Reliability Assessment of Shallow Foundations on Sand-over-Soft-Clay Strata Using Dual Criteria: Field Evidence from a Niger Delta Site

Abstract

Traditional shallow-foundation design on layered soils typically focuses on bearing-capacity failure and uses a single factor of safety (FS). 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) and checks it with Monte Carlo Simulation (MCS). The study applied these methods to field and laboratory data from a two-layer sand-over-soft-clay profile at Lolo Ama, Port Harcourt, Rivers State, Nigeria, characterised by three boreholes to a depth of 6 m and BS 1377 laboratory testing. Reliability indices were calculated for footing width-to-length ratios (B/L) of 0.50, 0.75, and 1.00, and FS values between 2.0 and 6.0, for both bearing capacity and settlement. The results show a clear difference: at FS = 2.0, the bearing-capacity reliability index (β = 2.42–3.15) approaches or meets typical ultimate-limit-state targets, but the settlement reliability index (β = −0.65 to −0.54) indicates a failure probability above 70%. To meet the EN 1990 serviceability target (β ≥ 1.5), a minimum FS of 5.0–6.0 is needed. Footing geometry has little effect on settlement reliability, which depends more on sand-layer thickness and clay compressibility. Regression-based design equations were also developed for both criteria, enabling engineers to size footings to meet target reliability without a full probabilistic analysis. These results suggest that using a single FS is insufficient for shallow foundations on Niger Delta sand-over-soft-clay strata, and offer a reliability-based design approach applicable to similar deltaic areas.