| With the continuous promotion of the national "One Belt,One Road" strategy,the traffic flow in the permafrost area of the Qinghai-Tibet Plateau is increasing day by day,and the construction of integral expressways has become a development trend.Due to the thermal sensitivity of permafrost in the permafrost region of the Qinghai-Tibet Plateau and the large scale heat accumulation effect of the integral subgrade,it is still a key problem to build a wide integral highway in the permafrost subgrade.At present,domestic frozen soil ventilation duct subgrade mainly uses natural ventilation,which relies heavily on natural conditions,and the effect of disordered air flow on cooling frozen soil subgrade is uneven.Based on this,this paper focuses on the integrated highway construction in the permafrost region of the Qinghai-Tibet Plateau,and studies the protective effect of mechanical ventilation on the permafrost underlying the subgrade,so as to achieve the long-term stability of permafrost protection and permafrost subgrade operation.Based on the climatic conditions and environmental geological conditions of the Qinghai-Tibet Plateau,establishing the earth-air coupling calculation model to study the influence of the integral subgrade on the heat of the underlying frozen soil under different temperature conditions.Then,the first type of three-dimensional mechanical ventilation tube subgrade model with temperature boundary conditions is established.The mechanical ventilation parameters under different temperature conditions are determined by adjusting ventilation parameters to achieve that the artificial upper limit of permafrost under the subgrade is no less than the natural upper limit.Based on the determination of ventilation parameters,the spacing of ventilation pipes is adjusted to achieve the stability of the artificial upper limit of frozen soil under the subgrade.The reasonable spacing of ventilation pipes is determined,and the ventilation pipe system is established based on this,and the energy consumption per kilometer of the ventilation system is calculated.Through the research,the main conclusions are as follows:(1)The changes of thawing plate and maximum melting depth of integral subgrade under three annual mean temperature(-3.5℃,-4.5℃ and-5.5℃)are compared.It is found that the higher of the annual mean temperature,the longer the annual existence time of thawing core under subgrade,which results in the greater the maximum melting depth of permafrost under subgrade.It is found that the higher the ambient temperature is,the greater the vertical heat flux of subgrade basement and the greater the depth of thermal disturbance.(2)Through theoretical analysis and numerical simulation,the optimal ventilation pipe layout height is determined to be 0.5 m above the ground;Two ventilation duct types suitable for integral subgrade structure are compared by numerical simulation,and it is determined that T-type ventilation duct structure is more suitable for integral permafrost subgrade.By calculating the pressure drop along the ventilation pipe,the diameter of T-type ventilation pipe is determined to be 0.2 m.Then,the model is used to calculate the reasonable ventilation parameters under different annual average temperature,and the ventilation wind speed under the conditions of annual average temperature of-3.5℃,-4.5℃ and-5.5℃ is 2 m/s,1 m/s and 0.25 m/s,respectively.(3)The stationarity of the artificial upper limit of the underlying permafrost is analyzed by using the numerical simulation to adjust the pipe spacing,and it is found that the optimal ventilation pipe spacing under the three annual mean air temperature is 2 m.The ventilation piping system is established and the energy consumption is preliminarily calculated.It is found that the total power required per kilometer is 9.93 k W when the average annual temperature is-3.5℃.Under the condition of average annual temperature of-4.5℃,the total power required per kilometer is 1.24 k W;Under the condition of average annual temperature of-5.5℃,the total power required per kilometer is 0.02 k W. |
