The project consisted of installing a helical tieback system to strengthen a new 80 feet long-driven steel sheet pile wall. The helical tieback system was designed to retain the wall during construction with cut excavations in front of the wall exceeding 15 feet. Construction on other portions of the hospital was to be ongoing during tie-back installation.
The helical tie-back system, designed by Thiele Geotech, Inc., called for installing one to two rows of 1.5″ square shaft helical tiebacks at about a 5 to 6-foot spacing through the driven sheet piles. The tiebacks generally consisted of either a double 10″-12″ diameter helix blade lead section or a triple 10″-12″-14″ diameter helix blade lead section. The tiebacks were installed to lengths on the order of 25 to 30 feet beyond the wall face.
Torque readings in the range of 3800 to 5500 ft-lbs were achieved during installation with ultimate capacities on the order of 38 to 55 kips, or 68 to 140 percent higher than the required design ultimate capacity of 22.5 kips. A total of nineteen (19) helical tie-backs were installed.
Despite limited working space and heavy construction traffic, the helical tie-backs were installed in less than 3 days.
Upon completion, BDB was required to conduct a static load test on each of the nineteen tiebacks. With the help of Grip-Tite’s Installer Development Manager, BDB loaded up each tieback to a capacity of no less than 15 kips for a period of 10 minutes. Quality control was conducted by Double D Excavating and engineers and technicians with Terracon Consultants, Inc.
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Grip-Tite® offers a 1.5-inch Square Shaft Helical and a 1.75 Square Shaft Helical suitable for anchor and tie-back applications. The Grip-Tite® Square Shaft Helical Anchor lead sections and extensions are available with helix blade configurations to meet project-specific requirements.