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Laboratory Tests For Determination of Strength of Rocks

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The following laboratory tests are commonly used for determination of the strength of rocks:
1. Unconfined compression test
2. Triaxial compression test
3. Splitting tension test
4. Beam bending test
5. Ring shear test

All these tests are briefly described as below:

1. Unconfined compression test
This test is the most popular and most commonly applied strength test in rocks. To obtainaccurate results, the unconfined compression test should be done with utmost care. The specimen should be in the form of a cylinder of length to width ratio varying from 2 to 2.5. The ends of the specimen should be flat, smooth and parallel. The ends should be exactly perpendicular to the axis of the cylinder. Cores obtained during explorations are usually trimmed for this purpose. The specimen is subjected to compression between the cross-head and the platen of a compression testing machine.

According to IS:9143-1979,the specimen should preferably have a diameter of 45 mm. in no case, the diameter should be less than 35 mm. the load should be applied continuously with a stress rate of 0.5 to 1.0 MPa per second.

The compressive strength (qu) is determined from the relation:
qu= P/A
Where P is the peak load and A is the initial area of cross section of the specimen.

Triaxial compression test
The triaxial compression test in principle is similar to that used for soil. The cylindrical specimen is first subjected to the lateral pressure and then deviator stress is applied. As the stresses are quite large, a special type of equipment is required for conducting the test.

The usual procedure is to first applying the confining pressure (p) all around the cylinder and then to apply the deviator stress when the confining pressure is kept constant.

The all-round pressure p increases the strength of the rock. However, the increase in strength is realized only when the specimen is enclosed in an impervious jacket. Normally, hydraulic oil is used as a confining fluid. The jacket is usually made of oil-resistant rubber such as polyurethane.

Splitting tension test
The splitting tension test is also called the Brazilian test. In this test, the rock specimen is split by applying the load along the diametric plane. The rock core having a length-diameter ratio of unity when loaded on its side in a compression testing machine splits along the diameter and parallel to the cylindrical axis. The horizontal stresses acting perpendicular to the loaded diameter of the instrument has to be uniform and tensile for accurate results. The tensile stress is given by

σ= 2P/Πdt

Where P is the applied load, d is the diameter of the cylinder and t is the thickness of the disc (i.e. the length of the cylinder). IS: 10082-1981 recommends that the diameter of the specimen should be at least 45 mm and the thickness of the disc shall be approximately equal to half the diameter. The load should be applied at the rate of 0.2 kN/sec. The applied load will be measured with an accuracy of 1%.

The splitting tension test generally gives a value of tensile strength higher than that obtained from a direct tension test. This is probably due to the less effect of fissures in the splitting tension test as compared to that in a direct tension test.

Beam bending test
The beam bending test is also called the flexural test. In this test, the rock beam is subjected to bending till failure occurs. It is usually conducted on rock cores obtained from rock exploration. Generally, 4-point flexural loading system is used. The bottom surface of the beam is loaded at the third points. This system produces pure bending (without shear) in the middle third of the beam.

The flexural strength (or modulus of rupture) is the maximum tensile stress at the bottom surface of the core corresponding to the peak load. It is calculated from the simple beam theory assuming that the material remains elastic right up to the failure. The flexural strength (Tmr) is given by

Tmr= 16 PL/3Πd3

Where P is the maximum load at failure, L= span i.e. distance between reactions on the lower surface and d is the diameter of the core

Ring shear test
The ring shear test is generally used to test in-situ rocks. It gives the shear strength of the rock as a function of the confining pressure. The rock core specimen in this test does-not require perfectly square and smooth ends. The confining pressure is applied by the load parallel to the axis of core. Since the load is applied to the plunger of the instrument, the two sets of entirely different complex fracture surfaces will form along the two planes of the imposed shear surface. The imposed shear strength can be calculated using the below equation:

τp=P/2A
Where P is the peak load and A is the area of cross-section of the specimen.

As in the case of the triaxial test, there is a substantial increase of strength due to the confining pressure.

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