Research On Three-dimensional Microstructure Evolution During Wetting And Collapsible Mechanism Of Loess

Loess Plateau in China,the cradles of Chinese civilization,is one of the most important areas for energy and agricultural activities,and functions as the junction of the East and West in the construction of“The Belt and Road”.However,geological hazards of various types are frequent in the Loess Plateau due to reasons such as the fragile geological environment,which seriously constrained the rapid and healthy development of economic and society in the Loess Plateau.Special structure in loess and strong water sensitivity are the primary intrinsic factors for these geological hazards,lots of findings related to loess collapsibility have been obtained during these years.However,the collapsible mechanism is very complicated,involving various physical processes and chemical reactions,there has not been an official theory to explain all the collapsible problems perfectly.Recognizing the loess collapsible mechanism thoroughly is the basic and important task to solve the geological hazards and engineering problems in loess regions.This dissertation,supported by the Key Program of National Natural Science Foundation of China and the Foundation for the Excellent Doctoral Dissertation of Chang'an University,took Malan loess in the newly developed district of Yan'an City as a case study.Based on extensive literature review,the author carried out research on the loess microstructure including particles,pores,contacts and arrangements in three-dimensional?3D?space,combining with field investigation,laboratory tests,micro-CT scanning and 3D reconstruction system.Based on the 3D microstructure of loess and micro sized hydro-mechanical consolidation system developed during the tests,the microstructural parameters variation and microstructure evolution during wetting were studied systematically.Combining the physical process and chemical reaction during wetting,the collapsible mechanism for Malan loess in Yan'an City was recognized thoroughly,which provide strong theoretical support for collapsible problems in loess regions.The main innovative findings are shown as follows.?1?3D microstructure for Malan loess in Yan'an City was first established based on serial micro-CT images and 3D reconstruction system,the microstructure in 3D space was identified comprehensively combined with energy spectrum analysis.The particles in loess can be classified into four types according to their morphologies,including flaky particles,plate-shaped particles,subangular particles and subrounded particles.The sphericity of 95%of particles varies in the range of 0.4 to 0.8,a quarter of particles distribute in the direction close to z axis,which forms the metastable structure in loess.Clay particles,calcite and iron oxide are the primary bonding components between particles,which provide high strength for natural loess and play important role in loess collapse.The pore numbers present gamma distribution,the pores in the diameter of 8 to 32 predominate in the total number with a percentage of 81.3%.The pore volume percentage present normal distribution,the pores in the diameter of 13 to 50 predominate in the total volume with a percentage of 92.5%.These spaced pores and inter-aggregate pores provide main space for loess collapse.?2?Based on the 3D microstructure of loess,the microstructural parameters variations during loading and wetting were explored.Under loading,the particles size and sphericity in loess showed no apparent changes,part of particles tended to distribute in the horizontal direction.When the loess collapsed under wetting,all the parameters related to particles presented no apparent changes.The pores varied greatly during wetting,the distribution of pore number and volume percentage were more concentrated.The number and volume percentage of large pores decreased and the ones of small pores increased,the change boundary is 23?m and 26?m respectively.?3?Micro sized hydro-mechanical consolidation system was first developed to realize the high resolution observation for the same loess sample before and after wetting.During wetting,the particles primarily moved down in vertical or inclined direction with little rotation in the XZ or YZ plane,which verified the results of Finding 2.The area or volume of space pores shrinked on the whole without structure collapse,the inter-aggregate pores also decreased in area and volume with poorer connectivity.When the loess was wetted,the water firstly distributed in the contact between particles,part of inter-aggregate pores and intra-aggregate pores.The clay particles in the cementation swelled and dispersed,which caused the movement of particles,whereas,as for direct contact,there was no apparent movement during wetting.Based on the observation of physical process,the clay swelling and dispersion during wetting can be considered the primary explanation for loess collapse.?4?The quantitative analysis and comparison for pores in the same loess sample was conducted,which verified the pores variation among different samples in Finding 2.According to the resutls,the pores in loess can be classified into three types,the boundary is10?m and 26?m,respectively.The pores larger than 26?m are the primary contributor for loess collapsibility.?5?Combining the collapse process described in Finding 3,PFC simulation and laboratory tests such as leaching test and collapsible tests with various solutions,the collapse mechanism was recognized thoroughly.Loose structure in loess is the prerequisite for its collapsibility,spaced pores and inter-aggregate pores provide sufficient space for collapse.The swelling and dispersion of clay particles in the cementation during wetting result in an increase of distance between particles and in a sharp decrease of strength,the particles therefore move down and collapse occur.The wedge of water film and decrease in capillary forces as well as the dissolution of soluble salts and CaCO₃ during wetting make little contribution to loess collapsibility.