Abstract:
The expansion of the Narragansett Bay Commission’s Bucklin Point Wastewater Treatment Facility in Pawtucket, Rhode Island, presented significant geotechnical challenges associated with deep excavation support, groundwater control, and foundation installation adjacent to a tidally influenced river. Keller was responsible for the design and construction of a sheet pile support of excavation (SOE) system, structural micropiles, tiebacks, toe pins, dewatering wells, and a contingency groundwater cutoff solution.
Initial SOE and dewatering designs assumed limited hydraulic communication through bedrock, with groundwater control achieved via sheet piling driven through the glacial till to bedrock refusal with conventional pumping. However, preconstruction testing and early construction activities—including micropile and tieback drilling—revealed substantial grout loss, groundwater communication between drill holes, and tidal influence through the fractured bedrock. Subsequent pump testing confirmed that bedrock permeability exceeded overburden soils by more than two orders of magnitude, resulting in hydraulic connectivity between the excavation and the Seekonk River and introducing a bottom stability risk driven by upward seepage gradients beneath the excavation.
To mitigate these conditions, a rock grout curtain was designed and implemented to cut off groundwater flow and isolate tidal influence. The grout curtain targeted the upper 30 feet of bedrock beneath and adjacent to the SOE using primary, secondary, and tertiary grout holes drilled around the perimeter of the excavation. Multiple grout mixes, including microfine and Type III cement with admixtures, were employed to achieve a target permeability reduction and meet defined Lugeon criteria. Grouting operations were closely monitored using real-time pressure, flow, and volume data, enabling refinement of the program and confirming grout penetration into fractured rock zones.
Following completion of the grout curtain, excavation stability was restored, dewatering demands were reduced, and structural micropile installation proceeded successfully. This case history highlights the importance of adaptive design, targeted subsurface testing, and integrated grouting solutions when managing groundwater risks in rock-controlled excavations for deep foundation and infrastructure projects.