Rock Socket Capacity Certification Using the Point Load Index Test-Paper

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The Foxwell Road Overpass project in Queensland used the Point Load Index (PLI) test to certify the capacity of bored piles socketed into sandstone and argillite. Unlike the usual subjective field descriptions of rock strength, PLI testing provided a quantitative measure during construction. The study compared 119 tests on irregular rock fragments recovered during pile excavation with 30 tests on intact core from the site investigation.<br /><br />Because PLI results were highly variable and often right-skewed, the authors compared normal and log-normal probability distributions. The log-normal model better represented both datasets and avoided physically impossible negative strength values. Using this model, construction-phase results were broadly comparable to investigation results; construction samples did not show the expected bias toward stronger rock. Analyses of the largest fragments likewise found no evidence that only the strongest material survived excavation.<br /><br />The authors recommend using the lower-quartile PLI value—not the mean or median—as a practical characteristic strength for construction certification. It provides a conservative, simple criterion and was close to the point where the fitted normal and log-normal distributions converged. Outliers can substantially inflate means, so they must also be considered when interpreting results.<br /><br />Plate Load Tests at the bridge piers provided a comparison with in-situ rock-mass stiffness. The results indicated that predicting stiffness from PLI-derived strength requires very conservative strength values, around the fifth percentile, and appropriate strength–stiffness relationships. Thus, PLI is more suitable for direct strength assessment than for reliably predicting rock-mass modulus.<br /><br />Overall, the case study supports using PLI tests on excavation-recovered fragments for rock-socket quality assurance at this site. The authors caution that further studies are needed to confirm the method’s applicability across other rock types and conditions.

Keywords

Foxwell Road Overpass

Queensland

Point Load Index

PLI testing

bored piles

sandstone

argillite

log-normal distribution

rock strength

quality assurance