Wake shielding and bistable interference in twin pile foundations under submarine debris flow

Submarine debris flows threaten offshore pile foundations, yet their interaction with twin piles on sloping seabed remains unclear. A three-dimensional multiphase numerical model based on the Herschel–Bulkley rheology was established and validated, to investigate the effects of pile spacing ratio G/D (0.5–8, G = center-to-center spacing, and D = pile diameter) and upstream pile aspect ratio h/D (2–8, h = upstream pile length) on twin-pile force and flow field evolution. Two interference regimes were identified: strong coupling shielding at G/D ≤ 3 (downstream pile peak drag coefficient rising by 200% from G/D = 0.5 to 3) and weak coupling independence at G/D > 5 and h/D ≥ 6 (downstream steady drag coefficient increasing by ∼2.5% per 1D spacing). Increasing h/D strengthens the upstream pile's shielding effect, fragmenting the downstream steady recirculation zone into discrete recirculation zones of varying intensities (dispersion index F minimum 0.355 at h/D = 6). Impact peak dynamic pressure is 1.67 times the steady-state value for the upstream pile and 2.31 times for the downstream pile.