Stiffness and strength anisotropy of overconsolidated Bootlegger Cove clays

Abstract

This paper presents the evaluation of the stiffness and strength anisotropy of overconsolidated (OC) Bootlegger Cove Formation (BCF) clays at the Port of Alaska, formerly known as the Port of Anchorage. The stiffness and strength anisotropic material response was evaluated based on triaxial samples equipped with internal instrumentation including a submersible load cell and three subminiature linear variable displacement transducers (LVDTs). Three sets of bender elements were used in this research to measure shear wave velocities for different propagation and polarization directions. The effects of reproducing the stress history of the soil deposit on the stiffness cross-anisotropic behavior of the material are discussed. The laboratory test results are compared with in situ measurements of shear wave velocities based on suspension logging and crosshole and downhole soundings. The results of the experimental program showed that BCF clay is a cross-anisotropic material. Mean stiffness anisotropy ratios ranged from 0.90 to 1.22 and 0.93 to 1.46 for lightly OC and OC conditions, respectively. Strength anisotropy ratios, defined as the ratio of undrained shear strength under triaxial extension to compression, varied between 0.8 and 0.5. It is found that reproducing the stress history of the OC soil deposit during the laboratory reconsolidation stage did not have a significant impact on the initial stiffness anisotropy ratios of the BCF clay.