Great Lakes Finishes Mystic River Job
Great Lakes Dredge & Dock Company was awarded the $31.9 million contract for the Boston Harbor Deepening project last May. The contract includes maintenance dredging, improvement dredging and berth deepening in a unique project where 794,000 cubic yards of maintenance material is being placed in cells excavated in the harbor bottom and capped with clean sand.
The channels and berths are being deepened from their present 35 feet to 38 and 40 feet, which will increase the viability of a port that served more than 2000 bulk and container vessels in 1997.
The project also includes dredging 1,325,000 cubic yards of clean material to deepen the channels to 40 feet from the existing 35 feet, 16,900 cubic yards of rock, which will be blasted, and an estimated 1,300,000 cubic yards of clean clay excavated from the disposal cells.
All new work and cell material has been approved for open water disposal, and is being placed in the Massachusetts Bay Disposal Site, (MBDS) which is a historic dredged material placement site 22 miles east of the harbor near the Stellwagen Bank.
Great Lakes began dredging the cells in the Mystic River in August, moving the overburden of silty maintenance material into a barge, digging the cell with Dredge 54, then placing the silt into the cell. They created four cells in this manner, and so far have filled three during the Mystic River channel maintenance dredging.
The plan calls for excavating 47 cells throughout the project, each measuring about 200 feet wide, 500 feet long and “as deep as possible,” (about 20 feet or more.). The cells will be capped with three feet of clean sand.
All the maintenance material is being contained in capped sites. While the discharge of sewage into the harbor has been stopped, there remains urban runoff and a large quantity of historic deposits that have accumulated and continue to move around the harbor, settling into the deeper areas, such as the navigation channels. There are no major inflows of sediments from rivers, which is why the port has a 10-year maintenance cycle. The channel and berth sediments contain PAH’s and minor concentrations of heavy metals and PCB’s.
Project specifications designated a sealed environmental bucket for digging fine sediments, in order to minimize turbidity during dredging. The 54 used a 40-cubic-yard, 25,000-pound Cable Arm clamshell for maintenance. The design of the Cable Arm reduces bottom suction, which creates little turbidity when rising through the water column. Material is loaded into four 4000-cubic yard scows and two 7200 cubic yard scows and placed in the cells with the aid of a GPS unit on the tug.
“The Cable Arm has worked out well because it discriminates between the silt and clay,” said Pete Jackson, project manager for the New England District, Corps of Engineers. “It skims across the top of the clay, removing only the silt. This is important because the cost of digging the material placed in the cells is about $23 per cubic yard, opposed to $7 for the clean material. This makes it important to dredge only the material designated unsuitable for open water disposal in this phase,” he said.
For deepening berths, Great Lakes is using a McGinnis bucket that was more maneuverable along the dock faces, said Richard Johnson, project manager for Great Lakes. “We made changes to the bucket to comply with the job specifications, and got it approved for this job by the Corps of Engineers,” he said.
To date, Great Lakes has excavated four cells and filled three, and is excavating a sixth, larger cell, called the Super Cell, using Dutra’s clamshell dredge Super Scoop. Because they are getting steeper side slopes and greater depths in the cells, it is likely that the maintenance material will fit into fewer cells, which would all be concentrated in the Mystic River. Clustering the cells in one area will make it easier to monitor and maintain them, said Jackson.
The next event was capping of the material in the cells with sand brought from a maintenance contract in the Cape Cod Canal by the hopper dredge Sugar Island. The hopper dredge is ideal for laying the sub-surface cap because of its maneuverability, and the ability to either bottom dump or pump out, said Jackson.
By November 1, the maintenance in the Mystic River was complete, and the 54 went to Portland, Maine for a maintenance job before returning to begin the deepening phase on the Mystic River channel. Dredging will continue 365 days a year until the project is completed.
The total cost of the project, including dredging, design, management and monitoring, is $47 million: $20 million for the maintenance, $20.3 million for the improvements and $6.7 million to deepen the berths. Of this, federal funding is $31.7 million, using Civil Works funds from fiscal years 1998, 1999, and a small amount in FY 2000. Scheduled completion time is December 1999.
There is an extensive water monitoring program during the placement and capping of the maintenance material, said Jackson. Once the first three cells are capped, and the fine-tuning is done, we expect smooth sailing for the rest of the project, he said.
The navigation improvements at Boston Harbor were authorized by the 1990 Water Resources Development Act, which approved the following changes:
Deepen to 40 feet in the Reserved Channel, except for the upper 1340 feet, which would remain at 35;
Deepen the existing 35-foot Chelsea River Channel to 38 feet;
Deepen the existing 35-foot Mystic River channel to 40 feet, except for an area at the upstream limit along the southern shoreline where existing users to not require greater depths;
Deepen a portion of the inner confluence 35-foot main ship channel leading to the confluence of the Mystic and Chelsea Rivers, and deepen the entire confluence to 40 feet to provide maneuvering area for vessels entering the Mystic River.
In addition to the WRDA authorized deepening, 10 berths will be dredged. The four terminals on the Mystic River are Distrigas (liquefied natural gas,) Prolerized (scrap metal,) Boston Autoport and Massport’s Medford Street Terminal. On the Reserved Channel, berths to be deepened are Black Falcon Cruise Terminal at Massport’s Conley Container Terminal, and the former army base berths. Along the Main Ship Channels, Massport’s North Jetty, Mystic Piers and other army base berths will be dredged. Privately owned berths will be deepened by their owners.
This project began in 1997 on a small scale when Massport contracted Weeks Marine to dredge berths 11 and 12 of the Conley Terminal. The Corps of Engineers provided design and construction management services to Massport, and the one-month project was completed in July 1997. Weeks used a 25 cubic yard Cable Arm bucket on their clamshell Dredge 550.
This initial project included all the components of the larger project, including the permit conditions. The cell used for disposal is just south of the Inner Confluence along the East Boston side in the main ship channel. About 66,500 cubic yards of material was removed and disposed in the cell, and clean material taken to the Massachusetts Bay Disposal Site. The disposal cell required removal of 102,500 cubic yards of clean material, which was taken to the MBDS. The lessons learned from this phase were applied to the plans and specifications for the current project.