Meso-structure evolution of cracked expansive soils

Abstract: The crack property of expansive soil is one of the important characteristics, which is different from other types of soils. The development of cracks greatly affects the engineering properties of expansive soils. In order to study the meso-structural characteristic of cracked expansive soil, the dry-wet cycle test, triaxial soaking test and isotropic loading test were carried out on the remolded expansive soil in scientific research station of Logistics Engineering University in Hanzhong city, Shanxi province, China. The data including volumetric strain and deviatoric strain were obtained by these tests. Using CT (computerized tomography), test samples were scanned nondestructively in real-time to obtain CT images. The whole evolving process of cracks in expansive soils was investigated under the action of water and external force from the microscopic view. The effect of the crack on the macro-meso deformation behavior of expansive soil was studied by the relationship between the micro data and macro physical parameters. Test results showed that, under unconstrained conditions of dry-wet cycles, the edges and the gathering holes area of expansive soil sample were easy to form crack. Volume shrinkage caused by dry-wet cycle of expansive soil had a stable asymptote line and would tend to a stable value. The volume change of the expansive soil had experienced 3 stages in triaxial soaking test with the water and external force. Firstly, in the initial stage of triaxial soaking test, expansive force caused by soaking water led to wetting expansive deformation of cracked expansive soil. Secondly, the softening effect appeared and the expansion force gradually decreased with the increase of water content, under the influence of confining pressure and deviatoric stress, the shearing shrinkage was followed. Thirdly, in the late period of soaking test, the shear failure of the specimen under the effect of deviatoric stress caused the phenomenon of slight dilation. Cracked expansive soil during isotropic loading process had obvious yield phenomenon, taking yield point as the cut-off point, the curve of scanning data and pore ratio with the load increases was divided into rapid and slow volume shrink period. The rapid volume shrinkable period was related to cracks and holes closure, and the slow volume shrinkable period was related to the formation of the new structure which had ability to resist the external load. From the CT-scanning images of triaxial soaking test and isotropic loading test, under the action of water and external load, the irregular cracks and holes were gradually evolved into a regular circular hole, and the circular hole tended to close. With only the action of the external force, the crack was more difficult to complete closure. The interaction of water and external force made the expansive soil crack closure effect better than that of only external loading. Therefore, water plays a key role in the effect of the crack 'development of expansive soil. The results would provide valuble information for the further understanding of the influence of cracks on the mechanical properties of expansive soil.