ABSTRACT

ABSTRACT: A three-dimensional numerical analysis produced the same overall stress change trends and magnitudes as was measured in situ at a number of locations in a rib pillar undergoing extraction in an underground mine.

1 INTRODUCTION

Results were obtained from three-dimensional numerical modelling and stress change monitoring of a rib pillar undergoing extraction at the Renison tin mine, Tasmania, Australia. The mine is highly mechanised and currently produces 850,000 tonnes of tin ore per annum primarily by the flatback Cut and Fill mining method. The pillar is situated at a depth of 210 m in the lowest of three tabular orebodies which dip at 30 degrees. The pillar separates two stopes and is overlain at either end by stopes located in the middle orebody. The pillar dimensions are approximately 10 m stope-to-stope, 10 m high and 100 m long. The modelling and monitoring covered about 44 m of pillar extraction.

2 NUMERICAL MODEL

A three-dimensional numerical model is needed to take account of the interaction of the various stopes located on the different orebody horizons. The tabular orebody geometry was modelled using a three-dimensional, multi-seam displacement discontinuity program, BESOL/MS221 (Crouch Research, Inc., 1986). Each orebody horizon was modelled by 24,300, 4 metre square elements covering an area 720 m by 540 m which allowed all of the stopes and sub-pillars located in the vicinity of the rib pillar to be defined. Stress predictions were obtained for a sequence of pillar configurations representative of the actual extraction sequence.

3 INSTRUMENTATION AND MONITORING RESULTS

The pre-extraction stress field was determined at three locations using the overcoring technique in conjuction with the CSIR0 hollow inclusion (HI) cell (Worotnicki and Walton, 1976). Stress monitoring instrumentation was then installed along the rib pillar at three locations. To compare instrumentation performance, CSIR0 HI cells and Yoke Gauges (Walton and Worotnoecki, 1986), vibrating wire stressmeters and a prototype biaxial "rigid" inclusion stressmeter were installed at one location. Further CSIRO HI cells were installed at the other sites. The instrument outputs, except vibrating wire stressmeters, were measured and recorded for 14 months by a CSIRO designed datalogger.

In broad terms, partial extraction of the pillar caused the stresses in the remaining part of the pillar to decrease by approximately 10 MPa in the direction along the pillar. The vertical stress initially remained constant and then increased by 2 MPa as mining progressed. There was also an initial stress increase of about 5 MPa across the pillar which then decreased as pillar extraction continued. The greatest shear stress component change was in the vertical-across-the-pillar plane.

4 COMPARISON OF RESULTS

Reasonable agreement was obtained between predicted and measured stress change components at the three in situ monitoring sites as mining progressed. For each stage of mining, the stress magnitudes along the footwall of the pillar were closely matched, whilst the cross pillar and vertical stresses were overestimated by a maximum factor of less than 2.

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