Stability Analysis Of Large-scale Marginal Ore-body Mining In The Open-pit

The open-pit mining is prevalent in China. Most large open pit mines were established in 1950-60's, of which about 80% will step into advanced. With the rate of 12-20m mining annually, some open-pit mines are in the transition period of turning to underground mining. Currently, the main features of open-pit are as follows:1. Before the commissioning of the underground mining, some alternate ore-bodies must be exploited to meet the needs of dressing due to the production drawdown when the open pit is approach to be closed. They are always border ores with complicated geological conditions in the open pit.2. The marginal ores mining or underground mining is simultaneous with open pit mining.3. When exploiting the marginal ore-body, the pillars and safety pillars were kept to prevent the influence on open pit slope as mining method of stable structure was adopted.As a result, the main questions are:1. Evaluation on mine pillar stability.2. Analysis of slope stability under stress field disturbance due to mining.3. Determination of the scope of subsidence area.With the background of SWW marginal ore-bodies in Ekou iron mine, the Block Theory, Vector Sum Analysis and Extend Finite Element Method (XFEM) have been introduced to study the above-mentioned three problems systematically in this dissertation, of which the key contents are as follows:1. Study the geometric and mechanical stability of ore pillar based on 3D Joint Network.(1) There existing shortcomings of two popular methods in searching the key block, which are overcome by combining them in this dissertation. First of all, searching the closed loops on the free surface of mine pillar by Tree Cut Method; then, the sub-region of every single closed loop, which is a range box of typeâ… joints, is cut by infinite plane of type I and other relevant joints to form many small blocks, the block which has the exact face of closed loop is the key block. The combined method has no requirement of intricate judgment when the block is complicated, also will not cause enormous computational cost when the joints are token as infinite plane, and can realize concave block searching without any special algorithm.(2) The material of ore pillar cannot be treated as continuative simply as the size of which is the same as or slightly bigger than joints. The pillar region was cut by infinite planes of joints to generate a series of convex polytopes, and then the non-exist face of polytope is deleted to make the combination of convex polytopes to form the blocks that can truly match the joints network. Sometimes, the joint cut and polytope combination take place only in sub-region which is generated by equally divided the pillar region to avoid forming large number of polytopes. The blocks of entire pillar region can be formed by piecing together all sub-region blocks. Mechanical analysis can be carried out once the formation of pillar block is finished. The block formation method is prior to 3DEC itself in which the joint is regarded as infinite plane to cut the blocks and lead to inconsistence with statistical parameters of the joint such as trace length.2. Calculating the FOS (factor of safety) of slip surface and searching the critical slip surface.(1) The Vector Sum Analysis can calculate the FOS of the slope based on the stress field without iteration, and it's more suitable for analyzing the slope stability with stress disturbance due to mining.(2) The Ant Colony Algorithm (ACO) was improved to search the slope critical slip surface without the discretization of the slope body by taking the pheromone as a continuous distribution variable. In addition, to overcome low speed of ACO owing to absence of original pheromone, as well as multiple redundancies and low efficient solving of genetic algorithms as a result of no feedback ability, the ACO was integrated with genetic algorithm to complement each other's disadvantages.3. Actually, mine subsidence is the failure process of the rock mass. Also, the ubiquitous-joint is equivalent to the weakness of the mechanical parameters of rock mass in large-scale mining except classâ…¢and above. The FEM and DEM which are commonly used to analyze the mine collapse can not reveal the above features very well. The XFEM was adopted to solve this subject. In order to avoid rebuilding the overall stiffness matrix, the XFEM was realized by explicit solution technique. The normal and shear stiffness of contact surface was used to reflect the relationship of jumping displacement and stress on Gauss points of the fracture segments. Moreover, the tip-enriched freedoms can be directly derived by normal and Heaviside freedoms using the sub-structural stiffness matrix which is based on sub-structure method and made up of tip element and its neighbor elements, in the prevention of too many convergence indicators and oscillation of system. Then, some numerical simulations of crack tensile, compression and shear as well as propagation in a plate were carried out to illustrate the effectiveness of the methods. Finally, the section of 18# prospecting line was used for analyzing the rock failure by exploitation of north ore-body.