Abstract: To study swell-shrink fissure evolution rule of expansive soil in the natural rainfall and evaporation process, wetting-drying cycles tests, and CT scanning tests have been done. The three-dimensional reconstruction of soil CT images and the extraction of fracture information are carried out using Matlab software. The relationship between three-dimensional space cracks and wetting and drying cycles is studied from qualitative and quantitative dimensions. The relationship between soil’s fracture in three dimensional space and d wetting-drying cycles are studied from the qualitative and quantitative dimensions. Qualitative studies have found that the development of fractures begins at the weak spots such as initial pores and microcracks within the soil; and thus extends to form fracture surfaces and fissure bodies; and finally forms a complex three-dimensional fracture network. As the wetting-drying cycles progresses, the area and volume of the fracture gradually increase; and finally tend to stability. Quantitative studies have found that the distribution of fracture area along the axial direction of the sample has obvious periodicity. As the wetting-drying cycle proceeds, the period gradually decreases, and the amplitude A increases first and then decreases. The development of fissure is divided into three stages: i.e, fissure gestation period, fissure rapid development period and fissure steady development period; and fissure development mainly occurs during fissure development period. The approximate relationship between radial fracture area, axial height, and cumulative dry shrinkage volume change is obtained. The fracture volume is defined as a function of the Logistic function. It can predict the development and distribution of fissures in the sample, and can provide references for the permeability characteristics of fissured expansive soil and the study of the overall and local stress and strain.

Key words: expansive soil, wetting-drying cycle, fissure, quantitative analysis