ABSTRACT:

This paper presents a study, on laboratory scale, of the deformation and strength characteristics of mine support pillars. The experimental approach used small-scale models of the pillars, shaped in such a way that the uniaxial compression tests, with a very stiff servo-controlled testing machine, were not affected by the contacts between the testing machine and the specimens. An elastic numerical analysis was carried out, in order to investigate the influence of some parameters on the pillar stability

RÉSUMÉ:

Cette communication presente une etude, à l'echelle de laboratoire, des caracteristiques de deformation et resistance des piliers de soutènement des mines. L'approche expuerimentale usa des modÉles des piliers à echelle reduite, ayant une forme telle que les essais de compression uniaxiale, avec une machine d'essais servo-contrôle très rigide, n'ont pas ete affectes par les contacts entre la machine d'essais et les specimens. Une analyse numerique elastique a ete faite, afin de chercher à investiguer L'influence de quelques paramètres sur la stabilite de piliers.

ZUSAMMENFASSUNG:

Dieses Referat stellt eine Untersuchung, im Labormaßstab, der Verformungs- und Festigskeits charakteristika von Bergwerksstuetzpfeilern vor. Der experimentelle Ansatz benutzte Modelle in verkleinertem Maßstab von den Pfeilern, die solch eine Form hatten, daß die einachsigen Druckversuche, mit einer sehr steif en servo-gesteverten Versuchsmaschine, nicht durch die Kontakte zwischen der Versuchsmaschine und den Spezimina beeinflußt wurden. Eine elastische numerische Analyse wurde durchgefuehrt, um den EinfluÉ einiger Parameter auf die Pfeilerstabilitat zu untersuchen.

1 INTRODUCTION

In practical terms the use of multipurpose pillars has widespread in underground mines. Particularly when pillars provide the main support for the roof, in spite of progress of knowledge in rock mechanics already permitting to approach the design of such pillars rationally, it still seems justified to carry on research to investigate the mechanisms governing the behaviour of pillars from loading to failure. From the rock mechanics viewpoint, the design of support pillars aims at ensuring that their sizes and those of spans may be enough to prevent failure as long as required. Their design demands a detailed quantitative analysis of failure conditions resulting from the interactions of the complex matrix of parameters that condition the overall mine design. This matrix has dynamic characteristics which are imported to the factor of safety of the pillars, according to the development of mining which often introduces changes in load distribution on the supports. This work presents the overall question of the design of support pillars in mines, the study of the behaviour of models of pillars following experimental and mathematical approaches by using a schistous metamorphic rock from a tungsten mine exploited by a method that sometimes employs overstressed support pillars. The experimental approach made use of pillars shaped in such a way that deformation and failure conditions were not affected by the contacts between the testing machine and the specimens tested. Resides the tests were carried out with a very stiff servo- -Controlled testing machine, able to produce fairly reduced strain rates. Particularly the effects of the dimensional relations of the pillar (width/height ratio) were analysed for the complete load - deformation curves, including the region beyond the pick strength where they have a negative slope. The numerical analysis was carried out by using a linear elastic three-dimensional boundary element model and the influence of the Poisson's ratio and the width-heigth ratio on the stability of the pillar.

2 EXPERIMENTAL STUDY

The aim of the study on the basis of this paper was an experimental simulation in laboratory of the usual work conditions of support mine pillars as well as a theoretical approach, in order to contribute to the knowledge of the behaviour of mine pillars, in particular in the period beyond rock elastic behaviour until complete breakdown. The laboratory testing was carried out in a 4000 KN quick-response servo-controlled testing machine with a stiff frame of l0l0N/m. The electronics pack age has three controllers monitoring three different control variables: position of the piston of the machine, axial displacement and axial load. The outputs are fed to a X-Y plotter which gives the load-deformation curve of the specimen being tested according to the conditions fixed with a ramp generator in which the sign varies monotonically with time. The different properties of the material of the platens and the rock provoke the well known end effects which can influence the results obtained from the tests. In order to overcome this difficulty all the pillar models tested were of the shape represented in Fig. 1. The relative dimensions of the cylindrical pillar models are such as to reduce the possibility of failure by bending of the zone above the top of the pillar. The rock used for the model pillars was a schistous metamorphic rock with high strength, whose schistosity varies slightly around the axes of the model pillars.

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