Discrete Element Modelling of Footwall Rock Mass Damage Induced by Sub-Level Caving at the Kiirunavaara Mine

Mikael Svartsjaern,David Saiang

doi:10.3390/min7070109

Abstract

The Kiirunavaara mine is one of the largest sub-level-caving (SLC) mines in the world and has been in underground operation for more than 50 years. The mine has been the focus of several case studies over the years. The previous works have either focused on the caving of the hanging wall, using the footwall as a passive support, or focused on the footwall using the hanging wall to apply a passive load. In this updated study the findings of the previous case studies are combined to study the interaction between the caving hanging wall, the developing cave rock zone and the footwall. The geological data for the rock types in the mine area are used to derive upper and lower limits for the geomechanical parameters calibrated for numerical models in the previous studies. The calibrated parameters are used as inputs to a numerical model constructed using Itasca’s Particle-flow-code (PFC) encompassing a mine-scale 2D section at the mid portion of the mine. The model captures the failure locations well in the footwall underground and indicates damage development without a coherent large-scale failure. The trend in subsidence data on the hanging wall is adequately simulated but the magnitude of deformation is underestimated. The input strength for the hanging wall was lowered to study the impact of hanging wall strength on footwall damage development. It is shown that when the footwall strength is kept constant, while lowering the hanging wall strength, the extent of damage and magnitude of displacements in the footwall increases. From these observations it is argued that the hanging wall and footwall cannot be studied independently for the Kiirunavaara mine since the cave rock zone significantly affects the damage development in both walls.

Highlights

A well-established fact is that large-scale mining affects the rock mass at a considerable distance from the actual mining area, especially in the case of large-scale underground mining
The strength footwall properties rock massestimated containsfor accessible underground infrastructure, which allows the
The equivalent rock mass input was estimated using a combination of measured data, rock mass classification results and back analysis data from previous studies

Summary

Introduction

A well-established fact is that large-scale mining affects the rock mass at a considerable distance from the actual mining area, especially in the case of large-scale underground mining. A common problem, is that even though the effect of the underground mining might be evident from surface settlements or lowering of ground water, the explicit behaviour of the rock mass can seldom be directly studied. The transition from open pit into underground mining by SLC at the end of the 1950s has had a significant impact on the ground surface footprint, with caving and surface settlement on the hanging wall and caving of a cap rock above the lake ore, which was not mined during the open pit operation (see Figure 1). Aside from the lake ore area, the surface impact on the footwall has been small, with tensile crack formation and discontinuous deformation confined to the Minerals 2017, 7, 109; doi:10.3390/min7070109 www.mdpi.com/journal/minerals oldopen openpit pitwalls walls[1]

Results

Discussion

Conclusion