SH20A to Airport: New Zealand’s First Project adopting Screw Piles and Diaphragm Walls as a Trench Combination

State Highway 20A (SH20A) is the primary route to and from Auckland’s international and domestic airports and forms a strategic link between State Highway 20 (SH20), State Highway 16 (SH16), the airport, and the greater Auckland area. The SH20A to Airport project comprises a new motorway interchange that separates the local and motorway traffic. SH20A is to be trenched below Kirkbride Road. The trench is approx. 450 m long and 30 m wide, retaining up to 7.2 m height of soil, and accommodating two carriageways with allowance for a future rail corridor. Diaphragm walls (D-walls) and screw piles are adopted as the trench supports, a combination never before used in New Zealand. This paper focuses on the trenching works, outlining the geotechnical design aspects for the trench, and the challenges through the design and construction to-date.