Abstract: Failure of buildings and structures due to the increase in pore water pressure (Uw) in soil is common in practice, and triaxial test under stress path of decreasing mean effective stress and constant deviator stress (CQT) is a frequently-used research method in related studies. However, to date experimental studies considering the influence of excavation-induced unloading during construction are rarely reported. In order to explore the effects of initial unloading on the mechanical response of granite residual soil during the increase in Uw, a stage of initial unloading was added in the conventional CQT and a series of modified CQTs were conducted, based on which the effects of initial unloading and unloading lag on the deformation characteristics, yielding behavior and strength properties of the soil were analyzed, and results of conventional and modified CQTs were compared to investigate the influence of initial stress path. The experiment results show that the deformation response to the increase in Uw can be divided into three different stages, negative increment of Uw emerges and mean effective stress increases instead as a result of rapid deformation and dilatation potential of the soil after yielding. The initial unloading brings about the degradation of the mechanical performance of the soil, which is manifested as the decrease in both stiffness and yielding strength with the increase in initial unloading degree. Compared with the conventional triaxial compression stress path, which is of “loading” type, initial unloading has a more significantly adverse effect. The degradation of soil performance intensifies as the unloading lag increases. The mobilized friction angle at yielding point decreases with the increase in the initial unloading degree, but is always greater than the mobilized friction angle at critical state.

Key words: granite residual soil, mechanical behavior, unloading, increasing pore water pressure, stress path, unloading lag