Study On Toppling Deformation Mechanism And Stability Of Tonghua No.1 Tunnel Entrance Slope Along Wenchuan-Maerkang Expressway

Toppling rock mass on Tonghua No.1 tunnel entrance slope is located in Tonghua town,Lixian County,Sichuan Province,and at the K71+100m of the newly built WenchuanMaerkang expressway,on the left bank of the Zagunao River.The toppling rock mass is 130~140m long along the slope,120~140m wide along the river,120~130m in height,and 10~40m in horizontal depth,usually 20~40m,the volume is 470 thousand cubic metres.The stability of the rock mass is one of the important engineering problems in the construction and operation of Wenchuan-Maerkang expressway.Through the geological environment investigation for the toppling rock mass,the spatial distribution and deformation damage characteristics were identified,and the toppling rock mass was divided into different groups according to degree of toppling deformation,and the possible instability modes were analyzed.Through the quantitative stability calculation and UDEC numerical simulation,the following understandings are obtained:(1)The toppling slope is mainly made up of phyllite whose foliation is highly developed.The rock mass stands steeply,and the trend of the bedrock and slope is parallel nearly,which is a reverse slope.With the earth's crust rising,the Zagunao river cuts downwards,and the rock mass of the slope bends,bends-toppling,bends-pulls,untill the bending fracture formed,finally,the toppling rock mass slides.(2)A reasonable division of the toppling rock mass was carried out through field surveys.The attitude of normal bedrock in the study area is 355°?77°.According to the degree of deformation of the toppling rock mass,the toppling rock mass is divided to three groups: the falling rock mass,the strong toppling rock mass,and the weak rock mass.The dip of falling rock mass is less than 17°,and the dip of strong toppling rock mass is between 17° and 45°,and the dip of weak toppling rock mass is between 45° and 70°.(3)Qualitative analysis in the field showed that there was a possibility of strong toppling rock mass slip along its bottom interface,especially in the ridges.The failure mode was retrogressive slide,the lower part slides first,causing the upper part to lose its support and slide along the bottom interface of the strong or weak toppling rock mass.Therefore,in the calculation of the stability,the bottom interface of the strong or weak toppling rock mass is taken as the slip surface for analysis and calculation.(4)The stability of the toppling rock mass under natural,heavy rain and earthquake(VIII degree)conditions were calculated using the limit equilibrium analysis method.The results showed that the toppling rock mass was generally stable under natural conditions.While in heavy rainfall or earthquake conditions,there is a possibility of overall slip along the bottom interface of the strong toppling rock mass.(5)Analyze displacement and stress of toppling rock mass using UDEC.The results show that the displacement under natural conditions is small,plastic yield does not appear,and the slope is generally stable.While under strong rainfall and earthquake conditions,the displacement of strong toppling rock mass increases and nearly evenly distributed,and the overall slip may occur along the bottom interface of the strong toppling rock mass,the stability of the slope is poor.(6)UDEC numerical simulation results show that the upper toppling rock mass is deformed step by step after losing the lower support.The overall destruction mode of the slope is “compression deformation at slope foot?upper toppling and gradually backwards?the slipping surface gradually penetrates ? overall slip of the toppling rock mass”.