Model Test Study On Collapse Failure Mode Of Vault Surrounding Rock Induced By Tunnel Construction In Karst Area Based On PIV Technology

In order to build a modern integrated transport system,China is vigorously building roads and railways in the central and western regions.Since the central and western regions are dominated by plateau and mountainous terrain,and there are a large number of karst landforms,it is very common that there are underground karst caves in the vault of the new tunnel.When the distance between the new tunnel and the cave in the vault is close,the rock between the tunnel and the cave may collapse under the action of blasting shock wave,and the surrounding rock of the vault of the tunnel will collapse into the tunnel under the superposition of gravity and cavity pressure,resulting in serious engineering accidents.Therefore,it is of great theoretical significance and engineering practical value to study the failure mode of tunnel vault surrounding rock collapse induced by tunnel construction in karst area and explore the failure range and gradual change law of tunnel vault surrounding rock collapse induced by construction disturbance.In this paper,the scale model test based on particle image velocimetry(PIV)is used to systematically study the failure mode and evolution law of vault surrounding rock collapse induced by tunnel construction in karst areas,combined with theoretical analysis and numerical simulation.The main research contents are as follows:(1)Based on the basic principle of elastic mechanics,the similarity ratio between the prototype values of physical and mechanical parameters of the model test and the model is derived.By selecting barite and river sand as aggregate,gypsum as cementing material and borax as additive,through similar material ratio test,the material ratio meeting the strength requirements of grade Ⅳ and Ⅴ surrounding rock is obtained.(2)Using the similar material ratio of grade Ⅳ and Ⅴ surrounding rock determined by material test to prepare grade Ⅳ and Ⅴ surrounding rock,the model test of tunnel vault surrounding rock collapse induced by construction is carried out.PIV technology was used to process the deformation images of surrounding rock taken by high resolution camera during the test,and the failure range and evolution law of surrounding rock collapse induced by tunnel construction in karst area were obtained.(3)Using the finite difference software FLAC3D,a three-dimensional finite difference model which is consistent with the model test conditions is constructed.By simulating the failure process of surrounding rock collapse induced by the construction of tunnel vault with karst cave,the numerical solution of the collapse failure surface of the middle rock layer between the karst cave and the tunnel is obtained,and the validity of the model test results is verified.(4)By using the upper bound theorem of limit analysis and variational principle,the analytical equation of collapse failure surface of surrounding rock of tunnel vault induced by construction in the presence of karst cave at vault is derived,and the shape of collapse surface is plotted.By comparing the collapse surface shape of the surrounding rock at the top of the tunnel obtained by the scaled model test with the theoretical calculation results,it is proved that the model test based on PIV technology in this paper can be used to analyze the failure mode and collapse range of the surrounding rock at the vault induced by tunnel construction in karst areas.