Lateral Resistance of Shallow Foundations

The resistance of shallow foundations to lateral loads is often relied upon to transmit the base shear forces from the ground to a building during an earthquake. Considering the importance of this link in the lateral load path it receives little attention in current design practice in New Zealand. An assumption is commonly made, either explicitly or implicitly, that the combination of sliding friction along the base of the structure and passive earth pressure acting against embedded foundation elements will have ample capacity to resist the design base shear. However, the actual mechanisms of lateral load resistance for shallow foundations are quite complex and poorly understood. Development of passive earth pressure requires significant plastic deformations within the soil mass and corresponding large movements of the structure. The required earth deformations may not be compatible with the structure’s geometry. Also, sliding friction may be limited by the use of polymer based damp proof membranes. Three possible failure mechanisms are commonly identified for shallow foundation systems (e.g. Clough and Duncan, 1991) as shown in Figure 1: “Wedge Failure”, “Flow-Under Failure”, and “Tip-to-Top Failure”. The “Wedge Failure” is based on classical Rankine passive earth pressure theory, and shows that vertical movement of the structure may be necessary to develop full lateral earth pressure against the foundation beams. The “Wedge Failure” figure also shows the inherent incompatibility between the mechanisms of sliding friction and passive earth resistance with development of the failure wedges lifting the structure off its base. Lateral Resistance of Shallow Foundations