Abstract: The undrained behavior of sand is mainly dependent on its initial state. To better understand and reveal this feature, the characteristic state lines, namely the loosest consolidation line (LCL), the static liquefaction dividing line (LDL) and the quasi steady-state dividing line (QDL), which are related to the undrained behavior of sand in the e-lnp plane, are presented firstly. Based on the theory of shear yielding and dilatancy, a simple method used to determine QDL is given, and the existence of LDL is demonstrated by introducing the normal compression line (NCL) of sand. Subsequently, the initial state of sand is divided into four characteristic subareas, which can be referred to as the static liquefaction subarea I, the strain softening subarea II, the quasi-steady state subarea III and the strain hardening subarea IV, and sand shows the similar undrained behavior when its initial state places in the individual subarea. Finally, the theoretical and engineering applications of subarea are analyzed.

Key words: sand, characteristic state line, undrained behavior, initial state dependence, subarea