Study On Large Deformation Of Surrounding Rock And Its Control Measures Of Strong Weathered Sandstone Tunnel

Strong weathered sandstone is widely distributed in Ningxia,Gansu,Sichuan and other areas in China.The strong weathered sandstone has high strength under the condition of dry state or very low water content.When excavating tunnel in such stratum,the surrounding rock has good self-stability and it is not easy to deform.The construction safety of tunnel can be ensured by using general support system.However,if the water content of strongly weathered sandstone increases,the strength of rock decreases rapidly,and it is easy to soften and disintegrate.When tunnels are excavated in the strongly weathered sandstone which with high water content,the disasters such as collapse,lining cracking and large deformation will easily occur.If the construction methods and supporting measures of tunnels are not changed,it will be difficult to continue and may create a threat to construction safety of tunnel.According to the characteristics of strong weathered sandstone,the construction control measures for large deformation of strong weathered sandstone tunnel and other weak surrounding rock tunnel at home and abroad are summarized.The article based on the Piancheng Tunnel of Guyuan-Xiji Expressway in Ningxia Hui Autonomous Region,according to the field monitoring data,the deformation law and characteristics of initial support are summarized,the failure mechanism and influencing factors of large deformation are analyzed,and the construction control measures for the large deformation of the surrounding rock are put forward.Using FLAC³DD finite difference software to simulate the construction process of Piancheng tunnel,to verify the control effect of surrounding rock reinforcement measures for Piancheng Tunnel,and to compare with the measured data.At the same time,using FLAC^3D software,the effects of surrounding rock reinforcement measures such as system bolt,long bolt and improving the overall stiffness of support system on controlling tunnel deformation are discussed.?1?The surrounding rock strength of strongly weathered sandstone is mainly affected by its grain composition,cementation,weathering degree and water-rich degree.The excavation of YK25+172^YK25+195 section of Piancheng Tunnel reveals that the surrounding rock is strongly weathered sandstone,there are two bedding permeable interbeds on the face of the tunnel and seeps under the interbedding.During the excavation,serious deformation occurred in the initial support.?2?The occurrence of large deformation of surrounding rock of strongly weathered sandstone tunnel can be divided into three categories according to its causes.The first is related to the physical and mechanical properties of rock,such as the strength of sandstone particles and the strength of cementation force between particles.The second is influenced by natural environment,such as the topography and landform of tunnel,the depth of tunnel,the magnitude of initial geostress,bias pressure and seepage,etc.Third,it is influenced by human factors,such as the size and form of tunnel section,excavation method,support system strength,construction time of final lining,etc.According to the analysis of large deformation of Piancheng Tunnel,low strength of surrounding rock,seepage between rock masses,bias pressure of tunnel,disturbance in construction process and weak strength of support system are the causes of large deformation of surrounding rock.?3?The control measures of large deformation of strongly weathered sandstone tunnel can be achieved by choosing tunnel section shape with better force,adopting excavation method with less disturbance to surrounding rock,applying the concept of flexible support,and properly improving the stiffness of support system.In construction,the reserved deformation and construction time of final lining can be adjusted according to the change of geological conditions and monitoring data,the surrounding rock deformation can be effectively controlled.After the large deformation of Piancheng Tunnel,the reinforcement measures such as temporary inverted arch,temporary steel arch protection and 6m?89 large lock-foot bolt were adopted to stabilize the deformation of surrounding rock.In the subsequent excavation,measures such as micro-blasting,increasing support system stiffness,large lock-foot bolt,and accelerating the construction progress of final lining and inverted arch are adopted.It made the tunnel construction go smoothly and ensure the construction safety of the tunnel.?4?The numerical simulation results of FLAC^3D show that the deformation trend of tunnel vault settlement and level convergence coincides with the actual excavation trend.The seepage has a great influence on the stability of strongly weathered sandstone tunnel.If there is no seepage,the maximum value of tunnel vault settlement is only 76.3 mm and the level convergence is 35.4 mm,while the calculation of tunnel vault settlement based on actual geological conditions is 193.2 mm and the level convergence is reached to 214.5 mm.Through numerical simulation,it is also found that the effect of system bolt is limited in strong weathered sandstone tunnel,while long bolt can play a very important role in the reinforcement of surrounding rock,and the deformation of surrounding rock can be better controlled by improving the stiffness of support system.