Meticulously Connecting the Dots

Slurry Microtunneling in Providence, R.I.A leading North American microtunneling contractor is nearing completion of a 19-month project comprised of 11,976 lf of microtunnel work in historic Providence, R.I.
The Woonasquatucket Interceptor will complement other infrastructure improvements, assisting in the separation of sanitary and storm water flows to eliminate pollution into the Woonasquatucket River, which flows to Narragansett Bay.

Estuary Narragansett Bay offers hundreds of miles of coastline to the City of Providence, features fresh and sea water and hosts a diverse ecosystem mix of species. Currently, 64 combined sewer systems in the surrounding districts are discharging pollutants into the Narragansett Bay and its tributaries to the tune of 2.2 billion gals each year during wet weather events that exceed the 100-plus-year-old infrastructure’s capacity. After many years of studies, workshops and project assessments, the Narragansett Bay Commission (NBC) put forth the Combined Sewer Overflow (CSO) Abatement Program. This program satisfies the regulatory requirements for the Federal Clean Water Act, the Rhode Island Department for Environmental Management and long-term cleanup goals for Narragansett Bay, while offering the lowest operation, maintenance and construction costs.

The full scope of the three-phase CSO design includes the construction of six miles of 26-ft diameter underground storage tunnels, five CSO interceptors, a wastewater treatment facility and sewer separation in 12 regions. Phase I took place from June 2001-October 2008. Phase II began in July 2011 and is currently in progress. Phase II, totaling $256 million, entails the construction of the Seekonk (7,200 lf) and Woonasquatucket (16,400 lf) CSO interceptors, two sewer separation projects and a wetland facility that will ease discharge from 17 current CSOs and a 1,800-ft connection adit to a Phase 1 tunnel. Construction on Phase II will continue through December 2014.

Slurry Microtunneling in Providence, R.I.The main contractor for the project is a joint venture of Barletta Heavy Division Inc. of Canton, Mass., and Shank/Balfour Beatty of San Mateo, Calif. Shank/Balfour Beatty was responsible for a 200-ft deep drop shaft and 1,800-ft adit and Barletta was responsible for the work of the microtunneled alignment. Barletta excavated shafts, installed precast manholes, backfilled structures and performed all open-cut pipe installations and final restoration. Hayward Baker performed shaft stabilization efforts and earth support system installation.

Subcontractor Super Excavators Inc. (SEI) of Menomonee Falls, Wis., is the microtunneling contractor. SEI is a predominant player in the microtunneling and pipe jacking industry. The contractor is known to tackle some of the most complex projects from all corners of North America and have recently set in motion plans to expand to serve Canada under the division name CRS Tunneling.

SEI mobilized a skilled microtunneling crew to Rhode Island, with an average crew size of 12 to 15, consisting of key personnel from its corporate headquarters and workers from the local labor unions. Combined, this team assumed single, double and three-shift scenarios to maintain the project schedule.
The entire City of Providence encompasses a total of 18.5 sq miles of land and is located at the tip of Narragansett Bay. The Woonasquatucket River, tributary of the Narragansett Bay, traverses 15.8 miles through Providence, and marks the location for the microtunneling work. Construction area for Phase II is located in the oldest parts of Providence, named the Federal Hill and Downtown regions. The streets are narrow, irregularly structured and surrounded by compact development. Proximity to buildings, erratic intersections and traffic constraints made detailed shaft planning a critical component to the microtunneling contractor’s success.

SEI’s superintendent and foreman had to be strategic and creative when staging mining equipment operations throughout the city. SEI project manager Shawn Stockwell stated, “The tight work easements required that we stack our equipment and MTBM control containers in order to fit within permitted areas. All of the RCP used on the project had to be staged along the streets, placed in median areas and/or parking lots then transported to the mining shaft because it could not fit onsite. At a few of the locations, the separation plant had to be setup at a remote location so there was enough room to handle the spoils.”
The Geotechnical Baseline Report was prepared by the project’s program manager, The Louis Berger Group Inc. of Providence. The GBR’s assessment of the region determined that the area consists of river bedrock containing sandstone, siltstone, shales, coal mixed with sand, silt, gravel, concrete and asphalt fill materials. The geology contains up to a 20 percent volume of cobbles and boulders and groundwater was encountered shallower than 10 ft below the surface.

SEI’s fleet of equipment includes many Akkerman microtunneling systems. SEI used three MTBMs, an SL36, SL60 and SL74, which were at times increased to accommodate the six pipe ODs. All MTBMs were outfitted with mixed ground cutter heads featuring disc cutters and drag bits to excavate the mixed geology.
SEI began work on the project in September 2012 and anticipates completion of 22 total runs in March 2014. These conduits along the interceptor include: 618 ft of 30-in. RCP in two drives; 149 lf of 36-in. HOBAS pipe in one drive; 4,423 lf of 48-in. RCP in seven drives; 743 lf of 54-in. RCP in two drives; 2,063 lf of 60-in. RCP in four drives; and 3,980 lf of 72-in. RCP in six drives.

Seventeen of the shafts were constructed using interlocked steel sheeting and concrete whalers. Two shafts, plus one more located along a sluice gate and screening structure, were constructed using interlocked steel sheeting and steel beam whalers. Two shafts were created using secant piles in geology with saturated bedrock. Due to the unexpected behavior of the challenging geographic profile, additional work was added to the contract for the installation of jet-grouted support for the MTBM as it entered and exited these shafts. Shaft depths varied from 21 to 42 ft deep, with an average vertical depth of 30 ft.

SEI crews assembled to begin in early September 2012. Right away, delays were experienced and the schedule of installations was altered due to sinking of the launch shaft floor and utility conflicts. Barletta  worked with the NBC on resolving these issues in a timely manner.

The first drive was selected due to shaft availability and was initiated at the center of the project alignment. This first run installed 149 lf of HOBAS pipe. Following this run, SEI crews tackled each conduit installation as opportunities became available. Stockwell described SEI’s approach as “meticulously connecting the dots” to complete all areas of the project.

Several additional change orders ensued, the result of unanticipated soil conditions. Stockwell reported,“Anytime the soil is disturbed, it liquefies, which has been proven problematic for all the work on the project.”

Most of 2013 was consumed with installations on the center and western most portion of the interceptor. Crews moved to the eastern most region of the interceptor after the beginning of 2014 where two 48-in. and two 72-in. installations will round the remaining runs on this project. Construction on the final drives tie into the shaft at the sluice gate and screening structure, which prevents solids from entering the channel to the Narragansett Bay during storm overflows.

SEI’s longest drive was achieved in mid-June, representing 1,073 lf of 48-in. ID RCP with the SL60 machine. For this drive, and others more than 750 ft length, SEI typically uses an intermediate jacking station to reduce jacking forces on the main jacking cylinders, distribute the thrust load and increase productivity. The average run on this project was 539 lf.

Stockwell reported, “The biggest challenges have been due to unstable soils and excessive water pressure. Throughout, we’ve had to make frequent adjustments to our equipment spreads, ground improvement plans and schedules to adapt to the poor ground conditions. The high water table has caused a lot of water pressure and has sometimes made for difficult launch and retrieval of the MTBM. The ground is so saturated that when the soils are disrupted, it loses all stability and at times won’t support the MTBM or the mining equipment.”

SEI’s vast experience in the microtunneling industry has made them adaptive to project adversity. This expectation on microtunneling projects is quite typical. Stockwell concluded, “Despite these issues, our team has been working diligently to overcome challenges and continues to move towards final completion, on schedule.”

Laura Anderson is director of marketing for Akkerman. Monique Mead, Shawn Stockwell and Justin Kolster with Super Excavators Inc. and Thomas Day with Barletta Heavy Division Inc. also contributed to this article.
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