Study On Creep Behavior And Microstructural Variation In Loess

The Loess Plateau is the most concentrated and largest loess sedimentary area on the Earth,and the main distribution area of loess in China.However,Loess Plateau is one of the regions with the most serious geological disasters in China due to the fragile geological environment.With the deepening of the national"Western Development"strategy,the advancement of the"Belt and Road"initiative,and the implementation of the"Yellow River Basin Ecological Protection and High-quality Development"strategy,the development of the Loess Plateau has entered the fast lane,and the scale of human engineering activities is unprecedented,which pose a considerable threat to human habitation and urban construction in the area.The creep characteristic is one of the important factors inducing geological disasters of loess.The essential reason for the generation and development of loess creep deformation lies in the its microsture change due to time.The research on microscopic mechanism of creep deformation is the basic premise of studying the response mechanism of engineering landslide and disaster warning.Supported by the sub-project"Loess geological structure and water cycle mode and medium catastrophe mechanical behavior"of the major project of the National Natural Science Foundation of China"The mechanism and prevention and control of major engineering disasters in the Loess Plateau"（41790440）,this paper selects Q₃ Malan Loess in the foot of a slope at a side in Nangou area in Yan’an city,Shaanxi Province,China,as the research object.A series of laboratory physico-mechanical tests,microstructure tests and picture processing technology were carried out to study the creep deformation characteristic,microstructure morphology and pore characteristics of undisturbed loess under different influencing factors,analyze the response degree of microstructural parameters to the creep deformation of loess,established the relation between the creep deformation and microstural parameters and reveal the micromechanism of creep deformation and failure in loess.The main findings are shown as follows:（1）According to the triaxial creep test results,it is found that the creep deformation of undisturbed loess is significantly affected by static load,loading time,moisture content,confining pressure and other factors.The creep deformation of loess under long-term static load can be divided into decelerated deformation stage and non-decelerated deformation stage,which are positively correlated with static load.The duration of each stage is related to soil type and static load.Under low water content,low static load and high confining pressure,the speak strain rate of loess is the lowest,the decelerated rate of strain rate-time curve is the fastest,and the time required for soil creep deformation to enter the stable stage is the shortest.The peak strain rate does not always increase with the long-term static load,and there may be three inflection points of the peak strain rate of loess with different long-term static loads,which can be regarded as the critical points of the transformation of the soil deformation state.（2）According to the results of triaxial shear test and triaxial creep test,it is found that the long-term cohesion and internal friction angle of undisturbed loess are smaller than the conventional shear strength indexes under the same conditions,resulting in the long-term strength of undisturbed loess is smaller than the shear strength.With the increase of moisture content and the decrease of confining pressure,both the shear strength and the long-term strength of soil decrease,and the long-term strength decreases less than the shear strength.The cohesion decreases exponentially with the increase of moisture content,while the internal friction angle remains unchanged.（3）According to the analysis of the shape,size and arrangement of the particles and pores of the undisturbed loess before and after creep deformation,it is found that the boundary of the skeleton particles and pores of the undisturbed loess is clearly visible,mainly angular particles,semi-angular particles,semi-circular particles and ground round particles,containing a small amount of peripheral clay particles and aggregate particles,and the soil pores are mainly support pores.After creep deformation,the soil particle pores are still clearly visible,and the soil particle edges and corners collapse or flake off.In the process of extrusion and collision,there are more clastic materials and clay particles in the ground soil,and the contact between soil particles is mainly indirect contact.The soil pores mainly consist of scaffolds and Mosaic pores.The content of large pores decreases,the content of small pores increases,and the total pore content decreases.The slender pores in the soil body decrease,and the smooth pores increase.After creep deformation,the particles and pores have obvious directional arrangement characteristics,and the dominant direction Angle is roughly the same as the fracture direction.The changes of static load,loading time,confining pressure and water content make the shape and size of soil particles and pores change in different degrees.The influence of water content is the most significant,followed by the static load and confining pressure,and the influence of loading time is the least.（4）According to the analysis of pore characteristics of undisturbed loess before and after creep deformation,it is found that the pores of undisturbed loess in its natural state have obvious bimodal distribution,and the bimodal do not disappear with creep.The small pore diameter peak is basically unchanged,and the large pore diameter peak obviously moves to the direction of small pore diameter.As compared to before creep deformation,the cumulative pore volume of undisturbed loess after creep deformation decreases,medium pores volume decreases significantly,small pores volume increases,and large and micro pores volume decreases and increases slightly respectively.In addition,the characteristic pore size of the soil decreases,the fractal dimension and the volume of the tubular pore increase after creep deformation.The changes of static load,loading time,confining pressure and moisture content make the soil pore characteristics change in different degrees,among which the moisture content has the most significant influence.（5）Grey correlation method was used to analyze the response degree of microstructural parameters to loess creep deformation.The response degree of pores to external environment was greater than that of particles.For soil particles,the response degree of particle shape to the environment is the largest,followed by the characteristic particle size,and the influence of particle size is small.For pores,pore morphology and fractal dimension are the most responsive to the external environment,followed by pore volume,pore size and characteristic pore.（6）Based on the above results,it is found that the creep deformation of undisturbed loess is essentially caused by a change in its microstructure.The creep process of loess is a process in which the loess microstructure adjusts to the external stress with time.The generation and development of loess creep deformation is can be regarded as the alternating process of interparticle bond damage（particle slip,structure collapse,particle damage）and particle recombination（compression,extrusion,rearrangement）.The change of external environment is an important factor affecting the creep deformation and failure of loess.