Rock mass strength at depth and implications for pillar design

Abstract

Construction of infrastructure in deep underground mines depends on an understanding of near wall rock behaviour as well as the ultimate load bearing capacity of confined rock, and thus on a reliable strength criterion for the rock near and far from the excavation. The topic of brittle failing rock i.e. rock failure dominated by tensile crack and fracture propagation even under low overall compressive conditions, is briefly summarised. Recently, it was suggested that the failure envelop for the entire confinement range of brittle rocks and rock masses is distinctly S-shaped. If validated, this impacts engineering problems such as pillar design where both wall instability and confined rock mass stability issues need to be considered. This paper explores the limitations and potential opportunities in pillar design. It is illustrated that current empirical design rules may be flawed and further studies are required to arrive at more economic designs for pillars at depth, or under high stress, and in brittle failing rock masses. When confined in the core of pillars, the rock mass may exhibit superior strength than typically assumed by Mohr-Coulomb or Hoek-Brown failure criteria as it will fail differently than near the wall. As a result, pillar strength may be underestimated based on field observations and if procedures of rock strength back analysis from near wall behaviour are adopted to determine the rock mass strength envelop. This means that the strength of pillars with width to height (W/H) ratios exceeding 1?5 to 2 may be systematically underestimated and may become burst prone, as the core may not yield as anticipated. Consequently, pillar designs based on current empirical rules may be inadvertently conservative and thus not optimal from an economic perspective. This aspect is of particular interest for block cave mines where drawpoint spacing may have a significant impact on cave propagation, recovery performance, and economics.

A Parametric Study of Blast Damage on Hard Rock Pillar Strength

“…Figure 9 shows pillars at three different heights (2 m, 4 m, and 6 m), each with a damage thickness of 0.5 m. Since this now incorporates different pillar heights, the strength of the normal pillars was analyzed at different pillar W/H ratios. Figure 10 shows that shorter pillar heights represent higher strength, while larger pillar heights lead to lower strength, in accordance with Kaiser [24]. Figure 11 shows that the blast damage has a significant effect on pillars with lower pillar heights.…”

“…The strength of the pillar decreases with the increase in pillar height at a constant pillar width-to-height (W/H) ratio [24]. Many studies have been conducted to understand the effect of size, which shows that at a constant W/H ratio, as the sample size increases, the strength of the sample decreases [25].…”

Coal Strength Development with the Increase of Lateral Confinement

“…This method was used by Amann et al [6] in studying the brittle behavior of clay shale. Even though the volumetric strain calculated from Equation (1) is not accurate due to the omission of high-order-terms, the volumetric strain calculated with Equation (1) has been confirmed to be an effective damage indicator with micro acoustic emission technique [6].…”

“…With the increase of mining depth, a number of new challenges have been encountered in underground coal mining [1]. In order to mitigate the hazards potentially threatening miners and make underground mining economically attractive, a thorough understanding of the mechanical behavior of coal under a wide range of confinements is essential.…”

The role of lithological layering and pore pressure on fluid‐induced microseismicity

“…This process involves forced fluid injection into hydrocarbon reservoirs to increase the rock permeability through the opening of fractures due to brittle failure and slip within the host rock [Pearson, 1981;King, 2010]. It is also applied to extract geothermal energy [Majer et al, 2007;Yoon et al, 2014] or to facilitate underground block caving in mines [Kaiser et al, 2011;Preisig et al, 2014]. A cloud of low-magnitude microseismic events (i.e., M < 0) develops during the fluid injection, and its analysis gives insights into the location, shape, time dependence, and failure mechanism of the developing fractures within the reservoir rock [Pearson, 1981;Phillips et al, 2002;Davies et al, 2012].…”