Experimental Study On The Treatment Of Collapsible Loess Foundation By Steam Humidification And Tamping Method

The distribution area of loess and loess-like soil in China is about 64,000 km2,accounting for 6.3% of the country's land area,among which collapsible loess accounts for about 60%.After encountering water,the strength of the loess will decrease,and the foundation is prone to subsidence,resulting in uneven settlement of the foundation and other phenomena.In order to solve the above problems,scholars at home and abroad developed the method of dynamic compaction to treat collapsible loess foundation.However,the water content of natural loess in northwest China is generally between 3% and 8%.Direct dynamic compaction under the current specifications will result in poor dynamic compaction effect and less effective consolidation depth.Therefore,in the construction,the method of water injection and humidification is usually used to make the soil near the optimal moisture content.However,using water injection to humidify will lead to slow humidification speed,uneven humidification,humidification range is difficult to control and other phenomena.In this paper,the remolded loess in lanzhou is taken as the research object,combined with the water transfer theory of unsaturated loess and the treatment method of collapsible loess foundation,A new method of collapsible loess foundation treatment steam humidifying ramming method is proposed.In the laboratory model test,the steam diffusion rod embedded in the soil was connected with the high temperature and high pressure steam to conduct the steam humidification test and compaction test on the soil.The law of water vapor diffusion and its influencing factors in unsaturated loess were analyzed by measuring temperature and volume water content.The change rules of compaction,compaction times,water content,dynamic stress and dry density of soil before and after humidification were analyzed.The main conclusions are as follows:(1)Through the introduction of high temperature and high pressure water vapor into the unsaturated loess,the law of water vapor uniform ellipsoidal diffusion in the unsaturated soil is obtained,and the position ±15cm radial from the vapor bar and the soil moisture content along the vertical depth of 10 cm are close to the optimal soil moisture content of 17.8%.The water in the unsaturated loess migrated together in the form of water and gas.In the process of water and gas migration,the water content in the soil increased with the increase of the wetting time.The water content in the soil in the early stage increased rapidly,while that in the later stage increased slowly,indicating that the water field of the soil had reached a stable state.(2)The transfer mode of temperature in unsaturated loess is mainly thermal convection and conduction.After the steam is introduced,the temperature of the soil 15 cm away from the steam diffusion rod changes first,and then the soil temperature changes successively along the steam diffusion rod 30 cm away from the steam diffusion rod,and the temperature has no obvious change trend at the furthest distance from the steam diffusion rod.The temperature warming rate of the soil 15 cm away from the steam diffusion rod is greater than that 30 cm away from the steam diffusion rod.(3)The variation of the ram's sediment mainly depends on the water content and the energy level.The cumulative ramming load increases with the number of ramming,and the click ramming load decreases with the number of ramming.In the later stage of ramming,with the continuous compaction of the soil,the variation range of the ramming amount decreases and finally becomes stable.For the same energy level,the cumulative compaction in the humidified area is greater than that in the unhumidified area when the ramming times are the same.According to the conclusion that the accumulative compaction amount accounts for 90%-95% of the total compaction amount,the accumulative compaction amount in the unhumidified area is optimal at the 7th stroke,and the accumulative compaction amount in the humidified area is optimal at the 10 th stroke.(4)As can be seen from the dynamic stress time-history curve,when the number of ramming increases from the first to the last,the dynamic stress of soil at different depths increases with the number of ramming at the center of the rammer.When the soil is rammed,the time for the soil to reach the peak stress is less than the attenuation time.With the increase of ramming times and energy levels,and the time for the dynamic stress to reach the peak is reduced.(5)The vertical stress changes with depth before and after humidification,the dynamic stress decreases with depth,and the attenuation law changes with power function.The curve slope of the last ramming stroke is greater than that of the first and second ramming strokes at the same energy level and at the same depth.With the increase of the ramming times,the upper soil mass is effectively strengthened,which enables the effectively strengthened upper soil mass to be transferred to the deeper soil mass through the action of ramming energy.(6)The peak change of dynamic stress depends on two aspects,one is the size of energy level,the other is the change of water content.The energy level depends on the rammer and the spacing.The higher the energy level,the stronger the compaction effect of the rammer on the soil.In the process of compaction,the dynamic stress is transferred from the shallow layer to the deep layer.The change of water content to dynamic stress peak is mainly manifested as: when water content is small,the elasticity of soil mass is large and the damping is small,and the energy generated is dissipated,which is not conducive to the continuous propagation of dynamic stress.When the water content is near the optimal water content,the elastic and damping ratio of the soil is equal during ramming,which is conducive to the continuous propagation of dynamic stress.(7)Under each energy level,the dry density at X=0cm is greater than that at X=10cm and X=20cm.As the energy level increases,the dry density changes with the depth.The variation of dry density with depth in the humidified region was greater than that in the unhumidified region.The dry density changes with the depth firstly decreases and then increases,and then decreases.The soil can be divided into loose area,plastic area and elastic area.