【正文】 oefficient of convective heat exchange at the surface of the tunnel wall .Therefore it is not practicable to define the temperature on the surface of the tunnel wall according to the outside air temperature .In this paper, we bine the air flow convective heat exchange and heat conduction in the surrounding rock material into one model， and simulate the freezethaw conditions of the surrounding rock material based on the in situ conditions of air temperature， atmospheric pressure， wind force at the entry and exit of the tunnel， and the conditions of hydrogeology and engineering geology. 重慶交通大學土木工程專業(yè)（隧道與城市軌道交通工程方向）畢業(yè)設計外文翻譯 3 Mathematical model In order to construct an appropriate model, we need the in situ fundamental conditions as a basis .Here we use the conditions at the scene of the Dabanshan Tunnel. The Dabanshan Tunnel is lototed on the highway from Xining to Zhangye, south of the Datong River, at an elevation of m, with a length of 1 530 m and an alignment from southwest to northeast. The tunnel runs from the southwest to the northeast. Since the monthlyaverage air temperature is beneath 0`}C for eight months at the tunnel site each year and the construction would last for several years， the surrounding rock materials would bee cooler during the construction .We conclude that, after excavation, the pattern of air flow would depend mainly on the dominant wind speed at the entry and exit，and the effects of the temperature difference between the inside and outside of the tunnel would be very small .Since the dominant wind direction is northeast at the tunnel site in winter, the air flow in the tunnel would go from the exit to the entry. Even though the dominant wind trend is southeastly in summer, considering the pressure difference, the temperature difference and the topography of the entry and exit， the air flow in the tunnel would also be from the exit to entry .Additionally，since the wind speed at the tunnel site is low， we could consider that the air flow would be principally laminar. Based on the reasons mentioned， we simplify the tunnel to a round tube，and consider that the air flow and temperature are symmetrical about the axis of the tunnel，Ignoring the influence of the air temperature on the speed of air flow, we obtain the following equation: 重慶交通大學土木工程專業(yè)（隧道與城市軌道交通工程方向）畢業(yè)設計外文翻譯 4 where t， x， r are the time， axial and radial coordinates。 T is temperature。 v is the kinematic viscosity of air。 L is the length of the tunnel。 D is the length of time after the tunnel construction。 f? , u? and fC , uC are thermal conductivities and volumetric thermal capacities in frozen and thawed parts respectively。 Lh is heat latent of freezing water。 ( ii ) substituting U0， V0 into eq. (2)， and solving (2)， we obtain p0。 (iv) solving the first and third equations of(1)， we obtain U2， V2。 then return to (ii)。 especially , the maximum monthlyaverage air temperature of 1979 was not for July but 重慶交通大學土木工程專業(yè)（隧道與城市軌道交通工程方向）畢業(yè)設計外文翻譯 8 for August. . Comparison of simulated and observed air temperature in Xiluoqi Tunnel in ,simulated values。2,observed values 4 Prediction of the freezethaw conditions for the Dabanshan Tunnel 4 .1 Thermal parameter and initial and boundary conditions Using the elevation of 3 800 m and the yearlyaverage air temperature of 3℃ , we calculate the air density p=0 .774 kg/m3 .Since steam exists In the air, we choose the thermal capacity with a fixed pressure of air ),./( 0 CkgkJC p ? heat conductivity )./( 02 CmW???? and and the dynamic viscosity )../(102 1 6 smkg???? After calculation we obtain the thermal diffusivity a= 1 .3788 sm /10 25?? and the kinematic viscosity， sm / 25???? . Considering that the section of automobiles is much smaller than that of the tunnel and the automobiles pass through the tunnel at a low speed，we ignore the piston effects， ing from the movement of automobiles，in the diffusion of the air. We consider the rock as a whole ponent and choose the dry volumetric cavity 3/240 0 mkgd ?? ,content of water and unfrozen water W=3% and W=1%, 重慶交通大學土木工程專業(yè)（隧道與城市軌道交通工程方向）畢業(yè)設計外文翻譯 9 and the thermal conductivity cmW ou ./?? , cmW of ./?? ,heat capacity ckgkJC oV ./? and duf W wC ????? 1 )(,duu W wC ????? 1 )1 2 ( According to the data observed at the tunnel site， the maximum monthlyaverage wind speed is about 3 .5 m/s ， and the minimum monthlyaverage wind speed is about 2 .5 m/s .We approximate the wind speed at the entry and exit as )/]()7(0 [)( 2 smttv ???? , where t is in month. The initial wind speed in the tunnel is set to be .0),0(),)(1(),0( 2 ??? rxVRrUrxU a The initial and boundary values of temperature T are set to be where f(x) is the distance from the vault to the permafrost base， and R0=25 m is the radius of domain of solution T. We assume that the geothermal gradient is 3%， the yearlyaverage air temperature outside tunnel the is A=3C0 ， and the amplitude is B=12C0 . As for the boundary of R=Ro， we fi