Wetting-induced Deformation Behavior Of Loess Soil

The traditional evaluation methodology of collapsibility of loess including single oedometer test and double oedometer test,are usually conducted on loess in saturated status.In practical engineering,additional deformation would generate before loess being wetted to saturation,which is call wetting-induced deformation.In recent years,some Q₁ and Q₂ loess layers are exposed or closed to the land surface by cutting loess hills to terraces,while a mount of high filled projected appears in gully regions.It probably occurs loess wetting-induced deformation by building constructions no matter on this intact loess layers or filled loess layers.In addition,the process of the loess saturation and wetting-induced deformation are so rapidly and difficult to be monitored.Meanwhile,the microstructure evolution during the wetting-induced deformation of loess is also hardly to measured accurately.These make the research for wetting-induced deformation behavior and mechanism become difficult.Thus,the paper conducted on the loess soil in Holocene?Q₄?loess layer,Upper Pleistocene?Q₃?loess layer,upper Middle Pleistocene?Q₂?loess layer,lower Middle Pleistocene?Q₂?loess layer and Lower Pleistocene?Q₁?loess layer located in Jiuzhou Tai loess profile in Lanzhou city,and introduced a graded wetting collapse test to simulate the dynamic process of loess wetted from initial saturation degree to saturated condition.On the other hand,a serious of tests was conducted on natural loess in different layers and remolded loess with different dry densities to study the unsaturated character,wetting-induced deformation and wetting-induced deformation mechanism of loess.In this paper,the main research contents and research results are as follows:1.The basic physical properties,chemical properties and microstructure of nature loess from different loess layersThe specific gravity,water content and plastic limit are in constant,while the dry density,liquid limit and plasticity index increase when the age of loess develops from younger to older.The mineralogical compositions of the loess are predominantly quartz,feldspar,mica and calcite,with a small amount of dolomite and chlorite.The major cation in loess is Na⁺,secondly are Ca²⁺,Mg²⁺and K⁺;The major anion in loess isSO₄^2-,secondly are Cl^-.The microstructure variation of loess from youngest loess layer to oldest loess layer is as follows:The angles of mineral grains are weakened and the particle size decreases;the type of particle connection develops from point-point contact to point-face point and even face-face contact;The volume of the intra-aggregate pores changes slightly,while the volume of inter-aggregate pores decreases significantly.2.The effect of loess structure on unsaturated characterThe effect of loess structure on soil-water characteristic curves is that:With respect to the condition that loess soil being in younger layer?or with lower dry density?,the pore size distribution of natural loess and remolded loess are similar,so as to the air entry value of natural loess and remolded loess are close;while the existence of cement bonds between the particles of natural loess obstructing the connectivity between pores,so as to the residual water content of natural loess is slightly larger than remolded loess.With respect to the condition that loess soil is in older layer?or with higher dry density?,the air entry value of natural loess is larger than remolded loess,depending on the volume of inter-aggregate pores of natural loess is smaller that remolded loess;while the residual water content of natural loess and remolded loess are similarly,because the effect of comment bonds on the connectivity between pores is reduced.The effect of loess structure on soil-water characteristic curves is that:With respect to the condition that loess soil being in younger layer?or with lower dry density?,the existence of cement bonds between the particles of natural loess obstructing the connectivity between pores,so as to the unsaturated permeability coefficient of natural loess is slightly smaller that remolded loess.With respect to the condition that loess soil is in older layer?or with higher dry density?,closely contact between particles of natural loess reduce the obstruction of cement bonds to pores,so as to the unsaturated permeability coefficient of natural loess and remolded loess are similarly.3.The effect of loess structure on wetting-induced deformation behaviorUnder loading and wetting,the initial microstructure of loess will collapse and then restructure,even for natural loess possess cement bonds between particles.The fabric of natural loess featuring a relatively nonuniform distribution of pores,so as to the microstructure adjustment,pore volume reduction and deformation of natural loess is larger than remolded loess under loading and wetting.In addition,depending on the structure difference between natural loess and remolded loess,the critical void ratio for wetting-induced deformation of natural loess is smaller than remolded loess,even the difference value increases with the increase of vertical pressure.In other words,natural loess enable to wetting collapse with a lower dry density when subjected to a constant vertical pressure.4.The wetting-induced deformation behavior of loessThe minimum saturation degree for remolded loess to begin wetting collapse is referred to as the critical collapse saturation degree in this study.For natural loess,the critical collapse saturation degree increases when loess changes from youngest loess layer to older loess layer,while it decreases as the vertical pressure increased.For remolded loess,the critical collapse saturation degree increased with increasing initial dry density,while it decreased as the vertical pressure increased.During the process of wetting collapse,the void ratio evolution of natural loess or remolded loess depends on the saturation degree and the vertical pressure,while it is unaffected by the initial dry density?or initial void ratio?.The void ratios of specimens with various initial dry densities decreased exponentially along the same curve with an increase of the saturation degree when subjected to a constant vertical pressure.Under a constant vertical pressure,there is a critical void ratio,where the natural loess or remolded loess collapses after being wetted to saturation if its initial void ratio is larger than the critical void ratio.The critical void ratio of natural loess or remolded loess decreases exponentially when the vertical pressure increases.5.The wetting-induced deformation behavior of loessThe size and volume of inter-aggregate pores decrease when loess collapse upon loading and wetting,while the particle size distribution,the size and volume of intra-aggregate pores change slightly.The dominant inter-aggregate pore size is the main factor for the collapsibility of loess upon loading and wetting.In addition,abundant intra-aggregate pores indirectly identify that the inter-aggregate pore structure in remolded loess is possibly unstable,which leads to the collapse of the soil.