Creating the foundations for a residential tower at Bullfinch Crossing in Boston saw Equipment Corporation of America (ECA) sell a package of machinery, including 21a Bauer MC 96 duty-cycle crane with a BC 35 trench cutter and a MAT DE 500-20 slurry handling system, to HUB Foundation as the contractor entered the slurry wall market. The sale was also the first package of this type of machinery sold by ECA in North America

ECA and HUB had an established relationship, but it was March 2017 at CON-EXPO/CON/AGG when the conversation started about breaking into Boston's slurry wall market. President and owner, Jim Maxwell knew that speciality equipment would be required and would come at a great cost.

HUB had established a reputation as a top-notch drilled shaft contractor. Although the contractor had developed expertise in bentonite slurry over the years, its leadership team had endless debates about whether to enter the slurry wall market. Bullfinch Crossing, a project HUB had under agreement, presented a timely opportunity that would make the decision clear. The geotechnical engineers had changed course. What started as a drilled shaft project would now include a slurry wall with load-bearing elements (LBE).

CHANGING BRANDS

Traditionally a loyal Soilmec customer, HUB had assembled an armada of its drilling rigs. The contractor ultimately concluded that Bauer was the global leader in slurry handling equipment.

Financing an investment of this magnitude was important. Bauer-Pileco's equipment finance specialist Anja Frazier entered the equation early on to make the numbers work. "We were able to keep the upfront costs at a minimum by including freight costs and sales tax into the financing," Frazier explained. "And with a longer financing term, HUB benefits from lower monthly payments which allow them to be more competitive in bidding jobs."

Anthony Sciortino, sales engineer for ECA Boston, and his father David, ultimately closed the deal along with Gordian Ulrich of ECA's Philadelphia office. Ulrich serves as Bauer applications engineering manager.

"This is unique because you only have a handful of customers that have made the investment to do this type of work," explained Sciortino. "You can count them on one hand."

"If you look at the numbers, the investment doesn't make sense," admitted general superintendent Greg Maxwell. "You're talking about upwards of US$6 million invested on a job (Bullfinch Crossing) that is less than the entire cost and you have two competitors in town fighting over the work already."

Before it received the machinery, HUB sent some of its staff overseas to maximise the investment before firing it up. "Any contractor can buy the equipment, but in order to make money with it, you've got to know what you're doing," explained Maxwell. "What we were trying to do was get a few rungs up that learning curve before we started doing it ourselves."

ECA and Bauer funded the trip to Europe so HUB could see the equipment in action and learn from some contractors that were using it. The plan was to visit projects in France and Germany, in addition to Bauer's Open House in Schrobenhausen, Germany.

HUB was onsite from December 2017 through May 2018, although the slurry wall work took place from February through April. HUB was to install a 36in slurry wall and LBEs down to 75ft to support the 45-story residential tower at Bullfinch Crossing.

SLURRY WALLS

The collection of massive Bauer machinery may look out of place in some American cities, but not in Boston. New York and Toronto design their excavation pits for secant pile walls, but Boston's geotechnical engineers prefer slurry walls. Slurry is effective for holding open excavations in these materials, which is why subsurface conditions in Boston are more conducive to slurry walls than its sandier counterparts like Connecticut or Rhode Island.

The city's complicated subsurface conditions play a role. Boston was originally an 800-acre peninsula (Shawmut). Today a large portion sits on man-made land due to a series of projects starting in 1630. The soil is a Boston blue clay underlain by glacial till and Cambridge argillite. The Cambridge formation underlies most of the downtown area.

HUB used the BAUER MC 96 with BC 35 to dig out a rectangular trench for the slurry wall. It also installed LBEs to support the load of the tower. There were 22 7 to 84ft panels, eight of which interlocked to form the core wall. Each was 3ft wide and 9.2 to 22.7ft long. The job also included 107 85 to 130ft minipiles and three 85ft long, 4ft diameter drilled shafts, and 42 piles for support of excavation.

The excavation was more shallow than anticipated, and due to the Boston blue clay, HUB started the excavation using another crane with a grab. The BC 35 was used to clean out the holes and finish the rock sockets, which ranged in length from 17 to 33ft.

"We felt very confident that the material was going to cooperate as far as the excavation process was concerned," explained Maxwell. "The conditions were almost perfect as far as mitigating risk with a slurry wall." HUB found that the clay was stiff, the argillite had minimal fractures, the glacial till had cohesive properties, and there were no large areas of sand or gravel in danger of collapsing.

SMALL FOOTPRINT

The rectangular Bullfinch Crossing site is wedged between the towering walls of the existing Government Center Garage and two busy streets. "It was kind of like a structured ballet," Maxwell said, referring to moving equipment and materials around on the job site. "The key to this job was all logistics."

The BAUER MC 96 was one of four cranes on site, in addition to a host of drilling and excavating equipment. HUB was restricted from accessing the core wall until late in the project, which hampered its efficiency and sequencing by confining crews to a small portion of the site. HUB chopped away at the slurry wall and LBEs one section at a time and shuttled machinery between working areas and parking spots.

The project was initially designed to include a combination of drilled shafts and micropiles, but geotechnical engineers Haley and Aldrich added LBEs to the mix.

The challenges of supporting a 45-story building on a small footprint became clear during the core design, according to Maxwell. He explained that these dimensions created tremendous shear load and uplift. The loads on a tower with a larger footprint would have been axial, transferring downward through the foundation.

"Once they [the engineers] got into the weeds of the core design they had a lot of difficulty getting those shear loads down into the shaft from the core wall," Maxwell recalled. "They had a lot more success pushing the loads into the slurry wall that we used because it was basically the same footprint as the shear wall."

THE EQUIPMENT

The centrepiece of the operation was a Bauer MC 96 duty-cycle crane. Its HTS (hose tensioning system) 58M enabled a cutting depth of up to 190.3ft. The BC 35 - a 41.3ft high by 9.2ft long unit with four cutter wheels - carved out the 36in wide trench for the slurry wall.

"Bauer is known not only for its power and its ability to cut and churn rock quickly but also its reliability," said Sciortino. "When you're on a high production slurry wall job, the reliability of the power comes into play."

The slurry handling system was a Bauer MAT DE 500, which consists of two BE 250-60 desilters and an ST H35 agitation tank. It was used to remove soil particles from the bentonite slurry used during installation of the LBEs.

 

Mobilising the equipment and keeping it running properly was critical and ECA facilitated a connection up front that resulted in HUB hiring industry expert Sal Arslan to keep Bullfinch Crossing running smooth. He helped to set up the slurry handling system, mobilise the crane and cutter, train HUB's operator, and share his knowledge of slurry wall construction.

HUB acquired the Bauer crane, cutter, and slurry handling system as a package, although it could have mixed and matched other brands. By doing so, the contractor was able to use the B-Tronic system, which collects data from dozens of sensors located at all critical points in the cutter. The display of the cutter's current position and the deviations in both X and Y direction enable the machine operator to adjust the steering plates and maximise performance.

SLURRY

HUB pumped bentonite from a hose within the cutter to support excavation for the slurry wall. The slurry spoils were then pumped into the Bauer desanding plant to remove soil particles. The clean slurry was pumped back into the trench for temporary excavation support.

Project delays put HUB on site during the winter months. Keeping the plant warm and slurry from freezing was a factor during repeated bouts of snowfall, sleet, and freezing rain.

Dealing with the slurry left over from the cutting operation was difficult on the compact site. The loadout area for the spoils and the rebar laydown area were one and the same.

"We trucked in as many cages as we could," Maxwell recalled. "We were able to build the cages for the LBEs offsite, truck them in as needed, and put them in the hole right away," explained Maxwell, noting that this kept the slurry loadout area open. That strategy ended when it was time to build the core wall. They were too big to haul so HUB's crews had to assemble them onsite. This resulted in periods of up to four days where slurry spoils were piling up, consuming nearly one-third of the site. Further complicating matters, the landfill only accepted two loads per day and would not take material during inclement weather.

Bullfinch Crossing was a successful maiden voyage for HUB. There were no equipment breakdowns and no major surprises underground. More importantly, HUB now has a successful slurry wall project under its belt.

"The clients will have more confidence in our ability to perform contracts," said Maxwell, noting that HUB was recently awarded another contract in downtown Boston. "We now have the experience, equipment, and people to do the work."

The company plans to concentrate on the New England and Upstate New York market it serves for now. Whether HUB's venture into the slurry wall market pans out remains to be seen.

"It was a bit of a gutsy move," admitted Maxwell. "One of the benefits is that we have a healthy minipile and drilled shaft market that has helped support the debt of this new venture."