Study On The Influence Of Extreme Climate On The Hydrothermal Environment Of Permafrost Foundation Of An Airport

The Qinghai-Tibet Plateau is a huge buffer zone between China and South Asian countries,and its unique geographical position makes the plateau easy to defend and difficult to attack,and it is easy to advance and retreat,so the construction project of Qinghai-Tibet Plateau is of great strategic significance,so the first military airport in the plateau area will be proposed to be built in Qinghai-Tibet Plateau to enhance the control ability of the border defense line.Since the permafrost area on the Qinghai-Tibet Plateau accounts for 80% of the total permafrost area in the country,such construction projects on the Qinghai-Tibet Plateau inevitably involve permafrost areas.At the same time,in the context of global warming,extreme climate occurs frequently,and the traditional engineering construction design of permafrost area does not consider the influence of extreme and sudden climate factors on the multi-year permafrost foundation in the engineering area.In this paper,based on the first military airport construction project on the Qinghai-Tibet Plateau,a hydrothermal coupling model of saturated-unsaturated soil freezing process is established using a combination of field exploration data and numerical simulation.Based on the natural site moisture,temperature and meteorological data in the area of the proposed airport project,the influence of the presence of ground water for 50 years on the hydrothermal state of the natural foundation is analyzed;according to the given foundation treatment plan in the project Based on the given foundation treatment plan,a coupled hydrothermal model of permafrost foundation with road surface structure was established to analyze the hydrothermal dynamic changes during the service period of the proposed flight area pavement and the stability of the road surface structure during the service period;the effects of short-term extreme high temperature events and different extreme precipitation events on the hydrothermal state of the proposed flight area foundation and the service performance prediction were studied,and the main conclusions are as follows:(1)Based on heat transfer and saturated-unsaturated soil percolation theory,the temperature and equivalent total water content are used as the basic variables to establish the controlling equations for the permafrost temperature field and the controlling equations for the moisture field under the effect of ground water accumulation.Comsol Multiphysics finite element software is used to solve the foundation model of the proposed airport engineering area.The comparison between the field surveyed foundation soil temperature and simulated temperature results is basically consistent,which verifies the validity of the theoretical model and numerical model,and the numerical calculation model can be used to simulate the hydrothermal state of the foundation soil under the flight area of the proposed airport.(2)The combined effect of concrete roadbed surface and climate warming gradually degrades the multi-year permafrost layer under the flight area and shifts the upper limit of permafrost downward.During the service period of the airport flight area pavement deep foundation soil appears perennial thawing area,deep foundation soil unfrozen water content increases,but there is still multi-year permafrost below the thawing area,the excess pore water can not be transported in a short period of time,so that the soil strength is reduced,which has a greater impact on the stability of the flight area pavement structure.(3)The extreme precipitation effect increases the water content of the active layer,and the accumulation of water at the shoulder of the flight area may cause damage to the flight area pavement structure such as freeze swelling and thawing during the freeze-thaw cycle.The increase of summer rainfall leads to the increase of surface latent heat,the reduction of surface soil heat flux,the reduction of surface temperature,and the mitigation of thermal disturbance of the flight zone pavement structure on the underlying permafrost;the infiltration of surface moisture,and the generation of moisture enrichment zone near the soil partition interface due to the different permeability coefficients and the existence of the freezing layer.The effect of extreme precipitation acting continuously for 3 years is basically the same as that of extreme precipitation acting for 1 year on the change of hydrothermal state of the foundation soil under the proposed flight area pavement,but the intensity of the change increases.(4)Short-term extreme high temperature events have a greater impact on the redistribution of the hydrothermal field of the foundation of the proposed flight area pavement,which is likely to lead to uneven subsidence and longitudinal cracks and other diseases in the flight area pavement structure due to the large longitudinal and transverse differences.The short-term extreme high temperature has a greater impact on the shallow surface,and the warming of the deep foundation soil is delayed and has a longer impact time;the melting area in the transverse and longitudinal range after the short-term extreme high temperature event occurs in the same time is greater than the melting range under the action of normal temperature,resulting in the reduction of the strength of the deep foundation soil,and the deformation of the proposed flight area pavement structure is mainly based on the melting and sinking deformation.