Experimental research on mechanical properties of “jacked pipe-frozen soil” composite structure in freeze-sealing pipe roof tunnel

As the key project of Zhuhai Connecting Line of Hong Kong‒Zhuhai‒Macao Bridge, the underground excavation section of Gongbei Tunnel has applied the freeze-sealing pipe roof method for the first time. Combining the advantages of the pipe roof method and the artificial ground freezing method, this method could form a “jacked pipe-frozen soil curtain” composite support system on the tunnel section, so as to effectively achieve the dual goals of “bearing” and “water-stopping between pipes”, thereby ensuring that stability and safety during tunnel excavation. To obtain the temperature, deformation and mechanical properties of the “jacked pipe-frozen soil” composite structure, a set of scaled model test system was independently developed based on the similarity theory and the experimental research was carried out. At the same time, a numerical calculation model was established using the finite element software COMSOL Multiphysics for simulation verification. According to the results, the freezing temperature field of the composite structure is characterized by uneven distribution due to the arrangement of hollow pipe, solid pipe and their internal freezers, and the formation rate of frozen soil slows down significantly in the later stage of freezing. Besides, the vertical frost heaving deformation increases sharply within 60‒160 min, the frost heaving amount increases with the depth, and the overall law of frost heaving is closely related to the distribution of the freezing temperature field. The frost heaving force caused by soil freezing has a great influence on the horizontal force of the pipe, so that the hollow pipe with relative low stiffness may be subjected to larger horizontal deformation. In the loading stage, the jacked pipe is mainly applied with force and deformed in the vertical direction. Due to the joint influence of the stiffness difference between the two pipes and the uneven thickness of frozen soil, the vertical deformation of the hollow pipe, including “bending” and “flattening”, has nonlinear characteristics, and the peak value of the vertical displacement at the bottom of the midspan section is about 1.6 times that of the solid pipe. When the load reaches 0.28 MPa, the frozen soil between pipes will be damaged at first, which leads to the failure of water sealing between pipes. In actual construction, the deformation law of hollow pipe, as well as the temperature change and integrity of the frozen soil curtain between adjacent pipes, is the key for monitoring. The research results can provide reference for the construction and monitoring of the freeze-sealing pipe roof method and verification basis for the thermodynamic coupling numerical calculation model.