ADVANCE ON THE ENGINEERING GEOLOGICAL CHARACTERISTICS OF EXPANSIVE SOIL

Expansive soil is considered as a problematic soil in engineering, because it usually results in various engineering geological problems and disasters. According to the published results on the engineering geological characteristics of expansive soil in recent years, the advances on swelling-shrinkage behavior, desiccation cracking, overconsolidation, strength, permeability and microstructure of expansive soil were summarized, and the following main knowledge was obtained. (1)The swelling-shrinking characteristics are mainly related to the fraction of expansive clay minerals, hydro-mechanical boundary conditions and initial state. Irreversible volumetric deformation would occur when the expansive soil is subjected to wetting-drying cycles. Until now, there is still not a uniform viewpoint on the intrinsic mechanism on swelling-shrinking behavior. (2)Desiccation cracking is one of the salient features to distinguish expansive soil from other soils. The presence of cracks in soil can significantly undermine the overall structure of the soil, greatly weaken the mechanical properties and lead to many engineering geological problems directly or indirectly. The formation of desiccation cracks is related to expansive soil mineral composition, microstructure and the development of internal stress during drying. (3)Overconsolidation makes expansive soil has a greater structural strength and horizontal stress. Stress-release cracks are easy to occur when the expansive soil slope is excavated, which can damage the overall structure of the soil and promote landslide.(4)In terms of strength, most of studies focused on the effect of wetting-drying cycles. Generally, the strength decreases gradually with increasing wetting-drying cycles and eventually reaches stabilization. The wetting-drying cycle induced presence of cracks and microstructure arrangement are the two factors responsible for the corresponding strength changes. (5)The hydraulic conductivity is largely controlled by cracks. It is therefore important to consider the crack effect when performing permeability test in laboratory.(6)Microstructure of the expansive soil reflects the formation conditions and stress history. It is a fundamental factor controlling the macro-physical and mechanical behavior. Microstructure analysis is also the important way and theoretical basis to explore the intrinsic mechanisms of the observed macroscopic properties. Furthermore, as for as expansive soil engineering treatment technology, this article focused on the physical treatment technology for embankment filled with expansive soil and flexible supporting technology for expansive soil cut slope. Based on the above understanding and current research shortages in this field, some important research topics that should be well investigated in future were proposed, including anisotropic behavior of swelling-shrinking, mechanism of the desiccation cracking, quantitative relationship between geometrical parameters of cracks and the indicators of macro engineering properties, the coupling of macro-micro mechanical model and the hydro-mechanical response characteristics of expansive soil under multi-field coupling condition, et al.