requires care and judgment. Sometimes it is better to

of the versus log p curve is the compression ratio, CR,

estimate two positions of this point-one as small as likely,

defined as

and the other as large as plausible, consistent with the

∆ε

data-and to repeat the construction for both cases. The

(3-4)

CR =

=

Cc

result will be a range of preconsolidation stresses.

log p2'

1 + e0

Because the determination of pc involves some

p1'

inevitable inaccuracy, the range of possible values may

be more useful than a single estimate which falls

where & is the change in vertical strain corresponding to

somewhere in the possible range. The higher the quality

a change in effective stress from p1' to p2', and e0 is the

of the test specimen, the smaller is the range of possible

initial (or in situ) void ratio. An approximate correlation

pc values. Approximate values of preconsolidation

between CR and natural water content in clays is given

pressure may be estimated from figure 3-8 or 3-9. Table

by the following:

3-2 can be used to obtain gross estimates of site

preconsolidation. This table and figures 3-8 and 3-9

CR = 0.006 (w - 12)(3-5)

should be applied before consolidation tests are

performed to assure test loads sufficiently high to define

d. Coefficient of volume compressibility. The

the virgin compression portion of e-log p plots.

relationship between deformation (or strain) and stress

for one-dimensional compression is expressed by the

c. Compression index. The slope of the virgin

coefficient of volume compressibility, m , which is

compression curve is the compression index Cc, defined

defined as

in figure 3-6.

Compression index correlations for

∆ε

∆ε

approximations are given in table 3-3. When volume

mv =

=

av

(3-6)

change is expressed as vertical strain instead of change

∆p

∆p (1 + e0)

1 + e0

in void ratio, the slope of the virgin compression part

mv =

0.434 Cc

(1 + e0)p'

Figure 3-8. Approximate relation between liquidity index and effective overburden pressure, as a function of the sensitivity

of the soil

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