Reliability of Shape Factors for Bearing Capacity of Square Footings on Spatially Varying Cohesive Soils

Abstract

Shape factors traditionally are used to relate the bearing capacity of square, rectangular, and/or circular footings to that of strip footings in geotechnical foundation designs, in order to take account of additional resistances developed in front of and behind strip footings. They traditionally are determined by experimental observations or deterministic analysis assuming uniform soil profiles. However, soils are spatially varying, and this variability can have a substantial influence on the bearing capacity of foundations resting on them. Therefore, the empirical shape factors used in foundation designs are quite uncertain, depending on the in situ soil variability. This paper investigated how soil spatial variability may affect the shape factors for the bearing capacity of square footings and the reliability of the traditionally determined shape factors. By quantifying the uncertainties involved, shape factors can be chosen based on site conditions and a target reliability. It was found that the traditional shape factor used in design has the lowest reliability for sites with intermediate soil spatial variability, and thus lower shape factors may be warranted to attain a required reliability in such cases.