ABSTRACT
Rock mass failure can take a form of a dynamic rupture, or progressive and transgressive deterioration and can take from a few seconds to many years. Dynamic failure could be as short as a few seconds. It starts with seismic activities in the form of rock noises, loud acoustic emissions followed by seismic shock. These activities start just before the collapse, reach the highest level during the brunt and slowly diminish over a number of hours or even days.
For progressive or gradual failure that takes a long time without a distinctive point of losing strength, it is often not possible to define a point of failure. During a process of progressive non-violent damage, stress transfer through the effected rock mass is slowly diminishing. The stress can be transferred through an abutment (rock mass in front of the mined-out void), remnants, pillars, or consolidated fragmented rock material in mining stopes. The rock mass can suffer substantial damage and excavations progressively lose their functionality but still can be able to transfer some stress, e.g. yielding pillars, progressive closure of secondary excavations or movement of open pit slopes.
Exposure to high stress (relative to the rock mass strength) usually caused by high abutment stress, can result in transgressive deterioration in ground conditions. Transgressive deterioration begins slowly, but then accelerates towards the eventual brittle failure.
From the first signs of high stress being noticed, deterioration in ground conditions can progress quickly and, with an increase in abutment stress, the damage can propagate exponentially, leaving weeks, if not days, before rendering an excavation unusable.
Three case studies on the effect of exposure to abutment stress on excavation deformation are provided: one in a deep copper mine using open stoping with pillar recovery where stress transfer took place through remnant pillars, the second in a nickel mine using sublevel caving methods where stress transfer took place by down-dip abutment, and the third in a copper mine using block caving method where stress transfer took place through compacted caved-in material.
