PAWTUCKET | US
sedimentary rock comprised of sandstone and siltstone, with lesser amounts of conglomerate, shale, and coaly deposits. Bedrock is moderately folded and faulted. Bedrock is overlain by a thin layer of glacial till and thick layers of glaciofluvial deposits. Glacial deposits are comprised mainly of sand, gravel, and silt, and are overlain by granular fill. The project area is within the Narragansett Basin,
which is approximately 55 miles-long, and 15 miles to 25 miles in width, and is made up of several thousand feet of non-marine sedimentary rocks that have been folded, faulted, and slightly to moderately metamorphosed. Sedimentary rocks of the Narragansett Basin have been divided into five formations, of which the Wamsutta Formation and the Rhode Island Formation occur within the tunnel horizon. Bedrock is overlain by glacial deposits consisting of till and glaciofluvial deposits as well as man-made fill deposits near surface. The Rhode Island Formation is widespread and
consists mainly of gray sandstone and siltstone, with lesser amounts of gray to black shale, conglomerate, and coaly rock. These sediments were deposited in medial to distal alluvial fan environments, and are comprised of meandering stream deposits, bank and flood plain deposits, and swamp deposits. Sediments of the Rhode Island Formation generally are finer-grained than those of the Wamsutta Formation. Rock types are laterally and vertically discontinuous, a characteristic of the environment in which they formed. The Wamsutta Formation occurs in the northern
part of the Narragansett Basin, where it underlies and partially interfingers with the Rhode Island Formation (Skehan, Rast, and Mosher 1986). The Wamsutta Formation consists of a sequence of red conglomerates, sandstones, and shales up to 3,000ft thick. Volcanic fragments are common. Conglomerate in the Pawtucket quadrangle contains boulders as large as 4ft wide (Quinn 1971). These sediments were deposited in an alluvial fan environment, and are composed of braided stream deposits, with related crevasse splay deposits
and flood plain deposits. Bedrock of the Wamsutta Formation is present along the northern 200ft to 400ft of the tunnel.
TUNNELING Notice-to-proceed with the Design-Build contract was issued at the end of 2020, triggering the TBM contract. The tunnel was mined with a Herrenknecht dual
mode (open/earth pressure balance (EPB)) machine fitted with a hard rock, 33.8-ft diameter cutterhead having 64 x 19” discs. The TBM used a conveyor in open mode, and screw conveyor in EPB mode. The structure and seals are designed for 72 psi of pressure, and minimum required curvature was 1,000ft for the 46ft-long shield. The TBM had 300-ft long trailing gear, comprised of four backup gantries. The Factory Acceptance Test (FAT) was completed
ahead of schedule in January 2022, and shipment delivery was April-June. The TBM began mining in November 2022, from the
launch shaft on the main site at the Southern end of the project area. Project challenges included: labor and material
cost escalations with supply chain disruptions during the pandemic; finding a home for the approximately 1,000,000 tons of excavated tunnel material; and pre-treatment of significant flow from excavation dewatering (ranging from 50,000 gallons per day to 120,000 gallons per day). The project parties collaborated to negotiate equitable sharing of escalated costs; beneficial re-use of more than half of the excavated tunnel material; and, adequate handling and pre-treatment of dewatering flows. TBM breakthrough was mid-February 2024 at the
receiving shaft. The entire tunnel was mined in open mode, the ground conditions not requiring the use of EPB mode. The project is to be substantially complete at the end
of this year. Thanks for information briefing to Stantec and AECOM.
PROJECT PARTIES List of the principal parties on the Pawtucket Tunnel project.
Project Owner: Narragansett Bay Commission (NBC). Lead designer, program and construction manager: Stantec, with Pare Corporation, for the Phase III program. Design Build Contractor: CB3A, a joint venture of CBNA, a U.S.-based subsidiary of French construction engineering firm Bouygues Travaux Publics, and local civil works contractor Barletta Heavy Division. Contractor’s prime engineer/designer: AECOM, and is Engineer of Record, with oversight of four sub-consultants (GEI, BETA, Gall Zeidler, and Mueser Rutledge) Timeline: Planning and alignment alternatives studies for Pawtucket Tunnel began in 2014. In 2019, NBC had industry outreach talks with contractors, then selected a Design Build delivery method to meet the regulatory project schedule and manage risk. Procurement was a two-step process: in 2019, Request for Qualification (RFQ); and, in 2020, the Request for Proposals (RFP) to short-listed firms to submit bids. The pandemic period of restrictions led to challenges for all. Contract award in late 2020. Construction started in Spring 2021, tunneling in late 2022. With the tunneling success, project completion is anticipated to be end of 2024.
Spring 2024 | 13
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