Flexural performance of composite pile–wall structures: Experimental investigation and stochastic fiber-beam modelling

Pile–wall integrated structures combine retaining piles with subsequently cast sidewalls to form composite load-bearing components in urban underground construction. Their flexural performance was investigated through four-point bending tests on four quarter-scale specimens and a stochastic fiber-beam model. The model incorporates a path-dependent interface slip-degradation law, correlated lognormal random fields for concrete tensile strength, initial interfacial shear stiffness, and slip threshold, and fiber-scale Weibull heterogeneity. Tests and simulations were compared in terms of moment–deflection response, sectional strain, stiffness degradation, interface damage, and failure localization. A total of 450 Monte Carlo realizations were used to assess ensemble response and sampling convergence. For the three continuously connected specimens, ensemble-mean predictions achieved NRMSE values of 4.16–5.57%, R ² values of 0.959–0.977, and peak-moment errors of 0.54–6.92%. For the weakened-interface specimen, the ensemble mean underestimated the test peak by 24.44%, while the predicted peak-moment COV reached 18.06%, indicating high sensitivity to spatial material–interface variability. Stiffness trajectories showed a progressive transition from integral to partial-composite action. Continuous interface reinforcement delayed slip localization and improved stiffness retention and post-cracking resistance. The stochastic formulation also quantified response dispersion and weak-zone co-localization beyond a single deterministic trajectory.