The Warnerville Pumping Station in Queens was commissioned by the New York City Department of Environmental Protection as part of the area’s new combined sewer overflow system. In early 2007, Nicholson Construction Co., headquartered in Cuddy, Pa., was awarded the contract to install a bottom plug for a 60-foot-diameter shaft.

The shaft was constructed by general contractor E.E. Cruz with driven sheet piles. Permeation grouting originally was specified for the shaft’s bottom plug, but Nicholson proposed and designed a less expensive and technically superior solution using jet grouting. After the sheet piles were driven, and micropiles were installed at the base of the shaft, Nicholson installed 209, 5.5-foot diameter jet grout columns to create a completely watertight seal at the base of the shaft. The fine sands present at the site combined very well with the jet grout to create a uniform end product.

The site’s high water table presented some challenges for Nicholson. As a result, work platform stability was a constant concern. E.E. Cruz prepared the platform each evening by grading off spoils. Platform stability problems diminished once 25 percent of the jet grout columns were installed. The high water table also meant the jet grout bottom plug had to withstand 55 feet of water pressure trying to displace it. Even one small leak would have caused major problems during shaft excavation.

Columns diameters were estimated from initial tests, and then replicated to avoid excessive overlap, which might have resulted in the shadowing of the adjacent column. The presence of dedicated onsite engineering staff ensured successful sequencing, layout, quality control and accurate calibration of mixing controls and drill rig instrumentation.

Since 1955, Nicholson has been an industry leader in performance and technical innovation in geotechnical drilling and construction. As the North American business unit of the Soletanche Bachy Group, Nicholson is part of a global network of geotechnical resources and expertise, specializing in deep foundations, earth retention, ground treatment and ground improvement.  ND