Study On Safety Risk Assessment And Countermeasures For Portal Excavation Of Collapsible Loess Tunnel

With the national strategy of "one belt and one road",Gansu,as an important pass of the Silk Road,has a direct impact on the overall implementation of the strategy.The construction of expressway will greatly reduce the resistance caused by traffic and will further improve the transportation network centered on Lanzhou.This paper takes Ziyunshan Tunnel of Dinglin Expressway as an example to carry out risk research.Considering that the overburden rock layer of Ziyunshan Tunnel is broken and collapsible loess is widely distributed,which greatly increases the difficulty of tunnel portal construction and there is a great risk in tunnel excavation.Based on the construction of inlet and outlet portal of Ziyunshan Tunnel and slope protection,the construction risk evaluation system for portal of Ziyunshan Tunnel is established by studying relevant data and existing documents.The risk category and weight proportion of each risk are found out and the risk level of tunnel portal during construction is evaluated.The research content and main achievements mainly include the following three aspects:Firstly,literature research and project investigation and design documents are consulted,relevant data of construction and management of Ziyunshan Tunnel are analyzed,risk sources of tunnel portal are clarified with risk management theory,major risk that may occur at the portal of collapsible loess tunnel are concluded.The risk index evaluation system is established by using the analytic hierarchy process from the aspects of design,hydrogeology,and engineering management.The risk factors of each index layer are ranked and the risk factors that should be paid attention to in risk sources management are determined.Based on the established risk index evaluation system,BP neural network is applied to evaluate the risk level of tunnel.Combining the monitoring data during the construction of tunnel portal,through training of neural network function,MATLAB programming calculation is carried out for the three major possible risk sources,the probability estimation and consequence loss estimation of safety risk are studied,the disaster-causing probability of the three main risk sources for the construction of Ziyunshan Tunnel portal is calculated,the overall risk value of tunnel portal construction is analyzed.According to relevant specifications and combined with specific engineering geology and construction environment,risk grade evaluation criteria are formulated to determine the risk grade for the portal construction of Ziyunshan Tunnel.According to the risk grade evaluation results of the portal construction of Ziyunshan Tunnel,it can be concluded that the excavation of the portal of Ziyunshan Tunnel belongs to medium-high risk projects.Combined with the project survey and design documents and the project construction and management,the risk control scheme related to large deformation of tunnel surrounding rock,collapse of portal,slope instability of portal is determined.The corresponding risk management scheme and on-site construction operation method are prepared to ensure the operability of risk sources prevention measures and emergency plan,to ensure that the economic loss of the project can be greatly reduced after the occurrence of the above risks,and to ensure the life safety of on-site construction operators.Through the risk management research of Ziyunshan Tunnel,the sticking point of excavation risk management of collapsible loess tunnel portal is determined by combining the risk management theory with the actual project.This paper summarizes a set of targeted project risk management methods to provide practical guidance for the follow-up similar projects.