(2) Clays. For simple total stress applications where the immediate
stability of foundations or embankments is of concern, the unconfined
compression test or UU triaxial test is often adequate (Chapter 1). For
very soft or sensitive soils, difficult to sample, the field vane test
(Chapter 2) is useful. For long-term stability problems requiring effective
stress analysis, such as landslides, CU triaxial tests with pore pressure
measurements should be used. Long-term stability problems in some highly
overconsolidated clays may require the CD test (see Reference 19, Stability
of Natural Slopes and Embankment Foundations State-of-the-Art Report, by
Skempton and Hutchinson).
(3) Silts and Mixed Soils. The choice of test is governed by
whether total stress analysis or effective stress analysis is applicable.
In cases of very soft silts, such as in marine deposits, the in-place vane
shear test is especially helpful in evaluating the shear strength and its
increase with depth. For some thinly layered soils, such as varved clay,
direct shear tests or simple shear tests are well suited for determining the
strength of the individual layers. Where partial drainage is anticipated,
use CU tests with pore water pressure measurements to obtain effective
strength parameters.
(4) Overconsolidated Soils. Frequently overconsolidated soils have
defects such as jointing, fissures, etc. The laboratory values of strength
which are obtained from a small test specimen are generally higher than the
field strength values which are representative of the entire soil mass.
The release of stress due to excavation and exposure to
weathering reduces strength over a long period of time. This effect cannot
be assessed by any of the laboratory tests currently in use. Most
overconsolidated clays are anisotropic and the degree of anisotropy may also
be influenced by their age. Effect of anisotropy can be determined in the
laboratory.
In highly overconsolidated soil which may not be fully
saturated, unusually high back pressure may be necessary to achieve full
saturation, thus making it difficult to perform CU tests. CD tests are more
appropriate.
b.
Type of Application.
(1) Total Stress Analysis. It is appropriate for the immediate
(during and end of construction) safety of foundations and structures
(embankments) consisting of or resting on clays where permeability is low.
It is also applicable to embankment stability where rapid drawdown can
occur. Use of unconfined compression tests or UU test is appropriate.
Sample disturbance has significant effect on shear strength in these types
of tests.
(2) Effective Stress Analysis. Evaluation of long-term stability of
slopes, embankments, and earth supporting structures in cohesive soil
requires the use of effective stress strength parameters, and therefore CU
tests with pore water pressure measurements or CD tests are appropriate.
Tests must be run at a slow enough strain rate so that pore pressures are
equalized during the CU test or are dissipated throughout the CD test.
Essentially all analyses of granular soils are made using effective stress.
7.1-150
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