Point load test

The point load test was developed as a small hand-portable test apparatus to provide an index for the strength classification of hard rocks in the field; it allows a quick and non-expensive on-site evaluation of stone strength of hard rocks.
Basically, the test method relies on the principle of inducing tensile stress into the rock by the application of a compressive force. The maximum tensile stress at the centre of the specimen may be related to the applied load and to the distance between the point loads according to the equation:
T = KP/h2
K constant, P applied load, h distance between loading points, T maximum tensile strength
This testing equipment consists of the systems for measuring the load required to break the specimen and for measuring the distance between the two platen contact points. Rock specimens in the form of core, cut blocks, or irregular lumps are broken by application of concentrated load through a pair of spherically truncated, conical platens.
From the 70’ it is a generalised and standardised test of the International Society for Rock Mechanics (ISRM, 1972) for obtaining rock strength data in a field environment.


Corrected point load strength can be given as:
IS(50) = (D/50)0.45 (P/D2)
D diameter of core samples, (mm) or equivalent diameter De for irregular test pieces or axially tested core samples
P applied load