| Along with the implementation of western development strategy, geological hazard of loess projects has occurred frequently to infrastructure construction, ecological environment improvement projects and construction of national defense in western regions. These problems can be solved only by the development of loess mechanics theory and engineering technology. On the basis of previous research, deformation properties of loess and damage localization of loess multi-arch tunnel are researched by theoretical deduction, experimental test, program composition, and numerical simulation. The main works of the thesis can be drawn as follows:(1) According to Q2 loess conventional triaxial compression test curves in different stress path, it can draw the relations of shear stress q and shear strainεs, relations of body strainεv vs shear strainεs. Besides, the change rules of Elastic constants against confining pressure, peak strength and remain strength against confining pressure are obtained.(2) According to Q2 loess conventional triaxial compression test results in different stress paths, the plastic potential function is adopted as hardening function and softening function for the reason that both the hardening parts and softening parts have the same approximate ellipse. Assume that plastic flow rule is accorded with the associative flow rule. The plastic potential function is adopted as hardening function and softening function, the elasto-plastic constitutive model of Q2 loess in strain space is set up.(3) With the deficiency of elasto-plastic constitutive model of Q2 loess, the total deformation is divided into elastic deformation, plastic deformation and irreversible deformation by degraded stiffness. Assume that the plastic deformation and irreversible deformation obeys Hyushin's postulate, and the plastic deformation is coupling with irreversible deformation, according to internal variable theory, elastic-plastic theory in strain space and damage mechanics theory, plastic work is calculated by separating strain of conventional triaxial compression test results. And the loading function is obtained with the plastic work as hardening parameter. And then, according to the damage potential determination method of Desai model, masonry model and Zhao Xihong model, damage yielding surface function is obtained with the parameter of damage energy density. Further more, evolution law of damage is deduced on the basis of correlative parameters simulated from test data. And the elasto-plastic damage constitutive model of Q2 loess in strain space is found.(4) Based on the plastic flow rule of FLAC3D and some corrections, the FLAC3D pattern of elasto-plastic damage constitutive model of Q2 loess is deduced. Utilizing secondary development function of FLAC3D, user-defined model is programmed with Microsoft Visual C++. The user-defined model is loaded in FLAC3D. The simulation results of modified Cam-Clay model and user-defined model to failure mode of test sample is compared.(5) The defining of surrounding rock damage degree (SRDD) is brought forward, and its computational expressions are deduced. It is then programmed with FISH language.(6) Based on project of Lishi loess multi-arch tunnel, the damage localization process of dynamic construction of loess multi-arch tunnel is simulated. By analysis on the SRDD distributing and its evolvement of two excavating scheme, the better scheme can be obtained. And it is compared with the practice construction of Lishi loess multi-arch tunnel to prove its creditability.(7) A simple evaluation system of surrounding rock stability is set up by adopting analysis methods and discriminated methods of tunnel engineering by combining with analysis and eyeballing of damage localization numerical simulation of tunnel engineering, and New Austrian Method monitoring of tunnel.
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