Characterization of the mechanisms underlying loess collapsibility for land-creation project in Shaanxi Province, China-a study from a micro perspective

Using the major land-creation project 'to bulldoze mountains to build cities' implemented in the city of Yan'an as an example, the collapse potential, microscopic observations, quantitative analyses and qualitative characterization of the microstructures of loess sampled from Yan'an were investigated to probe the fundamental mechanisms of loess collapsibility at the micro scale. The results show that loess in Yan'an exhibits a severe degree of collapse potential, as indicated by its maximum collapse index of 8.10% at a vertical stress value of 600 kPa. From a compositional and microstructural perspective, a large proportion of loosely arranged skeletal particles are widely separated by variants of clay fabric, forming a metastable structure with a high pore area ratio of 32.67% and demonstrating an unstable contact between particles in natural loess. These intrinsic microstructural features, with a high degree of collapse potential, act synergistically to augment collapse. When loess is saturated under a certain load, the initially high stiffness provided by the delicate clay fabric can be destroyed by its softening, dispersion or disruption. Unsupported skeletal particles are then rearranged significantly into a more stable structure. At the same time, pores larger than 60 mu m create favorable spatial conditions for wetting deformation, thereby spurring an abrupt reduction in pore size and area. Particles reorganize into a closely packed structure. More interlocking pores of < 20 mu m are subsequently generated. A greater understanding of soil microstructures and compositions can improve loess performance predictions and facilitate the remediation of collapsible loess for large projects. This paper provides a method of relating microstructure parameters to the mechanisms of loess collapsibility. The pore and particle information presented in this paper can also be used to build models for simulating loess behaviors. However, the applicability of the conclusions to other soils still requires verification, which will be performed in future studies.