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The Influence of Subsidence Cracking on Longwall Extraction

Underground » Mining Technology and Production

Published: August 00Project Number: C5016

Get ReportAuthor: George Klenowski | Australian Mining Engineering Consultants, Oaky Creek Coal

The aims of this research project have been to document recorded mine inflow rates, particularly where flooding of underground workings has occurred; to monitor the upward propagation of subsidence cracking; to conduct piezometric monitoring in actively subsiding areas; to analyse tensile fracturing and case histories using numerical methods; to establish criteria for calculating inflow rates into longwall mines; to design preventative and remedial treatment for open cut voids, internally draining spoil piles and water courses that are to be undermined within the critical tensile strain zone; and to design bulkheads and pumping systems for longwall mine flood protection.

Field work completed at the Oaky Creek and German Creek Mines between November 1995 and December 1999, has included collation of rainfall data, monitoring of subsidence fractures, installation and monitoring of piezometers in actively subsiding areas, measurements of rates of ingress of surface ponded water, determinations of spoil pile permeability in internally draining areas above panels, and monitoring effectiveness of remedial works by measuring mine water inflow rates during heavy rainfall events. Historical data from 1986 onwards have been analysed and included in this report.

Maximum recorded inflow rates into longwall mines through subsidence induced tensile fractures with increasing depths of cover were 140 l/sec at 70m, 83 l/sec at 100m, 45 l/sec at 120m, 27 l/sec at 140m, 30 l/sec at 160m and 2 l/sec at 175m. The values compare favourably with recorded inflows in the literature review.

The following site specific, empirical guidelines require comparative geotechnical assessment prior to being adopted at other longwall mines. For ponded water at cover depths of greater than 160m, remedial works are generally not required because standard underground pumping systems are capable of handling minor increases in flow. For ponded water at cover depths ranging from 160m to 120m, underground pumping systems should be capable of pumping a flow rate of at least 50 l/sec. For ponded water at cover depths ranging from 120m to 70m, flow rate increases from about 45 l/sec to 140 l/sec, and pre- and post-subsidence remedial works are recommended, as well as the installation of large capacity, emergency, underground pumps.

Computer modelling of tensile strain and flow through tensile fractures has been completed using the MAP3D program and the 3-D finite element program LUSAS. Calculated values have a good correlation with measured tensile strain and inflow values.

Surface remedial works which have been designed to reduce inflow rates through fractures in the critical tensile strain zone include blankets and spoil pile reshaping. Materials which have been investigated for use in compacted blankets include silty alluvium, wash plant rejects, weathered and fresh Permian spoil. Spoil pile reshaping techniques which have been researched to reduce inflow of run-off water into longwall mines include external drainage and pond landform. The cost of pond landform is about half the cost of external drainage.

Underground pumping systems which have been designed include conventional, submersible and borehole pumping systems. Safe welding procedures have been developed for borehole casing in hazardous environments where direct connection occurs to underground workings.

Methods for designing concrete and Tekblend underground bulkheads are described. Proper perimeter grouting around bulkheads is essential to prevent piping failure under increasing hydrostatic pressure.

Integrated water management systems have been designed and implemented at the German Creek and Oaky Creek Mines.

The final report details the aforementioned project outcomes.

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