GEOTECHNICAL CHALLENGES ASSOCIATED WITH CONSTRUCTION OF MACKAYS TO WAIKANAE DOUBLE TRACKING, WELLINGTON, NEW ZEALAND

Abstract

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 relevelling
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.
Keywords: peat, railway embankment, settlement, preloading, stabilising berm, trackbed.