Study On Mechanical Properties And Construction Technology Of Rock Pillar In Super Lager Section And Small Spacing Tunnel

In recent years,with the growth of the national economy and the acceleration of urbanization,a large number of two-way eight-lane super-large cross-section tunnels have emerged in China.Due to the limitation of terrain,they have to be designed as small spacing tunnel structures.The existing research shows that the stability of the middle rock pillar is directly related to the success or failure of the design and construction of such tunnels.Therefore,it is very necessary to study the deformation characteristics of the middle rock pillar,the reserved width of the middle rock pillar and the reinforcement method during the construction of such tunnels.Based on the Yantongshan two-way eight-lane tunnel project in Shahao section of Century Avenue in Zhongshan Torch Development Zone,this paper deduces the surrounding rock pressure acting on the middle rock pillar by using the process load design concept,establishes a simplified mechanical model of the middle rock pillar and gives the calculation process of the stress of the middle rock pillar.The extended SMP failure criterion is introduced to evaluate the stability of the middle rock pillar.A three-dimensional numerical analysis model was established to determine the reasonable range of the reserved width of the middle rock pillar and the reasonable reinforcement method.The deformation characteristics of the middle rock pillar are studied and compared with the field monitoring results.The following conclusions are obtained :(1)A simplified analysis model of the middle rock pillar is established.The stress calculation formula of any point inside the middle rock pillar is derived by taking the surrounding rock pressure derived from the process load design concept as the boundary condition.The theoretical analysis results are consistent with the numerical law,which verifies the feasibility of the theoretical model.(2)The extended SMP failure criterion considering the influence of intermediate principal stress is introduced,and the ultimate strength of the middle rock column under plane strain state is calculated.Compared with the actual stress calculated by the theoretical method in this paper,the results show that the middle rock column is in a stable state.(3)In the case of no targeted reinforcement of the middle rock pillar,the reserved width D of the rock pillar in the grade III surrounding rock should be greater than 0.5B(B is the excavation span);the reserved width D of rock pillar in grade IV surrounding rock should be greater than 0.6B.(B is the excavation span);the reserved width D of the rock pillar in the Vlevel surrounding rock should be greater than 0.7B(B is the excavation span).(4)For the small spacing tunnel with shallow buried depth of grade V surrounding rock,it is recommended to consider the combined reinforcement scheme of grouting and tension prestressed anchor.For the small spacing tunnel with higher surrounding rock level,one of the two can be selected from the perspective of economy and convenience.(5)Tunnel excavation has a significant spatial effect.The distance between the excavation guide hole and the middle rock pillar is directly related to the influence degree of the middle rock pillar.The closer the excavation guide hole is to the middle rock pillar and the larger the contact area with the middle rock pillar,the greater the influence degree.The deformation of the middle rock column vault is large,and the stress concentration is easy to occur at the wall waist position.The construction should be focused on,and local reinforcement should be carried out when necessary.