Study On The Three-dimensional Microstructure Evolution And Collapsible Mechanism Of Malan Loess In Different Regions During Collapsible Process

Collapsibility is one of the most special properties of loess and one of the important causes of geological disasters in loess area.Therefore,it is of great significance to understand the mechanism of loess collapsibility in essence to solve the geological disasters in loess area.In this paper,Lanzhou,Qingyang,Jingyang and Hengshan Malan loess are taken as the research objects.Based on micro-CT scanning,a three-dimensional microstructure quantitative model of Malan loess in different regions is established,and the microstructure parameters of Malan loess in different regions are analyzed.The change law of parameters,combined with the distribution and properties of clay,analyzes the reasons for the regional change of Malan loess collapsibility,and proposes the mechanism of Malan loess collapsibility.The main research results are as follows :(1)The collapsibility of the Malan loess is gradually weakened from northwest to southeast,but the law is not obvious in some areas,such as the Hengshan loess in the north is not collapsible;lanzhou and Qingyang Malan loess belong to strong collapsible loess,and the collapsibility coefficient reaches the peak at 400 k Pa load.Jingyang Malan loess belongs to medium collapsible loess.The collapsibility coefficient increases with the pressure and reaches the peak at 1000 k Pa.(2)The overall structure of Lanzhou and Hengshan Malan loess is relatively uniform.The microstructure of Malan loess in Lanzhou is mainly composed of overhead pores and loosely arranged mosaic pores.The pore size and pore volume are relatively uniform under different pore sizes,and the proportion of macropores to total pore volume is the highest.The clay particles mainly exist in the form of film and bridge.The Hengshan Malan loess is dominated by dense mosaic pores,and the pore volume distribution is the most concentrated.The proportion of small pores in the total pore volume is the highest,and the clay particles mainly exist in the form of film.There are many intragranular pores in Qingyang and Jingyang Malan loess,and clay particles mainly exist in the form of stable structure.Among the four groups of undisturbed Malan loess,Hengshan loess particles are more inclined to horizontal distribution and the structure is more stable,followed by Jingyang loess.Lanzhou and Qingyang loess particles tend to have a higher proportion of vertical distribution,and are more likely to form a metastable structure of loess.(3)Through the quantitative comparison of loess microstructure parameters,particle orientation angle,pore size,elongation,flatness and pore throat size in the process of collapsibility,it can be seen that the pore size distribution changes most obviously after collapsibility.After 400 k Pa load collapsibility,the number of pores larger than 26μm and pore volume of Malan loess decrease,and the number of pores smaller than 26μm and pore volume increase,indicating that pores larger than 26μm provide the main space for collapsibility.After400 k Pa load collapsible,the large pores of Lanzhou loess are more converted into small pores less than 12μm,and the large pores of Qingyang and Jingyang loess are more converted into medium pores of 11-21μm,which also explains that the small pore volume of Lanzhou loess does not increase significantly after collapsible.After collapsibility,the mosaic pores decrease uniformly,and one overhead large pore is converted into multiple medium and small pores after collapsibility.After the collapse of 400 k Pa load,the particle direction angle,pore elongation,flatness and pore throat size of Malan loess decrease,but the variation of microstructure parameters has no obvious rule with the collapsibility of Malan loess.(4)Based on the above results,combined with the distribution and properties of clay,the collapsibility mechanism of Malan loess is proposed : if the loess collapses,there must be sufficient pore space to provide conditions for particle slip,and the loose arrangement of overhead pores is conducive to loess collapsibility;secondly,the hydration expansion of clay cementation after soaking reduces the bonding strength between loess particles and provides power for loess particle sliding.When the clay particles are more bridged in the loess,most of the collapsible deformation will be completed under low load.When the clay content is more in the loess as a supporting structure,the collapsible effect under low load is not obvious,and the collapsible effect under high load is promoted.