Long-Term vs Short-Term Bearing Capacity of Shallow Foundations on Clays

Abstract

Determining the ultimate bearing capacity is crucial for the efficient design of shallow foundations. Design codes and standards generally provide guidance in estimating the ultimate bearing capacity of shallow foundations. These codes and standards suggest calculating the ultimate bearing capacity of shallow foundations on clays, considering that the undrained/short-term bearing capacity is more critical than the long-term/drained bearing capacity. This notion is true for normally consolidated to lightly overconsolidated clays. However, when shallow foundations are situated on heavily overconsolidated clays, the generated excess pore water pressure can either be positive or negative depending on the stress history of the soil and the imposed strain on the soil. Thus, the critical ultimate bearing capacity for heavily overconsolidated clays needs to be examined by comparing predicted and measured bearing capacities from actual load tests, as well as by comparing predicted undrained and drained ultimate bearing capacities using actual soil shear strength parameters. A database of shallow foundation load tests conducted on fine-grained soil was organized in a large spreadsheet called TAMU-SHAL-CLAY-Load Test. Information includes the geometry of the load test, soil shear strength parameters, in-situ field test data, and load-settlement data. The measured ultimate bearing capacity was obtained from the load-settlement data, while the predicted bearing capacity was calculated using the general bearing capacity equation with c’ and ϕ’ for the drained bearing capacity, and Skempton’s equation with S_u and direct strength equations with SPT N-value, PMT p_L and CPT q_c for the undrained bearing capacity. In order to validate the results of the analyses, information in the database was supplemented by c’- ϕ’ data of soils obtained from different sites around Houston, Texas. The comparison showed that the undrained bearing capacity is critical for clays with S_u < 120 kPa, while the drained bearing capacity is critical for clays with S_u > 120 kPa.