b. Recompression Index. The slope of the straight line in the
recompression range of the semilogarithmic pressure-void ratio curve is
defined by C+r,, where C+r, is equal to or less than C+s,. (See Figure 2).
6. COMPRESSION OF COLLAPSIBLE SOILS.
Such soils require a special test for
determining their collapse potential.
See Chapter 1 for test details.
7. COEFFICIENT OF CONSOLIDATION (c+v,). Those soil properties that control
the drainage rate of pore water during consolidation are combined in the
coefficient of consolidation.
a. Determination. Compute c+v, from the semilogarithmic
time-compression curve for a given load increment (bottom panel of Figure
2). Correct the origin for compression for the effect of air or gas in void
spaces by the procedure given in Reference 2.
b. Approximate Values.
Figure 4 may be used to determine approximate
values of c+v,.
8. SECONDARY COMPRESSION. After completion of primary consolidation under
a specific load, the semilogarithmic time-compression curve continues
approximately as a straight line. This is termed secondary compression
(Figure 2). It occurs when the rate of compression is no longer primarily
controlled by the rate at which pore water can escape; there are no excess
pore pressures remaining.
a. Organic Materials. In organic materials, secondary compression may
dominate the time-compression curve, accounting for more than one-half of
the total compression, or even obliterating the change in slope used to
establish the limit of primary compression.
b. Approximate Values. The coefficient of secondary compression
C+[alpha], is a ratio of decrease in sample height to initial sample height
for one cycle of time on log scale. See bottom panel of Figure 4 for
9. SAMPLE DISTURBANCE. Sample disturbance seriously affects the values
obtained from consolidation tests as shown in Figure 2 and below.
a. Void Ratio. Sample disturbance lowers the void ratio reached under
any applied pressure and makes the location of the preconsolidation stress
b. Preconsolidation Pressure. Sample disturbance tends to lower the
compression index (C+c,) and the preconsolidation pressure (P+c,) obtained
from the test curve.
c. Recompression and Swelling.
Sample disturbance increases the
recompression and swelling indices.
d. Coefficient of Consolidation. Sample disturbance decreases
coefficient of consolidation for both recompression and virgin compression.
For an undisturbed sample, c+v, usually decreases abruptly at
preconsolidation stress. This trend is not present in badly disturbed