The State Highway 20 Mt Roskill Extension project was completed in May 2009. This project, which creates another key link in the western ring route, involved extending the Southwestern Motorway 4.5km from Queenstown Road in Hillsborough through Mt Roskill to Maioro Street in New Windsor. Two new interchanges were constructed at Hillsborough Road and Dominion Road and a new roundabout was constructed where the motorway intersected Sandringham Road. Four new overbridges were constructed to separate local road traffic from the motorway traffic.
Bartley Consultants were part of the URS lead design team for this project carrying out the pavement design of the main motorway alignment, ramps and local roads. One of the main challenges for this project was associated with the extremely variable ground conditions which ranged from alluvium to Waitemata Series clayey silts, siltstones and sandstones, tuff, scoria and solid basalt rock