Mining props with active load setting are much in demand for the high productive coal winning technologies. This paper attempts to present salient features of mining props with active load setting capabilities. A discussion is made on merits and limitations of these props before conclusions are made. The paper also presents field trials of a unique active load setting prop which is completely mechanical in nature.
Props are one of the early forms of roof support systems which are still in use as an integral part of systematic underground mining. They have been in use to serve variety of functions under different mining scenario. Chokes/cogs are used to support larger roof area, while slender timber units which shows early warning signals through buckling/splitting are popular units known as ‘chowkidar’ in the mining industry. Steel props are now gaining more importance in application due to environmental concerns associated with the use of timber supports. Also, steel eliminates fire hazards associated with wooden props. The motivating factor for use of steel props is their small size, ease in installation, high load bearing capacity and long life. These props can be easily made to suit variable mining height and also seam inclination. Advantage of the fact that steel sections carries much higher load in compression is always used in designing the props. Props those can be tightly fitted between roof and floor through a tightening mechanism are in much need for the high productive mining technologies. High productive mining cycles means that there is a frequent requirement of setting props within the limit of blasting zone as well as in the depillaring areas carrying higher roof load due to insufficient roof fall time. The props with active load setting are resistant to blast impacts and they carry much higher load than the other form of supports being in use. The tightening mechanism helps in maintaining structural integrity as displacement between roof and floor is prevented. Figure 1 represents schematic three different load-displacement characteristics of three different popular mining prop systems. Figure 1(a) depicts characteristics of a wooden prop which carries smaller load bearing capacity (less than 5t) with a large allowable yielding between roof and floor. The higher yielding arise from bucking (before failure) and cracking/splitting (after failure) of the wood. Figure 1(b) is the characteristics of a steel prop without active load setting capacity. These props allow certain amount of convergence between roof and floor depending on the gap between roofs and prop or wooden ledge inserted between roof and prop to tighten the prop. There is hardly post-failure deformation to certain load carrying capacity (till the steel buckles) with these props once the prop capacity is reached (between 15–20t, depending on thickness of steel pipes). In contrast to the above mentioned two types of props, props with active load setting carries higher load bearing capacity (more than 25t, generally 50t capacity has enough portability) with minimal allowance for convergence between roof and floor.
