Variations in shear strength properties of clay-rich sediments over the effective stress range of 0.1 to 100 MPa

Tuesday, 16 December 2014
Brendan Casey1, John T Germaine2, Peter B Flemings3 and Brian P Fahy2, (1)Exponent, Inc. Natick, Natick, MA, United States, (2)Massachusetts Institute of Technology, Cambridge, MA, United States, (3)University of Texas at Austin, Austin, TX, United States
We conducted experiments on clay-rich sediments over the effective stress range of 0.1 MPa to 100 MPa and found that critical state friction angle and undrained strength vary systematically as a function of both stress level and sediment composition. We tested sediments from a diverse range of geologic backgrounds using a suite of triaxial systems specially designed for low, medium and high stresses. We used the resedimentation technique to prepare fully saturated samples of identical composition for testing. During the consolidation phase of triaxial tests specimens were subjected to one-dimensional compression, which produces a more realistic stress-strain response during shearing when compared to the more common approach of using isotropic compression. We measured friction angles as low as 12 degrees for a confining pressure of 63 MPa, and observed a decrease of undrained strength ratio by almost two-thirds when viewed over three orders of magnitude in stress. Clay-rich materials, particularly those with a large smectite fraction, display a more rapid decrease in both friction angle and undrained strength ratio with increasing effective stress when compared to more silty materials. We also observed similar trends for the horizontal-to-vertical effective stress ratio (Ko); Ko generally increases with effective stress level during normal compression, and this increase is most pronounced for smectite-rich sediments. We measured values of Ko as high as 0.90 at effective stresses approaching 100 MPa. This finding implies that high horizontal stresses can develop at depth solely due to normal mechanical compression, and that tectonic stresses, creep, or geologic unloading are not necessary to justify them. Furthermore, we establish, for the first time, a general relationship between Ko and the undrained strength of sediments. Higher values of Ko are associated with both lower friction angles and lower undrained strengths. Our experimental results reveal aspects of sediment behavior at high effective stress that have not previously been observed, and provide useful information on how both shear resistance and horizontal stress evolve within basins.