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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