World Trade Center

Service Provided: Dewatering

When Moretrench was called to the World Trade Center the week of September 11th, 2001 it was initially to provide extensive emergency-response dewatering services. The scope of the company’s involvement in the massive rehabilitation effort was soon extended to include a challenging program of urethane grouting to control seepage and ground loss through the exposed slurry wall joints and tieback penetrations.

The foundations for the World Trade Center towers sat on rock inside an area that covers six city blocks. The below-grade area of the site, referred to as “the bathtub” during the recovery operation, was as deep as 75 ft below street level, 66 feet below the water table, and was located inside a 3-ft-wide perimeter slurry wall that was severely damaged when the towers collapsed. Dewatering in this area was performed concurrent with the installation of new tiebacks (by others) to reduce the hydrostatic loading on the dangerously unstable wall. Slurry wall panel joints were spaced every 22 feet along the bathtub perimeter, and tiebacks were installed at both ends of the panel, several tiers deep.

As debris removal within the bathtub began, external pressure on the outside of the slurry walls, primarily due to groundwater loading, created significant movement of the wall panels, resulting in leakage between panel joints and the potential for significant ground loss.

In December of 2001, Moretrench initiated a six-month- long chemical grouting program to control the groundwater leakage into the bathtub through the joints between the slurry wall panels and also through pre-existing and new tieback penetrations. Grouting of the joints was performed upon exposure of the walls and again following installation and tensioning of the new tiebacks since this work had resulted in additional joint movement.

Hydrophilic Grout Provides Fast Seals

Grouting was accomplished using a single-component hydrophilic urethane grout that activated upon contact with water to form highly flexible and adhesive polyurethane foam. The process of sealing the joints and cracks in the slurry walls involved drilling into the structure and bisecting the crack at a 45° angle. The urethane grout was then injected into the wet cracks using a high-pressure injection pump. When it was necessary, dry cracks in the slurry walls were flushed with water to promote grout activation. The fast-reacting grout provided the seal needed to quickly halt the flow of water, which in places was as great as 100 gpm.

For the tieback penetrations, a specialized application method was implemented to slow the water and keep the chemical grout in place. Excess cable tendons at the tieback interfaces were cut and fitted with a custom fabricated steel housing and cap equipped with an injection port and relief valve. High- pressure pumps were then used to inject the same hydrophilic grout directly into the high-pressure flow, forcing it down the cable housing. This technique contained the leaks long enough for the expansion reaction of the chemical grout to take place and sealing of flowpaths to be effected.

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