Underground mine safety depends upon many factors, including special equipment that stabilizes the mine roof to prevent collapse during coal removal.
Known as pumpable roof supports, these collapsible, cylindrical bags are attached to the mine roof. Once attached, the bags are filled with cement through hoses threaded through holes in the mine roof. Wire bands encircling the bags maintain the cylindrical shape as the bags fill with cement.
Underground coal mines have used two types of pumpable roof supports for the past 20 years; however, little is known about differences in performance between the two main types. From the outside, the supports may look similar, but, on the inside, they differ considerably depending on what type of cement is used to fill them. Two types of cement are commonly used, one of which is calcium-based, while the other is limestone and clay-based.
To compare the differences between supports filled with the two different cements, NIOSH investigators tested pumpable roof supports more than 160 times over 7 years in the NIOSH Mine Roof Simulator. Located in Bruceton, Pennsylvania, the simulator provides a safe and controlled environment that mimics the conditions of an underground coal mine, including the thousands of pounds of load taken up by the roof supports once coal is extracted.
The tests showed that the calcium-based cement was significantly stiffer compared to the limestone and clay-based cement, according to the study published in the International Journal of Mining Science and Technology. Both types of cement decreased in strength as the load increased and they began to crack, which often occurs with brittle cements.
Up to a point, the bag contains the cracking material and the support can still bear significant weight. Eventually, however, an increasing load will cause the surrounding bag to tear, which leads to a rapid decrease in the pumpable support’s strength. Accordingly, the investigators now plan to study supports using less brittle materials than the two standard cements.