Research On The Stability And Design Optimization Of Jointed Rock Open Pit Slope Based On Reliability Theory

For open pit slope stability evaluation, determined safety factor was usually used to evaluate slope stability. Safety factor is obtained by determined method without considering the variability and uncertainty of geotechnical material parameters. The safety factor is greater than the design standard, but the reliability does not meet the stability requirements.The variability and uncertainty of strength-related parameters of rock mass have greater impact on slope stability, especially for the jointed rock mass slopes. This thesis took Muliashi Open Pit Copper Mine in Zambia as a research object, the reliability of jointed rock slope was studied by site investigation, laboratory tests, theoretical analysis and numerical simulation. The research revealed the characteristics and laws of jointed slope stability from the reliability perspective. The main research work and the corresponding results obtained are as follows:(1) Combination of the background information of Muliashi Open Pit rock mass, the results of rock mechanical tests and field surveying data, probabilistic models were constructed by applying uniaxial compressive strength (UCS), rock quality designation (RQD) and statistics data from joints survey as input parameters. In order to examine the goodness of fit, these probabilistic models were fitted by the chi-square test using the available data. On the basis of the generalized Hoek-Brown criterion, stochastic analysis methods, including MATLAB program and Monte-Carlo method, were used to get the probabilistic distributions of strength and deformability parameters of jointed rock mass. Then the ranges (minimum/maximum), mean values and standard deviations of the strength parameters of the different formations in souther slope in Muliashi Open Pit were obtained, which provided a reliable data basis for slope reliability.(2) On the basis of summarizing extensive research achievement, an adjustment factor was proposed by considering both the characteristics of non-coal open pit slope and the reasonable service life. And a target reliability index evaluation standard containing both the slope safety rating and design service life rating was further constructed by using the analogy method. Based on the results above, the acceptable target reliability index of overall slope of Muliashi Open Pit is chose as 1.86. Meanwhile, for non-critical areas, step slopes and other local places, the target reliability index could be 1.51.(3) Based on Latin Hypercube sampling technique and limit equilibrium method, the influence law of rock weathering on the slope reliability is analyzed in detail. And then the 6# and 5# cross-section limit equilibrium analysis models have been established and further used to evaluate the stability of the local and overall slopes on reliability perspective. The evaluation results obtained are as follows:the two typical profile overall slope reliability indexes are 1.937 and 2.228, respectively, which are greater than the target reliability index 1.86. The slopes stability are acceptable. But for the 6# and 5# upper parts and 5# middle part, the reliability indexes are 1.491,1.504 and-0.782, respectively. Locally instability are most likely to occur.(4) Rosenbluth point estimation method (PEM) coupled with jointed finite element method (JFEM) was applied in the evaluation of this jointed rock slope stability. A probability model was established, which puts the mean and standard deviation of the rock material strength parameters as input variables and the safety factors as output variables. And the point estimation state functions were solved by jointed finite element method. According to the on-site joints and structural surface survey, jointed slope model was built to solve the safety factors, and then the probability distribution of safety factors was obtained. The results of probability analysis are consistent with the instability situation on site. In this method, both the uncertainty of rock material and the properties of rock joints was taken into account thereby fully reflecting the role of the non-linear relationship among rock layers. It would make the rock slope stability estimation more reasonable.(5) Based on saturated-unsaturated seepage theory and slope reliability analysis method, combined with consideration of the probability distribution of rock strength parameters, the seepage field of 6# typical profile weathered rock slope was investigated under different rainfall conditions such as short heavy rain and sustained heavy rain. And then the change law of reliability indexes with the rainfall duration time were obtained. The control measures of rainfall infiltration in slope were finally proposed.(6) On the basis of slope stability analysis and evaluation results, a slope optimization program targeted at the Muliashi Open Pit southern slope was proposed. To evaluate slope state, reliability analysis method was used during the optimization process. After the optimizing, the reliability indexes of 6# upper part, 5# upper part and 5# middle part have increased to 1.527,1.523 and 1.550, respectively. Hence, the program reached the optimization purposes.