Geotechnical challenges associated with construction of MacKays to Waikanae double tracking, Wellington, New Zealand

Designed by Opus, Double Tracking and Electrification Works between MacKay’s Crossing and Waikanae at Kapiti Coast north of Wellington, New Zealand required 14 km length of up to 4 m high new railway embankments, ground improvement, up to 30 m high cut slopes and slope stabilisation works to be built alongside the existing North Island Main Trunk Railway embankment. Approximately 4 km length of the new railway embankments had to be built over a very soft peat within a narrow designation corridor located between the high steep eastern hills and State Highway 1. The project had to be built within a short timeframe and the existing Main Trunk Railway Line had to remain operational during the construction. The design and construction techniques used had to address a number of geotechnical challenges: construction on peat with undrained shear strength of 7 kPa to 15 kPa, insufficient room for stabilising berms, large (up to 1.5 m) settlements of the new railway embankments and frequent re-levelling of the existing railway tracks during construction, construction of upslope protection works to stabilise steep cuttings. Finite element modelling of the new and existing railway embankments was undertaken to predict the settlements and the required frequency of re-levelling of the existing railway tracks. Lateral and vertical deformations of the new and existing embankments as well as pore pressures in the peat were closely monitored during the construction phase. The project was successfully completed in February 2011.