Characteristic collapse domain of pile groups under eccentric vertical loads

Semi-probabilistic design codes, such as Eurocode 7, account for spatial variability in pile resistance through correlation factors applied to single-pile capacities. When piles are connected by a rigid cap, load redistribution reduces the likelihood that failure is governed by a single weak pile; however, this effect is represented through a fixed reduction in the correlation factors, thereby neglecting the additional contribution to characteristic resistance from larger pile numbers. A probabilistic–deterministic framework is proposed to evaluate the characteristic collapse domain of pile groups under eccentric vertical loads. Single-pile axial resistance is modelled as lognormally distributed and variability is propagated to the group by way of Monte Carlo simulation. Group collapse is described deterministically through limit analysis, capturing failure mechanisms and load redistribution induced by rigid pile caps. For each realisation of pile capacities, the corresponding collapse states are determined, and the characteristic collapse domain is defined as the 5th percentile of the ensemble, obtained by homothetic contraction of the mean domain. The resulting reduction factor for group resistance is smaller than for individual piles and decreases with group size, reflecting the increasing role of load redistribution. This provides a rigorous probabilistic interpretation and less conservative yet safe estimates of group capacity, offering a promising basis for future geotechnical design.