| The southeast coastal province of China, such as Fujian and Guangdong, are mountainous.With the rapid development of construction in these provinces, there are a large number of slopes. For example, there are 180 high slopes in Beijing-Fuzhou Expressway, which is just 200 kilometers. In the aspect of mineral development, open pit and tailing dam will be the formation of such a large number of high slope,for example the Zijinshan opencast mine will be as high as 880 meters.The slope stability problem has been brought by the formation of high slope, with a large number of engineering construction and mining. The improper slope excavation will lead to slope deformation and failure, thus the safety of construction would be affected. Especially in the large opencast mining engineering, because of the mining depth, if the slope stability was not paid attention to, a great loss of economics and life would be brought. Hence the study on the stability of high slope in opencast mine has very important economic value and engineering practice significance.If the society is built to be an environment-friendly and resource-saving society, the optimal design must be implemented in every projects. Optimum design is to save the cost as much as possible on the basis of slope stability requirement. In the aspect of mine slope, the main object of optimum design is slope angle. Hence the study on the optimal slope angle in opencast mine has very important economic value and engineering practice significance.Therefore, this paper took slope engineering of Zijinshan opencast mine as example and studyed the stability and optimal angle of high slope. This paper began with engineering geological conditions, then analysed the mechanism of deformation and failure, besides builds geological model. According to the result of examination and test, combining with Hoek-Brown strength criteria, rock intension was established. The stability coefficient of slope was calculated by limit equilibrium method, and stress and strain was analyzed by FLAC to validate stability of slope. Based on these, the optimal angle of slope was analyzed to guide the design and construction work.The main content as follows, (1) First, the engineering geological condition was analysed from the aspect of topography, geomorphology, lithology, hydrogeology, and physical geological phenomenon. In the qualitative analysis, this paper focused on the analysis of the characteristics of rock mass structure. The distributing character and scale of structural plane was researched by engineering geological survey. And rock mass structural character and slope deformation character was established by stereogram. Then, according to lithology and rock mass structure, the rock group division was done to provide material basis for the stability of the calculation.(2) Based on this, the rock mass physical and mechanical examination was done, the reliable parameter was get by Hoek-Brown strength criteria. Then the parameter was chosen to use in the slope stability analysis.(3) The failure mode was divided into two species that were partial failure and integral failure by qualitative analysis. According to the shape of sliding surface, the partial failure mode was divided into two types, that were arc failure mode and cuniform failure mode. And the integral failure mode was polygonal line controled by structural plane.(4) Based on qualitative analysis, calculation parameters and section plane were chosen, which accorded with the fact. The stability coefficient was calculated by Sarma method in all kinds of conditions, such as natural, storm, earthquake, storm plus earthquake. Then slope stability was evaluated by the stability coefficient. In the integral failure mode, the slope rock mass was divided by the 3rd structural plane. In partial failure mode, the automatic search method was used to search the sliding plane.(5) The quasi three dimensional calculation model was created according to petrofabric division. The stress distribution, failure mode and mechanical mechanism were established by finite difference numerical analysis under Mohr-Coulomb constitutive model. The plastic zone was found by FLAC3D to show the potential sliding surface, in order to give suggestions to monitoring and engineering treatment.Then the finite difference simulation results compared with the Sarma method that match the results from two methods to get slope deformation mechanism of the in-depth understanding of the slope failure mode verification.(6) The optimization design of Zijinshan open pit slope was done from the aspect of security and economics. The optimal angle of slope was determined, according to safety and economy. The objective function relationship between slope angle and engineering cost was built by constraint condition of partial and integral stability. Then linear programming was imported to confirm the optimal slope angle. Optimal results will be compared before study, and shows that after the original design slope angle of the optimization calculation, the stability of the slope to meet the requirements under the premise of the lower block structure of rock by increasing the slope of the slope angle, and slow down the upper fragmentation, rock slope of the granular structure slope angle, the total remains the same slope angle adjustment programs, achieve cost-saving purpose of excavation.Studies had shown that cost-saving purposes had been reached by increasing the lower angle and slowing down the upper angle.Therefore, there are some advantages as follow, (1) Because of the lithology of Zijinshan slope, this paper focused on the analysis of rock mass structure. The joints were grouped, and faults were analysed by the use of stereographic projection in order to analyse the stability of the slope. (2) In quantitative analysis, the integral and partial slope stability were analysed by the use of Sarma and FDM. (3) According to the rock mass structure of Zijinshan slope, the slope was divided into two parts at the level of 604 meters. The objective function relationship between slope angle and engineering cost was built by constraint condition of partial and integral stability. Then linear programming was imported to confirm the optimal slope angle.
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