C-26006 Leak Mitigation in G3/G4 Tunnels (Judlau Construction), MTACC, February/August 2016
Scope – Curtain grouting of station box, mezzanine, concourse, entrance and running tunnels.

Upon completion in 2016, Phase 1 of Second Avenue Subway will connect riders in an historic way with Manhattan’s Upper East Side – one of the most densely populated and heavily developed urban centers in the world. Although construction for Phase 1 broke ground in 2007, much of the planning and preliminary work for a Second Avenue line was done over the last century. The G3/G4 Tunnel (BMT-63rd Street Line) was originally constructed in the late 1980s as part of Route 131-A and runs north from 57th St. and 7th Ave. beneath Seventh Avenue, Central Park and 63rd Street before connecting with Lexington Av/63rd St. Station. This tunnel was designed to bring trains from the 7th Avenue Line to the eventual 2nd Avenue line. Since their construction the G3/G4 tracks have not been put into regular commercial service. Part of the work scope for Phase 1 of the Second Avenue Subway Project was the structural rehabilitation and restoration of rail systems in the G3/G4 Tunnel.

The G3/G4 Tunnel runs in very close proximity to the Central Park Pond. Over time the high groundwater table resulted in excessive leaking in this unused tunnel. In September 2015 Sovereign-Thyssen L.P. was invited by New York City Transit to assess the extent of these leaks and propose a plan for remedying them. After conducting several site investigations and referring to record drawings for the tunnel, it was decided that the sealing of 1,850 linear feet of the G3 Track and 900 linear feet of the G4 Track would be sufficient to prevent out-of-tolerance groundwater intrusions into the tunnel throughout the remainder of construction and revenue service. The proposed solution would be to drill a series of holes through the reinforced concrete tunnel liner and inject NOH2O polymer emulsion grout in order to create an impervious grout membrane between the tunnel and the surrounding ground medium.

Work began in February 2016 in the G3 track area with a crew of eight. It was decided that for the duration of grouting work, all other track and structural work would be done in the G4 track area so as to not interfere. A work train was used to bring all drills, pumps, grout lines, scaffolds and a sufficient supply of NOH2O and its additives to the G3 track. A crossover passage at the center of the 1,850 linear foot work limit would serve as a central pump room for grouting operations. For the G3 track, two to three drill locations were laid out along arch – one hole 2-ft from the crown, one hole 2-ft from the springline and (where applicable) one hole on the sidewall. This drillhole pattern would be spaced every 10 feet along the 1,850 feet of G3 track.

Holes were drilled off of a work scaffold which was rolled down the existing G3 tracks. Due to the need for expedited work and the high water pressures anticipated, a pneumatically-powered Mid-Western 83 Jackleg drill with a 1-inch carbide bit was used to drill through the concrete liner. The jackleg drill with carbide bit can drill around six times faster than electric drills with diamond core bits through reinforced concrete. Furthermore the jackleg will not short out if it gets wet, unlike electric core drills. This was significant, as some holes drilled through the liner discharged water at 20 gallons per minute. A separate scaffold would be used to install 1-inch mechanical packers in drillholes and perform grouting work. All grout and pumps would remain in the crossover, and grout lines would run out to the scaffold. This sequential method of work was performed along the entire alignment. Once the first round of grouting was complete, attention was given to treating leaks in the G3 crossover passage, where the leaks and cracks were most extensive. Working in this fashion, we were able to drill and grout 416 holes and inject 20,774 liters (5,488 gallons) of NOH2O polymer emulsion, providing a grout curtain for 40,700 square feet of tunnel along the G3 track in 24 shifts and less than four weeks.

The second phase of treatment began in August 2016, when the G4 track became available to conduct grouting operations. The G4 track transitions from running adjacent to G3 as a double-arch tunnel to running below G3 as a stacked tube. The 900 linear foot work limit addressed the double arch section of the G4 track. A work train transported equipment and a fresh supply of grout to a new pump location in the G4 track area, and a crew of four proceeded in a similar fashion to G3. By mid-September, we had drilled and grouted 132 holes and injected 7,030 liters (1,857 gallons) of NOH2O polymer emulsion, providing a grout curtain for 13,500 square feet of tunnel adjoining the G4 track in 5 weeks.

In this application, the primary constraint was schedule. The client needed a reliable product and an equally reliable solution which could be implemented flexibly and expeditiously. This sequential, hard-and-fast approach to positive-side curtain grouting with polymer-based emulsion yielded a production rate of 250 square feet per man-hour, including time required for mobilization, second pass and follow-up work. Providing a dry tunnel in this short span of time greatly helped the general contractor to push forward with track and systems work undeterred. Replicating these results using conventional crack-injection techniques would require dozens of workers and an extensive inspection and monitoring team be set up, and even then results would not be guaranteed as the water would retain its potential to migrate.

G3 Tunnel Before – 02/29/2016

G3 Tunnel Before – 02/29/2016

G3 Tunnel After – 03/05/2016