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This paper describes a recent study on using fly ash for backfilling abandoned room and pillar mines. Detailed investigations on fly ash properties such as the strength and stiffness of settled fly ash, flowability of fly ash grout, as well as chemistry and environmental aspects of fly ash backfill have been undertaken in the laboratory. Numerical modelling was also conducted to quantify the effects of fly ash backfill on the stability of underground pillars. The laboratory tests showed that with a solid concentration of approximate 50%, fly ash grout has an excellent flowability and very low viscosity. It is capable of penetrating and filling almost any voids underground if designed properly and settling as a reasonably stiff solid to provide support to the pillars. Several different types of strength tests proved that a consolidated fly ash should exhibit a friction angle above 42°. 3D numerical modelling on interaction between fly ash backfill and underground pillars has shown that fly ash backfill to 90% roadway height can raise the factor of safety (FoS) of a marginally stable area to above 1.6, which is the number often used in rock engineering design for long term stability. Chemistry and leachate analysis of representative fly ash samples from a local power station showed that the elemental concentrations in the fly ash solid sample are lower than the allowed contaminant threshold and specific contaminant concentration levels. Geotechnical monitoring in the high risk areas of an abandoned mine has been carried out as part of the risk management and control for potential subsidence. The monitoring has been very helpful in understanding the ground behaviour around the abandoned mine which can provide timely information to the parties concerned in order to make correct decisions to control the subsidence risk.

Conventionally, geotechnical information comes from cored drill holes. However, coring is expensive and most boreholes are drilled without, or with very limited coring. Alternative ways of obtaining geotechnical information need to be found. One approach is by geophysical logging, which is carried out routinely at coal mines. It measures various in-situ petrophysical parameters which are usually correlated with rock types and can be used for rock mass characterisation, litho-stratigraphic interpretation, orebody delineation and grade estimation.This paper presents examples of automated geotechnical characterisation from geophysical logs to identify the key strata responsible for caving behaviours during longwall coal mining at Southern Colliery in Central Queensland. The method is based on computer program ‘LogTrans’. Conventional logs such as density, natural gamma and sonic derived UCS (Uniaxial Compressive Strength) logs and full waveform sonic data are examined. The geotechnical units predicted from the geophysical logs are well matched with the original strata classification and core photographs. The results could assist site geologists, planning and production engineers predict and manage mining conditions on an ongoing basis.