Cohesive strength of clay-rich sediment

While studies of rock and sediment friction are common, cohesion is a component of the shear strength that is often ignored despite its potential importance for faulting, sediment transport, and other geomechanical processes. We directly measure the cohesion of clay-rich sediment by measuring its shear strength in a direct-shear apparatus with no applied effective normal stress (sigma'(n) = 0). We present measurements of cohesion for two cases: (1) After vertical consolidation only, and (2) after vertical consolidation followed by shear under applied normal stress. Under consolidation stresses of 90 kPa to 2 MPa, cohesion of both the unsheared and sheared cases depends linearly on the (previously) applied load. We interpret the cohesion measured after shearing under load to be the cohesive strength that exists throughout the shearing process, suggesting that for clay-rich materials the coefficient of internal friction should be used rather than the coefficient of sliding friction. Our data suggests that the proportion of shear strength resulting from cohesion can be as high as similar to 40% at low stresses. We attribute the cohesive strengthening to hydrogen bonding between adsorbed water molecules and clay mineral surfaces resulting from the atomic charge imbalance of the clays. Citation: Ikari, M. J., and A. J. Kopf (2011), Cohesive strength of clay-rich sediment, Geophys. Res. Lett., 38, L16309, doi:10.1029/2011GL047918.