Stress-strain behavior of surficial carbonate sediments from Key West, Florida, USA

A triaxial strength testing program was conducted to determine the stress strain and strength behavior of a surficial calcareous sediment from the Dry Tortugas study site west of Key West, Florida, which is part of the Coastal Benthic Boundary Layer Special Research Program. The sediment at the study site is comprised largely of Halimeda algal grains resulting in carbonate contents near 93% by mass. Sediment grains range from sand-sized shells and shell fragments to carbonate muds. Isotropically consolidated undrained triaxial tests were conducted at consolidation stresses no greater than 34 kPa on undisturbed and remolded sediment samples from box cores. It was found that the sediments behave like a loose sand during shear and are characterized by failure envelopes with friction angles between 37 degrees and 39 degrees with no cohesive intercepts. Initial elastic tangent Young's moduli, determined by the hyperbolic method, range from 130 kPa to 18,873 kPa, and exponential relations between the elastic moduli and confining stress are found to exist for the undisturbed and remolded sediments. Particle size distributions were found for the sediments at various stages of testing and results show that no particle crushing occurred due either to the stresses or the remolding procedures employed in this investigation. Although apparent overconsolidation was present, there was no evidence of cementation during shear, as a brittle failure mechanism was not present. Comparisons of the strength behavior of the undisturbed and remolded sediments suggest that particle mobility plays an important role as a mechanism of strength development.