US | PAWTUCKET
PROGRESS AT PAWTUCKET
Major milestone progress with TBM breakthrough at the Pawtucket CSO Tunnel project, in Rhode Island. The region has complex, fluvial-derived geology with structural weak zones
The Narragansett Bay Commission (NBC) has completed the excavation of the 2.2-mile long Pawtucket Tunnel, the centerpiece of Phase III of its Combined Sewer Overflow (CSO) Abatement Program in Rhode Island. Phases 1& II were focused on the Providence area and completed in 2008. Pawtucket Tunnel is located near the Seekonk and
Blackstone Rivers in Pawtucket, and is Phase IIIA of the CSO project. The project criteria is to provide a storage capacity of 58.5 MG during the 3-month design flow. The project includes an 80ft i.d. pump station shaft. Hydraulic criteria for the project specified use of vortex- style drop shafts (4No) able to convey peak flows from the 2-year storm. The tunnel alignment runs 115ft to 155ft (and slope
is 0.001 ft/ft) below the ground surface through complex geology, originating from fluvial deposits and predominantly consisting of sandstone and siltstone with some conglomerate and coal. The ground has structural features including folds, faults, and joints. The project was in planning and development since
2014 and was awarded as a Design Build contract (see box panel). Minimum rock cover is two tunnel diameters. A supplementary geotechnical investigation plan was developed. As the plans and design progressed, changes included during the contract raised the elevation by 25ft (during the Alternative Technical Concept process) to decrease the overall profile in rock; relocated the pump station away from a fault/shear zone; and, incorporated temporary support structures in the permanent design. The 30ft i.d. tunnel lining was erected by TBM and
has universal double taper rings (7 segments, of fiber reinforced concrete), 14”-thick and 6.6ft-long. Each segment joint, both radial and circumferential, is fully gasketed to achieve the water tightness criteria (Maximum allowable groundwater infiltration 1 gpm per 1000ft of tunnel and 0.1 gpm at any one segment).
Above: Breakthrough of TBM at Pawtucket Tunnel, RI IMAGE COURTESY OF AECOM
12 | Spring 2024
GEOLOGY The geologic history of the bedrock is complex. The rock originates from fluvial deposits, which characteristically are not laterally continuous. Sequential episodes of tectonic deformation have superimposed structural features including folds, faults, and joints. The Pawtucket Tunnel is constructed mainly in the Rhode Island Formation, a Carboniferous-age
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