PURPOSE:  Reduce the number of accidents and fatalities caused by mining in weak ground by developing accurate knowledge of how roof supports perform in weak rock, how fracturing is induced by excavation in weak rock, and how best to support adequately such ground or arrest the fracturing mechanism.

RESEARCH SUMMARY:  Weak rock is a significant factor in roof falls, which are a leading cause of fatalities and injuries in underground mines in the United States. From 1998 to 2002, the incident rate for underground fatalities was 56.2 per 100,000 equivalent full-time employees ; falls of ground accounted for 43.3% of all underground fatalities. During the same period, the incident rate for nonfatal injuries and illnesses was 7.8 per 100 full-time equivalent employees; fall of ground accounted for 1.25 per 100 full-time equivalent employees.

It has been shown that borehole roughness affects anchorage capacity. Anchorage capability is especially critical in weak roof where the rock is especially prone to fracturing. In this project, researchers seek to address the question of whether borehole roughness can be measured with a simple device to indicate the anchorage capacity of a fully grouted bolt. In this regard, two new tools have been developed at NIOSH to study rock strength: a rock strain strip (ROSS) and a miniature data acquisition system (MIDAS). A ROSS is grouted into a borehole to measure strain in the roof of a mine opening. A MIDAS is used to read the strain sensors of a ROSS without the need for long cables to hook up to a computer. Research to date indicates that ROSS's are able to measure anchorage slip in fully grouted bolts.

Numerical models are being used to study failure mechanisms in weak rock. Therefore, this portion of the research project is directed to studying whether numerical models adequately simulate the fracturing mechanism. Results to date support this hypothesis, but further advances in discrete element modeling are needed to build the case. With models that simulate these observed failure mechanisms, researchers can determine if fracturing can be limited with rock supports to reduce the rock falls.

It is expected that this research will improve the performance of roof bolts in weak rock and determine ways to arrest or minimize the propagation of mining-induced fractures around an underground opening. Such improvements should increase stability around underground openings in weak rock and reduce the number of roof falls.