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UBC Theses and Dissertations

Influence of a weak layer on depth of rock failure in underground limestone mines in high horizontal stress environments Metcalfe, Joshua Robert


High horizontal stress environments have plagued the aggregate industry in shallow room and pillar mining operations, particularly in the northeastern United States. With population density increasing around Southern Ontario and environmental regulations becoming more stringent, it appears that Ontario’s aggregate industry could be looking underground in the near future as the source of material to meet the ever-increasing demand. Given that the near surface horizontal stress conditions in Southern Ontario are uniquely high (σH/σV = 4-6 to depths of 200m; Lo, 1978), it can be expected that slabbing and buckling failures observed in similar mining operations with lower stress regimes in the U.S. will be exacerbated in Southern Ontario. In an effort to be proactive with this expected geotechnical design issue, a distinct element analysis using UDEC was carried out to understand the mechanisms driving failure in stratified rock environments under high horizontal stress conditions as well as to observe the impact of the high horizontal stress on the maximum depth of failure in the roof where the roof is composed of limestone interbedded with a weak shale layer. Accordingly, 116 models representing a variation of rock mass conditions subjected to stress ratios (σH/σV) ranging from 1 to 4 were simulated. A Voronoi tessellation was used to represent the intact rock mass directly above the excavation so that the failure profile through intact rock could be explicitly modelled. Key conclusions from the modelling were as follows: 1) A shale layer within a defined distance from the roof of the excavation could increase the depth of failure to three times what would normally be estimated for stratified rock masses of limestone only. 2) Failure driven by the influence of a weak shale interbed occurs through diagonal fracturing, supporting an experimental conclusion published by Stimpson and Ahmed (1992), as opposed to slabbing or buckling. Slabbing and buckling are the common failure mechanisms in stratified rock masses without weak interbeds. Therefore, it is critical to understand the interbedded nature of the rock mass comprising the roof over a mine opening so that a proper ground support design can be developed.

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