Mechanical tests on bearing capacity of steel pipe-frozen soil composite structure applied in Gongbei Tunnel

The freezing-sealing pipe roof (FSPR) as an innovative pre-supporting method in tunnel engineering has been applied to Gongbei Tunnel of Hong Kong-Zhuhai-Macau Bridge, which is the first application in the world. The definition of FSPR is that large-diameter steel pipes are laid out in a circle around the cross section of the tunnel in advance, and then the artificial ground freezing method is adopted to the freeze soil between steel pipes to form waterproof curtain. To study the bearing capacities of the steel pipe-frozen soil composite structure and the appropriate temperature of frozen soil between steel pipes in the actual project, the mechanical tests on steel pipe-frozen soil composite structure are conducted by using a uniaxial compression testing system of frozen soil with four different temperatures. The criterion for the ultimate deformation state of steel pipe-frozen soil composite structure under waterproofing is proposed. The bearing and deformation capacities of the composite structure under waterproofing are judged from the load-displacement curves in the tests. The results show that at relatively higher temperature, the deformation capacities of frozen soil following the steel pipes are better, but the bearing and deformation capacities of composite structure are relatively poor due to the low strength of frozen soil. Similarly, at relatively lower temperature, the results are relatively poor due to the poor plasticity of the frozen soil. When the temperature is moderate, the bearing and deformation capacities of composite structure are relatively strong, that is, the capability to withstand deformation and to bear the ultimate load is the largest. The ideal temperature of the frozen soil between steel pipes is about -10℃ and it has been adopted in the actual construction of Gongbei Tunnel, which is of reference value for similar construction.