By Ariel Wittenberg
May 16, 2014 12:00 AM
NEW BEDFORD — When finished, the retaining wall at South Terminal is expected to support almost anything, including New Bedford’s economic future.
The specially engineered retaining wall is what will ensure the port facility can hold the weight required to be a hub of offshore wind staging.
“This is really what makes this project unique,” engineer Jay Borkland said Thursday.
Made up of interlocking steel ovals and circles called cofferdams, the retaining wall will allow the facility to have an average weight-bearing capacity of 4,000 pounds per square foot. In some places, the facility will hold 18,000 pounds per square foot.
“You are going to have all of this weight transferring not just vertically but horizontally,” Borkland said. “We needed a retaining wall that would not blow out under the pressure.”
That’s partially because one of the major components of South Terminal construction is backfilling an area of the harbor just north of the Gifford Street boat ramp.
Once put to use, the manufactured land will have to hold hundreds of pieces of steel weighing up to 500 tons, as well as hulking cranes to carry the turbine components. The retaining wall ensures the facility doesn’t collapse under the pressure and into the harbor.
Choosing the retaining wall’s design was a process steeped in calculations. Borkland and fellow engineer Susan Nilson even conducted a “crane study” to help them, putting sensors in the ground under different types of cranes holding different types of equipment.
Borkland said measuring the crane pressure was particularly important because the cranes, which will be right up on the edge of the pier, will be monstrous in size.
With treads six feet tall and 20 feet wide, the cranes working at the completed facility will be four to five times larger than those helping to construct it.
“They are going to make these ones look like toys,” Borkland said.
Studying the cranes helped his team determine exactly how much pressure the retaining wall would have to withstand.
“When we say this facility is purpose-built for offshore wind, we mean we know exactly which cranes can pick up which equipment at which distance and be supported here,” said Bill White of the Massachusetts Clean Energy Center, which owns the facility.
Ultimately, the cofferdam design was picked from up to 20 choices considered by the engineer team.
“For the flexibility of the load, we had to throw out a lot of (the options),” Nilson said. “We found this was the best fit.”
Today, the circular structures of the cofferdam are still visible at the site. By the time the facility is completed seven months from now, the cofferdam will be covered by concrete platforms and blacktop. The finished facility will look like a parking lot.
“Now is the perfect time to see what holds this facility together,” Borkland said Thursday.
“The bones are out there now; that’s what gives it its strength. Soon it will be covered in skin.”