The Dirt on Mangere Motorway (SH20): Geotechnical Aspects of Design and Construction

The State Highway 20 Mangere Motorway Extension project completed the link to Auckland International Airport and Manukau City and was the first significant design-build roading contract let in New Zealand by Transit NZ . The $40-million project involved the design and construction of 7 km of four lane highway, nine vehicular bridges and four footbridges, fill embankments up to 10 m high over soft ground and 3 main cuts typically 6 m up to 10m deep. In addition, some 200 m of tied back pre-cast walls and 800 m of cantilevered walls were required to retain areas within the designated urban motorway corridor. This paper outlines the geotechnical aspects of design and construction including the results of settlement monitoring using a horizontal profilometer, lateral load testing of piles, pile driving, earthworks control, the stability of cuts and an outline of pavement conditions.