Point Load Strength Index and the UCS it Implies

A uniaxial compression test needs a cylinder ground flat and parallel to a tenth of a millimetre, a stiff frame, and a laboratory. The point load test needs a lump of rock, two conical platens and a hand pump you can carry down a drift. That is the trade the ISRM Suggested Method of 1985 formalises, and it is a good trade — provided you are honest about what the second test actually tells you.

Break the specimen between the platens, divide the failure load by the square of the equivalent core diameter, and you have $I_s$. For a core loaded diametrally $D_e$ is simply the diameter; for an irregular lump it is $\sqrt{4A/\pi}$, where $A$ is the minimum cross-sectional area through the platen contacts — a computed equivalent, not a width you measure with callipers.

The size correction is real and skipping it biases every non-standard specimen. Rock is weaker in larger pieces, because a larger piece contains more flaws and a bigger flaw is more likely to be among them. $I_{s(50)} = (D_e/50)^{0.45}I_s$ reports everything as though it had been tested at the 50 mm standard. A 30 mm specimen gets multiplied by 0.795 — a 26% reduction — and a shop that tests whatever core it has without correcting will find its "strength" varies with its drilling programme.

Now the part the older references get wrong, and it is the reason this page asks you for a number instead of supplying one. A great many textbooks quote $\sigma_c = 22\,I_{s(50)}$ as though 22 were a constant of nature. It is not. It is the centre of a scatter, and the real factor runs from about 10 to about 50 depending on lithology, anisotropy and moisture state: low for weak, porous, fine-grained rock, high for strong coarse-grained igneous rock, lower again for anything tested wet. That is a five-fold spread on the single number your entire Hoek–Brown assessment is scaled by, and a calculator that picks 22 for you is not saving you work, it is hiding a factor of five.

The only defensible $k$ is one calibrated on your own rock: run a set of point load tests alongside a set of proper uniaxial compression tests on the same material, and fit the ratio. It is a day's work and it is worth more than any published figure. Record it with the lithology and the moisture condition beside it, because a bare ratio a year later is undefendable.

Three notes on running the test. Anisotropy is a result, not an error — a shale or schist loaded across its bedding and along it can differ by a factor of three, and the ISRM method asks for both, reported separately with the loading direction stated. An invalid break is discarded, not averaged in: if the fracture ran through a pre-existing flaw rather than between the platens, the test measured the flaw. And the method asks for at least ten valid tests with the two highest and two lowest struck out, which tells you something about the scatter of a single result.