Environmental Engineering Reference
In-Depth Information
The repeated shearing is achieved in several ways:
-
Shear to limit of travel, unload (partly), rewind rapidly to start, then reload and reshear
etc. until a minimum is reached, or
-
Shear to peak, then unload partly, and wind back and forth rapidly until a total travel
of say 50 mm is achieved, reload and reshear etc. until a minimum is achieved, or
-
Shear back and forth under load at a slow strain rate until a minimum is achieved.
Further acceleration of achievement of the residual strength is sometimes accomplished
by cutting a slide plane into the sample prior to testing. The cut surfaces are sometimes
polished on a glass plate prior to testing.
Problems which arise in the use of the direct shear to assess peak strength include:
- It is difficult to ensure that the sample is properly saturated. Saturation is achieved by
soaking under a confining stress until there is no further volume change. Unlike triax-
ial testing, pore pressure cannot be measured, so it is necessary to rely on monitoring
the consolidation deformation of the sample during saturation and consolidation. The
best approach is to saturate under a low confining pressure until swell (or consolida-
tion) ceases, then load to the first test confining pressure, and monitor until settlement
ceases.
-Testing at a too high strain rate leads to overestimation of shear strength in a similar
way to triaxial testing, i.e. c
underestimated. This can be over-
come by monitoring the consolidation to obtain t
50
, t
90
or C
v
value, and then using for-
mulae developed by Akroyd (1975), Bowles (1978) or Head (1985) to estimate the
strain rate. Skempton (1985) showed however that the residual strength is not greatly
sensitive to strain rate provided reasonably low strain rates are used. This fact may be
used to speed up test rates, provided that when a constant shear stress has been reached
the strain rate is reduced, and the resulting shear stress is not significantly lower than
the previous minimum (see
Figure 6.17
)
.
-Testing at a too high confining stress range, and staged testing can lead to an overesti-
mation of c
is overestimated,
, underestimation of
in the same way as for triaxial testing.
Problems which arise in the use of the direct shear to assess residual strength include:
-
Repeated reversal of the direction of shearing is necessary, and this may partially
destroy the alignment of particles on the shear plane, preventing a true residual
strength being achieved;
-
Repeated reversal of the shear box often leads to soil squeezing out between the two
halves of the box. If this happens, or if the box tilts, interference can occur between the
two halves of the box leading to erroneous results.
The authors' experience is that one of the biggest problems is that laboratory personnel
(and the engineers supervising them) do not understand the definition of residual strength.
Figure 6.18
shows some of the all too common misconceptions.
6.1.4.3
Ring shear test
To overcome the problems of multiple reversals of a shear box and to obtain sufficient dis-
placement on the slide plane to achieve residual strength, Bishop et al. (1971) developed
a ring shear apparatus.
In the ring shear, an annular ring shaped specimen (
Figure 6.19
)
is subject to a constant
normal stress
n
confined laterally, and caused to shear on a plane of relative rotary
motion at a constant rate of rotation.
