Liquefaction as an energetic instability of saturated granular systems – Density control and static enthalpy equilibrium

Liquefaction of saturated granular materials is commonly interpreted within stress-based frameworks that rely on the existence of an intact grain skeleton. At the onset of liquefaction, however, the contact network collapses and effective stress ceases to be a meaningful state variable. This work reformulates liquefaction as an enthalpy-driven instability of the coupled grain–water system and introduces a stability concept based on Static Enthalpy Equilibrium (SEE). Within this framework, a saturated granular assembly occupies a local minimum of total enthalpy under the constraints of gravity, buoyancy and volume constancy. Liquefaction may be triggered by a single strong excitation and/or by multiple smaller excitation events occurring in rapid succession, provided that their cumulative energy input exceeds the porosity-dependent enthalpy barrier associated with SEE. Once this threshold is exceeded, the subsequent collapse of the grain skeleton proceeds spontaneously in a post-trigger sense, driven by internally released gravitational–buoyancy enthalpy rather than by continued external forcing. Two complementary stability controls emerge naturally from the enthalpy formulation. First, a density-controlled energetic limit defines a unique stabilised porosity that depends solely on grain density and follows directly from buoyancy constraints. Second, an energetic liquefaction curve specifies the minimum external energy required to destabilise a configuration at a given porosity. The intersection of these two conditions determines both the accessibility and the termination of liquefaction. The resulting framework provides a physically grounded optimization of admissible system states, derived from the first principles and valid across the entire transition from intact skeleton to fully liquefied suspension. It offers a consistent alternative to stress-based liquefaction criteria without invoking effective stress in regimes where it is not physically defined.