| During the engineerning design, rock mass (or clay) mediums are so complex for the engineer that it is difficult to choose the relational material parameters. With on-site test methods (such as the compression plate test), we are able to draw more satisfactory results. However, because of its own shortcomings of the method, engineering and technical personnel have to improve the traditional test methods and look for a new test method and technology. Therefore, how accurate solution to this issue has become the bottleneck problem that constraints rock engineering practice. Refering previous studies, the author utilize the displacement back analysis methodt, the finite element analysis menthod and the genetic algorithm theory to solve the parameters of rock mass materials, and to further explore the following issues, based of these, some conclusions are drawn:(1) Rock mass is a complex geological body, it is made up of rock and crannies, moreover, rock is indispensable skeleton parts. Rock deformation of the stress-strain relationship is the foundation of the rock mass stress-deformation. In this paper, the six typies rock deformation stress-strain curve are introduced, meanwhile, rock mass deformation behavior under complex stress are discussed too. The Constitutive Model is the essentialest difference amongs of the different materials, elasticity constitutive model of stress-strain relationship is deducted. Moreover, in the space contingency basis, Rock mass elasto-plastic constitutive model of the general formula are presented. Finally, the author points out using the elasto-plastic constitutive model on rock mass can generate better purpose.(2) The Finite Element method is often used in engineering numerical calculation. In the elastic-plastic finite element analysis, we must be study some key issues. For example: how to select the best suit the yield criteria, how toformat the elastic-plastic matrix, what is the essential of the incremental method on elastoplastic issue, and how to solve the nonlinear equations, and so on. These are basic issues about the elastic-plastic finite element analysis, and these are prerequisites to write the elastic-plastic finite element analysis program. Based on these, the author write the two-dimensional elastic-plastic finite element program (FEM.F90) for underground chamber caving, which is on the basis of the two-dimensional finite element analysis program (ZZYFEM.F90), which is written by Dr. Zhu Zhan Yuan who is studying in the HIT.(3) It is important for us which parameters should be selected as the object of the displacement back analysis, the concept of Parameter Sensitivity Degree is utilized. Based on this theory, the author suggests a new formulate to calculate the parameter sensitivity degree. At the same time, considering the usability, three points are selected to study the changes of displacement by operating every parameter at some rules. Finally, we can conclude from this caculating that: In four parameters, the Elastic Modulus of the largest displacement effects, the Poisson's smallest, the Cohesion and the Internal Friction Angle in the middle.(4) The Genetic Algorithm (GA) is one of the new good methods to resolve extreme value about complex functions. The essential theories of GA are the Selection operation, the Crossover operation and the Mutation operation, they control the genetic algorithm optimization performance, through three operations, new groups formed, and through calculating the fiteness function, and overall groups reach to evolutionary purpose. To enhance the Simple Genetic Algorithm optimization ability of the overall performance, the Niche Algorithm is introduced for improving the overall search function of the Simple Genetic Algorithm. Then, the authors used Fortran90 language to write the Improved Genetic Algorithms program (NIGA.F90), to solve complex function extremum problem. In order to verify the reliability and the practical of this program, the author design numerical experiments, this program is used to solve multiple-function and multi-function variables extreme function, such as the Shubert Function, and set the corresponding parameters: the number of group size is 50, the number of biggest evolution is equal to 200, the crossover probability is0.8 and the mutation probability is 0.1. After 153 generations cacaulation with this program, the results reach to the global optimum, and it is credible.(5) The key of the Elasto-Plastic optimization displacement back analysis is to make the objective function for the minimum solution. Based on the Least-Squares method, the author establish a new objective function J ( x ), it combines the Finite Element Method and the Genetic Optimization analysis methods. Meanwhile, the program (BAFEM.F90) of Improved Genetic Optimization Back Analysis is written for this issue. Moreover, the method of the Elasto-Plastic optimization displacement back analysis is proposed, and a hypothetical engineering example is caculated with this program, by contrasting all measuring points, the result shows that the error between them is very small.(6) The measured displacements of test hole in Shisanling Pumped Storage Station are utilized for this method, this paper set parameters for the optimization displacement back analysis program: the range of Elastic Modulus is from 5.0Gpa to15Gpa , the range of Poisson's Ratio is from 0.20 to 0.40, the range of Cohesion is from 1000Kpa to2000Kpa , and Internal Friction Angle is from 25°to 40°. Using the program (BAFEM.F90) to calculate the corresponding parameters, the results are: the value of the Elastic modulus is 8.46 Gpa , Poisson's Ratio is 0.357, the Cohesion is 1768 Kpa and the Internal Friction Angle is 28.94°, they approach to the test values. At last, these parameters are input in the finite analsis program, and the convergence displacement about every two test points are obtained, and we conclude that the measured displacements are close to the calculated displacement. Therefore, the accuracy and practicality of program of the optimization displacement back analysis are futher verified by the practice engineering.The analysises above all show that the method of displacement back analysis and the program (BAFEM.F90) based on the Improve GA are credible, it is also one of the ways to promote in engineering practice.
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