The most common method of shear testing is the direct shear test. It includes pushing a soil specimen to fail along a predetermined shear plane and testing the resistance to deformation. The Mohr-Coulomb law of shearing strength is based on a relationship between shear stresses and normal stresses on a horizontal slip plane.
Apparatus Used for Direct Shear Test
- Shear Box (made of brass or gun metal)
- Shear Box
- Proving Ring
- Dial Guage
- Grid Plates etc,
In a shear box made of brass or gun metal, a soil specimen is subjected to a direct shear inspection. It’s usually a square box with dimensions of 60 x 60 x 50 mm.
The dividing plane passes through the centre of the box, dividing it horizontally.
Locking pins hold the two halves together. Separation screws are often used to keep the two halves apart.
Grid plates, which are toothed and fit within the case, are included.
For undrained tests, the gripper plates are plain and for drained tests, they are perforated.
In drained tests, porous stones are mounted at the top and bottom of the specimen. A pressure pad at the top transmits the usual load. A steel ball bearing on the pressure pad transfers the usual load from the loading yoke to the top of the specimen.
Also Read : Vane Shear Test [ IS 2720 (Part XXX) – 1980 ]
The bottom half of the box is secured to the base plate, which is kept rigidly in place in a wide container. On a roller, the big container is supported. A geared jack that acts as a strain driven system will drive the container forward at a constant pace. The jack can be manually operated or driven by an electric motor.
To test the shear force, a proving ring is attached to the upper half of the package. The proving ring comes into contact with the set support. The proving ring tracks the shear force as the box moves.
A dial gauge attached to the container is used to calculate the shear displacement. To measure the change in thickness of the specimen, another dial gauge is attached to the top of the pressure pad.
On cohesionless soils, the direct shear test is performed.
Direct Shear Test can be performed on any one of three drainage conditions.
- Consolidated – Undrained Condition
- Consolidated – Drained Condition
- Unconsolidated – Undrained Condition
Procedure :
A soil sample measuring 60 x 60 x 25 mm is taken. It may be an undisturbed sample or soil that has been compacted or remoulded.
Grid plates and porous plates are chosen and positioned in the shear box depending on the type of test and drainage conditions. The specimen is then put inside the box or case.
A known standard load from the loading yoke is added to the top of the specimen after all other required changes have been made.
A constant rate of strain is applied to the shearing force. A dial gauge measures the shearing displacement.
The test is repeated with different standard load values to determine the corresponding shear load.
Normal stress and shearing stress at failure of the sample can be calculated by dividing normal load and maximum applied shear force by the cross-sectional area of the specimen at the shear plane.
Also Read : California Bearing Ratio Test【CBR Test】of Soil
Observation Table :
- Sample Size :
- Water Conduct :
- Proving Ring Constant :
- Shearing Data :
- Initial Density :
Results :
Failure Envelope :
After completing the given procedure we can obtain the failure envelope. It is done by plotting corresponding to shear stress (τ) as ordinate and normal stress (σ) as abscissa. From the above figure we can see that the inclination of the failure envelope respected to horizontal gives the value of Θ and its vertical axis intercept is equal to the cohesion intercept.
Mohr’s Circle :
- The shear plane is established in a direct shear test, but the principal planes are unknown. At any point F (σ, τ) draw a tension circle that touches the intensity envelope.
- A line FC perpendicular to the envelope is drawn to accomplish this.
- A circle is drawn with C as the centre and CF as the radius, intersecting the normal stress axis at A and B.
- Locate the pole P by drawing a horizontal line from P.
- The major and minor principal planes’ directions are PA and PB, respectively, and their inclination with the horizontal is β1 and β3, respectively.
Also Read : Effective Stress Concept in Soil Mechanics
Advantages and Disadvantages of Direct Shear Test
Advantages :
- Since the sample thickness is relatively thin, drainage is fast and pore pressure dissipates quickly.
- The test is quick and convenient, and preparing the sample is simple.
- It can be used to perform drained tests on a cohesionless soil.
Disadvantages :
- Only at the point of failure are the stress conditions shown. The circumstances leading up to failure are unknown.
- On the failure plane, the stress distribution is not uniform.
- As the test continues, the region under shear steadily decreases. However, since the corrected area cannot be measured, stresses are calculated using the original area.
- The failure plane’s orientation is set.
- Controlling the drainage situation is extremely difficult.
- It is not possible to calculate pore water pressure.
- The specimen is lateral restrained by the shear box’s side walls, which prevent it from deforming laterally.
For better understanding we have added a Youtube video which properly explain the detailed procedure about direct shear test. Do have a look!