2. TYPICAL VALUES. Coefficient of permeability is a property highly
sensitive to sample disturbance, and shows a wide range of variation due to
differences in structural characteristics. See Reference 14, Soil Mechanics
in Engineering Practice, Terzaghi and Peck, for correlations of permeability
with soil type. Permeability of clean, coarse-grained samples is related to
D+10, size (Figure 1).
Section 4.
CONSOLIDATION TESTS
1. UTILIZATION. One-dimensional consolidation tests with complete lateral
confinement are used to determine total compression of fine-grained soil
under an applied load and the time rate of compression caused by gradual
volume decrease that accompanies the squeezing of pore water from the soil.
See Figure 2 for test relationships.
2. TESTING PROGRAM. Consolidation tests require undisturbed samples of
highest quality. Select samples representative of principal compressible
strata. Determination of consolidation characteristics of a stratum
requires from two to about eight tests, depending on the complexity of
conditions. Select loading program to bracket anticipated field loading
conditions.
a. Incremental Loading (IL) With Stress Control. Ordinarily, apply
loads starting at 1/4 tsf and increase them by doubling 1/2, 1, 2, 4, 8,
etc., tsf. For soils with pronounced swelling tendency, it may be necessary
to rapidly increase loading to 1/2 tsf or higher, perhaps to overburden
pressure, to prevent initial swell. For soft, normally consolidated soils,
start loading at 1/16 or 1/32 tsf and increase loads by doubling the
previous value. (See Reference 2.) To establish the reconsolidation index
C+r,, and swelling index C+s,, include an unload-reload cycle, after P+c,
has been reached. Unload must be to 1/8 the existing load, or preferably
less. Reloads should be applied in the same manner as for the initial
curve.
b. Constant Rate of Strain (CRS). The specimen is subjected to a
constantly changing load while maintaining a constant rate of strain. Pore
pressure is continuously monitored to ensure that the primary consolidation
is completed at the applied strain rate. These tests can be performed in
shorter time than IL tests and yield more accurate values of
can be determined for very small load increments, but the test equipment is
more complicated and requires that estimates of strain rate and P+c, be made
prior to the start of the test. See Reference 15, Consolidation at Constant
Rate of Strain, by Wissa, et al., for guidance.
c. Gradient Controlled Test (GC). Drainage is permitted at the upper
porous stone while pore pressure is measured at the lower porous stone. A
loading control system regulates the application of load so that a
predetermined hydrostatic excess pressure is maintained at the bottom of the
specimen. This method as well as CRS has similar advantages over IL, but
does not require a prior estimate of strain rate. However, the equipment is
more complex than for CSR. See Reference 16, New Concepts in Consolidation
and Settlement Analysis, by Lowe, for guidance.
7.1-138
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