Studies on the undrained behaviour of Masado soil, a fill material which was chiefly responsible for the massive liquefaction in the reclaimed areas of Kobe city during the Hyogoken-Nambu earthquake, are presented. Results from multiple series of triaxial tests are used as a basis for behavioural characterization within the framework of the state concept. Reconstituted samples of three Masado soils with different gradation curves were used together with undisturbed samples recovered from Kobe Port Island to investigate the effects of the grain-size distribution, soil fabric, initial e-p' state (void ratio-confining stress state) and mode of loading on the undrained behaviour. The location of the steady state line in the e-p' diagram was found to be related chiefly to the gradation of the Masado soils. However, a unique pair of zero residual strength line and steady state line was found to exist in a modified e-p' diagram regardless of the gradation curve, fabric of the soil and mode of loading. It is shown that the state index I_s can be used to quantify the undrained behaviour when using these characteristic states as frames of reference. A procedure is proposed which enables an assessment to be made of the in-situ void ratio and undrained behaviour throughout the depth of a reclaimed deposit if the gradation curve of the fill material is known.