TM 5-818-1 / AFM 88-3, Chap. 7
CHAPTER 11
DEEP FOUNDATIONS INCLUDING DRILLED PIERS
foundation load recompresses the soil, and settlement of
11-1. General. A deep foundation derives its support
the foundation consists of elastic and consolidation
from competent strata at significant depths below the
components as shown in figure 11-1(b). Consolidation
surface or, alternatively, has a depth to diameter ratio
occurs along the recompression curve until the
greater than 4. A deep foundation is used in lieu of a
preconsolidation stress is reached, whereupon it
shallow foundation when adequate bearing capacity or
proceeds along the virgin compression curve. Calculate
tolerable settlements cannot be obtained with a shallow
the foundation heave and subsequent settlement using
foundation. The term deep foundation includes piles,
procedures outlined in chapter 5.
piers, or caissons, as well as footings or mats set into a
c. If the depth with respect to the type and
deep excavation. This chapter discusses problems of
shear strength of the soil is such that plastic flow occurs,
placing footings and mats in deep excavations and
loss of ground may develop around the outside of the
design of drilled piers. Drilled piers (or caissons) are
excavation with possible settlement damage to
simply large-diameter piles, but the design process is
structures, roads, and underground utilities.
somewhat different. An arbitrary distinction between a
d. The rate and amount of heave may be
pile and pier is that the caisson is 30 inches or more in
estimated from the results of one-dimensional
diameter.
consolidation tests; however, field evidence shows that
the rate of heave is usually faster than predicted. A
11-2. Floating foundations. A foundation set into a
study of 43 building sites found that the field heave
deep excavation is said to be compensated or floating if
amounted to about one-third the computed heave.
the building load is significantly offset by the load of soil
Where excavations are large and are open for
removed during excavation. The foundation is fully
substantial time before significant foundation loadings
compensated if the structural load equals the load
are applied, the actual heave may be close to the
removed by excavation, partially compensated if the
computed heave. Figure 11-2 is a plot of a series of field
structural load is greater, and overcompensated if the
results of heave versus excavation depth, in which the
structural load is less than the weight of the excavated
heave increases sharply with the depth of excavation.
soil. A compensated foundation requires a study of
An example of heave and subsequent settlement
expected subsoil rebound and settlement, excavation
calculations for a compensated foundation is shown in
support systems, means to maintain foundation subsoil
figure 5-4.
or rock integrity during excavation, and allowable bearing
e. The yielding of the excavation bottom can
pressures for the soil or rock.
be .caused by high artesian water pressures under the
excavation or by a bearing capacity failure resulting from
11-3.
Settlements of compensated foundations.
the overburden pressure on the soil outside the
a. The sequence of subsoil heave during
excavation at subgrade elevation. Artesian pressure can
excavation and subsequent settlement of a deep
be relieved by cutoffs and dewatering of the underlying
foundation is illustrated in figure 11-1(a). If effective
aquifer using deep wells. The pumped water may be put
stresses do not change in the subsoils upon the initial
back in the aquifer using recharge wells outside the
excavation, i.e., the soil does not swell due to an
excavation perimeter to avoid perimeter settlements or to
increase in water content, and if no plastic flow occurs,
preserve the groundwater table for environmental
then only immediate or elastic rebound from change in
reasons, but this operation is not simple and should be
stress occurs. If the structural load is fully compensated,
done only when necessary.
the measured settlement of the foundation would consist
f.
The likelihood of bearing capacity failure
only of recompression of the elastic rebound, generally a
exists primarily in clayey soils and should be analyzed as
small quantity, provided subsoils are not disturbed by
shown in chapter 14. A factor of safety, Fs > 2, should
excavation.
ideally be obtained to minimize yielding and possible
b. If the negative excess pore pressures set
settlement problems. A large plastic flow may cause the
up during excavation "dissipate, " i.e., approach static
bottom of the excavation to move upwards with re-
values, before sufficient structural load is applied,
foundation swell occurs in addition to elastic rebound.
(The original effective stresses will decrease.) The
11-1
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