Under the mountain – City Rail Link, tunnel portal temporary retaining structure design and construction challenges

S.A.B. Farquhar & Y.F. Thorp
Tonkin & Taylor Ltd, New Zealand

ABSTRACT
The City Rail Link (CRL) is the largest transport infrastructure project ever to be undertaken in New Zealand. It comprises a 3.45 km twin-tunnel underground rail link up to 42 m below the city centre. The Link Alliance are delivering the design and construction of two new stations, Te Waihorotiu and Karanga-a-Hape, redevelopment of the Maungawhau (formerly Mount Eden station) and bored twin tunnels between Maungawhau and Te Waihorotiu. This paper discusses the portal retaining wall at Maungawhau, where the tunnel boring machine started its journey to Te Waihorotiu station.
The portal wall is a complex reinforced concrete piled retaining structure up to 28 m high. In the order of 100 ground anchors provide stabilising tie back forces at four levels. 3D modelling was required to ensure no interaction between the bond lengths of the overlapping anchors, the tunnels, and the tightly constrained project boundaries. Three tunnels pass below the wall. The design was complicated by the presence of uncemented sandstone that was encountered at the mined tunnel face level, a 1.3 m diameter watermain that supplies a large area of Auckland inner city running just behind the wall, a street behind the wall that remained open for much of the construction period and a vibration sensitive television studio filming during construction across the road. This paper describes the design of the wall and monitoring results through excavation, tunnel mining and
backfilling phases of the wall. Instrumentation includes inclinometers, surveyed surface prisms and ground anchor load cells.