Participants

• Ole Madsen, Ph.D., MIT

• Sanjay Pahuja, Ph.D.,MIT

• John Germaine, Ph.D., MIT

Objectives

• To understand the temporal development of undrained shear strength in consolidating sediment.

• To obtain depth profiles of water content in consolidating sediment as function of time.

Approach

Proper sequestration of dredged sediment depends on the stability of the overlying sand/silt cap. This in turn requires that the underlying sediment have adequate bearing capacity to support the cap. Freshly deposited dredged sediment behaves like a cohesive soil in its formation stages. The evolution of macrostructural strength of such sediment depends on its sedimentation and consolidation characteristics. These characteristics remain poorly understood and exhibit wide variability depending on the composition and the initial state of the sediments. Despite extensive efforts to understand and predict sediment structure, there is no existing theoretical framework that will enable prediction of strength-vs-time behavior for a particular sediment.

We developed an experimental approach to understand the process of strength development during consolidation. Sediment dredged from Boston Harbor was allowed to undergo consolidation in sample cells for varying amounts of time, starting from different values of initial water content. The water content profile for each sediment sample was obtained by x-ray attenuation technique. The measurement of undrained shear stress was made using the Automated MIT Fall Cone Apparatus.

Status Report

Bottom sediment was procured from the Reserved Channel in Boston Harbor, in early summer (1998). After characterization, this stabilized sediment was stored as the source of material for the experimental research program. The Automated MIT Fall Cone Device was modified to measure shear strength in sediment samples up to 12 inches long. A remotely operated motor-driven carriage has been constructed in order to allow continuous profiling of sediment samples for x-ray water content measurements. The device was used to generate data and supports the need for long-term consolidation periods. Time varies with depth of materials and quality of sediments.

Participant Information

• Ole S. Madsen is Professor in the Civil and Environmental Engineering Department at MIT, with interests in sediment transport and coastal hydrodynamics.

• Sanjay Pahuja completed a Ph.D. in Environmental Engineering at MIT.

• John T. Germaine is Principal Research Associate in the Civil & Environmental Engineering Department at MIT, with interest in the geotechnical properties of soils.