Civil Engineering Reference
In-Depth Information
Fig. 4.11
Shear box assembly.
By means of additional transducers (fixed to the shear box) it is possible to determine both the horizontal
and the vertical strains of the test sample at any point during shear:
Movement of box
Length of sample
Horizontal strain
(%)
=
The load reading is taken at fixed horizontal displacements, and failure of the soil specimen is indicated
by a sudden drop in the magnitude of the reading or a levelling off in successive readings. In most cases
the computer plots a graph of the shearing force against horizontal strain as the test progresses. Failure
of the soil is visually apparent from a turning point in the graph (for dense soils) or a leveling off of the
graph (for loose soils).
The apparatus can be used for both drained and undrained tests. Although undrained tests on silts and
sands are not possible (because drainage will occur), the test procedure can be modified to maintain
constant volume conditions during shear by adjusting the hanger weights. This procedure, in effect, gives
an undrained state.
A sand can be tested either dry or saturated. If dry there will be no pore water pressures and the
intergranular pressure will equal the applied stress. If the sand is saturated, the pore water pressure will
be zero due to the quick drainage, and the intergranular pressure will again equal the applied stress.
Example 4.2:
Shear box test (i)
Drained shear box tests were carried out on a series of soil samples with the following
results:
Test no.
Total normal stress (kPa)
Total shear stress at failure (kPa)
1
100
98
2
200
139
3
300
180
4
400
222
Determine the effective cohesion and angle of shearing resistance.
Solution:
In this case, both the normal and the shear stresses at failure are known, so there is no
need to draw stress circles and the four failure points may simply be plotted. These
