The Cortland Street Station, which serves the R&W subway line in Manhattan, New York, is located immediately adjacent to the World Trade Center site. Constructed in the early 20th century using cut-and-cover methods, the station has two platforms and twin tracks situated within a typical New York City Metropolitan Transit Authority (NYCMTA) concrete arch box structure. Soils in the area, known locally as “Bull’s Liver,” soils are typically low-permeability, low-plasticity silts and have historically demonstrated high instability when saturated and subjected to construction-induced disturbance. This was the case when recovery and construction activities at the adjacent World Trade Center site created significant ground loss and subsequent loosening of the surrounding soils at the station site, resulting in settlement of the station structure up to three inches at the point of maximum deflection.
Overall, approximately 200 lf of subway alignment was disturbed, with maximum settlement generally limited to the southbound side of the station alongside the World Trade Center slurry wall. As a result of a test program initiated by the NYCMTA and implemented by Moretrench, compaction grouting was selected to remediate the loose and voided soil conditions beneath the southbound platform. This technique had earlier proved to be highly successful on the adjacent Dey Street Connector project in the same ground conditions and could be accomplished from inside the subway concurrent with active trains and without disturbing the adjacent excavation support system.
The work areas for both the test section and the final grouting program were characterized by extremely tight access and headroom of approximately eight feet. Given the severe space restrictions, the staging area, drill rigs, grout mixing and batching equipment, and grout delivery and monitoring equipment were necessarily accommodated within the confines of the station. For the test section, the drill rig was 'walked' up several flights of stairs to access the station platform.
Additionally, the multiple tiers of tiebacks that had been installed to support the damaged WTC slurry wall were still in use and had to be avoided during drilling and grouting activities.
Final Grouting Program
Following off-site injection testing of the design grout mix, the dry grout ingredients were pre-blended and supplied in super sacks. This provided an extremely high level of quality control and grout consistency, and minimized material handling on the very restricted site.
The final grouting program included 85 compaction grout elements installed from the southbound platform and the active southbound tracks. Moretrench used duplex drilling techniques to advance the 114-mm diameter grout casing to design depth at 50 feet below working grade. At each 1-foot stage, low slump grout was pumped until a volume of two cubic feet or 400 psi was reached.
Monitoring & Testing
Drilling and grouting operations were closely monitored with an automated system designed and built by Moretrench and capable of sensing the depth of the drilling/grouting operation and measuring the volume and pressures of the grout mix, along with relevant drill parameters.
An extensive automated survey system was installed to monitor the slurry wall, and frequent manual checks were also made. An additional survey crew stationed inside the subway tunnel also monitored structural columns for movement. Interaction between the grouting and survey crews was critical throughout drilling and grouting operations to ensure that any movement would be limited.
Cone penetrometer testing at prescribed locations to evaluate post-grouting conditions confirmed that the specified improvement had met the project objective.