Intake Structure - Amethyst Hydro Scheme

Description of Project

This is a 7MW hydro power generation scheme, with the intake located in the Amethyst Ravine at an elevation of 400m above the powerhouse alongside the Wanganui River. Water is diverted from the ravine, via the stilling basin into the penstock, down a 1.1 km tunnel and along the tunnel access road to the powerhouse.

Eliot Sinclair were initially engaged to design the access road for the tunnel construction, following this we were engaged to carry out the design of the reinforced concrete weir, intake chamber and stilling basin water retaining wall and flood wall, including the abutment and foundation rock anchorages. Eliot Sinclair Associate John Henry was the structural engineer, responsible for the structural design and detailing of these structures.

Factors Contributing to the Complexity of the Engineering Work

The overall design was made complex by constraints which were also construction constraints caused by rugged terrain, which did not allow for road access to the site, or crane on site. The nature of the rock foliations, or weaknesses, in the layering of the rock bedding planes, affected the design of rock anchors anchoring the main wall elements.

All materials, equipment and personnel were brought to site by helicopter (until the tunnel breached late in the construction). The space available for the helipad site was very restricted, with little room to store plant and materials, hence we designed for limited stages of construction, carried out using portable equipment and manual methods, which dictated the concrete pours  and complicated the jointing and detailing of the reinforced concrete. Although each construction step was made simple, the design process was far more demanding than for a normal site.

There was a limited window of time when the gorge water level was low enough to divert the flow for the main weir foundations to be built. This required an initial foundation pad that was simple and quick to execute but could be used to build the more complex structure on.  Even so, flooding and over-topping of the partially complete works occurred several times during the construction period due to storms and high rainfall, but with only minor setback.

Future Enhancements

Due to the complexity of the project and the conditions, Eliot Sinclair undertook careful construction monitoring, including adjusting the design as construction proceeded to suit the actual foundation conditions encountered. Complex projects such as the Amethyst Hydro Scheme allows Eliot Sinclair to display their versatility working in extreme conditions producing results to suit our client’s needs.