Frost Heaving Deformation Of Foundation Pit Retaining Structure In The Ming Tombs Subway Station

With the development of Beijing subway network,more and more subway lines extended to the suburbs and mountains.Then the foundation pit support project is facing the test of low temperature in winter,for the subway station foundation pit construction cycle generally across 2 to 3 winter.In recent years,the winter temperature in Beijing get lower compared to the same period.Extreme minimum temperature often appeared,especially in mountainous areas(extreme minimum temperature about-27??-29? in 2015).The duration of extreme minimum temperature often leaded to frequent damage to the foundation pit.To effectively prevent and inhibit soil frost heave of foundation pit envelope in winter,we must consider temperature,moisture and stress coupled model very much,which is an extremely complex problem concerning thermodynamics,hydrodynamics,fluid mechanics,physical and chemistry in the freezing process of the soil.Therefore,the frost heave deformation of the foundation pit retaining structure of the Ming Tombs scenic area was analyzed in this paper.The main work is as follows:(1)Based on Harlan model,temperature,moisture and stress coupled model considering the effect of water migration and water-ice phase change in the stress field was established.The Heaviside function is introduced to unify the unfrozen zone and the frozen soil and to eliminate the jump of characteristics between the frozen soil and the unfrozen soil.The PDE model in COMSOL Multiphysics is used to develop the temperature,moisture and stress coupled model.The coupling relationship between physical fields is defined as a variable in the model.(2)To prove the reliability of the model and the numerical method,the soil column tests were established by COMSOL Multiphysics.The simulations of the model reached experimental results very well.(3)Finally,the coupled model which was established in COMSOL to analyze the frost heave deformation of the foundation pit retaining structure of the Ming Tombs scenic area was used in the paper.It is revealed that the distribution of parameters in water,temperature and stress fields;the maximum vertical freezing depth is 1.2m,and the frozen depth is linearly increased with the square root of air freezing index;the maximum displacement of envelope with lateral support and without support is 3.6 cm and 7.9 cm respectively;it is indicated that the horizontal support can effectively reduce the frost heave deformation of the envelope;there are two peaks in the frost heave force of the foundation pit envelope with or without lateral support,the first peak position is at the position of 2.5m and 4.5m from the top of the pit,respectively,the second peak position is at the bottom of the foundation pit;the daily variation of soil temperature and the extreme temperature have great influence on the deformation of envelope.