| With the endeavor on the 13th Five-Year Plan and the Belt and Road Initiative,all kinds of long,high,large and difficult projects are increasing in loess area,which pose challenge to traditional loess mechanics.The macroscopic mechanics of loess closely related to the microstructure of loess.Thus from the perspective of microstructure to explain and predict the macroscopic mechanical behavior of loess have been the main research direction.Due to the microstructure of undisturbed loess is differ from remodeled loess,and both the geological disaster of undisturbed loess and fill engineering of remodeled loess vitally affect the life and property security,this thesis respectively research the structure characteristics of undisturbed loess and remodeled loess,aiming to reveal the structural mechanism of macroscopic deformation.This thesis builds the multi-scale study of loess with considering both the change of microstructure and the macro-mechanical response,in which the microstructural methods,solid mechanic methods and soil mechanic methods are incorporated.The results are as follows:1.By analying the Scanning Electron Microscope images,the microstructure of loess is divided into three parts:framework grains,cores and the connection between framework grains.According to the results of Mercury Intrusion Porosimetry and the application of fractal theory?the cores in undisturbed loess could be devided into four categories,namely,the miscopores(?0.06?m),fine cores(0.06-6?m),medium cores(6-16?m)and the coarse cores(?16?m).Both the size and grain composition of four types of cores have the self-similarity based on analysis of the Scanning Electron Microscope images,and the number and volume of four types of cores have the self-similarity from the view of fractal theory.Therefore the classification criteria of cores has clear physical meaning.2.Based on the data of confined compression test,the compaction,collapsibility and structure property have been anslysed.The structural yield stress of undisturbed loess and remodeled loess have been quantitatively researched by combining the graphic methods of Jose,Casagrande method,strain energy method and secant modulus method.3.Three mehods of the Scanning Electron Microscope image processing have been proposed to obtain the change rule of cores.Furthermore,the change rule of misrostructure charactistics,pores distribution and porosity have been researched.Then the structural mechanism of macroscopic deformation is revealed,in other words,for given loess,the structural yield stress under saturated water content is the initial collapse pressure of loess,and the pressure of maximum collapsibility depends on the structural yield stress of loess.4.The analysis is conducted on the loess in Luochuan including reserarch of physical index,compression characteristics,collapsibility and structural behavior.A test system for static pressure coefficient has been developed and a series of experiments have been conducted to build a model analysing the mechanics of gains.Reasearch shows that the static pressure coefficient of loess is controlled by the structure.When the vertical stress meets the structural yield stress,the static pressure coefficient will increase and the disaster will be induced.5.The split and expand mechanism of crack in loess has been analysed with drying-wetting cycles.The influence of static pressure coefficient change to slope stablity is summasized as:the existing crack on the edge of slope?the formation of wetting zone as rainfall infiltration?complex coefficient of friction decreases ? the static pressure coefficient increases?confined pressure develops?extrusion deformations of two side loess?unblanced force develops after water dissipation?crack begins to split and expand ? drying-wetting cycles ? crack expands to paleosol layer?water flow in paleosol layer?the lateral extrusion process of wetting zone starts?collapse or slope along the interface between the loess and paleosol layer is triggered.Furthermore,this mechanism could be used to explain the phenomenon of bowl shaped surface and the lateral displacement of layer in Large area field immersion tests.6.The structure is the natural characteristic of loess.All kinds of soils have structural features and the difference lies in the significance.Same to the undisturbed loess,based on the Scanning Electron Microscope images,the results of mercury intrusion porosimetry and application of fractal theory,the cores in remodeled loess also can be devided into four categories,namely,the miscopores(?0.06?m),fine cores(0.06?+?m),medium cores(6?12-32?m)and the coarse cores(?12-32?m).Different from the undisturbed loess,the miscostructure of remodeled loess is affected by the compaction.7.According to the phenomenon that the volume content for unit mass of pores,which diameter is less then 6?m,are constant for different compaction,the remodeled loess is assumed to be composed of cell grains.Further,the equation to calculate the terminated void ratio of remodeled loess has been derived.The terminated pressure for confined deformation also can be predicted by using the equation.8.The analysis is conducted to explore the influence of compaction degree on the loess collapsibility and structure.With high degree of compaction,loess is not collapsible and bounded back with water immersion.For case of medium compaction degree,loess bounded back under low pressure and is collapsible in medium and high pressure.The loess with low compaction degree have notable collapsibility.The initial collapse pressure is consistent with the structural yield stress under saturated water content.The peak collapse pressure also is consistent with the structural yield stress under optimum water content.It indicates that the collapsibility of remodeled loess with medium or low compaction degree is controled by structure.The structural yield stress of remodeled loess with medium or low compaction degree is lower than the remodeled loess with high compaction degree,and the significance of structural behavior is stronger comparing to the remodeled loess with high compaction degree.9.According to the contact model of Hertz,the force in ideal spherical particle accumulation have been analysed,which reveals the relation between strain and the secant modulus meets the power law.Further,the constitutive law of remodeled loess related to the compression deformation is derived with considering the miscrostructure.lt can be applied to calculate the deformation of compacted loess foundation after the construction and calculate the settlement for layer during the construction based on the monitoring data in site.10.Based on the in-situ investigation,geological engineering survey and laboratory tests,the failure mode of high sticking slopes is summarized:the differential settlement,which induced by consolidation of compacted loess and collapsible deformation of undisturbed loess under high pressure,will cause crack initiation in slope shoulder?crack extends as the deformation increasing?water infiltration in crack softens the compacted loess?the initial sliding surface forms in the middle of slope?water infiltrates the sliding surface?the front part of loess fails as the increased load and water infiltration?the back part of loess fails as the decreased load and water seepage?the locking section loess failes under increasing load?the whole slope fails. |
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