Permeation Grouting for the East Side Access Bellmouth Work Shaft, Queens, NY

The East Side Access Project will connect the Long Island Railroad to a new terminal beneath Grand Central Station in Manhattan, NY. Contract CQ026 in Queens, NY involved construction of a bathtub work excavation, approximately 30.5 x 91 m in plan, excavated through the granular overburden to rock at depths between 18.3 and 24.4 m below working grade. Design of the excavation was complicated by several factors related to construction in an urban setting, notably that one area of the excavation would extend down and uncover an existing subway tunnel that would remain active during the work, excavation would proceed to 15 m below the groundwater table, and that plumes of contamination existed nearby that could not be disturbed by off-site drawdown. The primary means of support of excavation were a structural slurry wall extending to rock and connecting to an existing slurry wall (installed during construction of the active subway), and steel sheetpiles which had been installed over the existing subway. In order to ensure that the joints between the different means of excavation support did not allow water infiltration, zones of soil immediately outside of the joints were permeation grouted prior to the start of excavation to complete the watertight bathtub. Additionally, a groundwater cutoff bulkhead was required around all four sides of the existing active subway structure. Permeation grouting was used to close the gaps and joints and create the bulkhead around the existing structure to limit offsite drawdown and the movement of contamination. The permeation grouting was re-designed after bid time to allow all of the grouting to be performed from outside of the excavation. This reduced the risks associated with the performance of the grouting and allowed the contractor to accelerate his schedule. This paper discusses the design, re-design and execution of the permeation grouting program, including significant quality assurance and quality control measures.