Experimental And Numerical Simulation Of Excavation Process And Microstructure Study

With construction and development of some large important engineering projects, such as high-rise buildings, subways, nuclear power stations and so on, deep excavation has been a critical question in geotechnical engineering. In unloading condition, the microstructure characteristics and macroscopic mechanical properties for a specified soil are the basis of determining parameters required to simulating analysis and design of deep excavation. For a long time, parameters of shear strength of soil used in design of excavation are obtained by normal triaxial test. Unloading condition in the excavation process is usually ignored. It results in the difference of measured earth pressure and displacement and calculated value on the supporting structure of excavation.Based on the reality characteristic of excavation, microstructure characteristics of soil after lateral unloading are studied by applying the optical microscope and the fractal theory. Compared with normal triaxial test results, triaxial tests and numerical calculation of simulating deep excavation have been studied. The main research content and achievement have been obtained as follows:1. According to the realistic stress path of deep excavation, normal triaxial apparatus of strain control was transformed and triaxial test conforming to deep excavation characteristic was designed. These tests were done with saturated soil that located in the same site and the different depth of Tianjin urban districts. The results of this triaxial tests were compared with that of normal triaxial tests. The study indicated that the parameters of shear strength of soil were different in the two types of triaxial tests, and that the different stress path will affect the parameters of shear strength of saturated soil in Tianjin urban districts.2. The shear strength of soil is closely related with its microstructure. Based on the microstructure of soil, pore size and distribution of soil in the different stress path tests were studied by optical microscope. The reasons of the difference of shear strength parameters were explained quantitatively according to the microstructure of undisturbed soil sample.3. Because the fractal theory is suitable for the study of complex and irregular figure, the functions of studying microstructure of soil were analyzed. The plane fractal dimension used to study the pore distribution was defined. The program was design with MATLAB and the plane fractal dimension of microstructure figures of soil were calculated. The fractal characteristic of soil was analyzed in the different stress path tests.4. According to the triaxial test results, typical practical example of deep excavation was analyzed by finite element program of Plaxis. In the program, Mohr-coulomb yield condition was used and the excavation process was simulated in several separate excavation stages. Finally, the calculation results were obtained with the different parameters of shear strength. Some useful suggestions were provided for selecting the parameters of shear strength of soil in the design of deep excavation.