The shear strength of soil as determined in UU tests corresponds
to total stress, and is applicable only to situations where little
consolidation or drainage can occur during shearing. It is applicable
primarily to soils having a permeability less than 10. -3- cm per sec.
(2) Consolidated-Undrained (CU) or R Test. In the CU test, complete
consolidation of the test specimen is permitted under the confining
pressure, but no drainage is permitted during shear. A minimum of three
tests is required to define strength parameters c and [phi], though four
test specimens are preferable with one serving as a check. Specimens must
as a general rule be completely saturated before application of the deviator
stress. Full saturation is achieved by back pressure. Pore water pressure
is measured during the CU test, thus permitting determination of the
effective stress parameters c' and [phi]'. In the absence of pore pressure
measurements CU tests can provide only total stress values c and [phi].
(3) Consolidated-Drained (CD) or S Test. In the CD test, complete
consolidation of the test speciman is permitted under the confining pressure
and drainage is permitted during shear. The rate of strain is controlled to
prevent the build-up of pore pressure in the specimen. A minimum of three
tests are required for c' and [phi]' determination. CD tests are generally
performed on well draining soils. For slow draining soils, several weeks
may be required to perform a CD test.
(4) Factors Affecting Tests. Triaxial test results must be
appropriately corrected for membrane stiffness, piston friction, and filter
drains, whenever applicable. The shear strength of soft sensitive soils is
greatly affected by sample disturbance. The laboratory-measured shear
strength of disturbed samples will be lower than the in-place strength in
the case of UU tests. In the case of CU or CD tests, the strength may be
higher because of the consolidation permitted.
d. Other Procedures. In certain instances, more sophisticated tests
are warranted. These may include triaxials with zero lateral strain
conditions, simple shear tests, and tests inducing anisotropic stress
3. TEST SELECTION. In determining the type of test to be employed,
considerations must be given to soil type and the applications for which the
test data is required. (See Chapter 4 for a discussion of total and
effective stress concepts.)
(1) Clean Sands and Gravels. Undisturbed samples are very difficult
to obtain and test properly, therefore sophisticated shear tests are usually
impractical. For simple foundation problems, the angle of internal friction
can be satisfactorily approximated by correlation with penetration
resistance, relative density, and soil classification (Figure 7).
Confirmation of the potential range of the angle of internal friction can be
obtained from shear tests on the sample at laboratory densities bracketing
conditions anticipated in the field. For earth dam and high embankment work
where the soil will be placed under controlled conditions, triaxial
compression tests are warranted.