Stability Analysis and Case Study of Shallow Tunnel Using Pipe Roof Support

Pipe roof system is widely used in the shallow tunnel as the support system. Although the diameter of pipe roof is diversity, the pipe roof system of small diameter pipe roof and small spacing steel grating is widely adopted in China. The stability mechanism of that pipe roof system is to form shed-scaffold system. In that shed-scaffold system, the function of pipe roof is to form micro-soil-arch in overlying soils between the pipes and to transmit the excavation loads. The steel gratings are the main bearing structure. To form the shed-scaffold system effect, three stabilities, stability of soils between pipes, stability of tunnel working face and stability of tunnel foundation, should be satisfied simultaneously. On that basis, the collapse accidents of Tianhengshan highway tunnel and Haide highway tunnel in China are analyzed. It shows that the stability of tunnel foundation would be overlooked easily and causes even overall collapse. At last, the measures to improve the stability are proposed.