MacKay’s Crossing was one of the last remaining at grade road over rail crossings on SH1 between Wellington and Auckland. A four-lane overbridge and 2km realignment of SH1 was constructed in 2005 to provide scale separation. The site adjoins wetland and is underlain by 6 m thickness of peat, presenting geotechnical engineering challenges of stability and settlement.
Transit New Zealand (NZTA) engaged Tonkin + Taylor to undertake the geotechnical investigations, options evaluation and scheme design. The project was subsequently tendered as a design/build and we provided specialist geotechnical services to the successful design build contractors, Fulton Hogan Ltd.
15m high cuts in alluvium were formed and an 8.5m high fill embankment was constructed on 6m of peat. Our design included wick drains, monitored stage loading and preloading to promote geotechnical stability and mitigate settlement. The design specifically allowed for the limited tolerance to differential settlement of a gas lifeline which crossed beneath the embankment and the railway line which immediately adjoins the embankment.
- Geotechnical investigations
- Design Options Evaluation
- Detailed geotechnical design
- Construction monitoring