Compaction Mechanism Of Silty Loess Based On Micro-nano Structural Analysis

China has the most extensive and thickest distribution of loess.With the development of economic,compacted loess has been widely used as the basement of highways,high speed railways and airfield.In order to guarantee the reliability,safety and comfort,high strength and strict settlement control of basement are required.Silty loess takes up a very large part in loess,and the silty particles dominant the properties of silty loess.As a type of manipulated soil,compacted loess inherits some of the properties of loess because of the similarity of chemical composition,particle size and distribution,meanwhile,the artificial variables(i.e.,compaction procedure,energy and water content)during the remolding and sampling make the properties of compacted loess different from undisturbed loess.The morphology,chemical composition and mechanical properties in nano scale(hardness and Young's modulus)of the particles from the test loess and the engineering properties(deformation,direct shear strength and CBR)and microstructure of compacted silty loess were tested under the supportion of National Natural Science Foundation of China and Postgraduate Innovation Foundation of Shanxi Province.Then the compaction mechanism of silty loess was analyzed by interpreting the interaction of the particles,water and air in pores.This study could contribute to the design,construction and hazard prevention and control of basement of compacted loess.The integration of experimental results,observations,and analyses lead to the following major conclusions:(1)The test of compacted silty loess samples compacted with different conditions(compaction energies and water contents)show that,with the increase of water content,the direct shear strength and CBR decrease,and the deformation of the compacted loess decrease.The sample compacted on the dry side of the optimum presents significant water sensitivity,dramatic drops of strength and bear ratio happen when the sample is soaked in water.A more dense and stable sample could be get by employing a higher compaction energy.(2)A sample preparation method for undisturbed soils was developed,a recipe of solution for impregnation of soils with different void ratio was determined.This method could avoild the disturbance of the samples during the sample preparation.(3)For the samples compacted with a given compaction energy,the packing of the particles varies with the compaction water content: on the dry side of optimum,the silty particles interlock with each other to form skeleton,while clay friction associating together to form pour clay aggregates attaching to some single silt particles;at optimum and wet side of optimum,the silt sized particles pack together with the clay particles filling the interparticle gaps,bonding the skeleton particles as cementing material or forming bridge-like links between particles.The change of the pore distribution with compaction water content is consistent with dry density.The samples compacted at optimum are densely compacted,while the dry and wet compacted samples are more heterogeneous,with planar pores irregular in shape,larger in size and interconnecting with each other.(4)Silty particles take up a fraction of 80% in the test loess.Mineralogical investigations have shown that the test loess consists of chlorite,muscovite,quartz,albite,and calcite.The test results of quantitative particle shape analyses show that the silt particles are more of a square in terms of form and angular and subangular in terms of roundness.Particles with different chemical composition are different in shape.(5)The newly developed technology Nanoindentation was employed to test the strength of the particles(with the size of several micron),and the strength of particles having different mineral composition were obtained.0.20-0.58 GPa for chlorite and muscovite,0.98-1.38 GPa for calcite,1.46-1.92 GPa for albite and 2.68-2.90 GPa for quartz.(6)The nano-mechanical properties of muscovite samples subjected to prior unidirectional compression and prior cantilever bending were tested using nanoindentation.The principle of hardness and elestical moduli changes with the increase of pre-stress/pre-strain were obtained,which could contribute to the further study of the influence of stress history,overburden stress or stress during compaction of the soil to the strength and displacement of particles with layered structure,especially for clay particles.(7)Based on the microstructural characteristics of particles before and after compaction,the compaction mechanism was interpreted using comceptual models established based on analyzing the interaction of particles,air and water in pores in the compaction procedure.How the compaction energy and water content influence the mechanical properties of the compacted silty loess and the origin of water sensitivity were also studied using the conceptual model.