Fractal evolution mechanism of rock fracture in undersea metal mining

Abstract

Through rock mechanics test, similar simulation experiment, borehole photographic observation of rock fissure, numerical simulation calculation of plastic zone distribution and deformation monitoring of rock mass during undersea mining, the fractal evolution mechanisms of rock fracture in undersea metallic deposits of Sanshandao Gold Mine were studied by fractal theory. The experimental researches on granite mechanics test in undersea deposit indicate that with the increase of load, the granite deformation energy and the fractal dimension of acoustic emission (FDAE) increase gradually. However, after reaching the peak stress of specimen, the fractal dimensions of acoustic emission (FDAEs) decrease and the granite specimen fails. Therefore, the fractal dimension evolution of rock failure can be divided into four stages, which are fissure inoculation stage, fissure growth stage, fissure expansion stage and fracture instability stage, respectively. By calculating and analyzing the damage photographs of rock specimens in Sanshandao Gold Mine, the fractal dimension of rock fissure is 1.4514, which is close to the average value of FDAE during granite destruction, i.e., 1.4693. Similar simulation experiments of undersea mining show that with the excavation proceeding, the FDAE in rock stratum increases gradually, and when the thickness of the isolation roof is less than 40 m, the FDAE begins to decrease, and meanwhile the sign of water inrush emerges. The numerical simulation researches on the plastic zone distribution of undersea mining in Sanshandao Gold Mine indicate that the fractal dimension of plastic zone (FDPZ) where the failure characteristics occur is 1.4598, close to the result of similar simulation experiment of 1.4364, which shows the sign of water inrush. Meanwhile, the thickness of the isolation roof for undersea mining should be more than 40 m, which is consistent with the results of similar simulation experiment. In Sanshandao Gold Mine, the rock fissures in undersea mining were observed by borehole photography and the rock mass deformation was monitored by multi-point displacement meters, and at the same time the fractal dimensions of strata borehole fissure distribution and energy release ratio (ERR) of rock mass were calculated by fractal principle, which are 1.2328 and 1.2685, respectively. The results demonstrate that rock deformation and fissure propagation are both in the second stage of fissure growth, and have not reached the fourth stage of fracture instability. Therefore, the conclusion can be obtained that the undersea mining in Sanshandao Gold Mine is safe at present.

摘要

本文通过三山岛金矿海底金属矿床开采岩石力学测试、相似模拟试验、岩层裂隙钻孔摄像观测、 塑性区分布数值模拟计算和岩层变形监测, 研究了金属矿海底开采岩层裂隙的分形演化机理。海底矿 床花岗岩力学试验结果显示, 随着载荷的增大, 岩石变形能逐渐增大, 声发射事件分形维数逐渐升高, 达到峰值应力后, 岩石声发射事件分形维数开始降低, 破坏特征显现; 岩石破坏分形维数演化分为四 个阶段:裂隙孕育阶段、裂隙发育阶段、裂隙扩展阶段和破裂失稳阶段; 计算得出花岗岩破坏裂隙分 形维数为1.4514, 与岩石声发射在破坏附近的平均分形维数1.4693 相近。海底开采相似模拟试验结果 表明, 随着开挖的进行, 岩层声发射分形维数逐渐增大, 当顶板岩层安全厚度小于40 m 后, 声发射 分形维数开始降低, 呈现突水征兆。 通过三山岛金矿海底开采塑性区分布数值的模拟得出,出现破坏 特征的塑性区分布分维数为1.4598, 与相似模拟试验呈现突水特征的裂隙分形维数1.4364 相近, 三山 岛金矿海底开采安全隔离层厚度需40 m 以上, 验证了海底开采相似模拟试验结果。 三山岛金矿海底 开采岩层裂隙钻孔摄像和岩层变形多点位移计监测结果显示, 岩层钻孔摄像裂隙分布和岩层变形能量 释放的分形维数分别为1.2685 和1.2328, 表明岩层变形与裂隙扩展均处于裂隙发育的第二阶段, 岩层 裂隙扩展与变形未达到破坏失稳的第四阶段, 三山岛金矿目前开采是安全的。