Transport for New South Wales faced the challenge of constructing a grade-separated southbound freight line beneath three heavily trafficked electrified suburban railway lines, without disrupting service to the North Sydney rail network.
It was necessary to navigate surface rock outcrops on the eastern embankments while ensuring minimal disturbance to surface settlement.
Mott MacDonald was appointed as technical advisor subconsultants for the concept design phase. We assessed several alternate underpass construction options with the primary aim of eliminating the previously selected cut-and-cover approach to reduce construction time and disruption to mainline services above the underpass.
Drawing on finite element analysis (2D and 3D) and the multidisciplinary engineering expertise of our team, we demonstrated the feasibility of the driven tunnel option.
The selected tunnel option included movement of the tunnel alignment north by about 200 feet (60 meters), resulting in more favorable geological conditions and a deeper tunnel situated directly beneath the operational railway lines.
Construction commenced by inserting arrays of canopy tubes 39 feet (12 meters) long ahead of the tunnel under the operational tracks above.
This allowed full construction of the driven tunnel with a one-pass shotcrete lining within an 8-month period, compared to two to three years for the cut and cover option, with no disruption to train services.