Abstract: The water retention characteristics of Nanyang undisturbed weak expansive soil were tested using the osmotic technique and the vapor phase technique. The hydrating path of soil-water characteristic curve (SWCC) in the suction ranging from 0.01 to 309 MPa was gained and fitted according to a model proposed by Van Genuchten. Scanning Eelectron Microscope (SEM) and Mercury Intrusion Porosimetry (MIP) tests were conducted to analyse the influence of microstructure modification on SWCC. Results showed that during hydrating process, the volume content of micro-pore and macro-pore increased while that of fine-pore decreased. Changes in micro-pore and fine-pore resulted in the continuous absorption of water by soils with no inflection point on the SWCC. The increase of macro-pore mainly influences the boundary effect segment of SWCC, but such influence is slight.

Key words: expansive soil, water retention characteristics, microstructure, pore size distribution, hydrating path, SWCC