Question

4. What are the three most frequently and widely assigned rock mechanics property measurement tests? Explain them. How are they each used in rock mechanics property measurement test?

Answer 1

There are many types of rock mechanics property measurement tests. Elastic theory provides the solution to a large number of problems that have direct applications in structure of rock mechanics. Many materials that are linearly elastic at low stress level deviate if they are put under higher stress or at prolonged loading system. The deviation may indicate an incomplete instantaneous recovery of strain upon removal of stress and variation in mechanical properties with application of stress. Three main important tests are

1.Unconfined compression test

2. Triaxial compression test

3. Splitting tension test

Unconfined Compression Test on Rocks:-

1. It is more commonly used test for rocks to determine its strength but it should be done carefully for accurate results.

2. The samples obtained during exploration are trimmed as per requirements.

3. The specimen should in the cylindrical form and length to width ratio of specimen should be varying from 2 to 2.5

4. The ends of specimen should be exactly perpendicular to the axis of cylinder and ends should be flat, smooth and parallel.

5. The dia of specimen should not be less than 35 mm. generally 45 mm dia is preferable.

6. The specimen is placed in compression testing machine and subjected to compression with a stress rate of 0.5 to 1.0MPa per second.

7. Note down the peak load value (P) where it fails and find out the strength of rock using below formula. Where A = initial c/s area of specimen. {q _u = P/A}

Triaxial Compression Test on Rocks:-

1. This test is similar to that used for soils. Here also cylindrical specimen is required and is first subjected to lateral pressure and then deviator stress.

2. As the stresses are quite large, a special type of equipment is required.

3. In the usual procedure, the specimen is subjected to confining pressure (p) and then deviatoric stress is applied when P is kept constant.

4. The confining pressure P increases the strength of rock. However, the increase in strength is realized only when the specimen is immersed in impervious jacket.

5. Generally hydraulic oil is used as confining fluid. The jacket is made of polyurethane which is oil resistant.

Splitting Tension Test on Rocks:-

1. This is also called as Brazilian test. In this test, the specimen is split along the diametric plane by applying load.

2. The specimen having length to diameter ratio of unity when loaded on its side it splits along the diameter and parallel to the cylindrical axis.

3. The horizontal stresses perpendicular to the loaded diameter are uniform and tensile.

4. The diameter of specimen should be at least 45 mm.

5. Thickness of the disc shall be approximately equal to half the diameter.

6. The load should applied at the rate of 0.2 KN/sec.

7. The load should be measured at least to an accuracy of 1%.

Advantages of  rock mechanics property measurement tests:-

1. In the construction field, rock materials are very important.

2. The rocks adopted for the preparation of construction materials should contain great properties.

3. To estimate the properties of the rocks, these tests are so essential.

4. Without testing the rocks, we should not keep them in the field.

5. If we don't apply the property tests on the rocks, we will face many problems in the designing the structures.

6. We can only estimate whether a rock is suitable or not for particular structure can be found by performing these tests on the rocks.

7. There are many destructive tests involved in the laboratory as well as the field.

8. These tests can be applied to determine the tensile strength, compressive strength,toughness, ductility and hardness etc.

9. We can adopt the particular tests as per our requirement. However, these tests are very advantageous  in rock mechanics property measurement test.