3.2.1.5 Uniaxial Compressive Strength (UCS) / Cylinder Loading
A uniaxial compressive test is one where a disc sample is axially loaded and is unconfined in all other orientations. A cylinder-loading test can be both point and line loading of a core sample under diametric conditions.
Kotte et al [1968] compared UCS, cylinder and triaxial tests. He observed that cracks formed parallel to the loading force and concluded that these tests gave good indications for the prediction of strength of rock materials.
Broch [1972] investigated the UCS method and pointed out the extensive sample preparation required to perform this test. The relationships he used depending on point or line loading, are given below:-
Where k is a constant, P is the applied load, D is the distance between loading points and T is the maximum tensile strength. The constant k was found to be from 0.5 - 1 and was a function of sample geometry. A correction factor was used to relate core sizes to a ’standard’ 50mm diameter core. This sample correction is annotated as Is(50) and when k=1 the following relationship can be assumed:-
Work by Chau and Wong [1996] investigated the relationship between UCS, point load and Brazilian test methods. They concluded that a constant (k) relating UCS to strength index is not to be taken as universal and therefore it is not always safe to use the following equation:
This was further confirmed by Bowden et al [1998].
Gunsallus and Kulhawy [1984] and Bearman [1999] investigated the relationship between UCS and cylinder tests to KIC as they considered the Mode I fracture toughness a good indication of rock comminution behaviour. They concluded that if the ISRM standards were adhered to then the UCS and cylinder methods were accurate in their results.