A complete thermodynamic theory is presented that is capable of explaining the dependence of yielding on the degree of saturation in brittle granular aggregates. Historically, constitutive models represented this coupling between mechanics and hydraulics only phenomenologically, by way of the incorporation of the loading collapse curve concept. This was done both for fine-grained and granular soils, and, for the latter case, without embodying the physical connection of yielding to elasticity, as motivated by fracture mechanics. Here, this connection is captured by the breakage mechanics theory, which underpins a grain-size scaling of the mechanical part of the Helmholtz free energy potential. In addition, an explicit reliance of this potential on hydraulic measures is explored, with another grain-size scaling inspired by the capillary theory. It is shown that through homogenisation these two scaling laws motivate a total macroscopic Helmholtz free energy that, together with the breakage dissipation, captures the salient couplings between mechanics and hydraulics properties, while showing a promising agreement with experiments.