Geotechnical Evaluation of Roof Conditions at Crinum Mine Based on Geophysical Log Interpretation

At the underground coal operators conference held in 2008, Payne described the experiences of crinum mine in characterising the weak roof strata at the mine. To a large extent, primary and secondary roof support strategies are based on UCS values determined from sonic logs. Consideration is also made of lithological units that can be identified on natural gamma logs. At crinum, UCS values in the roof strata tend to fall in the range 3-30 MPa. Through ACARP funded research, a new method for evaluating geotechnical conditions known as the Geophysical Strata Rating (GSR) has been developed. The GSR is based on the interpretation of sonic, density and natural gamma logs and is designed to provide a measure of strata properties on a linear scale similar to that used in the Coal Mine Roof Rating (CMRR). GSR values are largely based on sonic velocity measurements and some degree of similarity therefore exists with the UCS determinations at Crinum. A comparison between the conventional UCS results at Crinum Mine and the new GSR determinations is made. The basis for relationships between sonic velocity and UCS is also discussed. Compositional factors and the range of depths over which a relationship is applied are important. The GSR takes these factors into consideration and provides an alternative and robust approach to estimating rock properties