Analysis on Parameter Optimization of Shed-pipe Advance Support in Portal Section at Xiaoxiangsai Tunnel

Because the stability of the surrounding rock is poor,the degree of the slope is big,and the thickness of the overburden is thin,the long shed-pipe is applied in construction for pre-reinforced supporting.Based on the initial design parameters of shed-pipe and using the 3D elastic-plastic finite element method,the parameters of range,space in the circumferential direction and the diffused radius of grout are optimized.The study result shows that there is a optimized value on the range,space in the circumferential direction and the diffused radius of grout.If the value of design parameter exceeds the optimized value,the settlement of the arch crown will converge to a constant quickly.So the blind increase of the design parameters can not obtain the ideal effect.