Duluth Superior Harbor – Slips 3 & C Remedial Cap

Project Summary

CONTRACT VALUE: $3,000,000


This project involved the dredging of approximately 27,000 cubic yards of clean sand from an area of Duluth Harbor designated as “East Gate”. The dredging consisted primarily of clean, silty, sand. Very little soft, organic, sediment was encountered; it was White Lake’s goal to avoid organic sediments to the maximum extent possible. The material was utilized as the base layer of a remedial cap in Slips C & 3 in the Duluth Harbor.


White Lake removed several large debris items identified during baythmetric surveying of both Slips 3 & C. The coordinates and depths of all debris items were input into the machine control computer and the operator was able to easily retrieve them with the same Sennebogenn 840 material handler that was utilized for all capping operations. The debris items removed were treated as contaminated and disposed of at a proper landfill.


This project involved the construction of a multi-layer, engineered cap in Slip C and Slip 3 within Duluth Harbor. These slips are active shipping slips and had high levels of heavy metal impacted sediments.

The cap consisted, primarily, of a base layer of silty sand dredged from the East Gate borrow area and subsequently placed as the base layer of the cap. Following the placement of this sand, another layer of stone aggregate (varying in size) was placed atop the sand in varying thicknesses as per the specifications. In general, the sand cap was placed in two 12” – 14” lifts for a total of 24” – 28” of cap. Following the sand, varying sizes of stone aggregate was placed as the armor layer for the cap. The thicknesses of these layers varied but generally ranged from 10” – 12” in thickness.


Materials dredged from the borrow area were loaded into 30’ x 80’ material barges comprised of 10’ x 40’ sectional barges. These barges were capable of transporting approximately 160 cubic yards of sediment in a barge. These material barges were pushed approximately 1.5 miles from the dredging borrow area to the location of the capping rig where they were then unloaded and the materials used in the construction of the cap.

The sectional barges utilized during this project added an important efficiency to the project. One of the slips that was capped was too narrow for large material scows. In addition, when it came time to handle imported aggregate material, these size barges were easily loaded and maneuvered around the restricted laydown area that White Lake utilized for the project.


Because of the high concentration of heavy metal contaminates in the underlying sediment being capped, environmental controls were implemented by White Lake at the entrance to each slip. A turbidity curtain was installed at the entrance to each slip equipped with a remote controlled “turbidity gate” that would lower a section of the curtain as the material barges approached to allow transportation of clean capping materials between the borrow site and the capping site. These gates were kept closed at all other times upon entering or exiting. In addition, White Lake performed all capping operations on cable anchors mounted on the side walls of the slip; spuds could not be utilized out of fear of damaging the cap and re-suspending contaminated sediments.

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