Case Study: Port Chalmers Foundation Design


Aurora Energy is one of the largest electricity distribution networks in Aotearoa New Zealand. They own and manage the network that delivers electricity to some of the fastest growing areas and over the most diverse terrain in Te Waipounamu, the South Island.


Aurora Energy Ltd identified that the existing conductor on the 11kV Port Chalmers line was reaching the end of its useful life and would therefore need replacement. Aurora’s forward planning meant that there was a need to future proof this line so that it could be operated at 33kV in the future when demand warranted.

Second to this was the existing lattice towers were also in poor condition and would need to be assessed for the additional loads imposed by upgrading the conductor to operate at 33kV.

The location of the line posed a real challenge. The 11kV line traverses the harbour via two islands (Goat and Quarantine) to provide a direct supply for the Portobello Peninsula. Access to the tallest structures (48m tall) was by helicopter or boat.

Figure 1 Crossing Tower on the Portobello Peninsula


Key to the upgrade of the existing line was determining the condition of the existing towers and foundations. After several site visits to collect asset information on the 1960’s era lattice towers and foundations, our team analysed the structures for the increase in loading because of the upgrade to 33kV and by the application of more modern design codes.

This analysis identified that neither the towers nor foundations were capable of supporting the increase in loading without significant spend on strengthening. Based on a cost benefit analysis, the decision was made to design new steel poles to replace the existing structures and provide future proofing for the line.

Noting that the new poles would be built on two rocky outcrops, foundation design became a critical item. Utilising our experience in transmission line construction, health and safety and environmental impacts were key considerations. Our team identified that reducing the depth of any foundations would be beneficial to reduce cost in excavation and ensure worker safety during the foundation installation phase. As a result, shallow foundations utilising rock anchors were selected.

Critical to this foundation design, was the need to understand the existing rock type and capacity to support the anchor system. Our team returned to site and worked closely with a specialist contractor to undertake rock sampling and anchor testing to collect the data required for the foundation design.

Figure 2 Test Equipment being flown to Quarantine Island

Figure 3 Test Anchor Installed

Figure 4 Anchor Testing


The resulting foundations designed for this project saved significant construction time because of the reduction in excavation required and that meant there was less spoil, minimizing the environmental impact. This was particularly important for the island’s owners, The Department of Conservation New Zealand.

The design also meant that the anchor installers could safely work in the excavation during the anchor drilling and installation.

Although our design reduced costs, the more important impacts were the benefits to environmental and health and safety.

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