A Study On Key Problems Of The Multi-arch Tunnel Design

Since 1990s, there have been more and more highways constructed in the western China. In the mountainous area, the multi-arch tunnel becomes the engineer's first choice due to the difficulty of the route selecting and other factors influence. Since China is a mountainous country, the multi-arch tunnel with its own advantages will have a very broad application of highway construction in the future. However, the design and construction experience of the multi-arch tunnel is limited, and we do not have the robust mechanics of the multi-arch tunnel theory. Currently, there are some noticeable problems in the multi-arch tunnel. It becomes the obstacle to the wide application of the multi-arch tunnel. It is very important and urgent to do the theoretical research for the wide application of the multi-arch tunnel. The followings are included in this paper.The application conditions of the multi-arch tunnel with advantage and disadvantage are studied by comparison of the separate tunnel and neighborhood tunnel in terms of the technology and the economics. The application conditions of the multi-arch tunnel's cross-section with advantage and disadvantage are analyzed by both theoretical method and computational method. The conclusions of this research are the foundation of the application of the multi-arch tunnel.Considering the load design method of the separate tunnel as well as the criteria of the deep or shallow depth, the theoretical method and the numerical method are studied. When the load-structure method is employed to conduct the structure computation, the influence parameter of the multi-arch tunnel is considered. The load design computational method for the multi-arch tunnel and the criteria of the deep or shallow depth are the foundation of the multi-arch tunnel design.Based on the theory of basic structural mechanics and according to the specific characteristics of the multi-arch tunnel with the integral middle-wall and the three-layered middle-wall, the internal force model and the method for the primary support, secondary lining and middle-wall are developed by the load-structure method.By the finite element analysis, the computational method of the middle wall internal force in the multi-arch tunnel by the stratum-structure method are developed, which is the foundation of the middle wall design. The followings are studied in the detail:construction sequences of the drift heading, parameter of the stress release, dimension of the drift heading, thickness of the middle wall, the depth, and the influence of the middle wall load capacity by the contact condition between the middle wall and the top surrounding rock. The results from this method are the same as the results from the load-structure method in the engineering application.The effect on the middle wall structural behavior by the middle guide tunnel, thickness of middle wall, depth, and the contact condition between the middle wall and the surrounding rock was analyzed in detail. The results showed that the effect of middle guide tunnel on the middle wall is not significant; the axial force in the middle wall increased with the thickness of middle wall, and the depth; the earlier the contact between the middle wall and the surrounding rock, the larger force in the middle wall. So the material with large elasticity should refill the middle wall as soon as possible in the condition of the weak surrounding rock. However, for the strong surrounding rock, the appropriate clearence is needed on the top of the middle wall to postpone the middle wall under loading and exploite the rock's structural capacity.Using the 3D finite element analysis, the mechanic characteristics of the multi-arch tunnel with the three-layered curved middle-wall during the construction stage are researched in terms of the strength and the stability. The results shows that the stability of the middle wall is good and the temporary support of the middle wall is not needed. Since the stability parameter of the middle wall in the V surrounding rock is small, the integral middle wall shall have the temporary support in the V surround rock. Considering the distribution of the plastic zone in the surrounding rock, displacement of the surrounding rock, and the mechanical characteristic of the support structure, the construction requirements and procedures are developed.Based on the field testing of the separate tunnel, as well as considering the structural characteristics of the multi-arch tunnel, the mechanical characteristics and the development of the supporting system in the multi-arch tunnel are studied theoretically. Testing results shows that the supporting forces developed quickly and became stable during the construction stage. The capacity of the steel support is not exploited fully. The grid steel support should be the first choice in the condition of the stable surrounding rock. The middle wall in the supporting system is the main structural system. The stability of the middle wall is good enough. The refill or the support is not needed in the middle guide tunnel. The control criteria of the displacement convergence in multi-arch tunnel is developed by correction of the control criteria of the displacement convergence in the separate tunnel. The convergence computational method is corrected according to the structural characteristics of the multi-arch tunnel. The theoretical analysis is demonstrated by several field testings of the multi-arch tunnel. The theoretical computational method for the multi-arch tunnel is correct because the field testing of the middle wall internal force is the same as theoretical results. According to the measurement of the displacement convergence, the control measurement criteria for multi-arch tunnel is reasonable for the forgoing tunnel of multi-arch tunnel with integral middle-wall, but the different control criteria should be used for the following tunnel. According to the structural characteristics of the multi-arch tunnel, the method of field testing for the multi-arch tunnel is improved; the displacement convergence computational method is corrected, the displacement convergence criteria for the multi-arch tunnel is checked.Since the limitation of the literature review, some conclusions are not demonstrated fully and future research is needed. This paper analyzed the key problems of the multi-arch tunnel design in details, and the results will contribute to the wide application of the multi-arch tunnel.