Subsidence Control Using Coal Washery Waste

This report is the final report of the ACARP project - C16023 Subsidence Control Using Coal Washery Waste. The main objective of this project is to provide technical knowledge to support a trial of overburden grout injection technology to control mine subsidence in an Australian mine. The project includes comprehensive pre-trial feasibility studies at three targeted mines by means of field monitoring, laboratory simulation and numerical modelling.

Detailed research tasks completed during the two-year period of the project include:

· Comprehensive geotechnical monitoring and numerical modelling at West Cliff Colliery to address the key technical issues identified for a full scale trial, including determination of the safe and effective grout injection horizons and quantitative assessment of the effects of the protective improvements by grout injection on the stability of surface features;

· Technical feasibility assessments for Moranbah North Mine and Mandalong Mine, based on the new and existing measurement data at these mines and knowledge/findings of West Cliff Colliery;

· A cost analysis of grout injection operations in different site conditions to highlight the benefits and costs of the technology for each site; and

· Investigation of the feasibility of using a wide range of coal washery wastes as the injection material.

Favourite conditions for an overburden grout injection operation

Based on the results of the current and previous studies, a set of conditions that favours the application of the overburden grout inject is summarised and listed below:

· Mining depth is greater than 200m;

· One or several strong and thick competent strata exist in the overburden at a distance greater than 50m above the mining seam;

· Strata in the immediate overburden are soft and impermeable;

· Grout materials (fine coal wash rejects or flyash, water) are locally available and can easily be transported to site;

· The protected surface features are located in a limited area in each longwall panel.