To improve the water quality and health of Back Cove, the City of Portland developed a Long Term Control Plan for stormwater, including the Back Cove West Storage Conduit Project, a 2.25 million gallon storage conduit under Baxter Boulevard. Sargent undertook this important municipal project, incorporating innovative solutions, collaboration, and the navigation of numerous challenges to reach a successful completion.

From the outset, the Sargent team understood the complexity of the project, which involved significant geotechnical risks, the immense challenge of installing precast concrete storage boxes below sea level, extensive ledge removal, and a constrained work area. Superintendent Adam Tenan played a pivotal role in devising strategies to address these obstacles, ultimately creating an innovative value engineering proposal with a double-wide 8’ by 20’ box culvert design that was supported by 3D printed mockups and draft plans. Custom equipment, such as a specially designed excavator undercarriage and a gantry crane, also contributed to project success by facilitating the installation of the wider, heavier storage boxes for the self-performed Support Of Excavation (SOE) system. Sargent worked in collaboration with both the City of Portland and the Portland Water District to achieve a successful outcome, and was awarded additional work to replace an existing force main, or wastewater pipeline, as well.

 

COMMUNITY

Stormwater management is critical in Portland, where the sewer system collects both wastewater and stormwater. During heavy rainfall, the system often becomes overwhelmed, resulting in the discharge of combined sewer overflow (CSO) into Back Cove, one of the city’s most popular recreation areas. The Back Cove West Project, the third CSO storage project in Portland performed by Sargent, provides a significant reduction of CSO flowing into Back Cove and eventually Casco Bay. This system captures an additional 2.25 million gallons per significant storm event of the dirtiest first flush of a one-inch rain storm and stores it for future transport to the East End Wastewater Treatment Facility. Not only does reducing CSOs improve the habitat for local wildlife, it also increases the potential for recreational activities beyond walking and running at Back Cove. In addition, Sargent completed significant improvements to ADA accessibility, with updated traffic lights, pedestrian crossings, and ADA ramp installations at several intersections.

To celebrate the completion of the project and the reopening of Baxter Boulevard, Sargent participated in the Back Cove Neighborhood Association’s block party and made a donation that enabled the Association to scale up the party to include live music, food trucks, games, and a bicycle course for kids. Sargent also brought in a dump truck for the always popular Touch-a-Truck opportunity for kids. Additionally, since there was a significant amount of waste fill material, Sargent saw an opportunity to help out a neighboring Little League program. Sargent trucked the thousands of cubic yards of waste fill to the Town of New Gloucester at the location of a future Little League complex at no cost to the League.

 

INNOVATION & CHALLENGES

During the value engineering process with the city, it became evident that innovative construction methods were essential for the project. Expanding the precast concrete storage conduit from 10’ to 20’ posed challenges with the already narrow work area. To address this, Sargent procured a Mi-Jack 750 gantry crane, customizing it to span the 32’ width of the sheet pile cofferdam and transport the larger conduit sections. Sargent also commissioned a customizedCAT 336 excavator undercarriage, expanding its width from 6’ to 39’, enabling it to traverse the cofferdam’s full width and access the excavation area more efficiently. Equipped with a third arm attachment, this modified excavator, dubbed “hover-hoe” or “Frankenhoe,” garnered widespread attention and facilitated project success. Upon completing the precast storage conduit, Sargent’s supplier, Superior Concrete, recommended using TerraThane to seal joints, ensuring water tightness and reducing maintenance. Following successful tests at the precast plant, the City and engineer approved the product’s use, anticipating decreased maintenance throughout the conduit’s lifespan.

The field crews encountered a myriad of challenges during the project. Soil conditions ranged from soft marine clays to bedrock, necessitating creative solutions for the Support Of Excavation (SOE) system and the precast concrete storage conduit. Toe pins and hydraulic shoring were introduced to the design to address varying soil conditions, while nearly 10,000 cubic yards of bedrock had to be drilled, blasted, and excavated. Stringent safety measures were implemented during blasting, including pre-blast surveys and video documentation. The project’s location, situated near Back Cove in the middle of Baxter Boulevard, posed additional complexities. As a popular recreation area, Sargent put in strict erosion controls with weekly inspections, and  prioritized pedestrian and biker safety. To protect the historic Linden trees memorializing WWII soldiers that lined the boulevard, Sargent utilized specialized equipment to minimize disturbance to the trees and their root systems.

 

SAFETY

Given the complex nature of the project and site, safety had to be a major priority for Sargent on this project. To address safety challenges such as deep excavation, crane work, and fall protections, Sargent held regular training sessions and daily “Plan Of the Day” (POD) meetings. Sargent developed the Support of Excavation (SOE) system for the precast storage conduit in collaboration with GZA GeoEnvironmental and United Rentals, and incorporated crew feedback for continuous improvement. Blasting operations were meticulously managed with advance notifications to residents, blast matting, video recording, and seismograph measurements, resulting in no documented damage to nearby structures. The Sargent team worked over 93,000 hours without any OSHA recordable injuries or lost time incidents.

 

View the project video here – https://youtu.be/m6V4w-kJGgk

Project Title: Back Cove West Storage Conduit

Owner: City of Portland

Construction Manager: Sargent Corporation

  • Adam Tenan, Project Superintendent
  • Doug Morrison, Operations Manager

Engineer/Architect: Wright-Pierce

Key Partners:

  • GZA GeoEnvironmental
  • United Rentals
  • Superior Concrete