Study On Force Characteristics And Optimized Type Selection Of Open-cut Tunnels Under Debris Flow Impact

With the improvement of national economic and technological level,more and more tunnel projects have been conducted.Whether in new construction or in expansion,the tunnel engineering standards have been upgraded,causing the tunnel site selection more difficult.In some cases,a tunnel has to cross unfavorable geological sections,which results in hazards to open-cut tunnels caused by debris flow impact.Although scholars at home and abroad have done researches and achieved certain results in the formation and prevention of debris flow,detailed researches,on the whole,in the impact characteristics of debris flow to open-cut tunnels are relatively limited.Therefore,carrying on research in the force characteristics and safety issues of debris flow on different open-cut tunnel structures is of great significance for the structural forms and engineering applications of open-cut tunnels.In this study,a combination of field surveys,numerical simulations,and laboratory tests is taken to work out the impact characteristics of debris flow on open-cut tunnels,as well as the safety and economy of open-cut tunnel structures.The main conclusions are drawn as follows:(1)Based on field investigations and analysis of relevant geological data,the distribution of main sediment sources,slope changes,stratigraphic characteristics,meteorological and hydrological information in the tunnel site area is determined.Through research and judgment,it is found that the right side of the tunnel exit has abundant sediment sources,the overall area appears significant changes in topographic slope,and rainstorms frequent in summer seasons.The tunnel site area suffers the risky conditions for debris flows,including sediment sources,topographic conditions,and triggering conditions.(2)Based on the flow characteristics of debris flows,Massflow software is used and the Voellmy basal model is selected.The DEM raster data of the tunnel site area is converted through ArcGIS to ASCII format to establish a 3-D motion model of debris flows in the area.The impact characteristics of debris flows with different return periods are simulated and analyzed by using the modified storm flood method.As the return period increases,the debris flow depth,velocity,and impact force all gradually increase.The maximum impact force of debris flow in a-hundred-year return period is 160 kPa,with extremely-high-risk and high-risk areas covering 457 m2 and 2699 m2,respectively.In all return periods,the open-cut tunnel is located in an extremely high-risk area.(3)Indoor similarity tests are conducted considering variables such as different solid volume fractions and densities of debris flows.Six sets of different working conditions are established to study the flow process,morphology,velocity,depth,and distribution of impact forces on the open-cut tunnel structure.The process of debris flow impact on the open-cut tunnel can be divided into three stages:impact,overflow,and deposition.When the density of debris flow reaches 1700 kg/m3,the maximum values of debris flow depth,velocity,and impact force are 10.6 cm,4.8 m/s,and 32.61 kPa,respectively.Based on the distribution of impact forces,the main protection for open-cut tunnel structure against debris flows should be focused in the area 6.42 m below the height of the open-cut tunnel ear walls.(4)ABAQUS finite element software is used to establish three open-cut tunnel models:the double-ear-wall type,the cut-and-cover type,and the single-ear-wall type.And the force and displacement of all the three types of open-cut tunnel structures are analyzed under the most unfavorable impact force load of debris flow.The cut-and-cover type is found damaged,while the single-ear-wall type and double-ear-wall type meet the safety requirements.Economical analysis is conducted by using the engineering quantity bill pricing method,and the cost of the open-cut tunnel in single-ear-wall type is 5.88%lower than that in double-ear-wall type.Considering factors such as safety,cost,and construction difficulty degree,it is suggested the open-cut tunnel in single-ear-wall type to be the optimal option.