Environmental Engineering Reference
In-Depth Information
vs. log time curve for a desired load. Since the relationship between compression and time
on the semilog plot is essentially a straight line, it can be expressed (Perloff, 1975) as
∆
e
C
α
log (
t
1
/
t
2
)
(3.74)
is the coefficient of secondary compression represented by the slope of the time
curve,
t
2
the time at which secondary compression is desired and
t
1
the time at the start of
secondary consolidation.
Secondary compression is a form of creep.
where
C
α
Lateral Strains
Lateral strains, prevented in the one-dimensional consolidation test, can be significant
under conditions of relatively rapid loadings of soft clays. Analysis is based on triaxial test
data of Lambe and Whitman (1969), Lambe (1964, 1973), Skempton and Bierrum (1957),
Hansen (1967), and Davis and Poulos (1968).
Expansion of Clay Soils
General
Expansive clay characteristics are described in terms of activity in
Section 5.3.3
and in gen-
eral in
Section 10.6.2,
which includes tests for their identification.
Usually, clays with expansion potential are less than 100% saturated in the field; there-
fore, the consolidation theory does not apply. Tests are performed in the consolidometer
or the California bearing ratio mold to obtain measures of percent swell or volume change
under a given load, or the maximum swell pressure that may be anticipated.
Consolidometer Testing of Undisturbed Samples
Percent Swell Measurements:
Place specimen in consolidometer at natural moisture (
w
) content and provide
protection against changes in
w
.
●
Add loads in the same manner as the consolidation test, although initial loads
may be higher, measuring and recording compression until the final design
foundation load is attained.
●
Immerse specimen in water, permitting saturation, and measure the volume
increase as a function of time, until movement ceases.
●
Maximum Swell Pressure:
Place specimen in consolidometer under an initial seating load.
●
Immerse in water and add loads as necessary to prevent specimen from swelling
to determine the maximum swell pressure.
●
CBR Mold Testing of Compacted Samples
Procedures are similar to the percent swell test in the consolidometer.
Compression in Cohesionless Sands
Measurements
Cohesionless sands are seldom tested in the laboratory for compression characteristics
since undisturbed samples cannot be obtained; they are occasionally tested in the con-
solidometer as reconstituted specimens placed at various values of
D
R
.
Evaluations of compression are normally based on
in situ
test data such as obtained with
the pressure meter, SPT or CPT, or load test.
