Risk Evaluation And Surrounding Rock Stability Of Large-span Shallow Tunnels Under Unsymmetrical Pressure

The surrounding rock of shallow tunnel is cracked, fractured and unstable. For theasymmetry force caused by biasing effect, the early support even the secondary liningmay crack. Then the bearing capacity of tunnel construction decreased and the waterproof&drainage system is destroyed, which will lead the water leakage in tunnel. Itis often occurred that roof collapse, landslides, water gushing and other hazardsduring excavation process and water leakage during operation. Thus surrounding rockstability analysis and construction risk evaluation of large-span shallow tunnels underunsymmetrical pressure turn to the major issue to be solved.By Yangang tunnel in Guangshen expressway, the mechanical effect of shallow tunnelunder unsymmetrical pressure was analyzed. Based on extenics theory, the evaluationmatter-element model was established, and risk evaluation was performed. Applyinggeostatistical kriging method, reliability analysis of tunnel surrounding rock stabilitywas carried out. Among them, reliability analysis by Kriging method is the focuspoint in this paper. The main research contents are as follow:(1) By the numerical simulation through FLAC3D, it is found that, because of thebias effort, the size of plastic zone in deep buried region is obviously larger than thatin other place. When the bias rate of tunnel is1.09,1.13and1.22respectively, thearea plastic zone on the right side, particularly the right wall and haunch, is increasedwith the bias rate. When the rate is1.22, the range of the plastic zone reaches themaximum area. According to the analysis of the numerical simulation, to the plasticzone, the supporting measure must be reinforced to optimize construction and makethe surrounding rock remain stability.(2) Eight risk factors of large-span shallow tunnel with unsymmetrical forces weredetermined, including bias ratio, geology ratio, water condition, buried depth,integrality index of rock, attitude of rock, fracture between layers and rock level.Based on the elementary matter-element principle in extenics, the eight-indexcomprehensive matter-element evaluation model was constructed. The quantitativelyanalysis was preformed to qualitative index and the normalization preprocessing wasdone to evaluation index, which make the evaluation index is comparable.(3) The random variable was expressed by different variation function in Kriging method. With the quadratic orthogonal method, the reliability of tunnel lining wasstudied. The parameter obtaining method based on the tunnel seismic prediction wasproposed, overcoming the hard deterministic of tunnel mechanics parameter. Inpractice, the accurate reliability of tunnel lining can be obtained, proving therationality of the presented approach.