Evaluation of Underdrainage Techniques for the Densification of Fine-Grained Dredged Material.

The result of a large-scale field experiment to evaluate the dewateirngdensifying of fine-grained dredged material with underdrainage techniques is described. The techniques evaluated were gravity underdrainage, partial vacuum in an underdrainage layer, seepage consolidation, and seepage consolidation with a partial vacuum in the underdrainage layer. The experiment was conducted using five test sections having 30- x 30-ft bottom areas and 1v on 2H sideslopes. One section was used for each of the four techniques evaluated and a control section was not treated. Settlement, pore pressure, water content, and vane shear measurements were taken to provide a basis of evaluation. Initially, a 6-ft nominal thickness lift of dredged material was used then, after one year, a second 6-ft lift was added. The experiment ran for approximately two years. All of the techniques evaluated produced more densification than did the test section containing untreated material. Of the techniques evaluated the partial vacuum in an underdrainage layer was the most effective. This was true with both lifts of dredged material tested but was considerably more pronounced with the first lift than the second.