PURPOSE:  This project seeks to reduce injuries and fatalities from ground falls in underground coal mines by developing state-of-the-art design tools for three related ground control problem areas: 1) deep cover coal pillar recovery, 2) high horizontal stress control and 3) multiple-seam mining.

RESEARCH SUMMARY:  In the 10 year period from 1995 through 2004, ground fall fatalities have averaged about 8 per year and accounted for almost 44% of all fatalities in underground coal mines. To meet increased coal demand, the industry must extract resources under more extreme and potentially dangerous ground control conditions. Interaction with stakeholders reveals three recurrent ground control problems in extreme conditions, namely, pillar recovery at depth, high horizontal stress control and multiple-seam mining.

Traditional room-and-pillar mine design methods used for shallow mines (less than 750 ft) do not work well in deeper mines. NIOSH researchers developed design guidelines for deep cover pillar recovery that reduce the likelihood of an uncontrolled roof collapse or a massive pillar collapse. The recommendations have been incorporated into the Mine Safety and Health Administration-approved Roof Control Plans at more than 100 operating coal mines throughout the United States.

The magnitude of maximum horizontal stress and whether it will induce mine roof damage under particular circumstances has remained elusive. NIOSH researchers found that horizontal stress in a mine roof depends on the roof rock modulus. A field study critically examined horizontal stress variation and displacements in a mine roof. Numerical models were created that simulated the observed behavior. NIOSH researchers developed a modeling procedure that uses sufficient geologic detail and the proper horizontal stress variation to create realistic numerical simulations of observed mine roof behavior.

The multiple-seam mining portion of the project is developing design tools for evaluating and controlling potential multiple-seam mining interactions using a combination of statistical analysis of a case history database and numerical models of the failure mechanics. Over 200 case histories of multiple seam mining interactions have been carefully documented. Sophisticated computer models are being utilized to understand the failure mechanics resulting from multiple seam mining. Development of rational multiple- seam mining guidelines that are based on the failure mechanics and the case history database is underway.

Some expected outputs from this work are:

1. Suggested guidelines for sizing coal pillars for recovery under deep cover.
2. Techniques for evaluating and controlling multiple seam hazards in underground mines.
3. Design methodology and guidelines for multiple seam operations.