Strength Test Correlations and Anisotropy of the Main Sydney Region Rock Units

The typical strength descriptor and parameter used for rock is Uniaxial Compressive Strength (UCS). In many small to medium size projects laboratory tests to determine UCS are only conducted on a few rock core samples, if at all. Point Load Strength Index (PLI) testing is commonly conducted at regular intervals on rock core. Correlations between PLI and UCS are used to assess rock strength and as input to rock mass classification systems. PLI tests are usually made parallel (Axial) and perpendicular (Diametral) to the core axis. The Axial results are commonly used to estimate rock strength. The ratio of Axial to Diametral PLI results can be used to assess the degree of anisotropy.

It is common to compare UCS with PLI test results that are close together to establish site specific correlations but limitations in datasets often do not result in clear cut correlations. This paper presents a methodology for assessing UCS and PLI datasets and comparing these to the relationship between UCS and PLI assumed in AS1726 Geotechnical Site Investigation.

There are three main bedrock units encountered in the Sydney Region: Hawkesbury Sandstone, Ashfield Shale, and Bringelly Shale. This paper presents assessments of the relationship between UCS and PLI for these bedrock units. The degree of anisotropy of each of the main Sydney bedrock units is also assessed and compared with published values.