TM 5-818-1 / AFM 88-3, Chap. 7
stress beneath a circular area are shown in figure 5-3. If
curves from unconsolidated-undrained laboratory
the foundation consists of a large number of individual
compression tests are frequently only one-half or one-
footings, influence charts based on the Boussinesq case
third as large as the in situ modulus. This difference is
will greatly facilitate the computation of stresses.
due to disturbance effects, and the disparity may be
Programs for digital computers and programmable
even more significant if the amount of disturbance is
calculators are also available.
unusually large. The undrained modulus of the clay may
d. A structure excavation reduces stress in
be estimated from figure 3-20. The values of the K in
this figure were determined from the field measurements
foundation subsoils. The decrease in vertical stresses
and, therefore, are considered to be unaffected by
caused by the weight of excavated material is computed
disturbance. The value of Poisson's ratio is equal to 0.5
in the manner described in the previous paragraph. The
for saturated clays. For partly saturated clays, a value of
bottom of the excavation is used as a reference; vertical
0.3 can be assumed. Because immediate settlements
stresses produced by the weight of excavated material
occur as load is applied and are at least partially included
are subtracted algebraically from the original overburden
in results of laboratory consolidation tests, they are often
pressure to compute final foundation stresses.
not computed and only consolidation settlements are
considered to affect a structure.
5-4.
Settlement of foundations on clay.
a. When a load is applied over a limited area
5-5.
Consolidation settlement.
Consolidation
on clay, some settlement occurs immediately. This
immediate settlement, ∆Hi, , has two components: that
settlement of cohesive soil is normally computed from
pressure-void
ratio
relations
from
laboratory
caused by distortion or change of shape of the clay
consolidation tests on representative samples. Typical
beneath the loaded area, and that caused by immediate
examples of pressure-void ratio curves for insensitive
volume change in unsaturated soils. In saturated clays,
and sensitive,
normally loaded clays,
and
there is little or no immediate volume change because
preconsolidated clays are shown in figure 3-7.
time is required for water to drain from the clay.
Excavation results in a rebound of foundation soils and
b. Immediate settlements can be estimated
subsequent recompression when structure loads are
using methods given in chapter 10. Values of undrained
added. This
modulus determined from the slopes of stress-strain
Table 5-3. Empirical Correlations Between Maximum (∆) and Angular Distortion (δ/ϑ)
Approximate Value
of δ/ϑ fora
∆= 1 in.
Type of Foundation
Mats on sand
1/750(0.0013)
Rectangular mats on varved silt
1/1000 to 1/2000
(0.001 to 0.0005)
Square mats on varved silt
1/2000 to 1/3000
(0.0005 to 0.0003)
Mats on clay
1/1250(0.0008)
Spread footings on sand
1/600(0.0017)
Spread footings on varved silt
1/600(0.0017)
Spread footings on clay
1/1000(0.0010)
a δ/ϑ increases roughly in proportion with ∆. For ∆ = 2 in., values of δ/ϑ would be about
twice as large as shown, for ∆ = 3 inches, three times as large, etc.
(Courtesy ofJ. P. Gould and J. D. Parsons, "Long - Term Performance of Tall Buildings of New York City
Varied Silts, " Proceedings, International Conference on Planning and Design of Tall Buildings , Lehigh
University, Bethlehem, Pa., 1975. Reprinted by permission of American Society of Civil Engineers, New
York.)
5-4
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