The East Side Access Project, ultimately connecting the Long Island Railroad to a new terminal beneath Grand Central Station in Manhattan, involved construction of a work shaft, approximately 100 x 300 feet in plan, excavated through granular overburden to rock at depths between 60 and 80 feet below working grade. Groundwater was present at 20 feet below grade. The two primary means of excavation support were a structural slurry wall extending to rock and connecting to an existing slurry wall, and steel sheetpiles installed over the subway structure.
A contaminant plume identified within a nearby existing rail yard limited the amount of groundwater drawdown outside of the excavation to a maximum of two feet to minimize the potential for plume migration. This necessitated that the excavation support key into the underlying rock and be watertight, in effect creating a ‘bathtub.’ To achieve this, sodium silicate permeation grouting was specified to seal three interfaces or “corners” of the excavation, and to close gaps and joints and create an extensive grout bulkhead which would wrap around all four sides of the existing subway structure where it protruded through the bathtub.
The corner closures were required in highly variable and marginally groutable natural soils. All proposed bulkhead grouting was to be performed within the excavation in structural backfill. A significant portion of the work was anticipated to be in marginally groutable soils. With the specified scheme, the chemically grouted soil on the excavation side of the existing subway tunnel would be acting as an unrestrained structural member against more than 50 feet of hydrostatic head. Grouting of all of the specified zones was required to impart structural strength to the soils in addition to impermeability. This was a major concern particularly due to the significant groundwater pressures that the grouting was to withstand.
Redesign Offers Several Advantages
In light of these challenges, Moretrench offered a post-bid redesign centered around relocation of the bulkhead further back along the existing tunnel alignment and outside of the excavation area. Acceptance of this proposal resulted in several construction advantages.
The volume of soil to be grouted was reduced by approximately half. A fully restrained condition was created so that the grouted soil was no longer required to perform as a structural member, but acted solely as a water cut-off. Greater assurance was provided for meeting project goals for external drawdown. Relocation of the bulkhead section outside of the excavation also allowed its construction to be performed in advance of the completion of the structural slurry wall. This freed up valuable scheduling time as, under the original design, portions of the grouting would have had to be performed from within the excavation limits which would have impeded other planned construction activities.
Grouting Program Design and Implementation
Bulkhead grouting was accomplished first. Three rows of pipes were installed to create a six-foot thick grouted section, with grout pipes in each row spaced approximately three feet apart. This grout pipe spacing was typical for the project and used at each of the grouted corners as well as the bulkhead section. Following bulkhead grouting, Moretrench de-mobilized to allow the completion of the slurry wall and the installation of the tight steel sheeting for the bathtub and then re-mobilized to grout the intersections of the different excavation support systems.
In addition to a comprehensive QA/QC program, the site was also instrumented with piezometers to monitor the actual external drawdown resulting from the dewatering of the excavation. Piezometric data confirmed that, while the interior water level was lowered to the top of rock approximately 60 feet below its starting location, the exterior water level did not drop more than the specified two feet, demonstrating that the cut-off system utilized achieved the overall project objective of controlling offsite drawdown.