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Theory of Mixtures with Interfaces (TMI) is used to develop "eld equations governing the behaviour of unsaturated porous media under dynamic loading conditions. Interfaces existing between bulk phases in unsaturated porous media are explicitly considered in the TMI. Volume fractions and area densities are introduced as independent constitutive variables. A new de"nition for the total stress that explicitly includes interfacial e!ects is deduced. Thermodynamic restrictions are used to derive the constitutive relationships. It is found that there exists a well-de"ned potential (i.e. Gibbs' thermodynamic potential) to drive the #ow of the #uids. E!ective stress for unsaturated soils is theoretically shown to dependent not only on the degree of saturation but also on the stress path and soil type. Theoretical basis for the use of two stress state variables, net stress and suction, instead of a single e!ective stress is provided. The soil}water characteristic curve is shown to be none other than the linear momentum balance equation of all the interfaces. Realistic functional forms are suggested to take into account the variation of suction when the solid skeleton undergoes deformation. The "nal set of governing equations presented are easily implementable into "nite element methods and can be used to analyse problems such as the earthquake loading of compacted soil embankments.

[1] Soil water characteristic curves (SWCCs) represent the relationship between suction and water content in unsaturated soils. The SWCCs exhibit hysteresis during wettingdrying cycles; however, the empirical expressions used to describe SWCCs have typically ignored the hysteresis. Additionally, the shape of the SWCC will vary depending on the void ratio of the soil and changes resulting from soil skeleton deformations, which may also show hysteretic behavior under various loading conditions. Therefore, it is important to investigate, both experimentally and theoretically, the relationship between soil skeleton deformations and the SWCC for different soils. There is limited information in the literature that examines, both experimentally and theoretically, the complex coupling between the soil skeleton deformation and SWCC behavior, and generally, this behavior is not well understood. This paper presents laboratory test results of SWCCs determined under different confining stresses on similarly prepared samples of a silty soil; drying, wetting, second drying, and scanning curves were obtained. The influence of soil skeleton deformations on SWCCs is inferred from the curves measured in an oedometer under different stress conditions. An elastoplastic phenomenological constitutive model based on the bounding surface plasticity theory was utilized to simulate the coupled mechanical-hydraulic behavior of measured results. This research demonstrates that the model is capable of predicting hysteresis in SWCCs and soil skeleton deformation and the coupling between the hydraulic and mechanical behavior of unsaturated soils.