Stressed state of a rock mass with protective extraction in the wings of tectonic faults

Abstract

For the October deposit conditions extraction of the gently-sloping deposit of copper-nickel ores within the wings numerous faults intersecting the ore body is connected with increased danger of rock bursts and the requirement for carrying out a considerable amount of labor-consuming driving of dead ends. Mining is carried out in a continuous sequence by layers and chambers with stowing of the worked-out space with consolidating mixtures. Previously by driving and filling parallel cross drifts over the upper contact of the deposit a continuous concrete layer is built. The workings are orientated almost along the strike of tectonics fractures [1]. In seeking the possibility of reducing the volume of driving in zones of tectonic faults industrial experiments were carried out in two areas of the deposit intersected by faults. In contrast to the traditional scheme for constructing a protective slice there was simultaneous erection of sections of a protective slice within the wings of a fault before approaching the extraction front of the main reserves towards the tectonic fault zone. It was assumed that presence of an additional front of a protective slice leads to mutual shifting of the wings of a tectonic fault with loosening of the ore mass. As a result of this treatment of the reserves in the wings of the fault was accomplished without creating a protective slice beneath part of the ore bounded by the overlaying strata. The set of instrument and full-scale observations confirmed this assumption. In test areas both in stopes created beneath the protective slice and in the overworked part of the ore body no signs of increased mine pressure were noted [2]. However, industrial experiments were limited by the specific mining and geological conditions. Observations and measurements made in the test areas give information about the mechanical state of the rock mass at a limited number of points. Transfer of the technology to other conditions and depths requires additional substantiation and studies connected with considerable expenditure and organizational difficulties. In this situation it is possible most simply by means of mathematical modelling to reveal the most common features of stress field formation within the vicinity of a fault. Calculations of the stress-strain state for a rock mass were performed by the finite element method. In the mathematical describing material deformation the rock mass outside the plane of a tectonic fault was represented by elastic materials with corresponding mechanical properties. This assumption is correct in view of the fact that the rocks comprising the rock mass have high elasticity constants and the theological properties appear to an insignificant extent. The deformation of a tectonic fault was described by an elastoplastic material reflecting the main features of fault behavior under load [3, 4]. The original stressed state for the ore and surrounding rock masses is characterized by the presence of tectonic forces. Horizontal compressive stresses are 0.6-1.7"rH, and vertical stresses are 0.6-1.43,H (H is working depth). The most clearly expressed regions of disturbance of the original stressed state are confined to large amplitude faults. According to measured data in them both horizontal and vertical stresses may predominate which may lead to a marked difference in the mechanism of rock mass deformation within the vicinity of a fault on introducing mine working. In the present work the case is considered when within the vicinity of a tectonic fault horizontal stresses predominate. The original stress field in all of the calculations discussed below is adopted as follows: Cry = 3'H, a x = 1.53,H, rxy = 0. Corresponding to this at the outer boundary of the calculation region normal and tangential force components were prescribed. The stress -strained state of the rock mass was determined for the plane strain condition which is fulfilled here due to the considerable extent of a tectonic fault, and stoping and preparatory workings in direction perpendicular to the section in question. Values of mechanical properties for the ore body, the surrounding rocks, and stowing material were taken as typical for the October deposit conditions [5]. The corresponding values of Young's modulus were 5" 104, 8- 104 and 3.104 MPa, and for Poisson's ratio they were 0.25, 0.2, and 0.4. The ore body thickness was 25 m.