The longest single pipe installation in New Zealand using HDD
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Ever increasing development within the catchment pushed the existing trunk sewer servicing Birkdale beyond its capacity, resulting in severe overflows during wet weather conditions.
The preliminary design was based on a combination of micro-tunnelling and open cut, where the micro tunnel section was to follow the road reserve at depths of up to 17 metres and the open cut section was to follow the original trunk sewer route along a walking track surrounded by native bush.
A review of the proposed installation methodology considering latest advances in technology and the capability of local contractors resulted in the identification of an alternative alignment previously believed to be unconstructible.
The opportunity to install a gravity sewer utilising long distance (960m) directional drilling was identified after reviewing the potential alignment and its geology as well as equipment available to penetrate rock varying from soft to very hard at flat gradients at depths of up to 32 metres.
Long distance directional drilling is used around the world to install infrastructure but mainly in applications where grade is less of a concern, such as pressurised systems. The Birkdale trunk sewer has to achieve 1.2% grade over nearly half of the installation distance Therefore a key consideration was appropriate risk mitigation, through the design and procurement process as well as on site.
This paper covers the general technological aspects of directional drilling and the specific aspects relating to Birkdale C including procurement strategy and lessons learned.