∆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.

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