【正文】 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。 U， V are axial and radial wind speeds。 a is the thermal conductivity of air。 fS (t), uS (t) are frozen and thawed parts in the surrounding rock materials respectively。 and To is critical freezing temperature of rock ( here we assume To= ℃ ). 2 used for solving the model Equation(1)shows flow. We first solve those concerning temperature at that the temperature of the surrounding rock does not affect the speed of air equations concerning the speed of air flow, and then solve those equations every time elapse. 2. 1 Procedure used for solving the continuity and momentum equations 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 5 Since the first three equations in(1) are not independent we derive the second equation by x and the third equation by r. After preliminary calculation we obtain the following elliptic equation concerning the effective pressure p: Then we solve equations in(1) using the following procedures: (i ) Assume the values for U0， V0。 (v) calculating the momentumaverage of U1， v1 and U2， v2， we obtain the new U0， V0。2,observed values parison of simulated and observed air temperature inside The Xiluoqi Tunnel in ,simulated values。2,outside air temperature year when permafrost maximum thawed depth after Begins to from in different permafrost formed in different years Sections of the surrounding rock 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 11 4 .3 Preliminary conclusion Based on the initialboundary conditions and thermal parameters mentioned above, we obtain the following preliminary conclusions: r temperature at the entry and exit. It is warmer during the cold season and cooler during the warm season in the internal part (more than 100 m from the entry and exit) of the tunnel than at the entry and exit . Fig .1 shows that the internal monthlyaverage temperature on the surface of the tunnel wall is ℃ higher in January, February and December, 1℃ higher in March and October, and 1 .6℃ lower in June and August, and 2qC lower in July than the air temperature at the entry and exit. In other months the infernal temperature on the surface of the tunnel wall approximately equals the air temperature at the entry and exit. 2) Since it is affected by the geothermal heat in the internal surrounding section ， especially in the central part, the internal amplitude of the yearlyaverage temperature on the surface of the tunnel wall decreases and is 1 .6℃ lower than that at the entry and exit. 3 ) Under the conditions that the surrounding rock is pact , without a great amount of underground water, and using a thermal insulating layer(as designed PU with depth of m and heat conductivity ? = W/m℃， FBT with depth of m and heat conductivity ? =℃ )， in the third year after tunnel construction， the surrounding rock will begin to form permafrost in the range of 200 m from the entry and exit .In the first and the second year after construction, the surrounding rock will begin to form permafrost in the range of 40 and 100m from the entry and exit respectively .In the central part， more than 200m from the entry and exit, permafrost will begin to form in the eighth year. Near the center of the tunnel， permafrost will appear in the 1415th years. During the first and second years after permafrost formed， the maximum of annual thawed depth is large (especially in the central part of the surrounding rock section) and thereafter it decreases every year. The maximum of annual thawed depth will be stable until the 1920th years 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 12 and will remain in s range of 23 m. 4) If permafrost forms entirely in the surrounding rock ， the permafrost will provide a waterisolating layer and be favourable for munication and transportation .However, in the process of construction， we found a lot of underground water in some sections of the surroundin