||Weathered soils are used extensively as fill materials for constructing slopes in tropical regions such as Hong Kong. Safe and successful implementations of many slope stabilising measures often require the understanding of the fundamental mechanical behaviour of soils that are to be strengthened. Behaviour of loose weathered fill materials especially at unsaturated state is not often investigated and far from fully understood. The objective of this study was to understand the mechanical behaviour of loosely compacted unsaturated weathered fill materials through first studying the stress-strain relationship, the effects of suction and stress path, in particular the wetting path, on shear strength and volumetric behaviour; and second interpreting the behaviour by the critical state framework and the elasto-plastic constitutive model. Three series of triaxial tests were conducted on loose decomposed volcanic and granitic soils. The tests included consolidated undrained tests on saturated specimens, constant water content tests and wetting tests at constant deviator stress on unsaturated specimens. The last series of tests was designed to simulate the effect of rainfall infiltration in initially unsaturated slopes. During undrained shearing, the saturated loose decomposed volcanic soil shows a tendency to dilate and possesses ductile stress-strain behaviour. However, the saturated loose decomposed granitic soil shows a tendency to contract and possesses brittle stress-strain behaviour if the initial state lies on the loose side of the critical state. Hence, static liquefaction is very unlikely to occur for saturated volcanic soil but is possible for saturated granitic soil. On the other hand, the observed volumetric behaviour of the unsaturated loose weathered soils is influenced by the imposed stress paths and the stress conditions. For unsaturated specimens sheared at constant water content, both soils exhibit volumetric contraction for the range of net mean stress considered. This seems to imply that static liquefaction is possible if the rate of air escaped from the voids is slower than the rate of volumetric collapse of the soil structure, from which excessive pore-air pressure will be generated. Regarding wetting tests at constant deviator stress, both unsaturated soils behave differently. Decomposed volcanic soil contracts at low net mean stress but decomposed granitic soil contracts at high net mean stress. Since the behaviour of the unsaturated loose decomposed soils is stress and path dependent, only results from relevant stress paths and stress conditions should be adopted for the design of stabilising measures. An elasto-plastic constitutive model for unsaturated soils was proposed in this study. The proposed model is formulated using independent stress state variables and state-dependent dilatancy. Predictions from the model are in good agreement with experimental results, in particular stress-strain and axial-volumetric strain relationships, from undrained tests and constant water content tests on saturated and unsaturated loose decomposed soils, respectively. Regarding wetting tests at constant deviator stress on unsaturated specimens, the predictions underestimate suction and volumetric strain at failure. Despite this, the model is able to capture the stress and path dependent volumetric behaviour of the saturated and unsaturated loose decomposed soils.