The first problem of tunneling in permafrost area is how to excavate and how to lining at the same time

because the needed temperature for construction of liner is different from that of excavation. Excavating needs lower temperature for stability of wall rock

while construction of lining needs higher temperature for concrete strengthening. By finite element analysis of temperatures inside tunnel

four phases (safe phase

relatively safe phase

safety precaution phase and potential safety troubles phase) suitable for construction are put forward to guide excavation and construction of lining.