| With the national western development strategy for the implementation of the Qinghai-Tibet Railway, the West-East Gas Project, the Water Diversion Project and some other major construction projects are carried out, cold permafrost and cold regions tunnels will be more and more. If those projects are built as the general area of tunnel engineering, the frost is unavoidable, for example, 2th TianShan, QiDaoLiang Highway tunnel in Gansu Province, GuanJiao Tunnel of Qinghai-Tibet Railway and so on. In order to prevent frost from damaging the tunnel structure, thermal insulation layer is widely used in the tunnel engineering,for example, 5cm thermal insulation material has been laid between tunnel lining and the first support in Fenghuoshan tunnel and Kunlun mountain tunnel , Dabanshan tunnel of Xi (ning) Zhang (ye) Highway had used 8cm thermal insulation outside two lining. Whether the thickness and thermal conductivity of insulation layer is reasonable, economic and secure is unknown, Optimization problem is also not known. In order to solve this problem, thermal insulation layer must be optimized and designed.Therefore, this article has taken the Fenghuoshan tunnel Dabanshan tunnel of Xi (ning) Zhang (ye) Highway as examples and established optimal design theory of thermal insulation materials in cold region by means of finite element theory and optimal design theory. Simultaneously, this paper has carried on the optimized computation analysis. Application of large-scale finite element software ANSYS, Fenghuoshan tunnel in Permafrost Regions and Dabanshan tunnel in seasonal frozen soil regions are optimized and calculated, the mainly results as following:①Without taking into account of climate warming and considering the global warming situation in cold regions tunnel,the optimized result is completely consistent with the checking calculation result,the most optimal r design is also in the feasible territory and is the optimal solution,therefore this optimized method and the results are reliable;②Regarding to Fenghuoshan tunnel, the original design of the thermal insulation parameters are: H =0.50E-01m,λ=0.30E-01 W.m-1.K-1, the price is 5972.6 yuan per meter. The optimized result is following: when climate is unwarming the highest price is 7581.3 yuan per meter, when climate is warming the highest price is 7581.5 yuan,these parameters are H =6.64E-02m,λ=0.13025E-01 W.m-1.K-1; when climate is unwarmin the lowest price is 5752.7 yuan , The parameters of thermal insulation are H =4.66E-02m,λ=2.8491E-02 W.m-1.K-1; when climate is warmin the lowest price is 5812.4 yuan, the parameters are H =5.84E-02m,λ=6.5613E-02 W.m-1.K-1. Warming is a trend, to ensure the adjacent rock around the tunnel structure at zero temperature,if we established parameters are H =6.64E-02m,λ=0.13025E-01 W.m-1.K-1, although its price increased 26.9%, all around the surrounding tunnel structure can guarantee to avoid the effect of freezing and thawing, simultaneously, it explained the original design is basically reasonable.③Regarding to Dabanshan tunnel, the parameter of thermal insulation layer in original designion is H =0.135m andλ=0.341E-01 W.m-1.K-1, the price is 12256.8 yuan per meter. the optimized results is following: when climate is unwarming the highest price is 6787.5 yuan and the parameters are H =5.51E-02m,λ=1.0323E-02 W.m-1.K-1;when climate is warming the highest price is 7613.0 yuan and the parameters are H =6.60E-02m,λ=1.0227E-02 W.m-1.K-1;when climate is unwarming the lowest price is 6030.7 yuan and the parameters are H =4.52E-02m,λ=1.1027E-02 W.m-1.K-1;when climate is warming the lowest price is 6240.0 yuan and the parameters are H =4.77E-02m,λ=1.0226E-02 W.m-1.K-1。Warming is a trend, so in order to ensure that the rock around the tunnel structure is positive, if the parameters of the thermal insulation are H =6.60E-02m,λ=1.0227E-02 W.m-1.K-1,the price would reduce by 37.9%. Therefore, under the two kinds of conditions, although the temperature of the rock around tunnel structures is positive.The original desig of Dabanshan tunnel is inclined to conservative.④The results of calculation and analysis indicate that the rock's temperature have large difference between climate warming and unwarming. The global warming is an indisputable fact, so global warming must to be considered in cold regions tunnel engineering.⑤Along the tunnel, with the annual average temperature changed inside the tunnel, if the thermal insulation layer is installed with different parameters in different sections, better economic efficiency should be got.
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