SHEAR STRENGTH OF SOIL BY DIRECT SHEAR TEST
TO DETERMINE THE SHEAR STRENGTH OF A SANDY SOIL SPECIMEN BY DIRECT SHEAR TEST
Shear strength of a soil is its maximum resistance to shearing stresses. The shear strength is expressed as
Where c’ = effective cohesion, = effective stress, = effective angle of shearing resistance.
The shear tests can be conducted under three different drainage conditions. The direct shear test is generally conducted on sandy soils as a consolidated drained test.
1. Shear box, divided into two halves by a horizontal plane, and fitted with locking and spacing screws
2. Box container to hold the shear box
3. Base plate having cross grooves on its top surface
4. Grid plates, perforated, 2 Nos.
5. Porous stones, 6mm thick, 2 Nos.
6. Loading pad
7. Loading frame
8. Loading yoke
9. Proving ring, capacity 2kN.
10. Dial gauges, accuracy 0.01mm, 2 Nos.
11. Static or dynamic compaction devices.
1. Measure the internal dimensions of the shear box. Also determine the average thickness of the grid plates.
2. Fix the upper part of the box to the lower part using the locking screws. Attach the base plate to the lower part.
3. Place the grid plate in the shear box keeping the serrations of the grid at right angles to the direction of shear. Place the porous stone over the grid plate.
4. Weigh the shear box with base plate, grid plate and porous stone.
5. Place the soil specimen in the box. Tamp it directly in the shear box at the required density. When the soil in the top half of the shear box is filled upto 10 to 15mm depth, level the soil surface.
6. Weigh the box with soil specimen.
7. Weigh the box inside the box contained and fix the loading pad on the box. Mount the box contained on the loading frame.
8. Bring the upper half of the box in contact with the proving ring. Check the contact by giving a slight movement.
9. Fill the container with water if the soil is to be saturated, otherwise omit this step.
10. Mount the loading yoke on the ball placed on the loading pad.
11. Mount the dial gauge on the loading yoke to record the vertical displacement and another dial gauge on the container to record the horizontal displacement.
12. Place the weights on the loading yoke to apply a normal stress of 25 . Allow the sample to consolidate under the applied normal stress. Note the reading of the vertical displacement dial gauge.
13. Remove the locking screws. Using the spacing screws, raise the upper part slightly above the lower part such that the gap is slightly larger than the maximum particle size. Remove the spacing screws.
14. Adjust all the dial gauges to read zero. The proving ring should also read zero.
15. Apply the horizontal shear load at a constant rate of strain of 0.2mm/minute.
16. Record the reading of the proving ring, the vertical displacement dial gauge and the horizontal displacement dial gauge at regular time intervals. Take the first few readings at closer intervals.
17. Continue the test till the specimen fails or till a strain of 20% is reached.
18. At the end of the test, remove the specimen from the box and take a representative sample for water content determination.
Repeat the test on identical specimens under the normal stresses of 50, 100, 200, 400, etc. (The range of stresses selected should correspond to the actual field stress conditions.)
Fig: Direct Shear Test Apparatus
DATA SHEET FOR DIRECT SHEAR TEST
Size of the box =
Area of the box =
Thickness of the specimen =
Volume of the specimen =
Mass of soil specimen =
Bulk density =
Water content =
Dry density =
Void ratio =
Tare mass of hanger =
Mass of hanger
Total mass =
Normal stress =
Mass of box + base plate + porous stones + grid plate =
Mass of box + base plate + porous stone + grid plate + soil specimen =
Use separate data sheet for tests under normal stresses. Determine the shear stress at failure in each case. Summarise the results as follows.
Plot the Coulomb envelope between the normal stress as abscissa and shear stress at failure as ordinate.
Fig: Stress Strain Curve from Direct Shear Test
Fig: Failure Envelope
From plot, c’ =