Structural Stability Analysis Of Ultra-shallow Buried And Unsymmetrically Loaded Muti-arch Tunnel In Loess

In recent years,the transportation engineering in western China has developed rapidly.Due to the mountainous terrain in the region,tunnels have become a better choice for the construction of transportation facilities.The multi-arch tunnel is a relatively new and special structure form in highway tunnels,with the characteristics of multiple construction processes and complex changes in supporting structure forces.In recent years,many different forms of multi-arch tunnels have appeared in the loess distribution areas of central and western China.Based on the Zhuohe Highway Nieye Tunnel,this article selects typical cross-sections to study the stress and strain of the supporting structure through in-situ monitoring using soil pressure boxes,surface strain gauges,steel reinforcement gauges,and concrete strain gauges.The numerical simulation method is used to simulate the entire construction process of the tunnel,analyze and study the distribution laws of the surrounding rock stress and deformation and supporting structure stress,and verify the rationality of the numerical simulation by comparing it with the in-situ monitoring results.Based on this,the stress and deformation of the middle wall,the deformation of the tunnel structure,the differences between early-constructed secondary lining and in-situ construction methods,and the deformation suppression effect of rotary jet piles on loess multi-arch tunnels are analyzed and studied.Finally,the stability of the multi-arch tunnel is judged,and the influencing factors of the structural stability of the ultrashallow-buried and large-unsymmetrically loaded loess multi-arch tunnel are summarized.The results show that:(1)Surrounding rock pressure and steal arch frame stress increase rapidly within 10 days after excavation.The surrounding rock releases most of the load within 10 days.The maximum value of surrounding rock pressure,272.1 k Pa,appears at the right arch shoulder of the right tunnel,and the maximum value of steel arch frame stress,277.9 MPa,appears on the inner side of right arch shoulder of the right tunnel.The worst construction time for the two is the period from the excavation of the upper step in the right tunnel to the excavation of the upper step in the left tunnel.The proportion of load carried by the secondary lining is smaller than that of the primary lining.The contact pressure,axial force of the secondary lining steel bar,and concrete strain develop stably after the secondary lining trolley is removed.The upper structure construction of the secondary lining has little effect on the primary lining stress and the surrounding rock pressure.The most unfavorable positions of each monitoring item are all in the right tunnel.The secondary lining on the right side of the inverted arch in the middle of the right tunnel may crack and fail,so special attention should be paid during construction.(2)The construction of the middle guide hole and the middle wall has little influence on the stress and displacement of the surrounding rock in the main tunnel.The most unfavorable positions for the stress and displacement of the surrounding rock during the excavation of the upper step in the main tunnel are the middle step and the face of the lower step.The excavation of the upper step in the right tunnel has the most significant impact on the stress and displacement of the surrounding rock,with 50% of the stress released after excavation.The impact of the excavation of the left tunnel on the surrounding rock is relatively small compared to that of the right tunnel,but it will affect the horizontal stress of the soil around the right tunnel.The maximum surface settlement value of the target plane is 39.42 mm,appearing 24 m to the right of the central line of the structure.There is a slight difference between the numerical simulation results and the in-situ monitoring results,but the development and distribution laws are very similar.The numerical simulation results of this article have research reference value.(3)According to the numerical simulation,the construction of the main tunnel has almost no effect on the surrounding rock pressure and the internal forces of the supporting structure.For the shallow-buried and unsymmetrical loaded multi-arch tunnel in loess,the middle wall and the supporting structure of the middle pilot tunnel are always under biased pressure.To prevent the middle wall from overturning,the distance between the two tunnels should not be too large.The middle wall eventually has a deviation of 0.1°,which is mainly influenced by the biased terrain.During the construction process,the impact of the step excavation support on the internal forces and deformations of the middle wall is the largest.(4)The first three stages of step excavation are the most unfavorable time for rock deformation.The deformation of the surrounding rock develops rapidly,and the settlement of the arch crown exceeds 80% within 10 stages.The settlement of the right tunnel with greater depth is larger than that of the left tunnel.The effect of strengthening the foundation with rotary jet grouting piles is remarkable in suppressing the deformation of the loess tunnel.The convergence deformation of the right tunnel is reduced by 90.8% compared to that without reinforcement.The shallow-buried side convergence deformation is larger after reinforcement,which is about twice that of the deep-buried side,but the deformation does not exceed the control benchmark.Based on the safety level determination,the most unfavorable position of the steel arch frame is the right shoulder of the right tunnel.The secondary lining of the main tunnel is in a safe and stable.The stability factors of the multi-arch tunnel mainly include the mechanical properties of the surrounding rock,the stability of the middle wall,the safety and stability of the supporting structure,environmental factors,construction process,and construction methods.(5)Based on the analysis of in-situ monitoring and numerical simulation results,the construction of the secondary lining has little impact on the surrounding rock stress and deformation and will not cause instability of the surrounding rock.The construction of secondary lining has little impact on the stability of primary lining.The maximum stress growth rate of the steel arch frame after construction is 692.1%,but the stress value at that position is relatively small.The stress change is not significant or decreases in most positions.The lateconstructed secondary lining method used in-situ has a significantly better surface settlement performance.Compared to the timely construction of the secondary lining method,it shows little difference in surrounding rock stress and displacement and supporting structure.The secondary lining bears less stress and mainly serves as a long-term safety reserve.It can be considered feasible to complete the upper structure of the secondary lining for the entire section simultaneously after tunneling,primary lining,and the inverted arch are all constructed using the in-situ method.