Research On The Impact Of Loess-bedrock Landslides On The Structural Safety Of Operating Tunnels

Tunnel is a linear structure crossing regions with landslides or unstable geological formations.Engineering activities may disturb the rock and soil mass,altering their stress conditions,and triggering landslides that affect tunnel structural safety in serious cases.This thesis concentrates on operation tunnels for highways in the loess region of Shanxi Province.Field investigations,laboratory tests,theoretical analysis and numerical calculations were conducted to study the impact mechanism of soil-rock composite layer landslides on operational tunnels.The research resulted in the following findings:(1)A direct slip mechanical model based on energy conservation is constructed to address shear slip along the loess-bedrock interface,in addition to a landslide-tunnel mechanical interaction model.The mechanical model of the contact slip process was established using the Mohr-Coulomb strength criterion and taking into account the energy changes in the shear slip process at the soil-rock interface.To reflect the anisotropic characteristics of the contact surface,the two-dimensional interface was expanded to a three-dimensional scale,from which the corresponding mechanical model was derived.The study establishes a mechanical model of landslide pushover tunnels,taking into account the plastic behavior of foundation soil to enhance the elastic foundation beam model.The research analyzes the mutual feedback influence between loess-bedrock type landslides and tunnels utilizing an indoor large-scale landslide-tunnel model test system constructed independently.The mechanical model’s validation is achieved through the combination of numerical calculations and theoretical analysis.(2)According to the shear slip characteristics of loess-bedrock interface,a mechanical model of interface slip based on energy change is constructed,and the mechanical model of landslide influence on tunnel is put forward.Under the influence of geostress,the loess-bedrock interface exhibits different movement characteristics,of which time-dependent slip is a typical movement mode.According to the changes in the force characteristics of the landslide,the slip body undergoes a time-dependent slip motion along the soil-bedrock interface,which is defined as continuous slip and intermittent slip according to its different slip forms.Based on the rock body viscous slip mechanical model,the interface elastic-viscous-plastic shear slip mechanical model based on Maxwall creep model is constructed,and the theoretical curve of the mechanical model is expressed as a segmented function.On this basis,the energy damage model of landslide body-tunnel structure is established,and a structural damage variable method is proposed to evaluate the effect of landslide timeliness on tunnel structure.The mechanical model was verified through indoor modeling tests,combined with theoretical analysis and numerical calculations.(3)Through numerical simulation and theoretical analysis,the mechanism and deformation mode of loess-bedrock landslides are revealed,and the influence mechanism of landslides on the deformation of tunnel structure is proposed.The disaster-causing mechanisms of high-steep,shallow-buried and deep-buried slopes were investigated by means of satellite remote sensing image identification,on-site investigation and indoor experimental study.The seepage-stress catastrophe mechanism of the high steepness landslide,the laminar catastrophe mode of layer-by-layer destruction of the shallow buried landslide,and the progressive destruction mode and slip zone formation mechanism of the deep buried landslide were revealed.Under the influence of landslides,the tunnel structure shows the characteristics of cantilever beam deformation,with uniform and coordinated deformation on the section,overall downward or upward deformation,and the largest deformation in the region close to the soil-rock interface.The damage characteristics of outward tilting of the portal wall under the action of landslide occurred.In addition,the deep sliding will also induce the soil body at the top of the tunnel to have a secondary landslide,which is the main factor causing the deformation and damage of the tunnel structure.(4)Based on the deformation characteristics and mechanisms of landslides and tunnels,a technical system of anti-slip prevention and control for loess-bedrock landslides and tunnel structures has been established.Corresponding anti-slip protection measures are proposed for the landslide-tunnel system under the influence of intermittent sliding,including anchor cables,anti-slip piles,retaining walls,micropiles,etc.,and the controlling effect of various anti-slip protection measures on tunnel deformation is evaluated by numerical simulation.The landslide-tunnel support system with anchor cable as the core is proposed,and an adaptable system of auxiliary protective measures is constructed according to different construction conditions,project scale and geological conditions.Anti-slip pile support is prioritized in soil layers with high pile-side friction resistance,while gravity-type portal retaining wall is used to control the uneven deformation of the tunnel structure,and miniature piles are set at the portal foundation to reduce the settlement deformation of the foundation soil at the tunnel portal,and the effectiveness of the protection system is further verified by numerical methods and on-site engineering examples.The effectiveness of the protection system is further verified by numerical methods and field engineering examples,which provides certain guidance for similar projects.