An Engineering Assessment Of The Strength And Deformation Properties Of Brisbane Rocks

The use of standard relationships between point load index (Is(50) ), Uniaxial Compressive Strength (UCS) and elastic modulus values is commonly used within the geotechnical engineering community. The validity of these relationships is examined using data from several major projects carried out in Brisbane recently. Additionally the methods of Point Load Index testing is examined, particularly the impact of the direction of testing. It is established that in extremely low to low strength rocks, axial tests are likely to produce a higher point load index value. Based on our data set, the use of the standard multiplier of 24 (Broch and Franklin, 1972) to obtain UCS from Is(50) values is shown to be unconservative for several rock types. In terms of modulus values, in comparison with published recommendations (eg Deere, 1968), the established ratios are generally within the 100-500 times UCS ratio quoted, however the value for the DW metasediments is considerably higher.