Mathematical model of the consolidation/desiccation processes in dredged material
Cargill, Kenneth W.
Technical ReportAbstract: This report documents the development of a mathematical model of the consolidation/desiccation process in soft, fine-grained soils such as dredged material; provides for computer solution of the model; and verifies the solution through comparisons with field measurements. The consolidation process is modeled through the previously documented finite strain theory of consolidation which accounts for the large strains and nonlinear soil properties inherent in the very soft materials commonly found in maintenance dredgings. Pertinent equations necessary for process calculation are given. An empirical description of the desiccation process is presented in terms of the water balance in the uppermost crust in the dredged material layer and generally conforms with previous work. The first and second drying stages along with associated characteristic material properties (saturation limit and desiccation limit) are defined. Procedures for calculation of the effective depths of first and second stage drying, soil evaporation rates, and the surcharge induced by water table lowering are also given. The interaction of the consolidation and desiccation processes is discussed and the mathematical treatment proposed. The mathematical model is next rewritten for computer solution through the computer program PCDDF. The program uses an explicit finite difference scheme for solving the consolidation portion of the problem and makes monthly adjustments in the top boundary condition and boundary location in accordance with the amount of desiccation which has occurred. In addition to material settlement which comes from a calculation of void ratio distribution, the program also calculates the distribution of stresses and pore pressures through the layer, which can be indicative of soil strength. Any sequence of material deposition as well as consolidation in an underlying foundation layer can be considered. The model and computer solution are then tested through comparisons of predicted material settlements with measured settlements in three confined disposal areas. The areas include Canaveral Harbor where one layer of material was deposited, Drum Island where two layers were deposited about 1 year apart, and Craney Island where yearly depositions have occurred for a 24-year period. The results of these comparisons show that the proposed model and computer solution offer realistic indications of the material settlements under a wide variety of conditions. Appendices to the report include a detailed user's manual (Appendix A) for computer program PCDDF along with a program listing (Appendix B) and sample input and output data (Appendix C). Consolidation properties (Appendix D) and the design of a comprehensive field verification site (Appendix E) are also included.
Environmental Laboratory (U.S.)Engineer Research and Development Center (U.S.)
Dredged material; Soil consolidation; Soil compaction; Soil dessication; Soil moisture; Mathematical models; Numerical models; Computer programs; PCDDF
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