Abstract:

The characteristics of meso-structure evolution of expansive soil and loess were studied systematically with a CT-triaxial apparatus in various stress paths, wet-dry circle, soaking swelling and inundation-collapse. The expansive soil samples were taken from two sites located in the canal slope of the Middle Route Engineering of South-to-North Water Transfer Project in China. The loess simples include Q3 loess and Q2 loess, and the former was taken from the site of Pumping Station #11 in Ningxia Province, and the later was taken from the site of Pucheng Power Plant in Shaanxi Province. A number of CT images and CT data of meso-structure evolution of the samples were obtained. The research results show that the macro-behaviors of samples are closely related with their meso-structure changes and CT data. The fissures sprout and develop during loading and lateral unloading on undisturbed expansive soil and during wet-dry circles on remolded expansive soil in free state; and their meso-structure are damaged. However, the original fissure in remolded expansive soil is toward close under loading and immersion; and the meso-structure of the soil is repaired. The tendency of meso-structure change of intact loess in loading process is dependent on the stress and suction level. The holes and fissures of intact loess become gradually small even disappear during inundation; and the original structure of intact loess failure and a new homogenous structure would be formed. The macro-void of intact loess cannot be destroyed ether by high stress or by the combination action of low stress and immersion. On the basis of CT test data, the quantitative indexes describing the meso-structure of expansive soil and loess were defined; and the structure evolution equations for the soils under various test conditions were proposed; as well as a method to determine the yield stress in triaxial test was suggested. Thus, CT technology makes the research of meso-structure of soils to reach the quantitative stage; and the technique also provides a practice test foundation to establish the evolution equations of meso-structure of soils and corresponding structural constitutive model.

Key words: expansive soil, loess, meso-structure, evolution law, CT-triaxial apparatus, stress path, loading, lateral unloading, wet-dry circle, immersion