Test investigation on mechanical behavior of steel pipe-frozen soil composite structure based on Freeze-Sealing Pipe Roof applied to Gongbei tunnel

Freeze-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. To determine an appropriate temperature to freezing the soil mass between steel pipes with relatively good waterproofing performance in the project prototype of Gongbei tunnel, model test study on steel pipe-frozen soil composite structure was conducted with two types of pipe layout design at four different temperatures. The bearing and deformation capacity of the composite structure under the condition of waterproofing were judged from the bearing capacity and displacement in the test and ultimate vertical displacement relationship of the composite structure under the condition of waterproofing is proposed. The results show that at relatively higher temperature (−5°C), the coordinated deformation capacity and bearing capacity of composite structure are relatively poor due to the low strength of frozen soil. Similarly, at relatively lower temperature (−15 °C), the coordinated deformation capacity and bearing capacity are also relatively poor due to the poor plasticity of the frozen soil. When the temperature is moderate (−10 °C), the coordinated deformation capacity and bearing capacity are relatively better, that is, the capacity to withstand deformation and load under the condition of waterproofing is relatively good. Finally, some microcracks which are likely to produce water channel appear near the interface between frozen soil and steel pipes. According to the similarity rule, the appropriate freezing temperature of the frozen soil between steel pipes is about −10 °C and it has been adopted in the actual construction of Gongbei tunnel, which has reference value for similar construction.