A Coupled <i>u</i> – <i> p ^w </i> SPH Formulation for Hydromechanical Modeling of Retrogressive Landslides and Comparison With a Penalty‐Based Approach

ABSTRACT We present a strongly coupled displacement () and pore water pressure () version of the Biot–Zienkiewicz (–) equations in saturated porous media for the meshfree Lagrangian smoothed particle hydrodynamics (SPH) method. We propose two distinct formulations using a single particle layer, two‐phase framework, one based on a one‐step solution of a pressure Poisson equation (PPE formulation) and another allowing compressibility of the fluid resulting in an explicit rate equation for the pore pressure (PR formulation). We discuss both formulations from a numerical perspective and verify them using benchmark problems from poroelasticity, including Cryer's problem, as well as using undrained triaxial tests, where we additionally compare the results against a weakly coupled penalty‐based undrained framework for SPH. Beyond the formulations, the focus of this work is on modeling retrogressive landslides in saturated sensitive clays, which are particularly destructive due to their extended runout and fast movement. Our simulations emphasize performing strongly coupled hydromechanical modeling even for quasi‐undrained conditions, contrary to the predominant practices in the literature, as the structural features of the landslides change significantly when pore pressure dissipation and coupled effects are included. Additionally, spreads develop under a greater variety of slope conditions as opposed to the more fluidized flowslide type of retrogressive landslides captured when solely considering undrained behavior. Lastly, we apply the PR formulation to explore how slope steepness and height influence deformation modes and apply the framework to simulate the 1994 Sainte‐Monique landslide, recreating the topographic profile, runout, and deformation features post‐failure.