Study On Key Technology Of Tunnel Construction In Shallowly Buried Pebble Soil

Due to the rapid growth of China ’ s economy and the continuous advancement of the western development,China ’ s infrastructure projects have developed rapidly.By the end of 2021,more than 30,000 km of highway and railway tunnels have been built,of which there are more than 19,000 highway tunnels and the total length of tunnels has exceeded 19,000 km.And with the evolution of tunneling project,massive tunnels have been built in mountainous areas,in which the tunnel is widely used as the main construction method.There are a large number of pebble soil mountainous areas in the central and western regions of China.When the tunnel passes through the stratum and is in the shallow buried zone,there are many cracks in the surrounding rock,which are mostly broken.Under the condition of rainfall infiltration,it is easy to produce water gushing,mud gushing and other disasters.Due to the shallow buried depth and poor lithology of shallow tunnels,vault collapse and even roof fall accidents are prone to occur during tunnel excavation,which brings hidden dangers to the lives and property of construction personnel.Therefore,the study of tunnel construction technology in shallow pebble soil layer is particularly important.This paper mainly studies :（1）The common construction methods of shallow buried pebble soil tunnel,including excavation technology and pre-reinforcement technology,are discussed and compared to study the surrounding rock lithology applicable to each construction technology.Based on the engineering experience and qualitative analysis,the corresponding suggestions are given for the selection of construction methods under different geological conditions.The results show that the surrounding rock grades of shallow tunnels are mostly IV and V,and the surrounding rock conditions can be divided into extremely broken,more broken and more complete.The following conclusions are drawn : 1 Double side guide pit method and combined pre-reinforcement technology are recommended for the broken surrounding rock of extremely broken shallow tunnels of IV and V;2 Under the hypothesis of ensuring the security,the surrounding rock of IV and V grade relatively complete shallow tunnels is mainly cost-oriented,and it is recommended to use the construction method of ring excavation reserved core soil and combined pre-reinforcement technology;3Grade IV and V broken surrounding rocks between the above two,it is recommended to use the reserved core soil method or the partition method and the combined pre-reinforcement technology.（2）A large number of on-site construction monitoring measurements were carried out on the Shiyangling shallow-buried cobble soil tunnel,and the deformation law of the tunnel under the ring excavation reserved core soil method and the combined pre-reinforcement technology was obtained by analyzing the data.The results show that : 1 The vault settlement of the Shiyangling shallow-buried cobble soil tunnel section presents a ’ two-stage ’ change,that is,’ quick-slow-fast-slow ’ trend,which is due to the use of the ring excavation reserved core soil method for excavation.When the upper ring area is excavated,the vault surrounding rock is disturbed and the first rapid deformation occurs;when the left and right parts are excavated,the arch foot of the ring primary support is in a semi-suspended state,so the second rapid deformation occurs.2 The peripheral convergence shows a trend of rapid first and slow later,because the peripheral convergence line is set at the joint of the arch foot in the ring part and the excavation of the left and right parts.When the arch ring excavation and the excavation of the left and right parts are affected by the arch foot,the peripheral convergence value has large deformation.However,the vault settlement and surrounding convergence will gradually tend to be gentle with the end of the surrounding rock stress release and the erection of the initial support arch.（3）Midas numerical simulation analysis software is used to construct the calculation model of shallow buried pebble soil section tunnel in Shiyangling combined with the field practice.Based on this model,four excavation techniques commonly used in shallow tunnel construction are discussed,and the displacement cloud images are compared and analyzed.It is considered that under the premise of unified construction of combined pre-reinforcement measures,the “ eye construction method ” has the best effect on controlling tunnel deformation,and the ring excavation reserved core soil method has the worst effect.The two partition methods are between the two,but the results show that in the case of shallow buried pebble surrounding rock in Shiyangling.The double side heading method does not have the advantage of overturning in controlling tunnel deformation.Therefore,it is proposed that the tunnel should adopt the job practice of ’ ring excavation reserved core soil method + joint pre-support + temporary inverted arch ’.（4）According to the numerical model of Shiyangling shallow tunnel,the construction method is further studied,and it is concluded that the effect of surface grouting is greater than that of advanced small duct in the combined pre-reinforcement technology used in the tunnel;combined with previous studies,the excavation technology of the tunnel is further simulated,and it is suggested that the excavation length of the core soil on the site is 2.5 m.（5）According to the idea of time series,based on the monitoring data of Shiyangling tunnel,the feasibility of cubic exponential smoothing method to predict tunnel displacement deformation is discussed,and the method is optimized in order to predict more accurately.The results show that it is feasible to predict tunnel displacement deformation by this method,and the correctness of the optimization formula is verified by the comparative analysis of the measured value,the predicted value and the optimized value.