Reliability Assessment of Pile Foundations in Saturated Sand Using Dual Criteria: Field Evidence from a Niger Delta Bridge Site

Abstract

Traditional pile foundation design in saturated sand typically focuses on bearing-capacity failure and uses a single factor of safety (FOS). However, this approach may not ensure reliable performance against settlement at the same design load. This study measures the difference between these two criteria using the First-Order Reliability Method (FORM), which is checked with Monte Carlo Simulation (MCS). These methods are to Standard Penetration Test (SPT) N-values from two 30-metre boreholes at the Omoku-Okpasimini Bridge site in Rivers State, Nigeria. Reliability indices are calculated for pile diameters from 200 to 600 mm and FOS values between 2.0 and 6.0 for both bearing capacity and settlement. The results show a clear difference. At FOS = 2.0, the bearing capacity reliability index (β = 4.28) exceeds the Eurocode 7 target (β = 3.8), but the settlement reliability index (β = −1.37) indicates a failure probability of over 91%. To meet the EN 1990 serviceability target (β ≥ 1.5), a minimum FOS of 4.0 is needed. To meet the Eurocode 7 ultimate limit state (ULS) target (β ≥ 4.0), the pile diameter must be at least 350 mm. Pile geometry has little effect on settlement reliability, which depends more on axial load and soil stiffness. Regression-based design equations are developed for both criteria, enabling engineers to size piles to meet target reliability without a full probabilistic analysis. These results suggest that using a single FOS is insufficient for pile foundations in Niger Delta saturated sands, and the study offers a reliability-based design approach applicable to similar deltaic areas.