Placing 3.8 Million Cubic Yards of Concrete - Modern Continental and D&M Take on the Engineering Challenge of a Floating Freeway May 1999

At $12 billion, it's one of the largest infrastructure projects in the United States

Modern Continental, the general contractor responsible for Boston's Fort Point Channel Crossing project, has the enormous and unusual task of building a freeway tunnel that will float on water. Currently under construction, the floating concrete tunnel is a challenge never undertaken before in the United States, and one that requires great attention to detail with no room for error.

"I would never have imagined a floating freeway was possible in my lifetime, but I'm sure glad our company is a part of it," said Norman DeMello of D&M Equipment Company in Fall River, Mass. One of the largest projects ever tackled by D&M, the company is supplying all the concrete services for the structure. This includes pumping, placing and finishing with the help of three Putzmeister concrete boom pumps – the 38-, 43+ and 52-Meter.

Jack DeMello, also of D&M, said, "We use Putzmeister equipment because it's critical to keep everything on schedule with a project of this magnitude. We've depended on these high-performance pumps for years. To date, we haven't experienced one minute of downtime on this job."

The $301 million Fort Point Channel Crossing is just one part of the overall tunnel project, also known as the Big Dig. It will be made up of six 50,000-ton concrete tunnel sections, four of which are completed. The $12 billion Big Dig project will ultimately consist of eight miles of tunnels and underground highways that will replace Boston's heavily used elevated Central Artery, which divides the north end from the rest of the city.

Floating tunnels new to North America While floating tunnels are new to North America, similar projects can be found in China, Taiwan and England. Eric Cederholm, senior project engineer of Modern Continental, said, "The Fort Point Channel Crossing does not have the luxury of nice, equal symmetry as in these other applications. This project has the additional challenge of working above an existing 100-year-old non-reinforced tunnel."

Some ingenious engineering is required to make approximately 1,100 feet of the concrete tunnel float. The principle is somewhat similar to the construction of a ship – the ship is built in a dry dock, after which the area is flooded with water and the ship floats. With this project, however, many more complex variables are involved.

The first step, started in 1995, was to dig casting bin – a hole 55 feet deep. Tied-back slurry walls were installed on three sides and cofferdam cells were put in place next to the water. These six-sheet pile cofferdam cells, filled with stone, were positioned to keep the Atlantic waters from entering the casting basin. These cells also allowed the opening and closing of the area for the construction of the six freeway tunnel sections.

Special concrete mix required In August 1997, the first of 12 phases in construction of the tunnel began. The first four sections were completed in 14 months and the remaining two sections are scheduled for construction later this year. To pump the pre-cast immersed tunnel tubes, D&M's Putzmeister unit was positioned at the top perimeter of the basin and extended out and then down.

"We're pleased with the performance of the Putzmeister equipment for precise and uninterrupted pumping," said Edward McInnis, project superintendent for Modern Continental. "And we're very satisfied with the service and support that D&M has provided us in this complex engineering project. They have gone the extra mile to ensure their portion of the project meets our satisfaction."

Adding to the uniqueness of this job, Bardon Trimount, the ready-mix supplier, created a special 5,000 and 6,000-psi mix design. In order to keep the temperature of the mixture below 70 degrees during the hot Boston days, Bardon Trimount added ice to the mix. Several corrosion inhibitors were also added to prevent against road-salt problems, along with super plasticizers to strengthen the mix.

D&M estimates hauling 120,000 cubic yards of concrete over the course of the Fort Point Channel Crossing project. When building the first four sections, D&M carefully tracked the density of the concrete to precisely monitor the concrete mix of the tunnel and to ensure the structure is light enough to float.

The first four sections will float in 1999 with the help of temporary water-filled ballast tanks. These tanks will be positioned inside the tunnel sections to help facilitate movement up or down in order to position each of the four sections and tightly seal them. For three days, water will be pumped slowly back into the casting bin to avoid crushing the structure. Once the area is flooded, the coffers will be removed and each section will be dragged to sea for further positioning.

Global positioning systems required Modern Continental is planning to use conventional divers and high-tech global positioning systems to align the four concrete tunnel sections. Each of the four sections will be placed over 6-foot diameter concrete drilled shafts, fitted with gaskets in order to seal them. The 4-inch space between the bottom of the tunnel and the top of drilled shafts will be grouted through grout pipes cast into the tunnel section walls. The bottom of the shafts will be drilled 175 feet into bedrock. Modern Continental will repeat the process for construction of the last two sections, which D&M anticipates floating in late 1999.

There's a billboard clearly visible when entering the construction zone of the Big Dig that asks "Will it ever be finished?" The project is scheduled for completion in 2004, after nearly 20 years of planning and 13 years of construction. In the end, an estimated 3.8 million cubic yards of concrete will take the place of 16 million cubic yards of dirt and clay removed to complete the project.