Abstract: The freezing scheme of the freezesealing pipe roof method comprises circularmain freezing and limiting pipes arranged in a solid jacking pipe filled with concrete and a profiled enhancing freezing pipe arranged in an empty jacking pipe. To examine the freezing effect around the empty jacking pipe of this novel freezing system, an onsite prototype test is conducted in the Gongbei tunnel. The influence of external surface insulation of the profiled enhancing freezing pipe in the empty jacking pipe on the freezingsealing effect is investigated based on the variation in the thickness of the frozen soil curtain. The results show that the insulation of the external surface of the jacking pipe is not conducive to the formation of the frozen soil curtain, and the formation rate of the frozen soil curtain decreases with freezing time. In addition, the thickness difference of frozen soil between the insulated and uninsulated cases at 60 days in the synergistic freezing mode is approximately twice that of 20 days of freezing. The empty pipe jacking can be considered as a "big freezing pipe", and overall cooling inside can be achieved using the profiled enhancing freezing pipe, which is more conducive to the formation of the frozen soil curtain around the jacking pipes.

Key words: freezesealing pipe roof, profiled enhancing freezing pipe, freezing pipe insulation, field test, Gongbei tunnel