STRENGTH CHANGE INVESTIGATION OF THE PRESUMPSCOT CLAY FORMATION

The Presumpscot Formation is a soft to very soft sensitive marine clay deposit that is found along the New Hampshire and Maine seacoast areas. The deposit varies in thickness from location to location but its geotechnical properties are overall very similar. Numerous studies using various in situ geotechnical tests such as the field vane, the piezocone and the flat plate dilatometer have revealed a discrepancy at depth with regards to the undrained shear strength profile. The studies, dating back to the construction of the I-95 Interchange in the mid to late-60’s, conducted in coastal areas of New Hampshire (Portsmouth, Dover and Newington) and Maine (South Portland) all show a sharp reduction in shear strength at depth followed by a gradual increase nearly parallel to the rate increase of the upper clay portion. A strength change of such significant magnitude is unusual and appears to be a regional change that may exists at a common point in geologic time rather than at a specific depth or elevation. Using specimens collected in Dover, NH across the zone depicting the strength change, laboratory tests using laser light scattering techniques and hydrometer tests were used to evaluate particle size distributions. X-Ray diffraction analyses were also used to evaluate the clay mineralogy. This paper or poster will presents the field and laboratory results at the various sites. The results from the particle size analyses showed a clear evidence of significant differences in soil composition in terms of silt and clay fractions above and below the change in undrained shear strength. Possible explanations for the discrepancy include sea level changes, partial erosion of the Presumpscot clay formation or occurrence of a significant ancient seismic event.