Rocks are an important type of construction materials used extensively from primitive times. Various types of constructions such as stone houses, gabion walls etc are constructed throughout the world. Due to the immense use of rocks in construction activities, it is necessary to ensure that the rocks used have adequate strength and stability. Thus, several tests must be conducted on the rocks to determine their suitability for the particular type of construction. Some of the common laboratory tests on rock for determining the strength of rocks have been briefly explained below.
Tests On Rock
1. Unconfined Compression Test on Rock
Unconfined Compression test is the type of laboratory test that is used to determine the strength of rocks. Unconfirmed compressive strength of rocks may be defined as the maximum axial compressive stress that the specimen can withstand under zero confining stress. This test is used extensively in geotechnical designs and construction.
The samples obtained after the exploration are first trimmed according to the requirements. Preferably, the specimen must be cylindrical and the length to width ratio of the specimen must range from 2 to 2.5.
The specimen is then placed in the testing chamber. It must be noted that the ends of the specimen are exactly perpendicular to the axis of the cylinder and the ends must be smooth and parallel.
The load is applied continuously at the rate of 0.5 MPa/s to 1MPa/s until failure occurs.
The peak load value at which the failure occurs is noted.
The compressive strength is then calculated using the following relation:
qu = P/ A
qu = unconfined compressive strength
P = peak load value
A = cross-section area of specimen taken
2. Triaxial Compression Test on Rock
Triaxial Compression Test may be defined as the type of compression test in which the cylindrical rock specimen is encased in an impervious membrane and is subjected to a confining pressure and finally loaded axially to failure. It is very similar to the triaxial compression test carried out on the soil. Since the test procedure involves the application of lateral pressure and deviator stress thus a special type of equipment is necessary for encompassing the large stresses. In general, the specimen of rock is first subjected to confining pressure then gradually the deviatoric stress is applied to keep the confining pressure constant. The specimen is usually enclosed in a jacket that is made up of polyurethane which is oil resistant.
The general procedure of the triaxial test on rocks can be listed as follows:
The specimen is first trimmed to ensure the shape of the specimen is cylindrical.
The specimen is then gently placed on the apparatus. Then, confining pressure is applied using a hydraulic pump.
The axial load is applied at a constant rate.
The process is repeated for about three times for different values of the confining pressure. Each test provides one peak value i.e. strength on the failure envelope.
Then, respective Mohr’s circle are drawn and the value of the internal friction angle and apparent cohesion is obtained.
3. Beam Bending Test on Rock
The beam bending test on rocks is also commonly known as flexure test. It is conducted to determine the flexural strength of the rock specimen. In general, four-point loading system is used and the specimen is subjected to bending until failure occurs.
The flexural strength or the modulus of rupture is then computed using the following relation:
P = Load applied (confined)
d = Diameter of specimen
L = Length of specimen
4. Brazilian Test on Rock
Brazilian Test is conducted to determine the tensile strength of rocks. This test is extensively used in geotechnical investigations and constructions. It is also commonly known as splitting tension test on rocks. The Brazilian test apparatus usually has a diameter of 50mm to 100mm and the thickness of half the diameter. The apparatus consists of a loading frame and a hydraulic jack that is fitted at the centre of the base of the load frame.
The procedure of the Brazilian test includes the following series of steps:
The specimen of rock is first trimmed so that the shape of the specimen is disc-like.
The disc-shaped rock specimen is then loaded using two opposing normal strip load at the periphery of the disc.
The load is then increased continuously; increase at a constant rate until the failure of the sample occurs. The rate of loading generally ranges from 10 to 50 KN/min.
The tensile strength is then computed using the following formula:
Tensile strength of Rock = ( 2 P ) / ( π D t )
P = load at failure (N)
D = diameter of the test specimen (mm)
t = thickness of the test specimen measured at the centre (mm)
5. Ring Shear Test
Ring shear test is used to determine the shear strength of rock. The shear strength of rock is obtained by this test as a function of the confining pressure. In this test, the specimen is placed on the apparatus and load is applied parallel to the axis of the core. The load is applied along the plunger. The shear strength is then computed using the following formula:
Shear Strength of Rock = P / ( 2 A )
P = Load
A = area of cross-section of the specimen