Catastrophic Failures of Underground Evaporite Mines 

Jeff Whyatt, Mining Engineer
 

Floyd Varley, Acting Branch Chief
 

NIOSH - Spokane Research Laboratory
 

Spokane, WA
 

ABSTRACT 

Deformation of underground salt, trona and potash mines is 

generally time dependent, providing for gradual adjustment of 

strata to mining induced stresses. Time dependence can allow for 

higher extraction ratios provided eventual failure can be tolerated. 

However, this eventual failure can be violent if creep deformation 

can shift stress and potential energy to strong, brittle geologic 

units. The mine failure case studies reviewed here illustrate this 

process. Yield pillars and defects in bridging strata figure 

prominently in these cases. Yield pillars provide local and 

temporary support to the roof, temporarily delaying the cave; and 

allowing extraction ratios and overburden spans to increase 

beyond the long term capacity of overlying strata. Defects (faults, 

voids, thinning) of strong overburden strata reduce the critical 

span, sometimes to less than panel width. Analyses of many of 

these cases have focused on a cascading pillar failure mechanism, 

but recent work and this review point to failure of strong 

overburden strata as the essential element. The suddenness of 

failure and attendant seismic events pose hazards to miners and, 

in some cases, to those on the surface. Characterizing these 

failures is a first step towards recognizing and managing the risk 

of catastrophic collapse in underground mines. 

INTRODUCTION 

Recent mine disasters have prompted the National Institute for 

Occupational Safety and Health (NIOSH) to look for 

opportunities for reducing the disaster potential of mining. A 

number of other agencies and groups are addressing the 

regulatory and management frameworks for controlling disaster 

potential. While the details of these schemes differ in assigning 

responsibilities and liabilities, all require technical means to 

discern and minimize the likelihood of catastrophic mine failure. 

Deep and high extraction mining is possible only through 

management of rock mass failure. Generally, two types of failure 

may occur. First, yielding around mine openings shifts stress 

away from openings towards more confined rock. Second, caving 

shifts the weight of caved material from pillars and abutments to 

the floor of mined areas. 

These types of failure may occur in two modes – ductile and 

brittle. Ductile failure consumes sufficient energy to prevent its 

acceleration, or at least enough energy to remove any hazard to 

miners. That is, the failure process will be sufficiently slow to 

give miners time to retreat. However, the economic and 

environmental consequences (inundation, sinkholes, etc.) may 

still be catastrophic. The alternative is sudden, brittle failure. 

Brittle failures of any appreciable size around working areas 

can be hazardous. The unexpected transition from a controlled 

ground failure process (that is, ductile failure and/or brittle failure 

isolated from miners) to a brittle failure process that impacts work 

areas is a hallmark of many fatal accidents and mine disasters. At 

ordinary working temperature and pressure, salt rocks 

(evaporites) are considered to be the most ductile of all rock types 

(1). As such, mines extracting such rock might be expected to be 

immune from such failures. 

This paper examines catastrophic failures of evaporite mines 

(including salt, potash and trona) that show that violent failure 

does, in fact, occur. It also attempts to identify common factors 

and characterize the mechanism of failure. While these cases are 

limited to evaporite mines, it is hoped that results are sufficiently 

general to provide insight into similar cases in other mining 

sectors. 

CASE STUDIES 

Cases selected for this review involved panel or mine collapse 

events that were initiated by strength failure of solid material. 

Events driven primarily by gas (gas outbursts) or groundwater 

(inundations) in the absence of an initial collapse of strata were 

not considered. However, many of the cases reviewed do involve 

release of gas and/or water. 

Varangeville Mine Collapse (1873) 

This, the oldest collapse case reviewed, occurred in 1873 at the 

Varangeville salt mine. Salt was extracted by blasting and 

solution mining for an ultimate extraction ratio of over 82%, 

leaving pillars with a width:height ratio of about 1:1 (2). 

Significant subsidence was noted, beginning in October, 1873. 

Creation of a fissure in a building at 5 am on October 31st 

prompted evacuation shortly before a collapse that reportedly 

took less than 2 seconds to occur. Fractures were formed within 

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 

failures to the surface as a result of unsustainable ductile failure 

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 

surface soils. 

At the Belle Isle mine, extraction ratios between 50 and 59% 

initiated creep in the salt structure with minimal disturbance of 

the active mine. Creep transferred loads to more brittle cap rock, 

eventually resulting in formation of a 70 meter diameter sinkhole 

in 1972. The sinkhole developed over about 15 minutes and 

engulfed the lone shaft accessing underground workings (5). 

30,000 m3

 of debris flowed into the mine (3). Fortunately, and by 

chance, no miners were entrapped or injured. The shaft was 

backfilled, a new shaft sunk, and the mine returned to operation. 

Subsidence monitoring showed increased risk of a second, much 

larger event in 1983. The mine was then flooded to stabilize 

workings and prevent damage to nearby oil and gas production. 

At Weeks Island, parts of the original mine workings, dating to 

the late 19th century, were taken by the U.S. Department of 

Energy for use as a petroleum reserve (6). Mining operations 

were moved to an adjacent part of the dome and monitoring of 

surface subsidence and water inflow initiated. A number of 

production levels were established in the new mine and mining 

progressed without incident for more than a decade. As at Belle 

Isle, the brittle cap-rock became over-loaded and a sinkhole 

developed at the surface along the boundary between old and new 

mines (7). Increased leakage of surface waters into the reserve 

was detected. The progression of sinkhole and leakage was slow, 

but still posed a significant risk to the petroleum 

reserve. Grouting failed to stem progression of failure. The 

reserve was stabilized by brine flooding and abandoned. 

Saskatchewan Potash Mining Seismic Events (1962-1989) 

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 

under 900 to 1000 m of overburden. These deposits lie in a thick 

sequence of halite and anhydrite known as the Prairie Evaporite, 

which is overlain by roughly 40 m of strong, dense Dawson Bay 

limestone (Figure 1). 

Between the onset of mining in 1962 and the end of 1989, 21 

mining induced seismic events with magnitudes of 2.3 to 3.6 were 

recorded. Horner (9) attributes these events to “brittle failure or 

sudden rupture” in the competent carbonate rock of the Dawson 

Bay limestone. Rockburst damage has not been reported but 

noise, movement of air (minor air blasts) and falls of loose roof 

rock have. Hasegawa et al. suggest that salt overlying mining 

provides protection by attenuating dynamic shocks. 

Sepehr and Stimpson (10) created a simplified numerical 

model of strata failure, inspired by a potash mine near Saskatoon, 

Sakatchewan that had produced seven events with magnitudes of 

2.3 to 3.5. The model simulated mining of evaporite beds under 1 

km of overburden. They found failure propagation into the 

Dawson was “so rapid and extensive... that numerical 

convergence is not achieved, signaling a structurally unstable 

situation.” They concluded “such extensive and rapid brittle 

failure would certainly induce significant seismicity.” That is, 

they interpret failure of the solver algorithm in their finite element 

program as indicating physical as well as numerical instability. 

Figure 1. Idealized cross section through the long axis 

of the Prairie Evaporite structure (11; 12). 

Un-named Phosphate Mine Collapse 

Chen and Peng (13) report on a relatively innocuous collapse 

that occurred two weeks after completion of mining. The collapse 

occurred within a pillar retreat panel with strong roof and floor, 

mined under 60 to 400 m of overburden near a cliff. The roof had 

remained intact during secondary mining. The collapse occurred 

suddenly, crushing 25 remnant pillars, all less than 6 m in width 

and 1.6 to 1.9 m in height, over a roughly 90 m by 55 m area 

(Figure 2). The collapse was also evident in surface subsidence 

and large cracks in the cliff face. 

Cheng and Peng attributed the failure to “pillar sizes that were 

too small and uneven.” Implicit in this explanation is a failure of 

overlying strata to transmit overburden loading to pillars beyond 

the collapse area. Fortunately, the pillars succeeded in providing 

support during mining of the area. 

Retsof Salt Mine Collapse (1994) 

Major seismic collapse events can also be linked to other 

mining hazards, before and after the collapse. For instance, a 

chain of events at the Retsof salt mine, New York, began with 

ground control problems encountered under roughly 300 m (1000 

ft) of overburden, including a roof fall that caused two deaths in 

1990. In response, the yield pillar panel method of mining was 

tested and then implemented in two full production panels, 2YS 

and 11YW (Figure 3). Mining of these panels was initiated in 

1993 but 2YS was halted in October “for safety reasons and due 
to clearance problems for the mining equipment” (14). A sudden 

increase in closure rates in both panels 2YS and 11YW led to 

cessation of mining in panel 11YW on March 1, 1994. 

Collapse of the 2YS panel on March 12, 1994 produced a 3.6 

magnitude seismic event as a 150 by 150-m (500 by 500-ft) 

section of shale roof collapsed. Methane and hydrogen sulfide 

gases were detected and brine water began flowing into the mine 

at nearly 19,000 lpm (5000 gpm) (15). The collapse stabilized 

closure of 11YW panel but flooding could not be stopped, 

eventually leading to loss of the mine. 

Subsequent investigations found evidence of fracture zones 

and a brine and gas pool 50 m (160 ft) above the mining horizon 

(14; 16). The fractures and pool weakened the overlying “bridge” 

of strata. Fractures hydraulically connecting the pool to surface 

waters allowed recharging of the pool and maintenance of 

hydrostatic pressure. Since this pressure was exerted within the 

bridging arch, it could not be entirely supported by intervening 

strata – nor by the yield pillars for which such loading was not 

anticipated. 

Figure 2. Collapsed pillars (shaded) and surface cracking 

(dashed lines) at a Phosphate mine (13). 

Figure 3. Yield pillar panels at the Retsof mine. 

The 2YS panel collapsed on March 12, 1994, leading 

to inundation and loss of the mine (14). 

Solikamsk-2 Potash Mine Collapse (1995) 

Collapse of the Solikamsk-2 potash mine, Verkhnekamsky 

deposit, in the Upper Kama district of western Ural, Russia 

resulted in a 4.7 magnitude seismic event on January 5th, 1995 

and 4.5 m of surface subsidence (17). Underground, a “massive 

falling of the mine roof” was noted over a 600 m by 600 m area 

(Figure 4). The event released an estimated 900,000 m3

 of gas (a 

mix of methane, hydrogen, carbon dioxide, carbon monoxide and 

other gases). These release led to gas explosions the following 

day. Timely placement of a “large volume” of backfill is credited 

with preventing further catastrophic consequences. Malevichko et 

al. describe the potash as “almost incompressible, highly ductile 

and rather easily deformed by creep.” Potash and salt beds were 

overlain by carbonates and sandstones. 

Figure 4. Subsidence resulting from collapse of the Solikamsk 

-2 Potash Mine, Russia, in 1995 (Modified from 17). 

Sylvinite (potash) was extracted from two to three beds with 

10 to 16 m of total extraction height in the collapse area. The 

mine used a panel system of rooms and pillars under 200 to 400 

m of overburden. Rooms were 13-16 m wide and pillars 11-14 m 

wide by 200 m long. 

An investigation across the Upper Kama district found that 

surface subsidence typically reached 50% of excavation height 4 ­

8 months after excavation. This event occurred 15 years after 

mining began, and 7 years after mining was completed (18). The 

unusual delay in subsidence implies extensive bridging of 

overburden, consistent with its uncommonly high release of 

seismic energy. The next largest collapses occurred in 1993 and 

1997 with magnitudes of 2.6 and 2.8, respectively. 

3rd Bereznki Potash Mine Collapse (1986) 

Collapse of the nearby 3rd Bereznki potash mine in July 25, 

1986 (19) is another interesting case. Mining was active at depths 

of 235 and 425 m below surface, and was overlain by a 100 m 

thick “salt complex” of halite and carnallite beds, overlain in turn 

by clays, carbonates, aquifers and sediments. Mining formed 

yield pillars with 5.3 m wide rooms, 3.8 m wide pillars and a 5.5 

m mining height. After mining, conditions were described as 

“pillars crushed and roofs sagged.” 
At the collapse site, mining induced fractures connected with 

natural fractures in a fold structure, providing a conduit for 

flooding of the mine. Loss of hydraulic control in January of 1986 

led to creation of a large cavity beneath a sandstone/limestone 

sequence nearly 200 m thick that was relatively stable until July. 

Failure of this sequence began at 18:30 hours with “clearly felt 

underground shocks” culminating with a final collapse at 

midnight “accompanied by an explosion with flashes of 

light” (Figure 5). 

Figure 5. Collapse and explosion of cavity on July 25, 1986 at the 3rd
 

Bereznki Potash Mine in Russia (redrafted from 19).
 

Solvay Trona Mine Collapse (1995) 

A roughly 1 by 2 km section of the Solvay Mine, Wyoming 

collapsed on February 3, 1995, causing a 5.1 magnitude seismic 

event (20). Seismic first motion showed dilation (collapse) at all 

seismic stations. The “dominant movement of the ground” took 

place on a “time scale of a few seconds or less” (21). Miners 

described the event as “a rumbling, a big boom, and then a 

deafening sound lasting 5 to 6 seconds in all.” The collapse 

caused an air blast fed by methane and ammonia emissions as 

well as closure. The failure was believed to have released an 

eventual total of 3 million m3

 of methane from broken shales with 

a peak release of nearly 1 million m3

 / day. Ammonia (broken 

trona and oil shales) and CO (kerogen-rich oil shale in the 

immediate floor) were released in smaller quantities. The single 

fatal injury was attributed to ammonia poisoning. 

The failure occurred over a multiple yield panel section within 

the southwest portion of the mine but was contained by barrier 

pillars along the main entries (Figure 6). All 13 panels in the 

southwest section were “completely collapsed or extensively 

caved” (22). Panels 4W through 12W were “caved tight at or near 

the submain ends.” 

The panel was mined under 450 to 520 m (1500 to 1700 ft) of 

overburden, including the massive Tower Sandstone. Failure of 

yield pillars within panels occurred in the immediate floor and 

lower portion of narrow pillars, while the immediate roof and 

upper portion of pillars remained intact. Gateroad pillars were 

shattered. 

Figure 6. Schematic of Solvay mine in 1995 showing (A) 

southwest panels (1W through 12W) closed by the collapse, 

(B) location of the southwest panels within the mine and (C) 

detail showing barrier pillars that bounded the collapse 

 

 

 

Surface subsidence of 0.75 to 0.9 m (2.5 to 3 ft), with a 

maximum of 1 m (3.3 ft), was noted over the collapse area (22) 

and subsidence bounds have been described as sharp scarps (23). 

A number of mechanisms, including a chain reaction pillar 

collapse have been proposed (21; 22; 24). Most recently, Board et 

al. (23) attribute the failure to “violent shear failure of a thick and 
 

 

 

 

strong overburden bed that was capable of application of full 

overburden loading over the entire pillar geometry.” 

Pechmann et al. (20) successfully fit the seismic event to a 

crack closure (implosional) mechanism, the crack being the 

mined trona seam. Seismic energy released was about 10% of the 

potential energy lost by observed subsidence of the overburden 

(25). A near-vertical shear failure of the sandstone is consistent 

with both brittle failure and a sharp subsidence scarp. In such a 

case, both pillars support and the bridging Tower Sandstone 

failed, and failed quickly during the collapse. 

German Potash Mine Collapses 

German potash mines are often overlain by sandstone strata 

regardless of whether the evaporites are within stratabound or 

domal geometries as in Figure 7. Beneath the sandstone, there are 

typically several hundred meters of “yield, i.e., creepable, 

saliferous rock layers, which hold the minable seam horizons of 

the potash mining” (26). Failure is characterized as involving “the 

entire structure” with caving up to the surface (subsidence) and 

“tremors, similar to those experienced during an earthquake.” 

Figure 7. Example of German salt dome potash occurrence 

with capping sandstone strata. Salzdetfurth mine (29; 12). 

Many collapses have occurred during mining of carnallite 

potash seams. Generally, the lower the halite content of a rock 

(with carnallite replacing halite) the more brittle its behavior. 

Initiation of collapse is generally attributed to a blasting event that 

begins a chain-reaction failure incorporating both pillars and roof 

strata, resulting in significant surface subsidence and a large 

seismic event. These collapses, named after overlying villages 

(27), include: 

3⁄4 A collapse at Teutschenthal in 1940 under 730 m of 

overburden (26). Failure propagated 1300 m in 0.9 seconds 

through slender pillars with a width:height ratio less than 

one. Back-calculation estimated a local magnitude of 5 for 

the resulting seismic event. 

3⁄4 A collapse at Herringen with a local seismic magnitude of 

5. 

3⁄4 Collapses at Merkers on July 8, 1958 and June 29, 1961 

with seismic magnitudes of 4.8 and 3.7, respectively. 

3⁄4 Collapse at Suenna in 1975 with a seismic magnitude of 

5.2. This collapse was located in the Werra potash district 

near Suenna, Germany and involved crushing of carnallite 

pillars over an area of 3.35 km2 (12). Damage was entirely 

contained within known fault planes. 

3⁄4 The Teutschenthal potash mine collapse of Sept. 11, 1996 

involved failure of 700 long pillars over an area of 2.5 km2 

and under 620 to 770 m of overburden in approximately 2 

seconds. The collapse produced a 4.8 magnitude seismic 

event and 0.5 m of surface subsidence (28). 

An additional event, and the largest of these, is the March 13, 

1989 collapse of the Merkers mine, 750-900 m beneath the town 

of Volkershausen. The collapse involved an area of 6.5 km2

, 

produced a 5.6 magnitude seismic event* and caused 

“catastrophic” damage to the town (30). Two levels were mined. 

A primary level in carnallite was extensively mined, leaving 30 

by 6 m pillars with a width to height ratio of 4 to 7. A secondary, 

overlying seam was mined in hard rock salt to a lesser extent, 

initially leaving large, stable pillars. In 1987, mining in the upper 

seam was modified to leave yield pillars with a width to height 

ratio of 1.7:1. Heavily “working” rock and roof control problems 

were encountered. The actual event appeared to originate with 

blasting of carnallite in the lower seam within the pressure 

abutment of the upper seam. Failure of 3200 pillars within the 

lower seam occurred within a time span of 2 to 3 seconds, 

resulting in up to 1 m of surface subsidence. 

The ground subsidence area was described as “rather sharply 

bounded by known or inferred fault or shear zones, and pillars on 

the other side of these faults remained intact” (12). Damage to 

structures straddling the shear zone was extensive (Figure 8). 

These faults and shear zones apparently weakened the bridging 

Buntsandstein sandstone, about 200 meters above the mining 

level. Injection of wastewater into this and adjacent dolomite 

strata may have contributed to weakening of these features. 

Figure 8. Damage to structure in Volkershausen 

directly above the shear zone (27). 

* Descriptions of seismic event(s) vary. Garrett (12) describes the 

collapse as occurring in “three major shocks over a 5 second 

period of 4.4, 5.1 and 5.5.” Bennet and McLaughling (31) 

described the seismic event as having a seismic magnitude of 5.4, 

representing 0.5% of the potential energy lost through subsidence. 

They also found the seismic characteristics of this event to be 

quite similar to that produced by the 1995 Solvay event. 

Discussion. The value of a set of case studies like these lies in 

their ability, through a composite view, to more fully reveal the 

mechanisms at work. The most important observation is that there 

is a strong correlation between dynamic collapse events and the 

presence of relatively strong, brittle bridging strata above the bed 

being mined. Other characteristics of these events, including 
collapse mode, seismic energy and the interplay of creep, energy 

and dynamic failure are explored further. 

Figure 9. Relationship between strata movements over time and overburden stratigraphy above solution-mined salt seams.
 

Sudden subsidence caused by caving of stiff layer is often accompanied by a sizable seismic event. After Daupley et al. (4).
 

 

 

Collapse Mode 

Two collapse mechanisms have been evoked in explanations 

of these cases. The first is a cascading or chain reaction failure of 

mine pillars. The second is sudden vertical shear failure of strong, 

brittle overburden strata. 

Whether or not collapse is controlled by chain-reaction failure 

of pillars can be addressed most directly by examining cases 

where there are no pillars. For evaporite mining, such cases are 

conveniently created by solution mining. Daupley et al. (4) 

explored collapse mode for various overburden (Figure 9). They 

found that generally soft strata were associated with slow 

development of a subsidence trough while the presence of 

stronger strata was associated with sudden subsidence events. 

Failure of strong strata was found to occur on near-vertical shears. 

The vertical shear mechanism was briefly explored in a twodimensional numerical model (Figure 10). The numerical model 

used a strain softening constitutive law for the strong, brittle 

stratum. Softer, weaker overlying strata bend rather than fracture, 

leaving a more gradual slope. The vertical shear failure 

mechanism has also been explored in physical models (Figure 

11). 

Board et al. (23) used the ground reaction curve concept to 

explore this mode of strata collapse. They modeled the panel as 

supported by a pressure that was slowly reduced from in situ 

stress levels to pressures equivalent to pillar support with 

increasing extraction ratios. At some point, stable redistribution of 

stress to barrier pillars is interrupted by sudden failure of bridging 

strata (Figure 12). Board et al argue that panel pillar design must 

preserve the integrity of bridging overburden and avoid the 

unstable ground reaction curve associated with its failure. 

Seismic Energy 

The seismic event created by sudden collapse carries 

information on the source mechanism of the event. The most 

comprehensive seismic analyses in this review were conducted 

for the Solvay case and include two important findings. First, the 

first motion of the ground recorded by seismographs is 

downward, indicating a collapse or implosion often described as a 

horizontal crack closure motion. Second, the potential energy 

released through subsidence is sufficient to produce the observed 

seismic event. 
 

 

 

 

 

 

 

 

 

Figure 10. FLAC model showing development of vertical
 

shears along the face of abutments and tensile failure
 

in the center of the roof beam.
 

Figure 11. Physical model of vertical shear failure (32). 

Figure 12. Stable and unstable ground reaction curves for 

strata bridging a pillar mining panel. The unstable curve 

reflects failure of bridging overburden strata (23). 

The consistency of potential energy changes and seismic 

energy release was examined. Estimates of the relative change in 

potential energy, based on the volume and displacement of 

collapsing overburden, were made for several large collapses and 

compared to reported seismic energy. The linearity of these 

results (Figure 13), shows that seismic energy release scales 

consistently with changes in potential energy loss. This 

consistency suggests a similar consistency in failure mechanism. 

The crack closure (implosional) character of seismic records 

from these events does not provide good insight into whether the 

event was triggered by failure of a pillar or strata. The short 

duration and wide extent of these events suggest that strata failure 

occurred early, as either the initiator or immediately after the 

initiating failure, providing a shock load that rapidly drove failure 

throughout the collapse area. In some cases, as in the Retsof 

collapse, weakening defects in bridging strata were discovered 

during post-failure analyses. The importance of such defects 

highlights the critical role of the strata bridge. That is, these 

defects violate a fundamental assumption in the design of yield 

pillar panels; that overlying strata can bridge the panel, shifting 

all but immediate overburden loading to adjacent barrier pillars. 

Figure 13. Seismic magnitude, a logarithmic measure, plotted 

against relative change in potential energy of the 

subsiding block.
 

Creep, Energy and Dynamic Failure 

The energy approach begins with the fact that underground 

mining, by creating the opportunity for overlying strata to fall, 

creates substantial amounts of potential energy. This energy may 

be contained in perpetuity if sufficient support is provided. 

Otherwise, it must be expended either in gradual closure of mined 

spaces or in a sudden collapse. The magnitude of available energy 

can be immense, as demonstrated by the magnitude of seismic 

energy released during collapse events. Management of this 

energy and its release is essential to prevention of violent mine 

failure. 

Highly stressed evaporite minerals typically deform as a 

viscoplastic material, gradually dissipating potential energy. 

Creep deformation sheds load from highly stressed pillars and 

abutments towards other portions of the rock mass. This behavior 

is advantageous for yield pillar panels, realizing good initial 

support pressure with high extraction ratios, and has been applied 

widely in evaporite mining. However, the support capacity of 

evaporite pillars, particularly yield pillars, will degrade with time, 

particularly in the presence of groundwater. Since pillars are the 

main difference between the work of Daupley et al., (4) on 

solution mining and conventional underground mining, 

degradation of yield pillars will serve to increase their similarities 

over time. 

Most evaporites also accumulate damage during creep and will 

eventually lose cohesion and fail. Moreover, creep may shift 

stress to and through structures (faults, strong sandstone beds, 

etc.) that are inherently brittle. Differential deformation between 

the evaporite deposit and overlying strata can also form voids, 

which may fill with pressurized gas and/or fluid. Failure of brittle 

elements, particular failures that compromise bridging of 

overburden stress, can exert a sudden dynamic load on pillars, 

resulting in their sudden failure. 

In some cases, creep contributes to the heightened level of 

energy release by providing a temporary stability, albeit one that 

is inherently unsustainable. Temporary stability provides an 
 

 

 

 

 

 

 

 

 

opportunity to extract more resource, increasing the magnitude of 

the eventual failure. Properly managed, such a process can 

provide high extraction and completion of mining before collapse. 

However, projecting when collapse will become imminent is 

problematic and subject to many geologic uncertainties. Even if 

collapse is delayed until after the mine is abandoned, there may 

still be consequences for surface structures and ground water. 

Improperly managed, the consequences can be substantial, as 

these case studies have shown. Total extraction in a longwall 

configuration will also induce collapse, but the lack of pillar 

support will cause collapse to start earlier and occur in smaller 

increments. 

The relationship between extraction ratio and the potential 

violence of failure is illustrated conceptually in Figure 14. The 

“first cave” line represents the extent of mining before first cave. 

Overburden strata, depth, etc. will define this curve for a given 

site. Support (pillars, etc.) can extend this span, pushing the mine 

along this curve to the right. However, if these supports fail to 

provide indefinite support, the eventual failure will be more 

violent. Subsequent caves extend the cave zone as cantilevering 

strata fail. This caving is represented by horizontal lines 

extending to the right from the first cave line whose intensity is, 

once again, dependent on local overburden, support and stress 

conditions. Provision of temporary support with creeping pillars, 

etc. can extend the cantilever distance and increase the violence 

of eventual failure of this subsequent caving as well. In other 

words, the intensity of caving can be reduced by early and 

sustained caving. 

Figure 14. The intensity of energy release during first 

cave increases as a function of span. Early and sustained 

caving limits the intensity of cave events. 

Strong overburden also complicates longwall mining, 

interfering with the steady progression of caving. In such cases, 

caving may need to be induced artificially or, alternatively, a 

bridge of strata maintained over the panel. Permanent 

maintenance of bridging strata requires a substantial safety factor, 

purchased by a substantial loss of resource to supporting barrier 

pillars and a thorough geomechanical exploration of the 

overburden bridge. Backfilling and/or brine flooding may 

enhance long-term stability (33). 

CONCLUSIONS 

Undermining of strata creates a significant reservoir of 

potential energy that may be supported indefinitely, subsided 

gradually over time or released in sudden collapse. Maintaining 

the safety of miners depends on properly anticipating whether a 

sudden collapse is possible – and if so, proper planning of mine 

operations. 

The cases reviewed here show a clear association between 

violent collapse and strong overburden strata. In these cases, 

failure of bridging overburden creates a shock load that drives 

rapid failure of pillars, all within a very few seconds. These cases 

also show that collapse events can occur where the mined horizon 

is not brittle and pillars are properly sized. Brittle overburden is 

sufficient. Yield pillars in ductile seams may delay collapse, 

allowing further mining that increases the potential energy 

released in the eventual collapse. 

This conclusion concurs with a similar association recently 

reported for pillarless solution mines and a recently published 

analysis of the Solvay collapse. The conclusion applies to any 

mine operating under sedimentary overburden with strong strata. 

This conclusion is inconsistent with the cascading pillar 

collapse mechanism that has been proposed for some of these 

cases. The distinction between overburden and cascading pillar 

failure is an important one. The potential for cascading pillar 

failure is evaluated and controlled by application of pillar design 

tools. Strata failure, on the other hand, is addressed by panel 

design, especially analysis of bridging strata spans and loads. The 

design can either protect these spans or, in caving methods, assure 

early and sustained failure. In addition, the overburden failure 

mechanism identified depends primarily on the nature of 

overburden strata, not the commodity being mined. 

Recognition of the presence of strong strata and proper 

consideration of its potential for failure is essential to proper 

management of the potential energy reservoir created by mining. 

Such recognition is important since many contemporary design 

tools, especially those based on empiricism and boundary element 

formulations, address only the pillar run mechanism. 

ACKNOWLEDGEMENTS 

Ron Hughes, General Manager of the Solvay Mine, and Dr. 

Mark Board, Itasca Consulting Group are acknowledged for many 

helpful discussions in developing the focus of this paper. 

Assistance provided by Dr. Mark Larson (NIOSH), who 

contributed the collapse model, is also gratefully acknowledged. 

The views expressed herein are entirely those of the authors and 

do not necessarily reflect those of NIOSH or those who have 

provided assistance. 

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