SSC JE 2023-What is the Direct shear test of Soil? Procedure, Advantages, and disadvantages
Understanding the shear strength of soil is crucial in geotechnical engineering. The Direct Shear Test is a valuable tool in this regard, offering insights into soil behavior under shear forces.
In this article, we delve into the procedure, advantages, and disadvantages of this test.
SSC JE 2023-Direct shear test of Soil?: Points to Remember
This test measures the shear strength of the soil.
The soil sample for examination is confined within a metal box, which can have either a square or circular shape in cross-section and is designed to separate into two parts horizontally.
If the specimen is fully or partially saturated, perforated metal plates & porous stones are placed above & below the specimen to allow free drainage.
Solid metal plates are used if the specimen is dry.
A pressure pad is placed on the top and the entire specimen is placed in the trolley.
One half of the box is fixed & the other half is pushed or pulled relative to the fixed one.
A vertical load is applied to the specimen through a static weight hanger & the soil is sheared gradually by applying a horizontal force which causes the two halves of the box to be more relative to each other.
The shear is normally applied at ‘CONSTANT RATE OF STRAIN’
The magnitude of the shear load is measured by the mean of the providing ring.
The shear deformation, as well as vertical deformation, are measured during the test with the help of the dial gauge.
The process is repeated on 4 – 5 specimens, each subjected to a different vertical load.
Normal & shear stress at failure are obtained by dividing the normal force & shear force by the specimen.
Value of shear stress at failure are plotted against the normal stress for each test.
The shear strength parameter is obtained from the line of best-fit method.
SSC JE 2023-Direct shear test of Soil-Procedure, Advantages, and disadvantages
Advantages:
This test is a quick, inexpensive, and simple test of the sand.
Disadvantages:
Drainage conditions cannot be controlled & pore water pressure cannot be measured.
Hence, it is not of much use for fine saturated soil where drainage condition plays an important role in influencing the shear strength.
This test is not recommended for conditions other than FULL DRAINED.
As a result, its usefulness is limited to only free drainage soil, where effective stress parameters & shear strength are to be determined.
The failure plane is always horizontal in this test which may not be the direction of the weakest plane in soil.
There are stress concentrations at the sample boundary, leading to non–uniform shear stress distribution on the failure plane.
Failure of soil in this case takes place progressively from the edges towards the centre.
The area of the specimen under normal & shear loading does not remain constant for the duration of test, hence stresses are calculated on the basis of the constant nominal area of the specimen which are inaccurate.
Direction of the principal plane is not known at every stage of the test, It is only after the Mohr failure envelope is known, the magnitude & direction of the principal stresses can be determined.
There is rotation of the principal plane throughout the test, due to this the field loading situation cannot be simulated accurately in the lab sample.
While the Direct Shear Test proves quick and cost-effective for sandy soils, it faces limitations with fine, saturated soils and controlled drainage. Understanding these aspects can help engineers make informed decisions about its application in different soil scenarios. Ready to put your geotechnical knowledge to the test? Take our Free SSC JE 2023 Mock Test to challenge your skills!
Asad Yar Khan
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