When asked “How strong is that rock?” I usually respond with “what is the reason you want to know the rock strength?”. The approach we take to assessing rock strength will to some degree depend on that reason. In most cases it relates to some part of an engineering design where some degree of assessment can be made when recommending values. The more difficult area is when assessing excavatability particularly in the case of a contractual assessment of strength (e.g. you have a clause in the contract which may read “Rock with a UCS of >25MPa”).

Rock strength can be measured in a number of ways. Generally, the UCS (Unconfined Compressive Strength) is the most common type of assessment. Direct measurement is undertaken in a similar way to concrete cylinders i.e. a core of rock must be taken of the rock mass you are assessing. This requires a medium to large sized drilling rig capable of wash boring with coring ability. It can also be significantly affected by defects in the rock and the test can be ineffective at low rock strengths. However, this can be an expensive way of assessing only a few samples.

The most common alternative method (and less expensive) is to carry out point load index tests (PLT, PLI or Is50) on grab samples. This consists of the application of load between two points to the fragment of rock. This is a measurement of tensile strength rather than compressive strength. As such a factor then needs to be applied to the value to provide an estimate of UCS. Numerous papers have been written which discuss this relationship which is usually treated as a simple multiplication factor.

A review of these papers shows that the factor will vary considerably depending on rock type, weathering grade and even the strength range of the rock. Multipliers of between 5 and 20 have been published for rocks in the Brisbane area and we have even seen values greater in other areas. It should be noted that these tests are measuring the intact rock strength of the rock fragment it is not measuring the rock mass which is affected by defects within the rock and is therefore likely to be lower than the intact rock strength.

The best solution is to make sure the contract indicates what test is required, to what standard and if any other methods of testing are acceptable and then the required value. In some cases, the use of the PLT may cause less disputes than the UCS.