e. Coefficient of Secondary Compression. Sample disturbance tends to
decrease the coefficient of secondary compression in virgin compression
loading range.
Section 5.
SHEAR STRENGTH TESTS
1. UTILIZATION. The shear strength of soil is required for the analysis of
all foundation and earthwork stability problems. Shear strength can be
determined by laboratory and field tests, and by approximate correlations
with grain size, water content, density, and penetration resistance.
2. TYPES OF SHEAR TESTS. Many types and variations of shear tests have
been developed. In most of these tests the rate of deformation is
controlled and the resulting loads are measured. In some tests total
stress parameters are determined, while in others effective stress strength
parameters are obtained. See Chapter 4 for a discussion of total and
effective stress concepts. The following are the most widely used testing
procedures:
a. Direct Shear Test. A thin soil sample is placed in a shear box
consisting of two parallel blocks. The lower block is fixed while the upper
block is moved parallel to it in a horizontal direction. The soil fails by
shearing along a plane assumed to be horizontal.
This test is relatively easy to perform. Consolidated-drained tests
can be performed on soils of low permeability in a short period of time as
compared to the triaxial test. However, the stress, strain, and drainage
conditions during shear are not as accurately understood or controlled as in
the triaxial test.
b. Unconfined Compression Test. A cylindrical sample is loaded in
compression. Generally failure occurs along diagonal planes where the
greatest ratio of shear stress to shear strength occurs. Very soft material
may not show diagonal planes of failure but generally is assumed to have
failed when the axial strain has reached a value of 20 percent. The
unconfined compression test is performed only on cohesive soil samples. The
cohesion (c) is taken as one-half the unconfined compressive strength.
c. Triaxial Compression Test. A cylindrical sample is confined by a
membrane and lateral pressure is applied; pore water drainage is controlled
through tubing connected to porous discs at the ends of the sample. The
triaxial test (Figure 5) permits testing under a variety of loading and
drainage conditions and also allows measurement of pore water pressure. For
details on testing procedures, see Reference 2. Triaxial shear test
relationships are shown graphically in Figure 6.
(1) Unconsolidated-Undrained (UU) or Quick Test (Q). In the UU test
the initial water content of the test specimen is not permitted to change
during shearing of the specimen.
7.1-145
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