Objective

Determination of shear strength parameters of a silty or sandy soil at known density and moisture content.

Reference Standards

IS: 2720(Part 13)-1986- Methods of test for soils: Direct shear test.

Equipment / Apparatus

• Shear box
• Box container
• Porous stone and grid plate
• Tamper, Balance , Sieve(4.75 mm)
• Loading frame, Proving ring, Dial gauge.

Preparation sample

One kg of air dry sample passing through 4.75mm IS sieve is required for this test.

Procedure

1. Shear box dimensions is measured, the box is set up by fixing its upper part to the lower part with clamping screws, and then a porous stone is placed at the base.
2. For undrained tests, a serrated grid plate is placed on the porous stone with the serrations at right angle to the direction of shear. For drained tests, a perforated grid is used over the porous stone.
3. An initial amount of soil is weighed in a pan. The soil is placed into the shear box in three layers and for each layer is compacted with a tamper. The upper grid plate, porous stone and loading pad is placed in sequence on the soil specimen.
4. The pan is weighed again and the mass of soil used is computed.
5. The box is placed inside its container and is mounted on the loading frame. Upper half of the box is brought in contact with the horizontal proving ring assembly. The container is filled with water if soil is to be saturated.
6. The clamping screws is removed from the box, and set vertical displacement gauge and proving ring gauge to zero.
7. The vertical normal stress is set to a predetermined value. For drained tests, the soil is allowed to consolidate fully under this normal load. (Avoid this step for undrained tests.)
8. The motor is started with a selected speed and shear load is applied at a constant rate of strain. Readings of the gauges are taken until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 20% of the specimen length.
9. The moisture content of the specimen is determined after the test. The test is repeated on identical specimens under different normal stress values.

Calculation

• The density of the soil specimen is calculated from the mass of soil and the volume of the shear box.
• The dial readings are converted to the appropriate displacement and load units by multiplying with respective least counts.
• Shear strains are calculated by dividing horizontal displacements with the specimen length, and shear stresses are obtained by dividing horizontal shear forces with the shear area.
• The shear stress versus horizontal displacement is plotted. The maximum value of shear stress is read if failure has occurred, otherwise read the shear stress at 20% shear strain. The maximum shear stress versus the corresponding normal stress is plotted for each test, the cohesion and the angle of shearing resistance of the soil is determined from the graph.

Safety & Precautions

• Before starting the test, the upper half of the box should be brought in contact of the proving ring assembly.
• Before subjecting the specimen to shear, the fixing pins should be taken out.
• The rate of strain should be constant throughout the test.