Development of methodology for the dimensioning of large mining excavations

Research project P 13805 Design methodology for large mining excavations Horst WAGNER 28.06.1999 Environmental pressures and the exhaustion of surface deposits result in a strong trend towards the underground extraction of mineral deposits. In the case of industrial minerals and minerals for the building industry the cost of underground production has to the competitive to that of surface mining. The requirements of low production costs and protection of the surface from the effects of underground mining are met by the excavation of large underground openings separated by support pillars. To avoid unnecessary losses of the mineral resources the pillars have to be designed as small as possible. While the design of room and pillar systems has reached a high degree of perfection in coal and potash mining the state of the art in hart rock mining is still in its infancy. The objective of this research project is to develop a design methodology for permanent open stopes and support pillars for massive alpine mineral deposits. The study will concentrate on deposits of industrial minerals, but the results will also have application for other massive mineral deposits. In the course of the investigation it is planned to conduct a number of field experiments in working mines, to perform laboratory tests on drill cores and to carry out numerical simulations of real mining situations. The combination of field experiments, laboratory tests and numerical simulations is necessary in order to arrive at a realistic assessment of the behaviour of large rock structures and to account for the important scale effects. Using methods of back analysis a design methodology for the dimensioning of large mining excavations and support pillars will be developed. All information will be stored in a central data base.

The demand for minerals continues to increase world wide and in Austria has reached a level of 13 tonnes per head of population and year. The growing concern for environmental matters and the depletion of mineral deposits situated close to surface has led to an increasing tendency to extract deeper mineral deposits by underground methods of mining. This trend is particularly noticeable for industrial minerals. In underground mining there are two opposing demands on the mining system. For productivity and cost reasons the underground openings should be as large as possible whilst safety and environmental considerations demand that they are kept small. Up to now the dimensioning of mining excavations has been done largely on empirical grounds. This has led to design procedures which work well in particular mining environments but which can not be employed outside the range of conditions for which they have been developed. This aspect is true for the dimensioning of large underground openings as well as for the pillars which separate the openings. The difficulty stems from the fact that the rock mass in which the excavations are situated is far from being a perfect engineering material. The strength and deformation properties of the rock mass are adversely affected by the presence of bedding planes, joint systems and shear- and fault zones. Against this background it is not surprising that so far the design of mining excavations is largely based on local experience and not on well established engineering principles. This is particularly evident when the multitude of pillar strength formulas known from the mining literature are considered. In this research project the behaviour of support pillars has been investigated in a number of mines using a combination of subjective, empirical, numerical and geophysical methods Based on these investigations a methodology for describing and quantifying pillar condition has been developed. Using this methodology areas/pillars in mines having different geomechanical behaviour or strength capacity can be identified and classified. Particularly useful in this respect is the seismic wave velocity which decreases with increasing degrees of pillar deterioration. Seismic surveys of mine pillars are well suited to quantify pillar deterioration over time. Using this method critical pillars can be identified at an nearly stage. Based on this methodology it was possible to assess mine stability and to recommend remedial actions in several cases. As a next step it is envisaged to employ the methodology developed under this project to carry out back analysis of pillar performance on several mines and to develop an improved pillar design procedure.