Nonlinear Failure Criterion, The Upper Limit Of The Shallow Tunnel Surrounding Rock Pressure

How to determine surrounding rock pressure is always the key of tunnel structure design.The current calculation methods of shallow buried tunnel contain limit equilibrium method, Terzaghi theory, and method which the highway and railway codes have recommended.In the past few years, the limit analysis method which has been widely applied in geotechnical engineering is also gradually used in some aspects of tunnel engineering. However, it is well known that almost all these methods above are carried out in the precondition that soil and rock masses satisfy the linear criterion.But a large number of experiments have proved the fact that soil and rock masses obey nonlinear criterion. So it is necessary to do some work about the surrounding rock pressure of shallow buried tunnel in the nonlinear criterion.On the basis of limit analysis theory, by introducing the nonlinear criterion, the thesis has done the following work about surrounding rock pressure of shallow buried tunnel:Firstly, nonlinear criterions and their applications in geotechnical engineering were presented.Secondly, the limit analysis theory was described. Specially, specify of the upper bound theory of limit analysis and its application conditions were given out.Then the thesis list out concrete method and steps of how to use the upper bound theory to calculate surrounding rock pressure of shallow buried tunnel.Thirdly, two new failure mechanisms were given and the corresponding calculation formulas about surrounding rock pressure were deduced in the frame of nonlinear criterions.Fourthly, some basic optimization theories such as quasi-newton method, genetic algorithm, sequential quadratic programming method were elaborated. Especially, the process of how to practice SQP method by Matlab was mainly explained.By using Matlab and its optimization toolbox, the optimal upper bound solutions corresponding to different failure patterns were got.Besides,through comparisons with existing analytical results and an illustration of practical engineering,it is found that the two patterns are reasonable for limit analysis of surrounding rock pressure.Finally, influences of nonlinear coefficient and other rock parameters on surrounding rock pressure were discussed.It was found that these parameters have significant effects on the shape of fracture plane of tunnel.Besides,another nonlinear failure criterion named Hoek-Brown criterion was applied in the calculation of pressure.A list of value of upper bound solutions and corresponding angles were got.Then the relationships between the Hoek-Brown parameters and upper bound solution, and the shape of failure plane were analysised.