Open Cut

Investigations For Open Pit Geomechanics Using Geophysical Logs

Open Cut » Geology

Published: May 13Project Number: C20025

Get ReportAuthor: Peter Hatherly, Terry Medhurst,Binzhong Zhou | Coalbed Geoscience, PDR Engineers, CSIRO

With the move towards deeper open cut coal mines, there is a need for on-going developments in technologies which allow management of geotechnical risk and the maintenance of high levels of productivity. Areas where development is required include highwall stability monitoring, semi-automated highwall mapping, 3D modelling and effective overburden characterisation. This project is concerned with the last of these and is based upon a new approach to characterising rock masses using conventional borehole geophysical logs.


Of particular interest are the composite rock mass failures that involve intact material as well as fracture and shear on existing defects. Composite failure has been identified as a key mechanism for many of the highwall failures that occur. While extension and stress provide the triggers for the failure, it is also clear that better characterisation of the overburden is required. In this regard, traditional defect logging alone often does not provide adequate information and face scanning methods require a geomechanical context against which to assess their results. Integration of highwall photogrammetry and image processing results with other datasets is difficult.


The approach to using geophysical logs has been developed through previous ACARP projects for underground coal mining and has resulted in a new rock mass rating called the Geophysical Strata Rating (GSR). The GSR was developed to address similar problems in underground mining where 3D models of the rock mass are needed to improve the assessment of roof stability, cavability and the interaction of longwall equipment with the surrounding strata.


The geophysical logs allow objective measurement of rock properties within a borehole and at great detail. At coal mines, geophysical logs, primarily natural gamma, density and sonic logs are obtained in most exploration boreholes. The GSR is an empirical rock mass rating scheme based on an interpretation of the logs. It aims to provide a measure of rock quality at every depth point in the geophysical log which is independent of rock type and depth. In this way, GSR is fundamentally different to the UCS estimates often made from sonic logs.


Given the large amount of data that is available, it is possible to create 2D and 3D models of the in-situ rock mass. In addition to models of GSR, it has been found that models showing the clay content of the strata that are present provide a very useful map of the lithological variation in the rocks forming the interburdens between the coal seams of interest. Within these models, it is possible to place additional information rock mass properties and defects in a meaningful spatial context.


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