There's a light at the end of the tunnel!

Project CARE is North Shore City Council’s strategic wastewater improvement project being implemented in a densely populated, urban environment, with iconic beaches and reserves and amenity much valued by the residents. There are many challenges affecting the implementation of this ambitious improvement project which necessitate an innovative approach to problem solving. A Principal Consultancy of SKM with subconsultants Opus was established at the beginning of 2000, to assist in implementing Project CARE. The project team is involved in various major capital projects arising from Project CARE, enabling an efficient, consistent approach to works, along with continuous learning and associated improvements for the ten years of the project life so far.

North Shore City Council has always considered holistic costs and impacts when installing new trunk sewers; with ease of consents, level of community disruption, effect on trees, all factoring into the installation methodologies chosen. A significant amount of the works undertaken in developed areas is installed using trenchless technologies. This reduces disruption and environmental damage. The risks associated with directional drilling and micro-tunnelling technologies are well understood, but opportunities continue to be identified.

A project has recently been completed to install a new trunk sewer in the Birkdale catchment of the North Shore to address capacity issues in the existing line, and to allow for future development and population growth in the catchment. The project was separated into three stages for ease of buildability – the lower section of large pipe on flat grade was installed by micro-tunnelling, and is the subject of another paper. The middle section was originally planned to be installed by directional drilling, and the upper section by a combination of microtunnelling and open cut.

At design stage for the upper section, a review of the proposed installation methodology, latest advances in technology and the capability of local contractors resulted in the identification of an alternative alignment previously believed to be un-constructable because of plant limitations. The new alignment required installing a gravity sewer utilising long distance (960m) directional drilling through rock varying from soft to very hard at flat gradients at depths of up to 32 metres.

NSCC, as client, decided to embrace new (to New Zealand) technology and equipment to install the trunk sewer, once the potential improvement in system performance, potential cost savings and reduction in disruption and environmental damage were identified.

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, something many directional drilling contractors struggle to achieve on even short distances. A paper was presented at the conference last year on key risk mitigation, through the design and procurement process and early lessons learnt on site. However, the use of such innovative technologies will always bring challenges.

This paper will cover the differences between ‘standard’ directional drilling projects for gravity sewers and long distance directional drilling that have been encountered. It will identify specific technological aspects of long distance directional drilling, such as steering technology, which were used on the project, plus the outcomes of NZ’s longest on-grade pipe drill, and the lessons learnt.