BC40 hydro cutter mounted on MC96 crane The 41.5 ft (13 m) deep excavation is supported by a 3 ft (1 m) thick, rebarreinforced diaphragm wall (D-Wall), which is typically restrained by three levels of tiebacks. Two hydro cutters and one hydraulic grab Tiebacks were typically installed at 5 ft (1.5 m) on center and were installed at 30-degree angles with sufficient unbonded length such that bond zone reached the competent upper layer sediments that are below the young Bay mud. The tiebacks were installed through pipe block-outs that were pre-tied in with the diaphragm wall rebar cages. Drilling the level two and level three tiebacks below the groundwater table and risking excessive water and soil inflow to the excavation was a challenging part of the construction. However, the team used a wal l mounted preventer system and developed cased drilling and grouting techniques to minimize the amount of soils and water that entered the excavation during the tieback install. Ultimately, hydroactive grout was pumped into the locked-off tieback wedge plate to stop the leaking water. All tiebacks were either performance or proof tested prior to lock-off. To allow for future site build out all tiebacks were required to be detensioned. It was very important that the drilled shaft foundation piles were installed first to avoid the risk of drilling through tensioned tiebacks. The diaphragm wall and the 100 ft (30 m) long tiebacks had to be drilled straight in order to miss the already existing pile foundations. 82 * DEEP FOUNDATIONS * JAN/FEB 2023