Constitutive properties for undisturbed marine sediments in support of the subseabed disposal program

A laboratory material property investigation was conducted on newly acquired undisturbed samples of Pacific illite in order to quantify the effective-stress, stress-strain and strength properties of the material when subjected to high-pressure static and dynamic loading conditions. The laboratory testing program consisted of triaxial shear (TX), hydrostatic compression, and uniaxial strain (UX) tests. Consolidated-undrained static TX tests with pore pressure measurements resulted in an effective-stress Mohr-Coulomb friction angle and cohesion intercept of 28.60 and 0.004 MPa, respectively. Comparable static and dynamic tests indicated that the undrained shear strength of undisturbed Pacific illite increased nominally 200% for an increase in strain rate from 10-6/second to 1/second. In terms of total stresses (up to 35 MPa), Pacific illite was observed to behave as a phi = 0 material in shear. Constrained modulus values derived from consolidated-undrained test results exhibited an expected dependence on specimen density. UX tests conducted to investigate loading rise time effects, however, produced divergent results. Two dynamic tests performed on virgin specimens exhibited negligible increases in undrained constrained modulus compared to static test results. In contrast, two multiply-loaded test specimens showed a modulus increase of 20% by the third load cycle. Drained UX tests were successfully accomplished to stress levels in excess of 50 MPa and strains in excess of 60%. Representative static and dynamic laboratory test results were selected for future use by constitutive modellers in characterizing the response of undisturbed Pacific illite. At the request of the sponsoring agency, several drained hydrostatic creep tests were also conducted as part of this investigation