Model Test And Stress Analysis Of Tunnel Pipe Roof Pre-supporting In Weak Surrounding Rock

The problems some special and difficult areas face, such as the condition with weak broken surrounding rock, the condition shallow buried and in large eccentric compression, the condition with strong groundwater activity, can be solved effectively by the leading pre-supporting technology of pipe roof which is the main method of tunnel excavation. However, there are still a lot of problems existing in the engineering practice. For example, the supporting force and the supporting effect of pipe roof are not quantized and the design of pipe roof pre-supporting technology is not optimized. So theoretical analysis, model test and numerical simulation are all applied for the study and the main work and achievements are listed as follows:(1)The mechanics model and the calculation parameters are determined when the factors are considered such as the tunnel depth, the conditions of pipe roof and the mechanics features of surrounding rock. The mechanics models of pipe roof are respectively established on the longitudinal and lateral direction. The method on the longitudinal direction is on the basis of Pasternak elastic foundation beam theory and the method on the lateral direction is on the basis of micro-arch theory to duduce the limit supporting force of pipe roof.(2)The stereo similar experimental model system is autonomously designed to realize the longitudinal excavation process. The inside model experiment which includes36pieces divided into12groups, is applied to analyze the failure process of the surrounding rock during the tunnel excavation under different arrangements of pipe roof, different cover-span ratios, different construction methods and different footages. Besides, The PIV technology is applied and proved in the experiment.(3)It is proved trough the experiment that the collapse can be avoided and the maximum ground settlement is decreased by59.5%～76.8%with the use of pipe roof pre-supporting. The width parameter of settling tank K is suggested in the Peck formula for ground settlement trough the experiment. The K is increased0.15under the pipe roof and it is larger in some range when the arrangement of pipe roof is tighter. The failure process of the surrounding rock during the tunnel excavation and loading is analyzed.The load surrounding rock can bear under the supporting force of pipe roof is increased by once and the failure form is changed from the collapse of vault to the extruding spread of sidewall. (4)The numerical simulation Flac3D is applied to analyze the horseshoe tunnel of Nanjing metro north-south line phase diagram from Zhujianglu Station to Gulou Station to Xuanwumen Station. It is proved that with the use of pipe roof pre-supporting the maximum ground settlement is decreased by74.3%and the maximum vault displacement is decreased by69%. Besides, the results of numerical simulation are the same as the model experiments. And it is proved that the ground deformation can be effectively controlled by pipe roof pre-supporting compared with the field test data. The plastic zone development and the initial support force of surrounding rock are different whether considering pipe roof or not. The internal force of pipe roof at every construction step can be obtained.The results show that the failure form of the surrounding rock, the ground settlement and the limit loading are obviously influenced by the different arrangements of pipe roof when the cover-span ratios, the construction methods, and the footages are different.