Work is well underway on the $27 million Back Cove West storage conduit project set in this scenic, quiet, residential area of Portland, Maine. As work progresses, it is not unusual to hear the sounds or see the sights of a gantry crane hovering over a 70,000-pound precast concrete box.

Sargent from Stillwater, ME is heading up the project – the third combined sewer overflow (CSO) storage project for Portland. The outcome will provide temporary storage for the future treatment of 2.25 million gallons that would otherwise end up in Casco Bay.

In 2013, Sargent completed the Back Cove North storage conduit and is working concurrently on the Back Cove South storage facility. When completed, the projects will provide the city with 7.75 million gallons of streamlined storage.

Upon initial review of the bid plans, the Sargent operations team determined that reducing the installation depth of the storage conduit would also decrease the amount of costly rock removal on the project and that shortening the length of the precast concrete storage conduit would reduce the quantity of the complicated support-of-excavation (SOE) system.

The bid design, had a total length of 3,150 feet of 8-foot by 10-foot precast conduit and 620 feet of 8-foot by 12-foot precast conduit. Working with the city and their design team, Sargent was able to reduce the overall precast conduit length from 3,770 feet to 1,689 feet, making up the difference in storage volume with a wider precast box combined with 2,035 feet of 60-inch ADS Sanitite pipe that could be installed at a shallower depth.

This change to pipe storage, resulted in reduced trench width and the ability to install with traditional trench box shoring methods. The changes to the storage conduit also required some modifications to the storm drain, electrical, and instrumentation and controls designs, which the project team was again able to work through with the city team.

While leading the value engineering effort, Southern Maine Operations Manager, Doug Morrison and Project Superintendent, Adam Tenan were able to have custom 8-foot by 20-foot storage conduit boxes designed and manufactured by Superior Concrete of Auburn, Maine. With Baxter Boulevard being lined by large historic trees however, Morrison and Tenan discovered that it was not feasible to work alongside the trench with a typical off-road or crawler crane. The two also determined that the project would require specialty equipment such as a Mi-Jack 750 gantry crane and the widened Cat 336 excavator.

“A gantry crane allowed for the offloading, transportation and setting of the 35-ton precast storage conduit box culvert sections, all while spanning the 36-foot trench width,” Tenan said.

Working with Tim Richards from the equipment department, Morrison and Tenan were able to locate and negotiate the purchase and transportation of a Mi-Jack 750 gantry crane from a company in Texas. A truck transported the crane in pieces up to the Sargent’s Stillwater shop, where it was overhauled, customized, assembled and tested before being disassembled and transported to Back Cove for use on the project.

“The custom equipment played a critical role in this project,” said Morrison. “These unconventional concepts were vetted by the actual field employees tasked with implementing the work. Their overwhelming buy-in to the strategy and confidence in our management team to implement the plan were key factors to the overall success of the project.”

Sargent’s Stillwater shop is noted for coming up with resourceful solutions for any given project.

“The whole maintenance team works so hard together. Our crew takes care of a lot of mechanical issues to keep jobs running during the season,” said Eric Gifford, Maintenance Foreman. “All the mechanics deserve a lot of praise, they put in a lot of overtime and weekends to make sure our field crews have what they need to keep pushing forward. In the shop, we are kind of in the shadows – people don’t always see everything we are working on – but next thing you know a piece of equipment is back up and running when it wasn’t the day before.”

Gifford said the maintenance team, machinists, welders, engineers and the transportation department all pitched in to get several unique machines ready for this project.

“The crews took on the challenge of putting together the huge gantry crane needed at the site. Four other cranes coordinated assembly at the shop.” said Gifford. “After a complete overhaul of the electrical and safety systems to pass inspection, it was dismantled and sent to the project.”

Another challenge that Morrison and Tenan addressed was how to excavate and backfill the storage conduit trench. Whatever excavator they chose, the same room constraints that the crane faced would apply. After seeing an image shared by a Sargent employee on social media, they formed the idea of customizing a 336 excavator into something that could span the entire trench. Sargent contacted Foresight Engineering of Lincoln, Maine and soon received plans of an excavator with a 39-foot span between the tracks - a typical Cat 336 has a span of just over six feet.

Sargent’s fabrication crew built the custom undercarriage at their Stillwater facility. After initial assembly and testing in Stillwater, the excavator was disassembled and mobilized to the project and reassembled there.

“The maintenance crew jacked up the machine safely, minus the undercarriage, and then put a new extended undercarriage on,” Gifford said. “They lengthened hydraulic lines, extended the hoses to the traveling motors and added another stick to the original. Chains and sprockets were replaced on the undercarriage to minimize the possibility of breakage due to torque in the span underneath.”

When completed, the maintenance teams went over the entire machine to check for any mechanical issues. Performing maintenance over a hole is not possible, so things had to be done correctly from the start. Field mechanics set up the gantry crane and the 336 CAT on-site and both proved to be invaluable equipment for the job.

Sargent excavator operator, Darrick Trueworthy is primarily working with the machine which they affectionately refer to as the “hover-hoe” (a combination of hovercraft and backhoe).

“You get a lot more flex, similar to a rubber-tire excavator,” Trueworthy said. “A big benefit is that you don’t have to worry about over-digging yourself like in normal pipe-laying activities where you can’t access the entire trench.”

“It was nice to see the operations, maintenance and welding sides coordinate things together – good communication was key,” Gifford said. “There was no division – this was a real team effort to produce a problem-solving, money-saving, working solution. Every employee-owner of Sargent has a unique skill set. While working together, the shop teams build on each other’s strengths.”

Shawn Newton, one of three Sargent employee-owners trained to run the gantry crane, said that the machine is a unique piece of equipment that can be both easy and quite complex to operate.

“It’s doing exactly what we want it to do and more,” Newton said. “We have been moving all the precast boxes, plus the mega-brace system (parts of the shoring system internal bracing) with it.”

The shoring system used for the box culvert installation is another unique feature of the Back Cove West project that involved a significant amount of design and review. The Sargent staff worked closely with the consultants from GZA and United Rentals to develop a 37-bay system. This was installed in a “leapfrog” sequence that would allow for continuous work for the sheet pile driving and pulling operation, along with storage conduit excavation, installation and backfilling.

Sargent is self-performing all the sheeting and shoring work, utilizing an SP-100 sonic vibratory hammer mounted on a standard width Cat 336. Sargent is supplying the steel beams for the lower-level framework of the internal bracing system and United Rentals is providing hydraulic wales, struts and corner braces for the upper-level framework.

While CSO storage is the focus, Sargent’s commitment also includes areas of full utility replacement, electrical upgrades, sewer, water, gas mains, full-width box cut, cobble gutter installation, gravel, paving and striping.

Project completion is on schedule for late summer 2023.

Photos courtesy of Cody Chiasson