Comparison of Piezocone and Laboratory Soil Parameters for Bridge Foundation Design on Clays: Case Study in Massachusetts

Results of a case study to demonstrate the use of the piezocone (CPTU) for selection of soil parameters for design of bridge foundations at a clay site in Massachusetts are presented. The test site is the location of several timber-supported bridges that have suffered significant deck and foundation deterioration and need replacement. The soil profile generally consists of 1 to 10 m of fine sand and silt, followed by 45 m of a low-plasticity marine clay that rests on a very dense fine to coarse sand. The site investigation consisted of traditional drilling and sampling techniques, field vane testing, and CPTU profiling. Shelby tube samples were used for laboratory tests, including basic soil characterization and measurements of stress-strain-strength behavior through oedometer and K₀ consolidated direct simple shear tests. Results from laboratory tests showed that some of the tube samples had suffered moderate to high levels of sample disturbance, making them unreliable for laboratory stress-strain-strength testing. A comparison of the laboratory results with the CPTU profiles showed that the CPTU provided excellent information on soil stratigraphy and good comparison with the laboratory-determined stress history and undrained shear strength profiles. The CPTU test program involved less time and expense than the laboratory test program and was not susceptible to sample disturbance effects. This case study suggests that state departments of transportation should consider the CPTU as the instrument of first choice in site characterization programs for design of bridges founded on soft clays.