SUMMARY:

The Dolphin Orebody of King Island Scheelite is wide and shallow. The ore is well jointed and mining is by post-pillar cut and fill mining. Mining of the first large stoping area at 130 m to 110 m depth has been extensively monitored by extensometers. Stress measurements have been carried out prior to mining and, as. mining progressed, in the post pillars, in the ground above the stope and in the abutments. An arch formed above the stope as mining progressed. Actual pillar loads were very low and a zone of destressed rock above the stope, supported by the pillars and boundary shear forces, was evident. From the investigations a maximum span was determined for the stope. The general problem of the size, spacing and support of the pillars and the determination of arching capacity and stability are examined in the light of the investigations.

RESUME:

Le filon metallifère Dolphin de King Island Scheelite est large, peu profond, bien disjoint et l'exploitation est faite par chambres et poteaux-piliers et remblayage. L'exploitation de la première surface d'abattage, situee 130 à 110m, fut auscultee au moyen d'extensomètres. Les contraintes furent mesurees, avant et durant l'exploitation, dans les piliers, dans le sol au dessus du chantier d'abattage et dans les butees. Une voûte se formait au dessus du chantier d'abattage au fur et à me sure de l'exploitation. Les contraintes des piliers etaient basses et une zone de stabilisation au dessus du chantier d'abattage, supportee par les piliers et les contraintes de cisaillement sous-jacentes, etait evidente. Une portee maximum fut determinee pour le chantier d'abattage. La dimension, l'espacement, support des piliers, la capacite de former une voûte et la stabilite sont examines.

ZUSAMMENFASSUNG:

Der der Fa. King Island Scheelite gehorende Erzkorper ist breit, flach, gut abgesondert und der Abbau wird durch, Kammer- und Stoβbau durchgefuehrt. Der Bergbau der ersten groβen, bei 130 bis 110m liegenden Abbauflache wurde mittels Extensometer kontrolliert. Spannungsmessungen wurden vor dem Abbau und bei fortschreitender Arbeit auch in den schlanken Pfeilern, im ueber dem Abbauort leigenden Boden und in den Auflagern durchgefuehrt. Mit fortschreitendem Abbau bildete sich ein Gewölb ueber dem Abbauort. Die Pfeilerbelastungen waren sehr gering, und eine ueber dem Abbauort liegende, von den Pfeilern und den Grenzscherkraften abgestuetzte Spannungsentlastungszone lieβ sich feststellen. Eine maximale Spannweite wurde fuer den Abbauort festgestellt. Das Problem der Gröβe, des Abstandes und der Abstuetzung der Pfeiler, sowie die Bestimmung der Aufwölbungsfahigkeit und der Standfestigkeit werden besprochen.

1. INTRODUCTION

Many wide orebodies are mined by underground mining techniques which are simply variations of room and pillar mining methods. Optimum extraction in noncaving situations generally requires primary mining panels with small pillars between larger barrier pillars for regional support (for example, Kravchenko and Pforr, 1976). A recent detailed review (Salamon, 1974) of room and pillar mining emphasises the interaction of the pillars with the surrounding strata in determining individual pillar loads, individual pillar stability and overall stability of each panel. Even if pillars yield, a panel itself may be stable. For many situations, Salamon (1974) postulated that stability could be determined by the elastic analysis of the stress/ deformation behaviour of mine openings incorporating non-linear pillar load/deformation relationships preferably determined in situ. However for room and pillar mining situations in jointed rock, the techniques for the determination of stability, maximum panel widths and pillar sizes are not well advanced. This paper describes a situation where the mining method is a variation of room and pillar mining: namely post-pillar cut and fill mining. A typical post-pillar mining layout is shown in Figure 1. Mining proceeds by taking a series of horizontal slices completely across the orebody for the extent of each panel. A regular pattern of pillars remains with typically 70% to 90% of the ore extracted. The stope is partially filled before mining the next lift above. progressive mining generates pillars typically with height to width ratios greater than two. As conceived (Cleland and Singh, 1973), the pillars are designed to yield and fail in a gradual manner below the fill level. This investigation concerns post-pillar mining in a shallow mine within a rock mass whose behaviour is governed by its geological structure. The aim of the investigation was to determine the factors that govern the stability of the post-pillar operation so that rational choices of panel span, of spacing and dimensions of pillars and of pillar support could be made.

2. SITE CONDITIONS

The Dolphin Orebody of King Island Scheelite is situated on the south-east coast of King Island, Australia. The scheelite deposit originally outcropped on the shore of the island and continued below the seabed at a mean dip of 300 to approximately 300 m depth. Mining of the upper portions of the orebody was undertaken as an open cut operation and was completed in 1974.

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