Catastrophic Failures of Underground Evaporite Mines Jeff Whyatt, Mining Engineer Floyd Varley, Acting Branch Chief NIOSH - Spokane Research Laboratory Spokane, WA ABSTRACT acceleration, or at least enough energy to remove any hazard to miners. That is, the failure process will be sufficiently slow to Deformation of underground salt, trona and potash mines is give miners time to retreat. However, the economic and generally time dependent, providing for gradual adjustment of environmental consequences (inundation, sinkholes, etc.) may strata to mining induced stresses. Time dependence can allow for still be catastrophic. The alternative is sudden, brittle failure. higher extraction ratios provided eventual failure can be tolerated. However, this eventual failure can be violent if creep deformation Brittle failures of any appreciable size around working areas can shift stress and potential energy to strong, brittle geologic can be hazardous. The unexpected transition from a controlled units. The mine failure case studies reviewed here illustrate this ground failure process (that is, ductile failure and/or brittle failure process. Yield pillars and defects in bridging strata figure isolated from miners) to a brittle failure process that impacts work prominently in these cases. Yield pillars provide local and areas is a hallmark of many fatal accidents and mine disasters. At temporary support to the roof, temporarily delaying the cave; and ordinary working temperature and pressure, salt rocks allowing extraction ratios and overburden spans to increase (evaporites) are considered to be the most ductile of all rock types beyond the long term capacity of overlying strata. Defects (faults, (1). As such, mines extracting such rock might be expected to be voids, thinning) of strong overburden strata reduce the critical immune from such failures. span, sometimes to less than panel width. Analyses of many of these cases have focused on a cascading pillar failure mechanism, This paper examines catastrophic failures of evaporite mines but recent work and this review point to failure of strong (including salt, potash and trona) that show that violent failure overburden strata as the essential element. The suddenness of does, in fact, occur. It also attempts to identify common factors failure and attendant seismic events pose hazards to miners and, and characterize the mechanism of failure. While these cases are in some cases, to those on the surface. Characterizing these limited to evaporite mines, it is hoped that results are sufficiently failures is a first step towards recognizing and managing the risk general to provide insight into similar cases in other mining of catastrophic collapse in underground mines. sectors. INTRODUCTION CASE STUDIES Recent mine disasters have prompted the National Institute for Cases selected for this review involved panel or mine collapse Occupational Safety and Health (NIOSH) to look for events that were initiated by strength failure of solid material. opportunities for reducing the disaster potential of mining. A Events driven primarily by gas (gas outbursts) or groundwater number of other agencies and groups are addressing the (inundations) in the absence of an initial collapse of strata were regulatory and management frameworks for controlling disaster not considered. However, many of the cases reviewed do involve potential. While the details of these schemes differ in assigning release of gas and/or water. responsibilities and liabilities, all require technical means to discern and minimize the likelihood of catastrophic mine failure. Varangeville Mine Collapse (1873) Deep and high extraction mining is possible only through This, the oldest collapse case reviewed, occurred in 1873 at the management of rock mass failure. Generally, two types of failure Varangeville salt mine. Salt was extracted by blasting and may occur. First, yielding around mine openings shifts stress solution mining for an ultimate extraction ratio of over 82%, away from openings towards more confined rock. Second, caving leaving pillars with a width:height ratio of about 1:1 (2). shifts the weight of caved material from pillars and abutments to Significant subsidence was noted, beginning in October, 1873. the floor of mined areas. Creation of a fissure in a building at 5 am on October 31st prompted evacuation shortly before a collapse that reportedly These types of failure may occur in two modes – ductile and took less than 2 seconds to occur. Fractures were formed within brittle. Ductile failure consumes sufficient energy to prevent its the subsidence zone in two concentric circles with 160-m and 350 -m diameters, respectively. Daw and Pollard (3) describe geology elsewhere in the deposit as having a surface stratum of 42 m of marl underlain by layered salt and marl. More recently, solution mining in the deposit has been conducted under 200 to 300 m of cover (4). The thick surface marl forms a brittle cap with increased calcium carbonate content. Apparently, there was a transition from ductile to brittle failure as caving entered the marl which formed a brittle cap with increased calcium carbonate content. Louisiana Salt Dome Failures to Surface (1972-1992) Rapid, but not seismic, collapses have occurred in Louisiana salt domes. The south coast of Louisiana contains five salt domes that have been mined since the beginning of the 20th century (5). More recently, two of the domes have experienced large-scale Figure 1. Idealized cross section through the long axis failures to the surface as a result of unsustainable ductile failure of the Prairie Evaporite structure (11; 12). long after mining of pillars in the affected area. The salt domes are massive deposits with discontinuous jointing and inferred shear zones overlain by a more brittle "caprock" followed by noise, movement of air (minor air blasts) and falls of loose roof surface soils. rock have. Hasegawa et al. suggest that salt overlying mining provides protection by attenuating dynamic shocks. At the Belle Isle mine, extraction ratios between 50 and 59% initiated creep in the salt structure with minimal disturbance of Sepehr and Stimpson (10) created a simplified numerical the active mine. Creep transferred loads to more brittle cap rock, model of strata failure, inspired by a potash mine near Saskatoon, eventually resulting in formation of a 70 meter diameter sinkhole Sakatchewan that had produced seven events with magnitudes of in 1972. The sinkhole developed over about 15 minutes and 2.3 to 3.5. The model simulated mining of evaporite beds under 1 engulfed the lone shaft accessing underground workings (5). km of overburden. They found failure propagation into the 30,000 m3 of debris flowed into the mine (3). Fortunately, and by Dawson was “so rapid and extensive… that numerical chance, no miners were entrapped or injured. The shaft was convergence is not achieved, signaling a structurally unstable backfilled, a new shaft sunk, and the mine returned to operation. situation.” They concluded “such extensive and rapid brittle Subsidence monitoring showed increased risk of a second, much failure would certainly induce significant seismicity.” That is, larger event in 1983. The mine was then flooded to stabilize they interpret failure of the solver algorithm in their finite element workings and prevent damage to nearby oil and gas production. program as indicating physical as well as numerical instability. At Weeks Island, parts of the original mine workings, dating to Un-named Phosphate Mine Collapse the late 19th century, were taken by the U.S. Department of Energy for use as a petroleum reserve (6). Mining operations Chen and Peng (13) report on a relatively innocuous collapse were moved to an adjacent part of the dome and monitoring of that occurred two weeks after completion of mining. The collapse surface subsidence and water inflow initiated. A number of occurred within a pillar retreat panel with strong roof and floor, production levels were established in the new mine and mining mined under 60 to 400 m of overburden near a cliff. The roof had progressed without incident for more than a decade. As at Belle remained intact during secondary mining. The collapse occurred Isle, the brittle cap-rock became over-loaded and a sinkhole suddenly, crushing 25 remnant pillars, all less than 6 m in width developed at the surface along the boundary between old and new and 1.6 to 1.9 m in height, over a roughly 90 m by 55 m area mines (7). Increased leakage of surface waters into the reserve (Figure 2). The collapse was also evident in surface subsidence was detected. The progression of sinkhole and leakage was slow, and large cracks in the cliff face. but still posed a significant risk to the petroleum reserve. Grouting failed to stem progression of failure. The Cheng and Peng attributed the failure to “pillar sizes that were reserve was stabilized by brine flooding and abandoned. too small and uneven.” Implicit in this explanation is a failure of overlying strata to transmit overburden loading to pillars beyond Saskatchewan Potash Mining Seismic Events (1962-1989) the collapse area. Fortunately, the pillars succeeded in providing support during mining of the area. Some evaporite mines are a significant source of mining induced seismic events. Hasegawa et al. (8) reviewed large seismic events produced by mining of potash in Saskatchewan Retsof Salt Mine Collapse (1994) under 900 to 1000 m of overburden. These deposits lie in a thick sequence of halite and anhydrite known as the Prairie Evaporite, Major seismic collapse events can also be linked to other which is overlain by roughly 40 m of strong, dense Dawson Bay mining hazards, before and after the collapse. For instance, a limestone (Figure 1). chain of events at the Retsof salt mine, New York, began with ground control problems encountered under roughly 300 m (1000 Between the onset of mining in 1962 and the end of 1989, 21 ft) of overburden, including a roof fall that caused two deaths in mining induced seismic events with magnitudes of 2.3 to 3.6 were 1990.