Several laboratory tests are used to determine the strength of rocks. In this article, we will focus on four of them in particular: Uniaxial Compressive Strength (UCS) test, Brazilian Tensile Strength test, Triaxial Compression test and Point Load Strength Index test.
The Uniaxial Compressive Strength (UCS)
The UCS test determines the uniaxial compressive strength of rock specimens by applying axial compression without lateral confinement. Cylindrical specimens, typically with a length-to-diameter ratio between 2 and 2.5, are subjected to increasing axial load until failure occurs. The maximum load at failure is divided by the cross-sectional area to calculate the UCS, which is a critical parameter for various engineering applications (Huang, n.d.).
The Brazilian tensile strength test
In the Brazilian test, a disc shape specimen of the rock is loaded by two opposing normal strip loads at the disc periphery. The specimen diameter shall preferably be not less than NX core size (54 mm), or at least 10 times the average grain size. The thickness/diameter ratio should be 0.5 to 0.6. The load is continuously increased at a constant rate until failure of the sample occurs within few minutes. The loading rate depending on the material and may from 10 to 50 kN/min. At the failure, the tensile strength of the rock is calculated as follows:
Strength =2P/(pDL)
P = applied load, D = diameter of the sample, L = thickness of the sample
The above equation uses the theory of elasticity for isotropic continious media and gives the tensile stress perpendicular to the loaded diameter at the center of the disc at the time of failure. If the sample is anisotropic and exhibits weakness planes (preferred orientation of minerals or stress history), the specimens should be prepared in such a way that both directions parallel as well as perpendicular to such planes can be tested (axis of the cylinder parallel to the plane) (Brazilian Test, n.d.).
The Triaxial compression test
Triaxial tests are widely used in geotechnical engineering both in soil and rock mechanics. Specimens are axially loaded to failure while a confining pressure is constantly applied. As a result, the behavior of geomaterials is investigated in a three-dimensional stress state.
The principal stresses (the maximum and minimum normal stresses acting on a plane at which the shear stress is zero) in 3-dimensional objects are three (σ1> σ2> σ3). In nature, the principal stresses may differ. However, in laboratory triaxial tests, the intermediate stress σ2 is equal to σ3. Conducting laboratory tests in which all applied principal stresses differ is challenging and is not widely used. Such a procedure would be referred as polyaxial or true triaxial test. Moreover, research has shown that the effect of the intermediate stress is minor (Triaxial Compression Test in Rock, n.d.).
The point load stress index test
Point Load (PL) is a test that aims at characterizing rock materials in terms of strength. It is an index test, meaning that it can be performed relatively quickly and without the necessity of sophisticated equipment to provide important data on the mechanical properties of rocks. Index tests have been widely used in soil mechanics (ex., particle size distribution, relative density, Atterberg Limits, water content) but are recently introduced in rock mechanics. The strength of a rock material is a critical factor especially in designing civil and mining engineering projects. Deriving this critical property is not an easy task, since it requires laboratory tests and special equipment (loading machines, core drilling and sampling techniques). Point Load test is an alternate method that can be used to adequately predict the uniaxial compressive strength of a rock material using a portable and simpler equipment. Schmidt Hammer Rebound test is also a technique used for this purpose but, it is admitted that its results are far more variable and affected by testing methods (Point Load Test, n.d.).
Reference
Brazilian Test. (n.d.). Retrieved March 5, 2025, from http://www.geotesting.org/geotest/brazilian-test
Huang, J. (n.d.). Laboratory Testing and Interpretation of Rock Properties.
Point Load Test. (n.d.). Retrieved March 5, 2025, from https://www.geoengineer.org/education/laboratory-testing/point-load-test
Triaxial Compression Test in Rock. (n.d.). Retrieved March 5, 2025, from https://www.geoengineer.org/education/laboratory-testing/triaxial-compression-test-in-rock
