The
test is carried out on a soil sample confined in a metal box of square
cross-section which is split horizontally at mid-height. A small clearance is
maintained between the two halves of the box.The soil is sheared along a
predetermined plane by moving the top half of the box relative to the bottom
half. The box is usually square in plan of size 60 mm x 60 mm. A typical shear box is
shown.
If
the soil sample is fully or partially saturated, perforated metal plates and
porous stones are placed below and above the sample to allow free drainage. If
the sample is dry, solid metal plates are used. A load normal to the plane of
shearing can be applied to the soil sample through the lid of the box.
Tests
on sands and gravels can be performed quickly, and are usually performed dry as
it is found that water does not significantly affect the drained strength. For
clays, the rate of shearing must be chosen to prevent excess pore pressures
building up.
As a vertical normal load is applied to the sample, shear
stress is gradually applied horizontally, by causing the two halves of the box
to move relative to each other. The shear load is measured together with the
corresponding shear displacement. The change of thickness of the sample is also
measured.
A number of samples of the soil are tested each under
different vertical loads and the value of shear stress at failure is plotted
against the normal stress for each test. Provided there is no excess pore water
pressure in the soil, the total and effective stresses will be identical. From
the stresses at failure, the failure envelope can be obtained.
The test has
several advantages:
• It is
easy to test sands and gravels.
• Large
samples can be tested in large shear boxes, as small samples can give
misleading results due to imperfections such as fractures and fissures, or may
not be truly representative.
• Samples
can be sheared along predetermined planes, when the shear strength along
fissures or other selected planes are needed.
The disadvantages of the test include:
• The failure plane is
always horizontal in the test, and this may not be the weakest plane in the
sample. Failure of the soil occurs progressively from the edges towards the
centre of the sample.
• There
is no provision for measuring pore water pressure in the shear box and so it is
not possible to determine effective stresses from undrained tests.
• The
shear box apparatus cannot give reliable undrained strengths because it is
impossible to prevent localised drainage away from the shear plane.
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