Mechanism Of Structural Evolution Of Initial Loess Deposit Driven By Environmental Factors

Loess geohazards prevent the infrastructure construction and safe services in central and western China.It is thought that the weak structure of present-day loess is responsible for such geohazards.The present-day loess structure is gradually evolved from the initial loess deposits subjected to various environmental factors(such as temperature fluctuations,wetting and drying cycles,freezing and thawing cycles and overlying loads)during the long geological history.Understanding the evolution trajectory of loess structure is an important scientific basis for deciphering the mechanism of loess geohazards.On the other hand,it can be used to predict the future evolution trend of loess structure,and provide theoretical and methodological guidance for land spatial planning and utilization and effective prevention and control of loess geohazards in loess area.Previous understandings on the formation mechanism of loess structure are speculations based on the observation of the present-day loess structure,lacking evidence,and there are obvious disputes among the existing views.The previous research on loess structure evolution experiments mainly focused on today’s undisturbed loess,remolded loess,or compacted loess.However,the initial structure of loess is far different from that of present-day loess,so the relevant research results cannot be simply applied to the initial loess deposits.Therefore,it is still unclear how the structure of present-day loess is formed,which environmental factors play a key role and what the mechanism is.In order to answer the above scientific problems,this paper reconstructed the initial loess deposit by simulating the aeolian environment.Physical simulation experiments of structural evolution of initial loess deposit under the action of different environmental factors were carried out to roughly present the formation and evolution process of the early structure of aeolian loess.Combined with CT scanning,high-definition photography,discrete element numerical simulation and theoretical analysis,this paper discusses the initial structure of aeolian loess and the structural evolution characteristics and evolution mechanism of initial loess deposit under the action of various environmental factors.The main conclusions are summarized as follows:(1)The typical Malan loess was selected as the raw material,and the initial loess deposit was fabricated in the laboratory by simulating the deposition process of dust particles by using the airfall method.With the help of CT scanning,image analysis technology and three-dimensional pore structure parameter extraction technology,the structure of initial loess deposit was quantitatively characterized.The results show that the initial loess deposit was extremely loose with a dry density of 0.88 g/cm~3 and a void ratio of 2.07.The overhead pores were well developed,and most of them were arranged vertically or nearly vertically.The pores existed in isolation from each other and were evenly distributed in the space.(2)The physical simulation of the structural evolution of initial loess deposit under the action of ambient temperature fluctuation was carried out in the test chamber(DHTH-500-40-P-SD),the coupling relationship between vertical deformation,soil temperature and ambient temperature was clarified,and the corresponding structural evolution mechanism was revealed.The results show that both soil temperature and vertical deformation of soil sample fluctuate with the ambient temperature,but lag behind.The vertical deformation exhibits a synchronous fluctuation with the soil temperature.With temperature cycles,the soil sample keeps contraction,and the soil sample experiences elastic-plastic deformation and then elastic deformation.(3)The physical simulation of the structural evolution of the initial loess deposit under wetting and drying cycle was carried out by rainfall and evaporation devices.To record the structural evolution,photos of the top and sides of the sample were taken at regular intervals and the sample after experiment was scanned by CT device.The results show that under the action of wetting and drying cycle,the initial loess deposit had evolved from the original loose and homogeneous structure to an anisotropic structure composed of vertical cracks and vertically arranged dense blocks.This structure is consistent with the vertiloess structure of present-day loess,which reveals the key role of wetting and drying cycle in the early structural evolution of aeolian loess.Comparison of structures at different stages of the evolution indicates that,contrary to the common belief,it is the wetting rather than drying that initiates and drives the structural evolution while drying preserves and expands the structures created by the wetting.(4)Based on the monitoring results of volumetric moisture content,matric suction,particle composition and soluble salt content of initial loess deposit,the controlling factors of structural evolution of initial loess deposit under the alternating action of wetting and drying were analyzed.The results show that the migration of fine particle and dissolution and migration of soluble salt in the process of wetting and drying have little impact on the structure evolution of the initial loess deposit due to the small amount of migration,while the capillary force generated in the wetting and drying process is easy to lead to soil shrinkage and cracking,which is the main driving force for the formation of vertical cracks and dense blocks in initial loess deposit.The numerical simulation of structure evolution of initial loess deposit under capillary action was carried out via Mat DEM program,and the results further confirmed that the major role of capillary force.(5)Based on the results of physical simulation,numerical simulation and theoretical analysis of the structural evolution of initial loess deposit under the action of various environmental factors,a model of the structural evolution mechanism of aeolian loess was proposed.The model indicates that the loose structure and the development of overhead pores of initial loess deposit provide material conditions and structural basis for the evolution of loess structure.The wetting and drying process plays an important role in the structural evolution of aeolian loess.The capillary force generated during wetting and drying,especially in the wetting,is enough to overcome the resistance and pull adjacent particles close to each other,modifying the overhead pores into vertical cracks.Temperature fluctuation,freezing and thawing cycle and the overlying load caused by later deposition tend to homogenize the aeolian loess structure,which are not conducive to the formation of the anisotropic structure of present-day loess and play a secondary role in the structural evolution of aeolian loess.