Uplift Control of Jet Grouted Bottom Plug by Friction Mobilized along Embedded Anchoring Elements

Jet grouted horizontal plug was used to seal the bottom of 20.5-m deep TBM launching shaft constructed for a new underwater road tunnel in Gdańsk. The plug was positioned right below the excavation level, and anchored with hollow bar steel micropiles 76 mm in diameter and 15-m long, installed centrally in 1-m diameter and 10-m long jet grout (soilcrete) columns. The columns were arranged in a triangular grid with side length of 2.1 m, and constructed as part of a single production cycle together with the plug columns that were 3.1 m in diameter and 3.5 m in length. The pull-out capacity of micropiles was determined by the frictional resistance mobilized at the tendon/soilcrete interface, and the measured average bond to soilcrete strength ratio was 0.25. A novel aspect was a combined use of the soilcrete plug as structural and watertight element. The adopted design solution contributed to reduction of bending moments and horizontal deformations of diaphragm walls, to more safer and controllable execution of works, and to cost and construction time savings.