| Under the unique generation and storage environment in the northwest China,unsaturated loess has the special physical characteristics of silt,porous structure and vertical joint development.In addition,the pore structure of unsaturated loess has the characteristics of bending,un-orientation and randomness,resulting in the water-vapor-heat coupling transport process accompanied by complex phase transition and coupled transport problems.The combination of water vapor humidification technology and dynamic compaction method to treat loess foundation is a new technology and method for foundation treatment in large thickness loess area.However,the key problem to be solved urgently is to explore the heat and moisture migration mechanism under the influence of multi-physical field and phase transformation when water vapor migrates between micro-framework pores of soil,and to study the laws of water and heat migration of steam in unsaturated soil.Developing a steamhumidified tamping method for loess foundation treatment will promote the development of heat and mass transfer in unsaturated soil,which has far-reaching influence on the development of discipline and technology.This thesis takes the law of water and heat transport when water vapor humidifies unsaturated loess as the research object,considers the coupling effect and influencing factors of water-gas-heat transport in porous medium.The method of indoor air permeability test,model test,numerical simulation and theoretical analysis were used to carry out model test of unsaturated loess humidified by steam under different influencing factors,which aims at the permeability characteristics of unsaturated loess moistened by water vapor,influence factors and hydrothermal transport law.The model test results are compared with theoretical analytical solutions and numerical calculation values,and the laws of water and heat transport when water vapor humidifies unsaturated loess are discussed.The influences factors of steam pressure,seepage diameter,soil compactness and moisture content on the variation laws of air infiltration characteristics,temperature field and moisture field during steam humidifying loess are analyzed.Combining steam humidification technology with dynamic compaction method,the model test of treating loess foundation by steam humidification and compaction method is carried out,and the influence of steam humidification on dynamic compaction is analyzed.The main research contents and innovations are as follows:(1)Aiming at the complex transport mechanism of water vapor humidifying unsaturated loess under the influence of multi-physical field and phase change,a series of soil column tests of water vapor humidification unsaturated loess were carried out by using selfmade air permeability equipment,considering steam pressure,permeability path,initial moisture content and dry density of soil,and the water vapor permeability characteristics in unsaturated loess were studied.It is found that the initial infiltration law of water vapor in unsaturated loess conforms to Darcy’s law.With the increase of wetting time and migration path,the Klinkenberg effect of water vapor infiltration characteristics appears.The relationship between water vapor permeability and vapor pressure under different conditions was analyzed,and the stable permeability of unsaturated loess was determined.The effects of initial moisture content,dry density and ventilation time on soil permeability were discussed.A water vapor permeability model based on dry density and moisture content of soil was established,and the permeability test results were compared with the predicted values.Considering the Klinkenberg effect,the test results of water vapor permeability in unsaturated loess are analyzed,and it is found that the Klinkenberg effect increases the permeability of soil.(2)Aiming at the macroscopic law of water and heat transfer during steam humidifying unsaturated loess,taking Lanzhou remolded loess as the research object,the model tests of wetting unsaturated loess with spherical and columnar steam sources are carried out in the indoor filling model,and the water and heat transfer law under the coupling influence of steam pressure gradient,temperature gradient and moisture content gradient during steam humidifying unsaturated loess is discussed,and the influences of soil compactness,moisture content and steam pressure on temperature field and moisture field when steam humidifies loess are analyzed.It is found that the effective humidification range of water vapor is dominated by high temperature steam heat transfer,and the temperature migration rate belongs to the rapid migration stage.Water migration is dominated by water vapor migration and liquid water migration driven by pressure gradient.The moisture content of soil after humidification is close to the optimal moisture content,and the humidification effect is better.The temperature migration outside the effective humidification range is dominated by heat conduction caused by temperature gradient,and the water migration is dominated by liquid water migration driven by temperature gradient and moisture content gradient,which belongs to the slow migration stage.With the increase of infiltration diameter,the vapor pressure slowly decreases in the range of 0-10 cm,and rapidly decreases in the range of 10-20 cm.The water vapor infiltration rate decreases with the increase of dry density and infiltration time,but increases with the increase of initial moisture content,and the larger the initial moisture content and the smaller the dry density,the more obvious the change trend of soil infiltration rate.(3)Based on the governing equation of two-dimensional heat and moisture migration in wet capillary porous media and the boundary conditions of model test,the analytical solution of water and heat migration when the unsaturated loess is humidified by spherical and cylindrical steam sources is obtained by the method of additive separation of variables.A two-dimensional axisymmetric model is established by using Comsol numerical calculation software,and the water and heat transfer law in unsaturated loess under steam humidification is analyzed.The measured values of the model,theoretical analytical solutions and numerical calculation results are compared and analyzed.It is found that when water vapor humidifies unsaturated loess,the heating rate and humidifying rate of each measuring point in the radial direction decrease with the increase of radial distance,and the humidification effect and humidification range decrease with the increase of soil compactness.Larger vapor pressure can significantly increase the heating rate and humidification rate,and the vapor pressure has the greatest influence on the soil humidification effect,followed by the initial moisture content,and the compactness is the smallest.The humidification range and degree can be controlled by controlling the humidification time and steam pressure.In practical engineering,the humidification parameters with steam pressure of 0.05 MPa or 0.10 MPa and humidification column spacing of 1 m can be selected according to the humidification efficiency and soil properties.(4)Combining steam humidification technology with dynamic compaction method,the new technology and method of treating loess foundation by steam humidification and compaction method are developed.Relying on the engineering example of dynamic compaction foundation treatment,the model test size and dynamic compaction parameters are designed according to the similar principle,and the model test of steam humidification compaction method for loess foundation treatment is carried out.Four kinds of tamping energy were used to carry out dynamic tamping tests in water vapor humidification zone and non-humidification zone.The variation laws of tamping energy,tamping settlement and dynamic stress with depth and tamping times during dynamic compaction are compared and analyzed.The dry density of soil in humidification area and non-humidification area after dynamic compaction is analyzed,and the effect of steam humidification on dynamic compaction reinforcement is evaluated.It is found that compared with the non-humidifying area,steam humidification can not only increase the single-click tamping settlement,cumulative tamping settlement and effective tamping times,but also increase the dynamic stress at different depths,indicating that steam humidification can significantly increase the influence depth and reinforcement effect of dynamic compaction. |
PDF Full Text Request