Board, M, Damjanac, B & Pierce, M 2007, 'Development of a Methodology for Analysis of Instability in Room and Pillar Mines', in Y Potvin (ed.), Proceedings of the Fourth International Seminar on Deep and High Stress Mining
, Australian Centre for Geomechanics, Perth, pp. 273-282.
Since 1994, a number of major roof falls and panel collapses have occurred in room and pillar trona mines
within the Green River Basin of Wyoming, USA. Trona is a relatively strong and brittle evaporite mineral
that occurs in a flat-lying, 3 m (approximate) thick bed at about 490 m depth. The floor of the seam is
composed of thinly-bedded, weak shale and the roof materials of interbedded shales, marlstones, mudstones
and sandstones. The largest of these events, a 5.2 Richter magnitude event, occurred at the Solvay Mine in
February, 1995. This event, induced by the collapse of the 1 SW panel (over 2 km2 area), occurred in
roughly 5 seconds with a simultaneous surface subsidence of approximately 1 m.
This paper describes a methodology for estimating the potential for major panel collapses developed
through back-analysis of several collapse incidents. The method is similar to the “ground reaction curve”
approach for ground support design, and involves separate analysis of the stress-strain response of the
primary mechanical components of the system: a) the pillar/floor system for a particular pillar design and
extraction ratio, and, b) the overlying roof strata for given panel widths and barrier pillar dimensions. The
stress-strain response of the pillar/floor system for a given panel geometry is estimated through back-
analysis of instrumented case studies and observations of pillar/floor punching. The stiffness and yield
response of the overburden is estimated numerically by replacing the panel pillars with an equivalent back-
pressure, and then incrementally removing the pressure to allow closure of the seam. The pressure-
displacement response of the two components are superimposed to determine whether an equilibrium state
can be achieved, and to estimate the potential violence of the failure response if equilibrium cannot be
achieved. Back-analysis of the collapse of three panels at one mine is shown. The method allows rapid
analysis of the impacts of adjusting panel extraction ratio, panel span and inter-panel barrier pillar
dimensions on global stability, and thus provides a reasonably-simple design analysis tool.
Fugro, Inc. (1997) Numerical analysis of trona-mining induced subsidence known sodium lease area, Green River,
Wyoming. Prepared for Joint Oil/Gas & Trona Industry Development Group, Green River, Wyoming, Rept. No.
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Pariseau, W.G. and Eitani, A. (1976) Laboratory rock properties: Alchem Mine. University of Utah, Rept. to Allied
Chemical Corp., October.
Pechmann, J.C., Walter, W.R., Nava, S.J. and Arabasz, W.J. (1995) The February 3, 1995, ML 5.1 Seismic event in the
trona mining district of Southwestern Wyoming, 66 Seismol. Res. Letters 25.
Terra Tek, Inc. (1996) Physical and Mechanical Properties Characterization of OGT COEX 2, Sweetwater County,
Wyoming. Prepared for the Joint Industry OGT Research Project, Rept. No. TR97-06.
0 0.1 0.2 0.3 0.4
Roof Displacement (m)
Development of a Methodology for Analysis of Instability in Room and Pillar Mines M. Board, et al.
282 Deep Mining 07, Perth, Australia