Long-Term Stability Evaluation and Pillar Design Criterion for Room-and-Pillar Mines

Abstract

Abstract:The collapse of abandoned room-and-pillar mines is often violent and unpredictable. Safety concerns often resulted in mine closures with no post-mining stability evaluations. As a result, large amounts of land resources over room-and-pillar mines are wasted. This paper attempts to establish an understanding of the long-term stability issues of goafs (abandoned mines). Considering progressive pillar failures and the effect of single pillar failure on surrounding pillars, this paper proposes a pillar peeling model to evaluate the long-term stability of coal mines and the associated criteria for evaluating the long-term stability of room-and-pillar mines. The validity of the peeling model was verified by numerical simulation, and field data from 500 pillar cases from China, South Africa, and India. It is found that the damage level of pillar peeling is affected by the peel angle and pillar height and is controlled by the pillar width-height ratio.

Subsidence Mechanism and Stability Assessment Methods for Partial Extraction Mines for Sustainable Development of Mining Cities—A Review

“…Besides the pillar strength, the width-height ratio and the pillar stress, long-term stability evaluations should also consider the time-dependent strength degradation and size reduction of coal pillars (pillar peel behavior) that is induced by multiple factors, such as weathering and the intrusion of underground water, so that the long-term stability of the pillar can be known, and the potential pillar failure time can be estimated. Recent research shows that fractured coal fragments can peel from the pillar and accumulate by pillars during time-dependent pillar size reduction, and may finally form a stable fragment pile that helps to restrict continuous pillar size reduction [108], thus, time-dependent pillar size reduction may not be limitless for some partial extraction mines, and these mines may still have the ability to remain stable long-term, although their stability has degraded due to multiple factors. However, the current literature on long-term stability evaluations is relatively limited.…”

“…The analysis of pillar creep behavior is another method to study time-dependent pillar stability. The pillar stress may increase with time due to progressive pillar failure, the pillar stress should never exceed the creep pillar strength to keep the long-term stability [103,[107][108][109]. The pillar stability assessment should not only depend on the safety factors and yielding pillar design methods but should include these methods to correlate rock discontinuities and long-term pillar behavior to pillar support capacity.…”

“…Both damage mechanics and fracture mechanics connect the discontinuities in material-to-pillar failures and are closer to realistic rock behaviors. The progressive pillar scaling down [103,[107][108][109] and strength degradation [121] are associated with discontinuity expansion and damage evolution, such damage accumulations will result in pillar rib spalling, a reduction of effective pillar size, etc. [101,105,[107][108][109]143], leading to the long-term instability of pillars.…”

“…A logical explanation is that when the coal pillar is slender, the load condition is more likely to be in a uniaxial state than a triaxial state due to the lack of lateral stress to pillar core. Besides the fact that the pillar strength and the pillar failure model are affected by the pillar shape, the pillar scaling progress is also affected by the pillar shape [108].…”

“…However, such pillar scaling (or pillar peeling) is not limitless. The coal fragments can peel from the pillar rib and accumulate around a pillar during the pillar scaling progress [103,108,109], and stable coal debris piles may finally form around the scaled coal pillars, providing horizontal confinement to the pillar rib and preventing the pillars from further scaling down [108,109].…”