Deep Excavations In Soft Clay On Brisbane Airport Link Project

This paper presents a case history of a 22 m deep excavation in deep soft clay on the $5.6 billion Brisbane Airport Link Project, one of the biggest infrastructure projects in Australia. A 25m deep cut and cover tunnel was constructed using a bottom up sequence to connect twin driven TBM tunnels at Toombul. Diaphragm walls with two layers of slabs and two layers of horizontal steel struts were adopted to support the deep excavation and to control the ground movements and ground water drawdown in order to ensure a safe excavation and to protect the adjacent existing residential buildings, utilities and North Coast Railway. Two-dimensional (2-D) finite element analyses were conducted during detailed design to predict the retaining wall displacement, ground settlement behind the wall and pore water pressure variation during construction and in the long term. Inclinometers, settlement markers and piezometers were installed to monitor the behavior of the diaphragm walls and the retained ground, pore water pressures during construction. The monitoring data is compared with the design predictions and comments offered on the success of the deep excavation and accuracy of predictions.