| The Loess Plateau in Northwest China is the most concentrated and largest loess distribution area worldwide.The ecological environment in the Loess Plateau is extremely fragile,and various loess geological disasters occur frequently.With the comprehensive promotion of various national and regional policies（“One Belt and One Road”,“Western Region Development”）,large-scale major construction projects such as“to bulldoze mountains to build cities”have ushered a new peak period,which can induce new loess disaster problems and pose a considerable threat to the geological environment and human habitation in this area.However,the essential reason for geological disasters lies in the unique microstructural characteristics of loess.The macro mechanical behavior of loess is significantly affected by its microstructure,and the microstructure can evolve due to external environmental changes on loess soil.The multiscale research combining the microstructure variation and macro mechanical response in loess under external action is the basis and key to further understand and solve its engineering geological problems and catastrophic effect.Supported by the key project of National Natural Science Foundation of China,“mechanism and prevention and control of major engineering disasters on the Loess Plateau”,this paper selects Malan Loess（Q3）in Yan’an New District,middle Loess Plateau of China,as the research object.A series of laboratory physico-mechanical tests and microstructure tests were carried out to investiagate the wetting deformation behavior（compression,collapsibility and wetting collapse）,shear properties and the variations in microstructure morphology and pore characteristics.In addition,the influence of initial microstructure on the macroscopic mechanical behavior of loess was analyzed,and the relationship between the mechanical behavior and the microstructure variation was established.Finally,the microstructural effect of loess deformation and failure and the underlying catastrophic properties under loading and wetting were revealed.These findings can provide theoretical basis for loess disaster prevention and geological engineering construction in loess area.（1）The compression and collapsibility of natural and remolded loess are affected by the initial water content,vertical pressure and dry density.The initial collapse pressure of loess at all initial water contents is close to the yield stress of the soil in its saturated state,while the peak collapse pressure is positively correlated with the yield stress of the soil itself.The initial wetting collapse pressure of natural loess decreases in the form of a power function with increasing wetting water content.The wetting sensitivity,the minimum wetting water content and the critical void ratio required for collapse are related to the vertical pressure.The whole wetting deformation process of natural loess under loading is divided into nonwetting deformation,unsaturated wetting deformation,wetting collapse deformation and no deformation.（2）The stress-strain relationship of natural and remolded loess changes from strain hardening to strain softening with decreasing confining pressure（<100k Pa）,increasing water content（>30%）and dry density（1.51g/cm3）,while the influence of stress ratio is relatively small.With an increase in water content,the cohesion of natural loess decreases exponentially.With an increase in dry density,the cohesion of remolded loess increases nonlinearly.However,their internal friction angles change little.The four different types of failure modes of the loess specimens under CU triaxial tests are shear failure,homogeneous compaction,delamination fracture and lateral expansion.The saturated natural loess under different confining pressures has potential liquefaction and flow sliding failure capacity.（3）In the natural loess structure,coarse particles,coarse particles coated in clays and aggregates are randomly arranged and connected to each other with clay,carbonate and iron oxide cementations,rendering the loess soil an open,metastable and bimodal structure characterized by intra-aggregate pores（<0.05μm）and inter-aggregate pore populations（>0.05μm）.The bimodal pore structure is not eliminated with loading,wetting and shearing.Loading natural loess results in an overall reduction in pore volume,while the pore size decreases slightly by 2μm.Wetting and shearing conditions can lead to a significant decrease in the volume of mesopores（2.5-14μm）and macropores（>14μm）and an increase in that of small pore（0.05-2.5μm）,while the volume of micropores（<0.05μm）changes little;meantime,the total pore volume and characteristic pore diameters all decrease significantly;high vertical pressure,confining pressure,water content and low stress ratio facilitate the variations in pore structure in different degrees.In comparison,the influence of stress ratio is smaller.（4）The pore size distributions of remolded loess transform from a bimodal distribution to a trimodal one under hydro-mechanical consolidation and triaxial shearing,while the trimodal feature disappears after shearing under higher dry density.After loading and wetting,the volume of mesopores（3.5～2.5μm-20～9μm）and micropores（≤0.06μm）decreases,while the volume of small pores（0.06-3.5～2.5μm）and macropores（≥20～9μm）increases.For high dry density remolded loess with dense initial microstructure,leaching enhances the connectivity and development of macropores,leading to the increase of total pore volume and characteristic pore diameters.The variation in pore structure of remolded loess during shearing is similar to that of natural loess,which depends on the dry density.（5）Compared with natural loess,remolded loess has more large-sized particles and inter-aggregate pores and less interparticle cementation.The unstable structure of remolded loess is more prone to yield under loading and wetting,and the space for microstructure adjustment is larger,leading to a larger macroscopic deformation,higher hardening degree,and smaller pore water pressure and shear strength.（6）The compression deformation of natural loess is mainly attributed to the overall reduction in pore volume with increasing vertical pressure,especially the complete shrinkage of active and unstable overhead pores.The wetting collapse of loess under loading is a macroscopic compaction manifestation under the change in its microstructure system from the initial open,metastable and non-water-stable structure（i.e.,overhead structure）to a relatively stable and close structure（i.e.,interlocked-cementation structure）.This process is dependent on the initial water content,vertical pressure,dry density and wetting level.This collapse chain reaction is manifested by cementation softening,disintegration and regroup,particle sliding and rearrangement,macropores and mesopores volume and size reduction,small pores volume increase,and macroscale volume contraction.For remolded loess,the changes in pore and particle levels are more significant.（7）The macroscopic shear deformation and failure of loess depend on the variations in different microstructure levels and elements.This variation process is related to the confining pressure,stress ratio,water content,dry density and failure mode of the specimen.This condition is manifested by the softening,dispersion and recombination of the cementation,rotation,breakage,sliding and rearrangement of the particles,decrease in the dominant size and volume of macropores and mesopores,increase in the volume of small pores,and the generation and development of cracks under certain conditions.（8）The microstructural variation is consistent with the macroscopic mechanical responses of both natural and remolded loess under loading and wetting.The response of different microstructure levels and elements to the changes in external conditions and their effects on the mechanical behavior of loess are different（positive,negative,no significant correlation）.In terms of the change in pore levels,it is mainly reflected in the inter-aggregate pore populations（reduction,reduction and mutual transformation,increase and mutual transformation）,while the inert and stable intra-aggregate pores（i.e.,micropores）remain almost unchanged. |
