DHARM266 GEOTECHNICAL ENGINEERINGat any stage and volume changes take place. It may require 4 to 6 weeks to complete a single
test of this kind in the case of cohesive soils, although not so much time is required in the case
of cohesionless soils as the latter drain off quickly.
This test is seldom conducted on cohesive soils except for purposes of research. It is also
called the ‘Slow Test’ or ‘consolidated slow test’ and is designated CD, S, or Sc test.
The shear parameters c and φ vary with the type of test or drainage conditions. The
suffixes u, cu, and d are used for the parameters obtained from the UU-, CU- and CD-tests
respectively.
The choice as to which of these tests is to be used depends upon the types of soil and the
problem on hand. For problems of short-term stability of foundations, excavations and earth
dams UU-tests are appropriate. For problems of long-term stability, either CU-test or CD-
tests are appropriate, depending upon the drainage conditions in the field. For a more detailed
exposition of these and other related aspects, the reader is referred to ‘‘The relevance of triaxial
test to the solution of stability problems’’ by A.W. Bishop and L. Bjerrum—Proc. ASCE Res.
Conf. Shear strength of Cohesive soils, Colorado, USA, 1960.
A fuller discussion of the nature of results obtained from these various types of tests and
the choice of test conditions with a view to simulating field conditions, is postponed to a later
section.
8.8 SHEARING STRENGTH TESTS
Determination of shearing strength of a soil involves the plotting of failure envelopes and
evaluation of the shear strength parameters for the necessary conditions. The following tests
are available for this purpose :
Laboratory Tests- Direct Shear Test
- Triaxial Compression Test
- Unconfined Compression Test
- Laboratory Vane Shear Test
- Torsion Test
- Ring Shear Tests
Field Tests - Vane Shear Test
- Penetration Test
The first three tests among the laboratory tests are very commonly used, while the
fourth is gaining popularity owing to its simplicity. The fifth and sixth are mostly used for
research purposes and hence are not dealt with here.
The principle of the field vane test is the same as that of the laboratory vane shear test,
except that the apparatus is bigger in size for convenience of field use. The penetration test
involves the measurement of resistance of a soil to penetration of a cone or a cylinder, as an
indication of the shearing strength. This procedure is indirect and rather empirical in nature