Jet Grouting for Groundwater Cutoff at Tie-in between Existing Tunnel and New Construction-Paper

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The paper describes a jet-grouting groundwater cutoff constructed where the new 96th Street Station transition tunnel connects to an existing tunnel in Manhattan. The work was complicated by abandoned steel sheet-pile bulkhead sheeting of unknown depth, limited access, nearby utilities, and the risk of damaging the existing tunnel.<br /><br />After investigations revealed the bulkhead’s location, the team installed most of the jet-grout elements through predrilled holes in the narrow space between the sheeting and tunnel structure. This single-phase approach formed both a cutoff below the tunnel and a base seal. Columns and panels were installed in a planned primary-secondary sequence, with diameters from 4 to 8.5 feet. Smaller primary columns reduced shadowing risks; larger secondary columns provided overlap. Grouting parameters were adjusted to field conditions, and lower jetting energy was used near the tunnel to limit over-pressurization.<br /><br />Drilling deviations caused by the bulkhead, previously installed columns, and concrete obstructions threatened continuity. Monitoring identified potential gaps, which were addressed by changing column sizes and adding a compensation column and panels.<br /><br />Following excavation and removal of the bulkhead, seepage was controlled and the excavation remained dry with pumping. Groundwater monitoring indicated limited drawdown outside the excavation. The case demonstrates how flexible jet-grouting layouts, field monitoring, and targeted adjustments can create an effective cutoff in a constrained tunnel tie-in while protecting existing infrastructure.

Keywords

jet grouting

groundwater cutoff

96th Street Station

transition tunnel

Manhattan

sheet-pile bulkhead

tunnel tie-in

primary-secondary sequence

excavation dewatering

monitoring and seepage control