Loess is susceptible to loading effects such as significant changes in strength and volume variation caused by loading and wetting. In this study, considering the different connection states of pore water and gas in loess fabric, the gas phase closure case is incorporated into a unified form of the generalized effective stress framework, introducing a damage parameter considering the effects of closed pore gas. The loading effects of unsaturated loess under wide variations in saturation are described in a unified way, and the model performance is verified by corresponding stress and hydraulic path tests. The results indicated that the collapse response involves the initial void ratio of loess, and the coupled outwards motion of the loading-collapse (LC) yield surface under loading enhances its structural strength. Suction-enhanced yield stress requires a greater "tensile stress" to counteract its structural stability. The nucleation of bubbles at high saturation causes a decrease in yield stress. The loading effect exhibits a smaller collapse behavior when the influence of closed gas is considered, whereas the suction path does not cross the LC in the stress space under hydraulic action for the same parameters, which amplifies the influence of closed gas on loess deformation.

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Keywords: Unsaturated loess, Closed gas, Generalized effective stress, Loading effect, Constitutive model