The mechanical behavior of heavily overconsolidated resedimented Boston Blue Clay

Abstract

Geotechnical engineers encounter some of the most challenging problems in heavily overconsolidated soils. Clays under this condition originated in nature or man-made construction. This thesis investigates the mechanical behavior heavily overconsolidated Resedimented Boston Blue Clay (RBBC), as an analog test material. The uniformity, reproducibility, and extensive knowledgebase on this clay permit the investigation of only the effect of overconsolidation. A series of undrained triaxial compression shear tests have been performed on specimens that were Ko-consolidated and swelled to overconsolidation ratios (OCR) ranging from 5 to 62 on RBBC, as well as on Intact Presumpscot Maine Clay. Specimens were consolidated to maximum stress levels between 1.1 and 5.4 MPa. The undrained shear behavior revealed an important relationship with overconsolidation ratio. The triaxial compression test results exhibit a significant increase in the undrained strength ratio (su/[sigma]'vc) when normalized to the minimum consolidation stress. It also shows an important increase in the stiffness, increase in the strain ([epsilon]f) to mobilize the peak resistance and a significant increase of friction angle ([phi]') with overconsolidation. The applicability of the SHANSEP equation and impact on its equation parameters are also discussed in detail. A comprehensive comparison of the new results with prior normally consolidated and lightly overconsolidated data is presented, showing the distinct behavior of heavily overconsolidated specimens. The failure envelope displays a non-linear behavior and the undrained shear strength (su) is reduced up to about 80% from a normally consolidated clay to a heavily overconsolidated clay with an OCR = 50. These observations demonstrate the importance of selecting appropriate design values in the presence of overconsolidated soils.