Undrained Bearing Capacity Factor Nc for Ring Foundations in Cohesive Soil

A ring foundation is a cost-effective circular beam foundation that is used to support tall and heavy circular onshore industrial structures, such as chimneys, cooling towers, storage tanks, and silos as well as offshore structures, such as wind turbines, and annular platforms. Compared with circular footings, ring foundations are more suitable and economical because they use less construction material. This study focuses on obtaining an undrained capacity for a ring foundation. A parametric finite element (FE) analysis will be carried out for circular and ring footings to evaluate the bearing capacity factors (N-c), which could be used by practicing engineers when designing foundations for onshore and offshore circular structures in cohesive soil. Variations in the geometry of ring foundation such that the ratio of inner to outer radius (R-i/R-o) was 0.2, 0.4, 0.6, and 0.8, the footing side roughness factors (alpha) of 0.2, 0.5, and 1, the foundation embedment depths (D/B) of 0, 0.25, 0.5, and 1 and the linearly increasing soil heterogeneity (kB/S-u) of 0, 2, 5, 10, and 30 will be considered in this study. The results show variations of N-c with the varying R-i/R-o, roughness coefficient, embedment depth, and increasing shear strength of the soil. Based on the results of the analyses, a simplified equation is proposed using dimensional analysis to evaluate the undrained bearing capacity of a ring foundation. From the sensitivity analysis, the undrained shear strength of the soil was the most significant parameter followed by footing side roughness factor and R-i/R-o ratio in the evaluation of undrained bearing capacity.