Correlation between point load strength index and uniaxial compressive strength in Sunshine Coast sandstone

K. Scott Roche and Adrian B. McCallum

Strength is an important engineering parameter for determining excavatability and load bearing characteristics of rock. Because of the low test cost, it is desirable to identify a multiplier relating the Point Load Test Strength Index (Is(50)) with strength derived using the more expensive Uniaxial Compressive Strength Test. Samples from eight test locations on the Sunshine Coast were tested using both techniques and data were analysed using multiple regression incorporating differences between the eight locations. A multiplier of 17 is observed to produce the most accurate results when inferring UCS from Is(50). This compares with the typical local multiplier of 15. UCS prediction from Is(50) results in substantial errors in some cases and complete reliance on Point Load Strength is not recommended. We recommend that UCS testing should be carried out for each engineering project and multipliers be determined on a site specific basis.