Abstract: Completely decomposed granite (CDG) and granite residual soil (GRS) are prone to trigger engineering accidents and geological disasters in South China. Despite that the weathering degree and engineering properties of CDG and GRS both vary remarkably, it is still a challenging task to distinguish CDG and GRS in practical geological investigation, in which the field geological appearance and the measured hit number of standard penetration test according to existing specification are taken as the major indicators for identification. In this paper, we discussed the relevant clauses about identifying CDG and GRS in some current specifications. In view of the difference in gradation between of CDG and GRS, i.e., the former is usually clayey sand, while the latter sand clay, we suggest to take grading characteristic as the subsidiary index of identifying CDG and GRS.According to physical properties test and triaxial shear test of soil samples from a foundation pit, we found that GRS sample had a larger pore ratio with a poor gradation lack of some intermediate particle sizes. Both the undisturbed CDG and GRS present initial structural features reflected by shear zone with strain localization in shear deformation, displaying drum-shear zone failure mode; the remolded samples present a standard drum-like failure mode. The friction angle of drainage shear of undisturbed CDG is slightly higher than that of GRS, and the cohesion obviously higher as CDG boasts a stronger structure; the cohesion of remolded CDG and remolded GRS are close due to the lack of original structure. Under low confining pressure, the structural strength of undisturbed soils contribute up to 60%-70% of the total shear strength, and the compression action of high confining pressure can completely destroy the initial structure of soils, in which case the structural strength will be eliminated.

Key words: completely decomposed granite, granite residual soil, physical and mechanical property, structural strength, shear band