Study On Risk Assessment Of Rockfall Impact At Tunnel Entrance Section And Energy Dissipation Shed-tunnel Structure

Rockfall disaster is one of the three major geological disasters in China,and the tunnel entrance section belongs to the frequent area of rockfall disasters,once it happens,the consequence will be more serious.In this paper,theoretical derivation,probability analysis,numerical simulation,software programming,case checking and other methods are used,the establishment of rockfall risk assessment system in tunnel entrance section,prediction of rockfall impact trajectory,accurate calculation of impact force and penetration depth,and design and optimization of energy dissipation shed-tunnel structure are studied.(1)Firstly,a preliminary evaluation card of rockfall risk is constructed from two aspects of "rockfall potential" and "rockfall historical events".Secondly,the RRES evaluation system of rockfall risk assessment is established by combining the five aspects of "topographic factor","geological factor","meteorological and hydrological factor","dangerous rock factor" and "highway factor".The risk assessment system of "collapse-impact" of rockfall hazard at tunnel entrance is established by using AHP-FUZZY evaluation method,which includes "evaluation system of slope dangerous rock collapse" and "evaluation system of the collapsed rockfall impact to tunnel entrance section".The probability and loss calculation formulas of seven disaster-bearing bodies hit by the rockfall are given.Considering casualties and economic losses by rockfall disaster at tunnel entrance,the "S-N judgment method" for life loss and the "J-M judgment method" for economic loss are established.“Disaster Prediction-Risk Assessment-Loss Assessment System for Rockfall Disaster at Tunnel Entrance Section”(DRLSRTES V1.0 software system)is compiled.(2)Based on Guan Baoshu’s method,the sinusoidal integral method is used to obtain the solution of the rockfall impact force and penetration depth at the tunnel entrance.Based on the rockfall impact the soil cushion of the shed-tunnel,the cavity expansion method and energy conservation method for calculating the impact force and penetration depth are given.Considering the influence of soil cushion material,cushion thickness,rockfall mass and rockfall velocity,120 groups simulation experiments were carried out.The magnification factor of impact force is obtained by comparing the numerical simulation results with the results of Guan Baoshu’s method,and the LS-DYNA formula of impact force is given.(3)Four stages of punching shear failure of shed-tunnel roof caused by rockfall impact are put forward.The maximum stress formula of shed-tunnel roof is deduced by theoretical method.On the basis of the penetrating block formed in the roof,the formula of the retardation effect of the shed-tunnel reinforcement is deduced by theoretical method.ROCKFALL software is used to analyze the trajectory and impact energy of No.A1 rockfall.LS-DYNA numerical simulation software is used to study the mechanical response laws of the main structure of the shed-tunnel(including roof structure + column structure + column structure foundation + side wall foundation + transverse tie beam).The mechanical responses of shed-tunnel roof and column structures are studied under different impact velocities,impact time,impact positions and incident angles.(4)Using LS-DYNA numerical simulation method,Based on the composite shed –tunnel structures of "Sand+EPE+Roof" and "Sand + EPE + Roof + Rubber bearing",mechanical properties of different structural compositions under different impact conditions are analyzed systematically.(5)The design flow,division of design condition and recommended parameters of energy dissipation shed-tunnel structure at tunnel entrance are given,and it is applied to the project case.