease with time, as

pore water is drained and load is transferred to the soil skeleton, thereby

increasing effective stress. Guidance on estimating changes in pore water

pressure is given in Chapter 5.

f. Effects of Stresses on a Soil Mass. Analysis of a soil system

(e.g., settlement, stability analyses) are performed either in terms of

total stresses or effective stresses. The choice between the two analysis

methods is governed by the properties of the surrounding soils, pore water

behavior, and the method of loading. (See Chapters 5, 6, and 7 for further

discussion.)

Section 3.

STRESSES BENEATH STRUCTURES AND EMBANKMENTS

1.

SEMI-INFINITE, ELASTIC FOUNDATIONS.

a. Assumed Conditions. The following solutions assume elasticity,

continuity, static equilibrium, and completely flexible loads so that the

pressures on the foundation surface are equal to the applied load intensity.

For loads of infinite length or where the length is at least 5 times the

width, the stress distribution can be considered plane strain, i.e.,

deformation occurs only in planes perpendicular to the long axis of the

load. In this case stresses depend only on direction and intensity of load

and the location of points being investigated and are not affected by

elastic properties.

Shearing stresses between an embankment and its foundation are

neglected.

b. Stress Distribution Formulas. Figure 2 presents formulas based on

the Boussinesq equations for subsurface stresses produced by surface loads

on semi-infinite, elastic, isotropic, homogeneous foundations. Below a

depth of

Change 1, September 1986

7.1-162

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