Huntsbury Reservoir

Among Christchurch’s most hard-hit earthquake-damaged facilities was the Huntsbury Reservoir. Water drained from the shattered 35,000-cubic-metre storage basin, the city’s principal drinking water storage facility. Innovative design and prompt decision-making proved paramount in the rebuild process.

At 12.51 pm on February 22, 2011, a high-intensity earthquake emanating from under the Port Hills of Christchurch pounded the city.

Across the dry slopes, the damage was immense as the reverberations were felt far and wide. Among the most hard-hit was the all-important Huntsbury Reservoir. Water drained from the shattered 35,000-cubic-metre storage basin, disappearing into the cracked hills.

Ongoing tremors further fractured the city’s principal drinking water storage facility.

Built in 1952, the reservoir was badly battered. A reinforced concrete structure – measuring 77.4 metres by 63m – with a 7.25m water depth, the roof was overlaid with soil and grassed, and the walls ranged from fully buried to exposed, cut into a sloping site.

Responsibility for much of the damage lay with an undiscovered “shear zone” under the reservoir. As a result, not only was the main storage facility out of operation, there was extensive damage to the pump station.

Shear surprise

Complicating repairs was the shear zone. Geological probes utilised boreholes to confirm the surprise find. An inspection of the borehole material revealed interfaces where rock-to-rock faces had slid across each other. It was estimated that the last shear zone movement occurred 15,000 years ago.

To accommodate future shear zone movement, structural changes were needed.

Two trapezoidal plan-shaped reservoirs were built either side of the shear zone.

A floor slab was overlaid on the existing slab and a reinforced concrete foundation and walls adjacent to the shear zone were built, along with a reinforced concrete roof.

The existing perimeter walls remained, while the roof column supports were reused.

The roof slab was designed to allow a crane to operate on the surface during construction. It was overlaid with a fibreglass-reinforced PVC sheet membrane.

Under the pump

In a paper on post-earthquake work on the reservoir, contractors Beca and Fulton Hogan and the Christchurch City Council detailed the damage, including “a broken inlet/outlet pipe flanged connection, extensive dislocation and cracking of floor slabs, cracking of the roof slab and some movement at wall joints adjacent to the corners of the structure”.

However, the associated pump station was “damaged beyond repair”.

Filling the void

A nearby inspection revealed other issues. The inlet/outlet pipe traversed an adjacent road, Huntsbury Avenue, via a tunnel. A tunnel check revealed collapsed roof sections.

The inlet/outlet pipe proved to be intact. The void between the pipe and the tunnel was filled with foam concrete, protecting the pipe from any falling roof material and cushioning it from further movement.

The restricted site access, confined space, tunnel debris hazards and the risk of future earthquakes influenced the rebuild decision.

Stage one of the reservoir and a new pump station were commissioned in December 2011, allowing water to be pumped to areas above the zone served by the reservoir. Stage two was commissioned in November 2012.

Teamwork, prompt decision-making, astute risk assessment and community consultation were key elements in the success of the project. However, the need for reservoir resilience and the attention to design details were paramount.

Huntsbury Reservoir repairs timelapse: https://www.youtube.com/watch?v=pmOh_Uyf5j4

Where has all Huntsbury’s water gone? News story: https://www.youtube.com/watch?v=swVrPNB4q8o