OBJECT: To determine the shear strength parameters of sandy soil by Direct Shear Box Test.
OBJECTIVE AND SCOPE
The test is carried out on either undisturbed samples or remolded samples. To facilitate the remolding purpose, a soil sample may be compacted at optimum moisture content in a compaction mold. Then specimen for the direct shear test could be obtained using the correct cutter provided. Alternatively, sand samples can be placed in a dry state at the required density, in the assembled shear box.
A normal load is applied to the specimen and the specimen is sheared across the pre-determined horizontal plane between the two halves of the shear box. Measurements of shear load, shear displacement, and normal displacement are recorded. The test is repeated for two or more identical specimens under different normal loads. From the results, shear strength parameters can be determined.
STANDARDS
AASHTO T236-90
ASTM D3080-90
BIS1377:part 4
EQUIPMENT
- Direct shear box apparatus
- Loading frame (motor attached)
- Dial Gauge
- Proving ring
- Tamper
- Straight edge
- Balance to weigh up to 200 mg.
- Aluminum container
- Spatula
THEORY
The strength of soil depends on its resistance to shearing stress. It is made up of basically the components;
- Frictional – due to friction between individual particles
- Cohesive – due to adhesion between the soil particles
The two components are combined in Coulomb’s shear strength equation,
τƒ = σ + c tan Ɵ
where: τƒ = searing resistance of soil at failure
c = apparent cohesion of soil
σ = total normal stress on the failure plane
Ɵ = angle of shearing resistance of soil (angle of internal friction)
PROCEDURE
- Assemble the shear box.
- Compact the soil sample in the mold after bringing it to optimum moisture conditions.
- Carefully transfer the sample into the shear box.
- Place the loading plane on top of the upper porous plate. After recording the weight of the loading carrier, place it on the loading cap.
- Position all dial gauges and set the readings to zero. Remove the alignment screws which hold two halves of the shear box together.
- Tighten the remaining, two diagonally opposite screws, until there is a small gap between upper and lower boxes to reduce the frictional force.
- Apply the desired normal load. If there is any vertical displacement, wait till the dial gauges indicate a constant reading and then reset the dial gauge to zero.
- Check the screws have been removed and then start the motor to produce the desired constant rate of shearing.
- Take the readings of,
- Shear load from the proving ring
- Shear displacement (i.e. horizontal displacement)
- Vertical displacement at every 15-second increment.
- Stop the test when the shear load starts to reduce or remains constant for at least three readings.
- Remove the soil and repeat the procedure with different normal loads at least for another two samples.
OBSERVATIONS
Proving ring constant = 0.15 kg/dial
Horizontal deformation is at the rate of 2mm/minute
Weight of hanger = 4.5 kg
Sample: 1 (Normal Load =24.5kg)
S.No. | (1) Time (sec) | (2) Horizontal Dial gauge reading | (3) Vertical Dial gauge reading | (4) Shear force = (2) x 0.15 (kg) | (5) Shear stress = (4) / 36 (kg/cm2) | (6) Vertical deformation =(3) x 0.01 (mm) | (7) Horizontal Deformation |
CALCULATIONS
Area of sample= 6×6 = 36cm2
Proving ring constant = 0.15 kg/dial
Max shear force = max horizontal dial gauge x 0.15 =
Shear stress =
Vertical deformation = vertical dial gauges reading x 0.01 mm =
Sample 2 (Normal Load =44.5kg)
S.No. | (1) Time (sec) | (2) Horizontal Dial gauge reading | (3) Vertical Dial gauge reading | (4) Shear force = (2) x 0.15 (kg) | (5) Shear stress = (4) / 36 (kg/cm2) | (6) Vertical deformation =(3) x 0.01 (mm) | (7) Horizontal Deformation |
CALCULATIONS
Area of sample= 6×6 = 36cm2
Proving ring constant = 0.15 kg/dial
Max shear force = max horizontal dial gauge x 0.15 =
Shear stress =
Horizontal deformation is at the rate of 2mm/minute
Vertical deformation = vertical dial gauges reading x 0.01 mm =
Sample 3 (Normal Load =84.5kg)
S.No. | (1) Time (sec) | (2) Horizontal Dial gauge reading | (3) Vertical Dial gauge reading | (4) Shear force = (2) x 0.15 (kg) | (5) Shear stress = (4) / 36 (kg/cm2) | (6) Vertical deformation =(3) x 0.01 (mm) | (7) Horizontal Deformation |
CALCULATIONS
Area of sample= 6×6 = 36cm2
Volume of sample = 6x6x2 = 72cm2
Proving ring constant = 0.15 kg/dial
Weight of hanger = 4.5 kg
Max shear force = max horizontal dial gauge x 0.15 =
Shear stress =
Horizontal deformation is at the rate of 2mm/minute
Vertical deformation = vertical dial gauges reading x 0.01 mm =
RESULTS
S.No | Normal Load=P(kg) | Normal Stress=P/A(kg/cm2) | Dial Reading | Shear Load(Kg) | Shear Stress(Kg/cm2) |
Review Questions
- What is the shear strength of soil?
- What is the value of “C” for sandy soil?
- What are the advantages and disadvantages of this test?
