Effectiveness of Effective Area Method for Assessing Undrained Capacity of Shallow Rectangular Foundations

This paper investigates the effectiveness of the effective area method (EAM) for assessing the undrained load-carrying capacity of rectangular foundations under combined vertical ($V$), horizontal ($H$), and moment loading ($M$) by comparing with finite-element method (FEM) results. Predicted ultimate limit states under combined loading from the two methods are compared, and the equivalent rectangle derived from the EAM is compared with the contact region obtained from the FEM. For one-way eccentricity, good agreement in the $V\text{-}M$ capacity is achieved with the two methods despite differences between the effective area and actual contact area. However, $V\text{-}M$ capacity for two-way eccentricity obtained from the EAM is significantly smaller than that derived from the FEM, with the discrepancy becoming more pronounced with increasing mobilization of the vertical capacity. $V\text{-}H\text{-}M$ failure envelopes established by the EAM also fall significantly inside those determined using FEM. The contributing factors for the discrepancies are explored systematically in the paper.