Ground Improvement in Manhattan Bulls Liver Soils for the Rehabilitation of the Cortland Street Station

Abstract

The Cortland Street Station serves the R&W subway line in Manhattan, New York, and is located immediately adjacent to the World Trade Center site. Recovery and construction activities at the 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 76 mm (3 in.). The ground conditions consisted of highly sensitive Manhattan Bulls Liver soils. In 2008, following the successful compaction and fracture grouting stabilization of a leaning 16-story brick building along the adjacent Dey Street Connector project, the New York City Metropolitan Transit Authority (NYCMTA) embarked on a test program to evaluate the feasibility and effectiveness of both fracture grouting and compaction grouting techniques to remediate the loose and voided soil conditions below. Based on the results of the trial program, compaction grouting was recommended as the more economical selection for large scale remediation. In 2010, the Rehabilitation of the Cortland Street Station contract was put out for public bid and included the ground improvement package for the entire southbound subway platform. Grouting would take place adjacent to the World Trade Center (WTC) site which had been excavated approximately 21 m (70 ft) to rock. The execution of the ground improvement program involved the performance of low headroom compaction grouting from both the platform and track levels of the station. In addition to being technically complicated, this project was also logistically complex, involving both single-shift and round-the-clock work, with limited equipment and material access. Test program results are presented, together with an overview of production work and the technical, logistical and contractual issues surrounding the completion of the ground improvement program at this particularly complex site.