Comparison of Two Modeling Approaches for Simulating the Consolidation and Desiccation Behavior of Dredged Fill

Abstract

This paper presents a comparison of two modeling approaches that can be used to predict the consolidation and desiccation behavior of dredged material, using a case study of Poplar Island, Chesapeake Bay. As part of the Site Development Plan (SDP) for Poplar Island, it is necessary to accurately predict the behavior of hydraulically placed dredged material soon after placement, as well as several years following placement. The accuracy of the prediction is critical because the success of the intertidal wetland portion of the site is highly sensitive to final elevations. This paper will present the results of comparative analyses using the U.S. Army Corps of Engineers’ computer model PSDDF, and the University of Colorado computer model CONDES. In the PSDDF analyses, the desiccation was modeled simultaneously with the consolidation, but the settlement results are computed separately for each process. In the CONDES analyses the settlements from each component cannot be separated in a single run. Thus, in order to facilitate the comparison, CONDES analyses were performed initially for consolidation only and subsequently for consolidation and desiccation simultaneously. The results indicate that both models, PSDDF and CONDES, are comparable tools to predict the consolidation and desiccation processes of fine-grained dredged material. When the same conditions are imposed in both models the results are quite similar. This is not a surprise for the consolidation portion of the process, as both models are based on the same theory. For the desiccation analysis the crucial parameter is the effective desiccation rate. As long as that rate is the same for both models, similar results should be obtained. The effective desiccation rate in PSDDF is obtained as a result of the water balance calculation with a number of empirical factors affecting the outcome. In CONDES, the effective desiccation rate is the top boundary condition and must be specified as input data. For the CONDES analyses the site conditions affect only the boundary conditions for the layer, while the quantities within the layer (void ratio and porewater pressure) are obtained in the solution process.