Option of Laying Position of Insulation Layer for Tunnel in Frost Region

Insulation layer is widely used to prevent tunnels from being frozen. Different laying position of insulation layer makes different temperature change of surrounding rock. The finite element method was used to calculate temperature field of surrounding rock. There were 3 types of insulation applied: insulation on the surface,insulation between two linings and insulation off the wall surface. It is found that: to seasonally frozen ground through which the tunnel passes,it's useless to lay insulation layer to protect surrounding rock from being frozen due to dual effect of the temperature both from tunnel inside and outside; to permafrost,laying the insulation layer on the surface of the secondary lining has a similar insulation effect with laying the insulation layer between primary support and secondary lining,but heat insulation material become easy to failure when it is laid between two layers of lining due to extrusion and invasion of water,so laying insulation layer on the surface of the secondary lining is the better choice; to non-frozen ground,separating the insulation layer from lining's surface has the best insulation effect,while laying the insulation layer on the surface of the secondary lining has a better insulation effect than laying the insulation layer between primary support and secondary lining. But the drainage system at tunnel foundation should be insulated.