Numerical analysis of longwall mining layout for a Wyoming Trona mine

At Solvay Mine, located in southwestern Wyoming, a subhorizontal trona seam is mined at depths of between 460 and 490 m using mechanized room-and-pillar and longwall mining methods. The stratigraphy at the mine generally consists of horizontally laminated (i.e., bedded) sedimentary rocks comprised mostly of shales and sandstones with significantly contrasting mechanical properties. Most notably, a 43-82 m-thick massive, brittle sandstone unit (Tower Sandstone), located approximately 100 m above the mining level, has a tendency to promote stress arching within the overburden rock that can bridge over panel-scale mine instabilities and can lead to violent multi-panel collapse failure. One such violent collapse is the 5.1 magnitude seismic event due to a 1×2 km multi-panel failure on February 3, 1995. It has proven difficult to account for this arching behavior with conventional mine design methods, such as the tributary area method. Therefore, over the past two decades or more, Solvay Mine has been utilizing numerical modeling techniques along with field instrumentation/monitoring as part of an integrated program to gain an enhanced understanding of the complex response of the overlying stratigraphy (i.e., arching) to mining. In 2005 and 2006, several longwall panels in the northwest and southeast areas of the mine were instrumented and monitored during mining. Two- and three-dimensional numerical models were developed and calibrated on the basis of the instrumentation data, and these models were then used for mine design verification (e.g., pillar and panel dimensions). This mining case study illustrates the complex excavation response due to the contrasts in stratigraphy at Solvay Mine and presents a numerical modeling study that captures the dominant aspects of these conditions.