Abstract

Gabion-type support is a favoured option to restrain bulking in pillar walls of mine footprint tunnels. It uses closely spaced short reinforcements in tunnel walls (typically fully grouted rebar) in combination with surface support (rock fragment retention systems such as shotcrete, weld wire mesh, straps, etc.). The system is installed while the rock is still mostly intact and is conceived to maintain support capacity even when, the rock attains a fully fragmented state, acting then like a gabion or earth-reinforced type retaining wall. In this paper the interaction between the support system and the highly stressed pillar walls is investigated numerically by means of finite element analyses within the framework of displacement-based design. Because the material response should capture the passage from intact rock to fully fragmented state, an advanced elasto-plastic bonded constitutive model was adopted as a simulation framework. The model is calibrated to replicate the mechanical behaviour of Bursnip Sandstone and Amarelo Pais Granite. These two rocks were selected because of high quality triaxial tests results from the literature. After showing the good performance of the model to reproduce both low and high pressure triaxial compression behaviour an extensive parametric study investigating the effects of bolt types on gabion response is presented.

Introduction

Massive underground mining and particularly caving mines at deep levels (more than 1000m) is becoming ever more widespread. The development of cave mine infrastructure in such highly stressed environments may lead to excavation instability problems [1]. The way mine development sequencing and cave advance affect the re-distribution of induced stresses on mine infrastructure as mining progresses, needs to be considered for support design [2]. Rock support in burst-prone ground needs to resist large deformations due to rock "dilation", called bulking, during the violent failure of rock [1]. The term "bulking" is used to describe volume increases of the rockmass near an excavation due to geometric non-fit of fragments during the transition from competent to fractured and then to broken rock. Near excavations, bulking is unidirectional toward the excavation (perpendicular to the wall), a function of the applied tangential strain, and highly dependent on the confining stress. For this reason, gabbion-type support is a favoured option to restrain bulking in pillar walls of mine footprint tunnels. It uses closely spaced short reinforcements (typically fully grouted rebar) in combination with surface support (rock fragment retention systems such as shotcrete, weld wire mesh, straps, etc.). The system is installed while the rock is still mostly intact and is conceived to maintain support capacity even when, because of mining operations, the rock attains a fully fragmented state, acting then like a gabion or earth-reinforced type retaining wall.

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