Pore fluid pressure diffusion in defluidizing granular columns

Pore fluid pressure variations play an important role in the motion of natural granular flows like debris and pyroclastic flows. Pore pressure in a defluidizing air‐particle bed was investigated by means of experiments and numerical modeling. Experiments consisted of recording the defluidization process, measured as the decay of the basal pore fluid pressure in initially aerated granular mixtures. Mixtures were aerated to different degrees of fluidization by introducing a vertical air flux at the base of a granular column. The degree of fluidization was characterized by the parameter βo (pore fluid pressure/lithostatic pressure). Bed expansion occurred for βo > 0.8–0.9, with maximum expansions near 8% at βo ∼1. Pore pressure diffusion in our mixtures was modeled by a simple diffusion equation, taking into account a variable diffusion coefficient. When mixtures were expanded ( βo > 0.8–0.9), continuous consolidation introduced nonlinearities in the diffusion coefficients, which retarded the decay of pore pressure. In contrast, for non‐expanded mixtures, the diffusion coefficient remained constant (linear diffusion). Our results highlight that mixture compressibility can effectively reduce the pressure diffusion coefficient in initially expanded granular mixtures, thus increasing the duration of pressure diffusion. In our experiments, as well as for most self‐consolidating natural granular mixtures, changes in permeability due to mixture consolidation appear to be negligible for the defluidizing process, as they are counteracted by changes in porosity and because the fluid behaves as incompressible, even when the fluid is air.