One half is fixed and the other half is either pulled or pushed. the soil specimen ( height around 25 mm ) is placed inside the box and place two porous stones to allow the drainage (under dry condition or drained condition ). The specimen should be selected from the undisturb sample because we need to check the shear strength of the corresponding soil in actual composition and arrangement.
After setting up of the equipment P - normal force to the sample is applied. It is applied thorough the rigid loading cap of the shear box ( evenly distributed force ). The specimen is allowed to consolidate under this loading which we need to keep during the whole test cycle. once the soil is fully consolidated T - horizontal force is apply to the shear box. it will apply to the top half of the shear box and the force need to increase to fail the ( move top half over bottom half ) sample under shear.
Measurements
1). Horizontal force ( T )
2). Lateral displacement ( delta L )
3). Vertical displacement ( delta h )
This test need to perform number of applied normal stresses,
where,
sigma n = normal stress
P = applied normal load
A = initial cross section
where,
tau = shear stress
T = horizontal shearing force
Ac = Corrected area
Area correction
Ac = ( L - delta L ) * L
1). Shear stress (tau) Vs Shear strain ( graph b )
2). Vertical displacement (delta h) Vs lateral displacement (delta L)
3). Shear stress (tau) Vs normal stress (sigma n)
Advantages and disadvantages of the Direct Sear Test
Advantages1) Simple test - less time
2) Relatively low cost
3) Ease of sample preparation
Disadvantages
1) Drainage cannot be controlled and pore water pressure cannot be measured, thus undrained test are not possible.
2) Failure is forced along a predetermined plane
3) Shear stress along the failure plane is not uniform. stress concentration exist and failure occurs progressively from the edge towords the center.
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