TM 5-818-1-1 / AFM 88-3, Chap. 7
Q triaxial compression test is commonly performed on
foundation clays since the in situ undrained shear
b. Undrained shear strength-cohesive soils.
strength generally controls the allowable bearing
Approximate undrained shear strengths of fine-grained
capacity. Sufficient unconfined compression and/or Q
cohesive soils can be rapidly determined on undisturbed
tests should be performed to establish a detailed profile
samples and occasionally on reasonably intact samples
of undrained shear strength with depth. Undrained
from drive sampling, using simple devices such as the
strengths may also be estimated from the standard
pocket penetrometer, laboratory vane shear device, or
penetration test, cone penetrometer soundings, and field
the miniature vane shear device (Torvane). To establish
vane tests, as discussed in chapter 4. For important
the reliability of these tests, it is desirable to correlate
structures, the effects of loading or unloading on the
them with unconfined compression tests. Unconfined
undrained shear strength should be determined by R
compression tests are widely used because they are
(consolidated-undrained) triaxial compression tests on
somewhat simpler than Q triaxial compression tests, but
representative samples of each stratum.
test results may scatter broadly. A more desirable test is
a single Q triaxial compression test with the chamber
c. Strength parameters, cohesive soils. The
pressure equal to the total in situ stress. Unconfined
undrained shear strength of saturated clays can be
compression tests are appropriate primarily for testing
expressed as
saturated clays that are not jointed or slickensided. The
(Courtesy of W. N. Houston and J. K. Mitchell, "Property
Interrelationships in Sensitive Clays ," Journal, Soil
Mechanics and Foundations Division , Vol 95, No. SM4.
1969, pp 1037-1062. Reprinted by permission of
American Society of Civil Engineers. New York.)
Figure 3-15. Remoltded shear strength versus liquidity index relationships for different clays.
3-18
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