Abstract: Soil's matrix-water interactions play an unprecedented role in controlling the physicochemical behaviors and mechanical properties of soils. Such physicochemical processes, however, have long been ignored or inadequately expressed in the classical soil mechanics, leading to significant limitations of existing theories when dealing with coupled multi-physical problems, especially those involving clayey and chemical problems. To fill this gap, this paper innovatively integrates the continuum theory of porous media with the principles of classical physical chemistry to establish a theoretical framework explicitly treating the soil's matrix-water interactions. This framework is capable of quantitatively describing the coupled thermo-hydro-mechano-chemical (THMC) processes in both saturated and unsaturated soils. By defining the soil's matrix- water interaction potential, the concept of relative chemical potential for the species of pore water solution is proposed, and a general formula for the relative chemical potential, along with the equilibrium conditions for multiphase porous systems, is constructed. The generalized effective stress principle is then introduced to derive the generalized effective stress formula. Governing equations for multiphase multi-constituent seepage are established, revealing the mechanisms of thermal osmosis and chemical osmosis, and a theoretically self-consistent multiphase flow model is developed. The phase equilibrium conditions for pore water-ice/hydrate systems are determined, elucidating the intrinsic relationship between phase transition processes and soil shear strength. This research not only provides a unified theoretical foundation for modeling the mechanical behavior and constitutive simulation of multiphase porous media under coupled multi-physics fields but also offers a new approach to expand the theoretical system of soil mechanics and enhanceing its engineering predictive capabilities.