How to Draw Mohrs Circle for Direct Shear Test
The most common method of shear testing is the direct shear test. It includes pushing a soil specimen to fail along a predetermined shear aeroplane and testing the resistance to deformation. The Mohr-Coulomb law of shearing force is based on a relationship betwixt shear stresses and normal stresses on a horizontal sideslip plane.
Apparatus Used for Direct Shear Test
- Shear Box (made of brass or gun metal)
- Shear Box
- Proving Band
- Dial Guage
- Grid Plates etc,
In a shear box made of brass or gun metallic, a soil specimen is subjected to a direct shear inspection. It's usually a square box with dimensions of threescore x 60 10 l mm.
The dividing plane passes through the eye of the box, dividing information technology horizontally.
Locking pins concord the two halves together. Separation screws are oftentimes used to keep the ii halves autonomously.
Filigree plates, which are toothed and fit within the case, are included.
For undrained tests, the gripper plates are plain and for tuckered tests, they are perforated.
In drained tests, porous stones are mounted at the top and bottom of the specimen. A pressure pad at the peak 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 broad container. On a roller, the big container is supported. A geared jack that acts every bit a strain driven system will drive the container frontwards at a constant pace. The jack can exist manually operated or driven past an electric motor.
To examination the shear forcefulness, a proving ring is attached to the upper half of the bundle. The proving band comes into contact with the set support. The proving ring tracks the shear force equally the box moves.
A dial gauge attached to the container is used to calculate the shear deportation. To mensurate the change in thickness of the specimen, another dial gauge is fastened to the top of the pressure level pad.
On cohesionless soils, the straight shear test is performed.
Direct Shear Test tin 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. Information technology may exist an undisturbed sample or soil that has been compacted or remoulded.
Grid plates and porous plates are called and positioned in the shear box depending on the type of examination and drainage conditions. The specimen is then put within the box or example.
A known standard load from the loading yoke is added to the top of the specimen later on all other required changes have been made.
A constant charge per unit 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 respective shear load.
Normal stress and shearing stress at failure of the sample tin 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 Exam】of Soil
Observation Table :
- Sample Size :
- H2o Conduct :
- Proving Ring Constant :
- Shearing Data :
- Initial Density :
Results :
Failure Envelope :
After completing the given procedure we can obtain the failure envelope. It is washed by plotting corresponding to shear stress (τ) as ordinate and normal stress (σ) as abscissa. From the in a higher place effigy we can come across 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 airplane is established in a direct shear test, just the principal planes are unknown. At whatever indicate F (σ, τ) draw a tension circle that touches the intensity envelope.
- A line FC perpendicular to the envelope is drawn to achieve this.
- A circle is drawn with C equally the eye 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 force per unit area dissipates quickly.
- The test is quick and convenient, and preparing the sample is unproblematic.
- Information technology can be used to perform drained tests on a cohesionless soil.
Disadvantages :
- Simply at the bespeak of failure are the stress conditions shown. The circumstances leading up to failure are unknown.
- On the failure airplane, the stress distribution is non compatible.
- 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 ready.
- 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 virtually straight shear examination. Do have a wait!
Source: https://civilengineeringnotes.com/direct-shear-test-for-shear-strength-of-soil/
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