Underground Mining Methods: Engineering Fundamentals and International Case Studies. Hustrulid WA, Bullock RC, eds., Littleton, CO: Society for Mining, Metallurgy, and Exploration, 2001 Mar; :493-511
Catastrophic collapse or cascading pillar failure (CPF) is a potential problem faced by all room-and-pillar mining operations. CPF occurs when one pillar fails suddenly, which then overstresses the neighboring pillars causing them to fail, and so forth, in very rapid succession. Within seconds, very large mining areas can collapse via this mechanism while giving little or no warning. The collapse itself poses grave danger to miners. In addition, the collapse can induce a violent air blast that disrupts or destroys the ventilation system. Further grave danger to miners exists if the mine atmosphere becomes explosive as a result of CPF. This paper has documented over 21 collapses that have occurred in the past 20 years mainly in U.S. room-and-pillar mines. Most of these collapses happened in coal mines since substantial production tonnage still comes from room-and-pillar mines; however, huge collapses have also occurred in various 509 metal mines (lead and copper) as well as nonmetal mines (trona, salt, and limestone) . Many other similar collapses are known to have occurred around the world. CPF, also known as massive pillar collapse, domino-type failure, or progressive pillar collapse, is the likely mechanism underlying these mine failures. The three case histories given in the paper (Figures 59.17, 59.20 and 59.27) show that the risk of CPF is most acute where large arrays of developed pillars exist without interruption by substantial barrier pillars. Traditional strength-based design methods are not sufficient to eliminate the possibility of CPF in room-and-pillar mines, and the number of documented collapses in the United States alone provides mute testimony to that statement. Pillar arrays with large average strength safety factors can fail in a domino-type failure (CPF) if just a few pillars in the array begin to fail. Pillars with large strength-based safety factors (for example 1.5) still have a finite probability of failure, and if the number of pillars in an array is large, failure somewhere in the array can become a near certainty, and that failure could in turn initiate CPF. Traditional strength-based design begins by estimating pillar stress using tributary area method, boundary-element-methods or other numerical methods. Next, various empirical pillar strength formulas or rock mass failure criteria such as the Hoek-Brown criterion provide estimates of the peak pillar strength. Finally, a strength-based safety factor is computed as strength over stress. The traditional approach provides required panel pillar size and barrier pillar size for room and pillar layout; however, this approach does not provide panel pillar width nor does it give any consideration to what might happen if pillars somewhere in the array begin to fail. More advanced rock mechanics considerations such as the local mine stiffness stability criterion provide this design information and a rational basis to eliminate domino-type pillar failures or CPF. The mechanics of CPF are well understood. Strain-softening behavior is the essential mechanical characteristic of pillars that fail rapidly via this mechanism. Pillars that exhibit strain-softening behavior undergo a rapid decrease in load-bearing capacity upon reaching their ultimate strength. The strain-softening behavior of pillars depends on both inherent material properties and geometry. Pillars with low W/H ratio exhibit a greater degree of strain-softening behavior than pillars with a higher W/H ratio, which typically have elastic-plastic or strain -hardening material behavior. The local mine stiffness stability criterion developed by Salamon (1970) provides a means to distinguish between mine layouts that fail in a stable nonviolent manner and those that fail in an unstable violent manner via CPF. Simple quasi-three-dimensional boundary-element-method programs such as MULSIM/NL or LAMODEL with strain-softening material models can calculate local mine stiffness (KLMs) and evaluate the stability criterion. These computer programs apply to a wide variety of thin, tabular, bedded-type deposits amenable to room-and-pillar mining methods. Field data on the complete stress-strain behavior of full-scale mine pillars is limited. The best data is for coal (Zipf 1999)_ Nevertheless, enough is known to evaluate the stability criterion and assess various mine layouts for their potential to fail via CPF. Three case studies of CPF are examined, and all three probably violated the local mine stiffness stability criterion. Three stability-criterion-based design approaches are suggested to minimize the risk of CPF, namely, containment, prevention, and full extraction. If an array of pillars violates the local mine stiffness stability criterion, the containment approach applies as shown in Figure 59.11. Low W/H ratio panel pillars that violate the stability criterion are surrounded by high W /H ratio barrier pillars that shield the panel pillars from full tributary area stresses and "contain" panel pillar failure should it initiate. However, if all the panel pillars in an array satisfy the stability criterion, then the prevention approach applies. The panel pillars do not exhibit much strain-softening behavior because their W/H ratio is sufficiently high (probably greater than 3 or 4). In the full extraction approach, the stability issue becomes a moot point, because complete and controlled opening closure occurs immediately after the completion of retreat mining. Large mine collapses can pose enormous safety hazards to miners and room-and-pillar mining operations. Mining engineers can limit the danger of CPF through prudent application of the local mine stiffness stability criterion and the three stability-criterion-based design approaches suggested to decrease the risk of CPF.