Evolution of rockburst control in the Coeur d’Alene district over the past 50 years

A rockburst is generally defined as “a sudden and violent failure of overstressed rock resulting in the instantaneous release of large amounts of accumulated energy and expulsion of broken rock into nearby excavations.” The root causes of rockbursting are in situ stresses concentrated around deep excavations, as well as unstable slip on faults and other geologic features. Rockbursting is a common problem in deep mines worldwide, with damaging events typically having Richter magnitudes greater than about 1.5 but may be as large as 4 or more. The Coeur d’Alene district The deepest mines in the United States are found in the Coeur d’Alene district in northern Idaho, where deep, narrow silver veins have been mined since the late 19th century. In the past 60 years there have been 22 fatalities from rockbursts at five different mines in the district, with the greatest number in the 1980s as mines were reaching depths of more than 1,800 m (6,000 ft). Currently, the Lucky Friday Mine of Hecla Mining Ltd. is stopping at around 2,300 m (7,500 ft) depth, with the recently completed #4 winze reaching about 3,000 m (10,000 ft) depth. Narrow-vein, cut-and-fill mining method Until the 1980s, the preferred mining method was nonmechanized, overhand cut-and-fill using, typically, uncemented sand tailings as backfill. This method, which employed vertically advancing stope lines along strike about every 61 m (200 ft) (vertical) on the vein, resulted in the creation of sill pillars of diminishing height between primary levels. In the brittle ore and wall rock of the Revett formation, the high stress concentrations in these pillars often resulted in large pillar bursts that were, fortunately, typically triggered by stope blasting. However, there were numerous incidents of on-shift events accompanied by significant damage and injury. 1970s – Research aimed at understanding root causes of seismicity Coeur d’Alene mining companies, assisted by the U.S. Bureau of Mines, Spokane Research Center, expended considerable effort in the 1970s to understand the role of geology, in situ stress and mining method in the root causes of seismicity. The first computer-based, mine-wide microseismic systems were installed in several mines with on-line event source location. This information, aided by interpretation from early numerical modeling, was used to understand the basic source mechanisms of seismicity (pillar bursting and unstable fault slip) and identify the role of mining geometry in inducing these events. 1980s – Adjustment to mining methods Based on data developed in the 1970s, the 1980s were largely focused on adjustment of mining methods to reduce or eliminate the stress concentrations introduced by mining geometry. Hecla Mining pioneered the development of underhand longwall mining with engineered paste fill to eliminate the creation of sill pillars and produce a flat, downward-advancing overall longwall face that minimized stress concentrations.It was recognized in the pre-1950s time frame that drilling and blasting additional, longer holes in face rounds could create limited fracturing, which could soften the rock in advance of a tunnel, sometimes resulting in a change in material behavior from an elastic-brittle (bursting) response to a crushing, plastic mode characterized by larger deformations. Also, targeted, intensive blasting in volumes of highly stressed rock (such as pillars or faces of advancing drifts) with localized blasting will often “trigger” a potential seismic event. This technique, developed and extensively used in Coeur d’Alene mines, was originally termed “destress blasting” and has been used as a proactive method for rockburst control. In the late 1970s and the 1980s, the application of destress blasting to larger volumes of the orebody was developed as a proactive rockburst control technique in advance of cut-and-fill mining. The term “preconditioning” was used for this method. Although early promising results of this method was found at the deepest levels of the Star Mine, the inability to fragment significant volumes of highly confined veins (as well as cost and production downtimes) resulted in discontinued use of large-scale preconditioning. 1990s and 2000s – Advanced microseismic monitoring and analysis methods aid in further understanding of favorable mining approaches Significant advances in methods for stress prediction and seismic data assessment were introduced that aided in better understanding of seismic source mechanisms and the role of in situ stress redistribution around mine excavations. Full-waveform seismic monitoring systems were introduced into mining that provided for improved source location and source characteristics such as magnitude, seismic energy release and mechanism (such as explosive failure and fault slip). At the same time, numerical stress analysis methods, including boundary element, continuum and discontinuum (explicit modeling of faults) were developed for practical use on desktop computers, allowing mine engineers to back-analyze the improved seismic data collection to determine basic root causes of seismic source mechanisms, including the role of mining method and blasting in creating conditions conducive to triggering seismic events. These advances in seismic data assessment led to data-based development of seismic risk assessment methods, improved dynamic ground support use and personnel exclusion protocols. Today’s seismicity control at Hecla’s Lucky Friday Mine – Culmination of learnings over many years Use of learnings previously described has shown that the primary source of current damaging seismicity at the Lucky Friday Mine is unstable slip on relatively small-scale faults that strike roughly parallel to the vein and cut through the orebody at moderate dip. The occurrence of these faults is generally not known in advance and may slip ahead of or adjacent to the downward-progressing underhand stopes due to stress concentration redistributing ahead of the stope (Fig. 1). These events can have Richter magnitude of 2 or greater and often occur within about 15 m (50 ft) or less of the stope. Damage from these events can include dynamic floor heave and present a significant safety issue. Although these events typically occur with the small face blast rounds used to advance a stope cut along strike, there is a risk of on-shift events, which requires significant personnel exclusion and downtime from production. A strategy was developed to eliminate or minimize this rockbursting mechanism via a new, modified choke-blast mining method termed “underhand closed bend” mining. This method, which is a modified longhole technique, is based on drilling 9 m (30 ft) deep vertical blasthole rings into the stope floor for the length of the stope (blasts are limited to one half the stope or approximately 107 m (350 ft) on strike). Holes are loaded with pumped emulsion and initiated with electronic detonators sequentially from the stope end to the stope entry. Use of a lower mucking drive or drop raise to create breaking room, typical of longhole stoping, is not possible as they will likely induce rockbursting. Thus, high powder factors and casting of the muck up into the open stope is used to sufficiently break the ore. The ore fragmentation for approximately two stope widths into the floor has two primary impacts (Fig. 1): (1) faults near the stope that cut through the vein are exhumed instantaneously with the blast, thus eliminating their energy release, and (2) the large size of the blast and its rapid stress redistribution triggers slip on nearby faults in the wall rock or deeper below the fragmented vein, typically within minutes, resulting in minimal stope damage. A 12-hour exclusion period after the blast is enforced to ensure any time-dependent seismicity occurs with no personnel present. After the blast, which fragments rock for the subsequent two mining cuts, the broken rock is mucked horizontally along strike in a cut-by-cut fashion below cemented paste fill from the previous cut in the usual manner. After the two cuts have been mucked and filled, the cycle is repeated by drilling rings and blasting, creating a protective advanced, fully destressed zone. This method, fully employed in all primary stopes by the end of 2020, has shown significantly improved results. There have been no damaging seismic events since full implementation, with nearly all events triggered with the blasting and on-shift energy release reduced by more than 90 percent. Production in the primary 30 vein stopes has nearly doubled compared to the pre-UCB production rate. (The author would like to thank Hecla Mining Co. for approval to publish this note and acknowledges the contributions of Chris Neville, Lucky Friday Mine superintendent, Wes Johnson, Hecla Technical Services, and the employees of the Lucky Friday Mine.)

Evolution of rockburst control in the Coeur d’Alene district over the past 50 years

Authored by: Mark Baker, Jackling Award recipient

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