Experimental Study On Water Vapor Humidification Technology Of Loess Foundation

Loess is widely distributed in the northwest of China.It is necessary to conduct foundation treatment when building or structure in this area,and dynamic compaction is often used in engineering.However,the moisture content of unsaturated loess in natural state is generally 3% ～ 8%,and the moisture content is an important factor affecting the deformation and strength characteristics of loess foundation.Therefore,when the dynamic compaction method is used to treat the loess foundation,the compaction effect is poor,and the bearing capacity of the foundation cannot be met,so humidification treatment is needed.Conventional humidification methods,such as pre-immersion method and drilling water injection method,often face the problems of long construction period,high treatment cost,uncontrollable moisture content and uneven distribution after humidification,so how to humidifying unsaturated loess foundation has become the focus of research.In this paper,based on the theory of water-air migration in unsaturated loess,high temperature and high pressure water vapor is injected into unsaturated loess foundation through self-developed humidification equipment.In order to study the humidification effect of unsaturated loess under the action of water vapor,the indoor model test of water vapor humidification was carried out on the remodeled unsaturated loess in Lanzhou area.The changes of water vapor diffusion rate,temperature and gas migration with time were analyzed under different water vapor pressure gradients,the number of humidified flower tube openings and the spacing of openings.On the basis of laboratory model test,COMSOL Multiphysics was used to simulate unsaturated loess foundation with steam humidification,and the changes of water distribution field,temperature field and pressure field in soil were compared and analyzed.The purpose of this study is to provide some reference and basis for water vapor humidification of unsaturated loess foundation related projects.The main conclusions of this study are as follows:(1)During the test,the change of water vapor pressure has a great influence on the volume water content,temperature and air pressure.When the humidification tube is constant,the humidification degree and humidification range of soil increase with the increase of water vapor pressure,and the change rate of temperature and air pressure accelerate.It is found that the water vapor humidification under 150 k Pa pressure is more reasonable.If the water vapor pressure is too large,the soil volume water content will exceed the optimal volume water content within 20 cm near the humidifying flower tube,and the small water vapor pressure can not achieve the purpose of humidification.(2)When the opening distance of the humidifying flower tube is small,the influence on the volume water content,temperature and air pressure of the soil is greater than that when the opening distance is larger,especially when the distance from the center is longer.The smaller hole spacing shortens the distance of water vapor diffusion in the direction of filling height.Similarly,when the number of openings is large,the distance of water vapor diffusion is shortened in the same fill height plane.With the increase of the number of openings,the uniformity of the soil is improved.(3)Heat transfer in wet porous media and water transport in porous media modules in the finite element software of COMSOL Multiphysics were used for numerical simulation calculations combined with laboratory tests.It is found that in the process of water vapor humidifying unsaturated loess foundation,the water field,temperature field and air pressure field are uniformly and symmetrically elliptical in the center of the humidifying flower tube.With the increase of radial distance,the density distribution of isothermal lines,isothermal lines and isobaric lines is dense in the middle and sparse on both sides.In addition,at the same time,the distribution of isotherm and isobar lags behind the distribution of water field.In summary,compared with the traditional humidification methods,the use of high temperature and high pressure steam humidifying unsaturated loess foundation overcomes the shortcomings of the traditional humidification methods,and has the advantages of flexible construction,simple operation,controllable construction time,uniform distribution of volume water content in the soil after humidification,and controllable size.In view of this,the combination of steam humidification and dynamic compaction is an effective technical measure to treat large area unsaturated loess foundation.