Environmental Engineering Reference
In-Depth Information
tests. This is mainly due to the inability to saturate the soil properly, along with the prob-
lem that the direct shear forces failure on a pre-determined plane, which is not coincident
with the principal stress direction at failure (Saada and Townsend, 1981). This means that
care should be taken in using the direct shear results and the test should not be used as the
primary means to obtain peak strengths for cohesive soils. The direct shear is a good way
to obtain the residual strength.
True laboratory residual strength of clay soils only develops with significant displace-
ment. Skempton (1985) gives the “typical displacement values” reproduced in Table 6.1.
Most laboratory shear box equipment is 75 mm
60 mm, and the
maximum practical displacement is of the order of 6 mm to 10 mm, sufficient only to
measure the peak and possibly the softened strength.
Residual strength is therefore obtained with a direct shear machine by repeatedly shear-
ing the sample until the strength is not further reduced by further shearing (the “mul-
tiple reversal” method). Typical load displacement curves for “turbulent” and “sliding”
shear are shown in Figure 6.16 for tests which are progressing well. Most tests are more
irregular.
75 mm or 60 mm
Typical displacements at various stages of shear in clays having clay fraction (3)
Table 6.1.
30%
(Skempton, 1985).
Displacement mm (2)
Stage
Overconsolidated
Normally consolidated
Peak
0.5-3
3-6
Rate of volume change approx zero (1)
4-10
At residual
1° (
R
1°)
30-200
At residual (
R )
100-500
(1) i.e. fully softened strength.
(2) For
Notes:
600 kPa.
(3) Clay fraction
n
% finer than 0.002 mm.
Figure 6.16.
Typical load displacement curves for direct shear tests (Skempton, 1985).
 
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