Creep behavior and the microstructural evolution of loess-like soil from Xi'an area, China

Creep behavior of loess-like soil is closely related to microstructural changes. The microstructure of loess-like soil collected from the Xi'an area, China, was investigated using triaxial creep tests. Qualitative microstructural observations were made by examining vertical and transverse sections from the undisturbed and tested samples with a polarizing microscope. Quantitative data were acquired by image analysis of SEM images of those sections. The image analysis documented changes in particles and pore structure under different axial stress and confining pressures. The results show that: (1) Undisturbed samples have plank-macrospore cline-cementation structure with the particles distributed randomly. The number and area of four kinds of pores decrease in the decay creep stage. The orientation of particle long-axis gradually aligns (weakly) in the steady creep stage. The samples have a lightly cemented mosaic microporous structure and plastic minerals like illite significantly aligned in a preferred orientation, and rigid quartz and feldspar grains become slightly arranged along the illite preferred direction in accelerated creep stage. The orientation of particles and pores interconnections eventually result in the creep failure of sample. (2) The prefer orientations of pores are most significant in accelerated creep stage, and the pores have insignificant preferred orientation in creep failure stage. (3) Mesopores cover the largest area, and they are the most important factors leading to loess-like soil creep. Undisturbed samples contain the largest number of pores which cover the largest area among all of the samples. As creep progresses, both number and area decrease gradually before accelerated creep stage, and finally the pore area has its largest in creep failure stage. (4) The pores in these samples are relatively oblate in shape with rough outlines in undisturbed samples, and the greater the deviatory stress is, the slender the pores with smoother outlines will be.