Geotechnical investigation: Uniaxial Compressive Strength

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Introduction

A rock sample’s hardness is its fundamental quantitative engineering characteristic. When using numerical studies in rock mechanics, it is crucial to evaluate the strength and deformability of the rock mass. The greatest axial force that a sample can sustain without experiencing lateral loading is described by the geomechanical rock parameter known as uniaxial compressive strength (UCS). For this reason, it’s also known as limitless compressive strength.

Let’s go deeper

The uniaxial compressive strength (UCS) is classified as one of the basic mechanical properties of rocks, which is widely used in mining and construction practice. For example, it allows geomechanics to classify individual rocks and soils into groups with the same or similar mechanical properties (Determination of the Uniaxial Compressive Strength of Rocks from the Strength Index, n.d.). The rock is sampled by core drilling, and the sample that best represents the rock mass is selected. The minimum diameter of the sample is 47 mm, and it is 10 times larger than the largest grain in the sample. The ratio of length to diameter of the sample (L/D) must be between 2 and 2.5 according to the ASTM (American Society for Testing and Materials) standard, and between 2.5 and 3 according to the ISRM (International Society for Rock Mechanics) standard (Tomiša, 2022).

Grinding is used to create smooth, flat cylindrical surfaces. The sample’s ends and vertical axis must be levelled within a tolerance of 0.02 mm and 0.06 degrees, respectively. The sample should be subjected to a constant load so that failure happens five to ten minutes after the stress begins. The load is continuously increased from 0.5 to 1 MPa.

The following components make up the device used to assess uniaxial compressive strength:

• a compression machine that applies force continuously at a specific pace until the test is finished (Fig 1);
• steel plates known as pressure plates are employed to transmit the axial load to the sample’s ends. Hardened steel with a minimum Rockwell 58 (“C” scale) hardness is required for its construction. When the plates are new, the contact surfaces stay within the allowed range of 0.025 mm and do not stray from the plane by more than 0.015 mm;
• Tools for measuring stress.

Figure 1. Graphic representation of rock failure under axial load (https://www.usb.ac.ir).

Using the following scale, the rock’s uniaxial compressive strength can be categorised as very weak to strong:

Table 1: Classification of rock hardness (Attewell and Farmer, 1976)

Because of increased lithification and lower porosity, uniaxial compressive strength rises with age in the majority of sedimentary rocks.

Conclusion

UCS is a fundamental parameter in rock mechanics. It is useful in mining and civil engineering as it allows engineers to better understand rock mass characteristics for effective infrastructure design. However, it is important to note that this test does not account for in-situ factors such as discontinuities, faults, and large-scale wear, making it unable to accurately simulate actual rock strength. Uniaxial compressive strength testing is a common and straightforward test that provides data considered representative through a standardized technique. However, in-situ rock mass testing is rarely feasible or cost-effective.

Reference

Determination of the Uniaxial Compressive Strength of Rocks from the Strength Index. (n.d.). ResearchGate. Retrieved March 3, 2025, from https://www.researchgate.net/publication/366266892_Determination_of_the_Uniaxial_Compressive_Strength_of_Rocks_from_the_Strength_Index

Tomiša, A. (2022, November 10). Uniaxial compressive strength (UCS) test I Geotech d.o.o. Rijeka. Geotech. https://www.geotech.hr/en/uniaxial-compressive-strength-ucs-test/