TEST TITLE : DIRECT SHEAR TEST 1.0 OBJECTIVE To determine the parameter of shear strength of soil, cohesion, c and angle of friction, ø. 2.0 LEARNING OUTCOME At the end of this experiment, students are able to: Determine the shear strength parameter of the soil Handle shear strength test, direct shear test 3.0 THEORY The general relationship between maximum shearing resistance, Շ f and normal stress, σ n for soils can be represented by the equation and known as Coulomb’s Law: where:
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TEST TITLE : DIRECT SHEAR TEST
1.0 OBJECTIVE
To determine the parameter of shear strength of soil, cohesion, c and angle of
friction, ø.
2.0 LEARNING OUTCOME
At the end of this experiment, students are able to:
Determine the shear strength parameter of the soil
Handle shear strength test, direct shear test
3.0 THEORY
The general relationship between maximum shearing resistance, Շf and
normal stress, σn for soils can be represented by the equation and known as
Coulomb’s Law:
where:
c = cohesion, which is due to internal forces holding soil
particles together in a solid mass
Ø = friction, which is due to the interlocking of the particles and
the friction between them when subjected to normal stress
The friction components increase with increasing normal stress but the
cohesion components remains constant. If there is no normal stress the friction
disappears. This relationship is shown in Figure 1. This graph generally
approximates to a straight line, its inclination to the horizontal axis being equal to
the angle of shearing resistance of the soil, Ø and its intercept on the vertical
(shear stress) axis being the apparent cohesion, denoted by c.
4.0 TEST EQUIPMENTS
1. Shear box carriage
2. Loading pad
3. Perforated plate
4. Porous plate
5. Retaining plate
Figure 3: Loading pad, Perforated
plate, Porous plate, Retaining plate
Figure 1: Shear stress versus normal stress
Figure 2: Shearbox carriage
5.0 PROCEDURES
1. The internal measurement is verified by using the vernier calipers. The
length of the sides, L and the overall depth, B.
2. The base plate is fixed inside the shear box. Then porous plate is put on
the base plate. Next, perforated grid plate is fitted over porous so that the
grid plates should be at right angles to the direction shear.
3. Two halves of the shear box is fixed by means of fixing screws
4. For cohesive soils, the soil sample is transferred from square specimen
cutter to the shearbox by pressing down on the top grid plate. For sandy
soil, soil is compacted in layers to the required density in shear box
5. The shear box assembly is mounted on the loading frame
6. The dial is set of the proving ring to zero
7. The loading yoke is placed on the loading pad and the hanger is lifted
carefully onto the top of the loading yoke.
8. The correct loading is then applied to the hanger pad.
9. The screws clamping the upper half to the lower half is carefully removed.
10. The test is conducted by applying horizontal shear load to failure. Rate
strain should be 0.2mm/min
11. record readings of horizontal and force dial gauges at regular intervals.
12. Finally the test is conducted on three identical soil samples under different
vertical compressive strsses, 1.75 kg, 2.5 kg and 3.25 kg