Abstract The dynamic and non-dynamic interactions between a gravity-type quay wall and a backfill ground were investigated by centrifuge model testing, considering cases in which a rigidly cement-stabilised ground existed at varying distances from the quay wall. In conducting the centrifuge tests, the performance of the instrumentation applied to measure the pressure from granular soils was critically assessed. At non-dynamic states, the backfill confined between the quay wall and the rigidly stabilised soil block exerted smaller earth pressure at deeper locations, at both apparently active and transient states. The calculation, based on perfect plasticity and considering friction arching, was useful in explaining these results. A similar feature was also observed during the steady-state oscillations, in the case of dry sand backfill, and was associated with the system’s increased seismic stability. In underwater cases, the pore water fluctuations in the backfill dominated the total earth pressure behaviour, with the active pressure being smaller again from a confined fill than from a fully extending one. Despite the reduced active pressure, placing the stabilised soil in the proximity of the quay wall increased the wall’s permanent seaward movement, unless the two bodies were in direct contact. This ostensible association of smaller active pressure with greater instability in the underwater cases cannot be explained by the conventional, simplified conception of the active earth pressure as a unilateral cause of instability. The evaluation of such unconventional backfill conditions seems to require rigorous consideration of the simultaneous soil–water-structure interactions.