TM 5-818-1 / AFM 88-3, Chap. 7
∆e
where
Cs =
(3-8)
∆ε =
change in vertical strain
log p2'
∆p =
p1'
P2 - p; = corresponding change in
effective vertical stress
where Ae is the change in void ratio (strictly a sign
av
=
Ae/Ap
=
coefficient
of
applies to Cc, Cs., Cr, and mv; however, judgment is
compressibility
usually used in lieu of signs). The swelling index is
p'
=
average of initial and final
generally from one-fifth to one-tenth the compression
effective vertical stress
index. Approximate values of Cs may be obtained from
figure 3-10. The slope of the recompression curve is
The units of mv are the reciprocal of constrained
expressed by the recompression index, Cr, as follows:
modulus. Table 3-4 gives typical values of mv for
several granular soils during virgin loading.
∆e
Cr =
(3-9)
log p2'
e. Expansion and recompression.
If
p1'
overburden pressure is decreased, soil undergoes
volumetric expansion (swell), as shown in figure 3-7.
The semilogarithmic, straight-line (this may have to be
The value of Cr is equal to or slightly smaller than Cs.
approximated) slope of the swelling curve is expressed
Cr/Cs
are
associated
with
High
values
of
by the swelling index, Cs, as
overconsolidated clays containing swelling clay minerals.
(Courtesy of T. W. Lambe and R. V. Whitman,
Soils Mechanics, 1969, p 320. Reprinted by
permission of John Wiley & Sons, Inc., New York.)
Figure 3-9. Approximate relation between void ratio and effective overburden pressure for clay sediments, as a function of
the Atterberg limits.
3-10
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