Pressure Arch Theory Study Of Horizontal Bedded Tunnel Surrounding Rock

With the development of High-speed railway and subway construction in China, it has brought opportunities and challenges of tunnel and underground engineering. Pressure arch effect is one of the basic mechanical phenomena existed in process of underground excavation, although it is recognized in surrounding rock of tunnelling in the 19th century. The natural equilibrium arch theory is created in the early 20th century, but in the absence of means for study, the pressure arch theory has been developed slowly. In recent years, with the development of numerical analysis and computer software and hardware, it is possible for further study of the pressure arch theory. So, the pressure arch of horizontal bedded surrounding rock in single tunnel and parallel tunnels are further studied by numerical analysis method, and its related applications are actively developed.(1) The popular calculation methods of plane and three-dimensional models, the method of semi-infinite domain boundary treatment and the practical method of the initial stress field simulation were described for the simulation of underground construction. The discrimination method of the pressure arch body in horizontal surrounding rock was defined by cross-sectional shape of cavity and the magnitude of the horizontal stress. The closed-loop body of pressure arch is formed just when the maximum displacements of the major positions of cavity after excavation of tunnel is similar, which was proved by a large number of numerical models. There are two criterions to evaluate the similar maximum displacements in the major positions of cavity:first, the ratio of the maximum settlement in the crown and the ratio of the maximum uplift displacement in the invert to the maximum horizontal displacement in the side wall are both less than 1.6 when the maximum horizontal displacement in the side wall is less than the maximum settlement in the crown and the maximum uplift displacement in the invert; second, the ratio of the maximum uplift displacement in the invert and the ratio of the maximum horizontal displacement in the side wall to the maximum settlement in the crown are both less than 1.6 when the maximum settlement in the crown is less than the maximum uplift displacement in the invert and the maximum horizontal displacement in the side wall. Whether pressure arch is a closed-loop body or not is depending on whether increases zone of strain energy within the surrounding rock can cover the entire cavity cross section, and whether the distribution of critical line of shear stress is cross-shaped. This conclusion has been confirmed by a large number of numerical models. The outer boundary of the pressure arch is a flow line (a segment curve or closed curve) of the maximum principal stress vector through some specific point. The basic criterion of drawing the approximate outer boundary of pressure arch was proposed:basically the outer boundary distribution of pressure arch and the maximum principal stress vector distribution in the surrounding rock must be tended to the conformity. The laws of pressure arch distribution within single horizontal bedded surrounding rock were studied in the aspects of the cavity burried depth, excavation width of tunnel, rock mass mechanics indices and construction methods.(2) The critical arching depths of single horizontal surrounding rocks were studied. The relationship between the surrounding rock classifications, the excavation section widths of tunnel and the natural or artificial critical arching depths was discussed, and the fitting equations between the natural or artificial critical arching depths and the excavation section widths of tunnel were established. The effect of physical and mechanical indices of surrounding rock on natural or artificial critical arching depth was analyzed. The overburden stratum coefficient (fu) and underlying stratum coefficient (fd) were proposed to study the effect of overburden or underlying stratum distributions of tunnel on the natural or artificial critical arching depths respectively. The values of overburden stratum coefficient (fu) and underlying stratum coefficient (fd), and the natural or artificial critical arching depth of main layer surrounding rock are used to study the natural or artificial critical arching depth of multi-layer horizontal bedded surrounding rock. The main factors affecting the values of overburden stratum coefficient (fu) and underlying stratum coefficient (fd), and the laws between overburden or underlying stratum distributions of tunnel and the natural or artificial critical arching depths were analyzed. The index formulas of critical depth for shallow burial and deep burial tunnels are established based on the critical arching of single horizontal surrounding rock for gradesâ…¡-â…¤surrounding rocks. With the example of horseshoe-shaped tunnel with 7.0m excavation section width, the study on critical depths for shallow burial and deep burial tunnels was carried out based on the critical arching of multi-layer horizontal bedded surrounding rock.(3) With the example of grade V surrounding rock and horseshoe-shaped tunnel with 7.0m excavation section width, pressure arch effect or arch beam action distribution of horizontal surrounding rock in parallel tunnels and effect of thickness and mechanical behavior of middle rock wall on pressure arch effect of surrounding rock or arch beam action in the case of 10-50m buried depths of tunnel were studied by using the finite element method. Arch beam structure and pressure arch are mechanical models which are abstracted from the angle of stress field. The distribution character of maximum principal stress and the law of load transfer are the same in the arch beam structure and pressure arch, but not the same formation mechanism, the role and effectiveness are different, the former is the beam action, and the latter is the pressure arch effect. The further study of compare between on arch beam structure and pressure arch was carried out. The thickness condition of middle rock wall in the formation of arch beam structure was proposed for deep buried tunnel. The concepts of union pressure arch effect and union pressure arch were proposed, and the thickness condition of middle rock wall in the formation of union pressure arch was pointed out. The feasibility of the various indices was discussed for the neighborhood tunnels classification, and the specific calculation methods of three classification indices were proposed. "Al class" subdivision below "A class" of neighborhood tunnels was proposed, which make it possible that deep buried neighborhood tunnels make use of union pressure arch theory to guide bolt and shotcrete supports design.(4) The ground displacement control principles were demonstrated by a large number of numerical models analysis based on ground loss control. The construction program optimization of the metro tunnel beneath the river and bridge and the method of safety evaluation of existing underground structures caused by adjacent construction were studied by numerical analysis method. In construction program optimizations, an integrated approach combining the use of engineering experience and mechanical analysis of tunnel construction based on numerical analysis method is emphasized. In the safety evaluation of existing underground structure, it has different features according to finite element and finite difference programs, a different analysis procedures are chosen and the results confirm each other is a very good idea.