Study On Water-thermal Migration Of Unsaturated Loess Humidified By Water Vapor

With the establishment of the long-term goal of 2035 and the advancement of the "One Belt and One Road" initiative,it has become particularly important to promote the development of the western region to form a new pattern in the major direction of gradually optimizing the spatial distribution of China’s land,promoting coordinated regional development and new-type urbanization.Therefore,the infrastructure construction in northwest China is one of the important measures to promote the new development pattern in the west.However,the loess distribution in northwest China has a large area and lots of diseases.In the process of engineering construction,the loess with low moisture content has high strength and strong bearing capacity,but its strength weakens and deformation increases after being soaked by water.Therefore,many highway,railway and water conservancy investment projects put forward higher requirements for foundation treatment technology in loess area.Based on the research of domestic scholars,this paper proposes a new method of humidifying soil by high temperature and high pressure steam.Taking the remolded loess of Q3 strata in Lanzhou as the research object,the indoor model test under different initial moisture content,different compaction degree and different steam pressure were carried out by using control variable method to study the change law of moisture and temperature in unsaturated loess.On the basis of the laboratory test,using COMSOL Multiphysics software to establish a two-dimensional axisymmetrical model for hydrothermal coupling calculation,optimize the model design and layout,so as to compare and analyze the change law of soil internal moisture and temperature under the action of water vapor,to provide basis and reference for engineering applications.The main research conclusions of this paper are as follows:The initial moisture content affects the diffusion rate of steam in soil and the absorption of steam molecules by soil.When the soil is humidified by water vapor,the soil with a higher initial moisture content needs a shorter time to reach the maximum moisture content,and the diffusion rate of water vapor in the soil is faster;The compaction degree has a great impact on the temperature transfer of soil during the steam humidification process.The close contact between soil particles with high compaction is conducive to the heat transfer of soil skeleton,and the water diffusion distance of the soil with low compaction is father than that of the soil with high compaction.The steam pressure has a great influence on the temperature migration and water diffusion during humidification.The temperature of 0.1 MPa steam pressure increases faster than that of 0.05 MPa steam pressure during humidification,and the water diffusion range is larger.When comprehensively considering the influence of various factors on the humidification of unsaturated loess,the steam pressure has a greater influence on the humidification of soil,followed by the initial water content,and the compaction degree has a smaller influence on the humidification effect.The numerical results show that the distribution of temperature field lags behind that of moisture field in the process of steam humidification.The distribution of the moisture at the steam diffusion inlet is approximately a long ellipse,and the moisture at the boundary tends to be straight with the water migration.and the iso-wet lines at the boundary gradually tend to be straight with the migration of water.The moisture at the farther distance has no difference in the depth direction.Under the continuous influence of high temperature and high pressure steam at entrance,the area of high temperature area is distributed greatly at the end of humidification,the temperature response is faster in the numerical calculation process,and the temperature response time of distant soil is longer in the experiment.The results of numerical calculation show that the water content change increases rapidly in the early stage and then slowly,and the growth rate of each point is different in the early stage.It is found that the water content change of the distant soil in the process of humidification has an initial stage.According to the comparative analysis of indoor test and numerical calculation,it is concluded that the steam pressure of 0.1 MPa has a higher humidification rate than that of 0.05 MPa,which can quickly strengthen the soil’s absorption and migration of water.In engineering practice,the steam pressure of 0.1 MPa can effectively humidify the soil.In summary,steam humidification method as a new technology of soil humidification,compared with the traditional soil humidification method,has the advantages of humidification time and range control,good uniformity,if combined with high-energy dynamic compaction method,it is bound to have a wide range of applications in the large thickness collapsible loess region foundation treatment.