Feasibility of controlling the stability of the edge of a rock mass in recovery mine workings at the bottom of a block during the action of horizontal tectonic forces

Abstract

Reserves of apatite-nepheline ores in the Khibina massif, which are being worked by the underground method using a system of forced collapse in stages, are represented by ore bodies with a dip of up to 25-30 ~ for an average horizontal thickness of 180-200 m and extent of 1800-2000 m. Ores of average strength having a uniform structure are broken up by systems of fractures into large and small blocks, and under load are inclined to brittle failure. The deposits are being worked horizontally in separate areas, blocks 70 m high, 150 m wide along the strike, and with a length across the strike over the entire horizontal thickness of the ore body. As the ore reserves are extracted, an open cleared space is formed in the shape of trenches, filled with the collapsed overlying rocks of the hanging wall. The maximum depth of the mine workings is 350-400 m at the present time.The stress state of the rock mass is determined by the action of horizontal tectonic forces and the weight of the overlying rock. The size of the tectonic component may reach 600 kg-sec/cm 2 [1], and its destructive effect on an element of the system at shallow mineworking depths, on the order of 70-100 m from the surface, is the predominant influence. IL is manifested in the form of scaling, exfoliation, and cracking of the rocks. In the latter case, slabs of rock weighing up to 3-4 kg often snap off the edge of the mineworking with an explosionllke sound, which is a great danger to the workers. The destructive effect of the gravitational component begins to appear at a depth of~S00-350 m.As the ore body is extracted beneath the hanging wall, the overhang of the cantilever of the overlying rocks causes a counterpressure on the rock mass being worked, the concentration coefficient of which may reach 3.0-5.0 before the collapse of the cantilever [2, 3]. In such cases, the gravitational component at the working depth reached causes stresses in the elements of the system which are equal to or exceed the compressive strength limits of the rock.The combined action of tectonic and gravitational forces is especially unfavorable for the stability of the most important element of the system, the bottom of the block, which provides for the execution of the main and auxiliary processes during ore extraction. Up to 60-70% of the total volume of the block working is located at the bottom, and this dissection is the cause of severe strain In its elements. Under conditions of the combined action of tectonic and gravitational forces, cases of crushing occur at the bottom of the blocks and even smallrock shocks, which comprise a safety hazard in the mining work and cause an increase in the loss of ore broken out and additional expenses in erecting reinforcement and sinking new shafts into the bottom. Thus, today, these expenses have already reached 20 kopecks/ton of ore extracted, i.e., 12% of its cost. But in some cases, even the most reliable forms of support cannot prevent the destruction of the bottom element for the construction being used. In zones of high rock...