Numerical Simulation of Crack Evolution Mechanism and Subsidence Characteristics Effected by Rock Mass Structure in Block Caving Mining

In the block caving mining, the significant rock mass deformation and surface subsidence will be formed with the continuous extraction of ore. However, the internal crack evolution mechanisms in rock mass and associated subsidence characteristics present one of the key issues in rock mining engineering. Although block caving method has been used for many years, current knowledge of the crack evolution mechanisms, the subsidence characteristics under the influence of rock mass structure and subsidence prediction capabilities are limited. Based on the rock mechanics model provided by CEMI, crack evolution mechanisms and subsidence characteristics effected by the rock mass structure in block caving are numerically investigated using RFPA 2D, a numerical code based on FEM. Crack formation, propagation and coalescence in the overlying strata and the stress-balancing arch evolution in the stress field are represented visually during the whole process of extraction. The numerically obtained crack evolution shows that the stress-balancing arch has a significant influence on the fracture development of rock mass, and directly determines the slump form and rate of the rock mass. After understanding of the crack evolution mechanism in rock mass, the characteristics of surface subsidence are analyzed. Numerical experiments emphasize the geometrical configuration of joints and faults about mechanisms of subsidence development, including joints orientation, faults location and inclination, which can provide significantly meaningful guides for investigation of subsidence mechanisms and implementation of remedial measures.

Instability Mechanism of Extraction Structure in Whole Life Cycle in Block Caving Mine

In the whole life cycle of the extraction structure in block caving mine from the beginning of roadways excavation to the end of ore drawing, there are many factors affecting the stability of the extraction structure. The investigation in the mine site shows that the extraction structure often presents the law of repeated instability. In order to reveal the mechanism of repeated instability of the extraction structure, the whole life cycle of extraction structure can be divided into three stages, namely, the formation stage of extraction structure, the undercutting stage without initial caving, and the ore caving and drawing stage. The three-dimensional finite difference software FLAC3D was used to establish the numerical model of the extraction structure in the whole life cycle in the block caving method. The process of ore caving and ore drawing was replaced by manual excavation of the caving area above the undercut space and applying stress on the major apex. The stress and displacement evolution laws of the extraction structure in three stages of the whole life cycle were studied and compared with the instability characteristics of the extraction structure on mine site. The whole life cycle instability mechanism of the extraction structure in Tongkuangyu mine is revealed; the research results show that the extraction structure near the advancing undercut front is prone to producing compressive stress concentration under the action of the surrounding rock stress arch in the stope; if the rock mass shear failure condition is reached, the instability of the extraction structure occurs. The extraction structure near the advancing undercut front is gradually transferred to the area under the undercut space with undercut increase, and the tensile stress concentration gradually appears in the sidewall of ore loading roadway and the tip of major apex; if the tensile strength of the rock mass in the extraction structure is exceeded, the instability occurs again. It is helpful to reduce the probability of the instability of the extraction structure to promote the overburden ore caving as soon as possible after the undercutting begins.

Study on Instability Mechanism of Extraction Structure under Undercut Space Based on Thin Plate Theory in Block Caving Method

The instability of extraction structure under the undercut space in the block caving stope presents specific characteristics: rib spalling and floor heave in ore-loading roadway and collapse of major apexes. In order to study the stress and displacement evolution law of extraction structure under undercut space and reveal the instability mechanism of extraction structure, the numerical simulation model of block caving stope was established using the finite difference software FLAC3D. According to the postundercutting strategy in Tongkuangyu Mine in China, extraction structure was formed first in the simulation process, and then the undercut level was divided into eight units for excavation step by step. The stress and displacement of extraction structure after each step of undercutting were monitored and analyzed. Based on the thin plate theory, the mechanism of stress change and deflection deformation of extraction structure was revealed. The research results show that, under the action of high horizontal tectonic stress and vertical stress, the extraction structure under undercut space produces vertical upward bending deformation after undercutting during the block caving. The tension stress concentration gradually appears in the side wall of the ore-loading roadway and the tip of the major apexes; with the increase of the undercutting area, the degree of tensile stress concentration gradually becomes strong; when the tensile strength of the rock mass in extraction structure is exceeded, extraction structure presents instability. It is necessary to make the overlying ore collapse on extraction structure as soon as possible after undercutting, which is beneficial to release the tension stress in the extraction structure under undercutting space.

Enhancement of the technology of mining steep ore bodies applying the “floating” crown

When mining ore bodies in Kryvyi Rih iron ore basin, underground mines apply open stoping or bulk caving systems in proportion of 55% to 45%. Most of underground mines prefer stoping with pillar caving. Yet, rock pressure contributes to growth of costs for workings maintenance and deterioration of extraction indices. Rock mass extraction indices can be enhanced by application of a protectve structure in the upper part of the block that will enable additional decrease in load on the draw level. There are a great many of methods for determining parameters of constructive elements of the protective structure that help keep its integrity for the whole period of block mining. The article suggests methods for determining parameters of the protective structure when mining steep ore bodies. The research conducted demonstrates that with the inclined protective structure, increase of unit load on it from 200 to 1200t/m2 leads to decrease of its thickness from 6.3-20.9m to 5.5-18.4m and increase of the crown length from 40m to 60m. The developed block caving system with application of the protective structure when mining steep ore bodies enables overall decrease of ore dilution in the block by 3%, increase of iron content in the mined ore by 1.3% without significant mining costs growth and decrease of loads on the workings of the receiving level.