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| Failure of mine ventilation stopping from inadequate transverse load capacity |
Background
Stoppings play a key role in controlling ventilation throughout an underground mining complex. The law requires that they provide a minimum transverse load capacity (the force acting on the face of the stopping) of 39 pounds per square foot of area. This requirement is based on a freestanding analysis of the stopping, meaning that, by itself and without any restraint on the ends of the wall, the wall must be able to take pressure without failing. Investigators believe that this assumption inaccurately addresses the in-mine service load condition and as such, endangers mine workers by allowing structures with low transverse load capacities to be employed anywhere in a mine. These stoppings are more prone to failure from pressure spikes caused by roof falls or ground pressures that squeeze the stoppings beyond their capacity. In addition, premature failures of stoppings pollute mine air with dangerous gases and dusts that can contribute to catastrophic explosions. If an explosion does occur, widespread damage to inadequately designed stoppings can make it more difficult to contain the fire. Efforts to reestablish ventilation can also put miners and rescue workers in danger following an explosion.
NIOSH is conducting research to show that arching is a more valid loading condition for stopping design and is working to develop a test protocol to evaluate the true loading capacity and limitations of block stoppings under arching conditions. Research to date has shown that transverse loading under arching conditions can be an order of magnitude higher than that considered in freestanding wall analysis and is directly dependent on block strength and wall geometry, not sealant properties. Since the current trend is toward the use of lightweight block materials to reduce the amount of physical strength required during construction (which in turn reduces handling injuries), it is imperative to understand that lower-strength, lightweight block materials will also significantly degrade the transverse load capacities of stoppings.
Potential Outcome
Proper design guidelines will ensure that stopping capacity matches the conditions in which they will be used in a mine. The impact of these guidelines will be to move the United States toward ratings for stoppings that are more indicative of their true transverse load capacity rather than one design for all applications as now permitted by mining law. Ultimately it is expected that the mining law itself will be modified to recognize the design factors (currently ignored) needed to achieve a more appropriate design goal. Another future impact of this work will be the development of strain-softening materials that can preserve the transverse load capacity of the stopping at an acceptable level.