Specifying an Overdredge Allowance for Environmental Clamshell Dredging in Puget Sound

Abstract

Environmental dredging costs in Puget Sound range from about $25/cubic meter (m 3 ) to $235/ m 3 ($21/cubic yard (cy) to $180/cy). Most of the costs are associated with the handling, treatment, and disposal of the contaminated sediments. To hold costs in line, engineers are being asked to reduce the overdredge allowance by eliminating excessive volumes of dredged sediments. Reducing the overdredge allowance, however, tends to slow production rates and increase the time and cost to complete the dredging project. This paper examines the overdredge tolerances associated with clamshell dredging in Puget Sound. The accuracy of the hydrographic survey, traditionally used to measure the overdredge allowance, is also reviewed. Methods of optimizing the overdredge allowance, given the inherent inaccuracies of the process, are discussed. Overdredge design and specification recommendations are made as well. Using three environmental dredging projects from Puget Sound as examples, this paper discusses the need for improvements in the vertical control of the bucket cut, investigates the limits of overdredge tolerance reduction, suggests contractual strategies for minimizing the overdredge allowance, and illustrates the difficulties of trying to minimize overdredging. An overdredge depth of 1 foot was specified for environme ntal dredging in the three project examples: the Puget Sound Naval Shipyard, Bremerton; the Cascade Pole Remediation Site at the Port of Olympia; and Eagle Harbor, Bainbridge Island, Washington. A penalty clause for excessive overdredging was included in the contract documents for the first two projects. All three projects involved an adjustment in instrumentation, equipment, or procedures to meet the overdredge allowance.