A range of Marlborough District Council stipulated outcomes have been successfully delivered in a challenging project to upgrade and provision for another 50 years’ working life for the Wither Hills (Weld Street) Reservoir.

The upgrade was undertaken as the opportunity as the approximately 50-year-old reservoir was beginning to suffer durability issues and pose seismic resilience concerns.

Rather than demolition and replacement, a more sustainable approach involving upgrades and additions to the existing 36 metre diameter prestressed concrete structure was adopted.

The construction had many challenges such as a tight programme, as-built details that differed from the drawings, along with existing (and new) concrete issues.

These challenges were overcome through a collaborative and innovative problem-solving approach between Marlborough District Council, CH2M Beca and Fulton Hogan.

The upgrade was undertaken as the approximately 50-year-old reservoir – which stores 5.3 million litres of unchlorinated, potable water for Blenheim – was beginning to suffer durability issues and pose seismic resilience concerns.

Amongst issues targeted for remedy in the reservoir’s upgrade, which is a critical regional water resource following earthquakes and for firefighting, were:

• Re-profiling design of the slope above the reservoir with a flatter angle to minimise the potential impacts of slope failure or earthquake debris on the facility.
• Placing a new reinforced concrete topping slab to provide a roof diaphragm, which also sealed the reservoir contents from rainwater ingress.
• Installing external post-tensioning to strengthen the roof tee beams for the increased mass of the topping slab and to also maximise reuse of the existing structure.
• Fitting a new reinforced concrete roof ring beam to allow for thermal movement of the roof while accommodating a seismic shear load transfer from the roof to the walls.
• Installing an internal reinforced concrete floor ring beam, with dowels cored through the walls, around the perimeter connected to the existing floor slab and foundation with reinforcement starters to provide an enhanced seismic base shear transfer mechanism.
• Installing external wall post-tensioning to compensate for a shortfall in the hoop tension capacity under Ultimate Limit State seismic loading, while also providing new active hoop compression to the wall joints for leak mitigation at the vertical construction joints.
• Fitting new, larger-dimensioned stainless-steel pipework to provide additional flow capacity, with a new chamber constructed to house sensor-controlled seismic isolation valves.
• Undertaking remedial work, including internal surface remediation of wall and columns, floor slab sealant replacement and crack injection of active leaks in the walls.

Other challenging aspects of the construction phase included:

• Cutting a large access hatch into the roof at the beginning of construction to provide ventilation, lighting and access for scissor lifts and other large construction equipment. This also meant the reservoir could be classed as a ‘restricted space’ instead of a ‘confined space’ due to alternative access/egress routes and additional ventilation.
• Staged external roof post-tensioning and roof topping slab pours to maintain stress limits in the existing roof tee beams. A survey was carried out at each stage to monitor deflections which were compared with predicted.
• A unique roof topping slab pour shape and sequencing of a series of concentric rings to achieve tight construction tolerances and complement external roof beam post-tensioning.
• Casting post-tensioning anchorage pockets into the concrete using three dimensional printed inserts.

The key target for successful construction was recommissioning the reservoir before the summer peak demand.

To address challenges which arose during construction, the following remedies were devised:

• Unique communication tools to allow for rapid problem solving on issues that arose due to the tight construction programme. One example was a virtual reality model of the existing and proposed structure created by Beca and used by Fulton Hogan prior to construction to help planning and highlight potential health and safety issues. Another example was the use of digital calling software to enable each party to share screens for sketching issues and allow remote, live brainstorming.
• Raising the dead-end anchorage at the inner end of the roof tee beam as the level of the bottom existing reinforcement was found to be higher than shown on the shop drawings. This meant the beam needed to be reassessed and external post-tensioning redesigned to address a flow-on clash with some of the existing prestressed strands.
• Revising the construction sequence developed for the roof topping slab due to issues created by the undulating nature of the existing roof surface and a tight slab thickness tolerance.
• Crack injecting blemishes in the roof topping slab; the performance of which were confirmed by subsequent water-proof testing.
• Re-finishing the wall surface with acid-resistant repair mortar to address cement paste loss.

A water drop test confirmed that water levels in the upgraded reservoir were dropping about 3 mm in seven days, well below the 10 mm drop performance limit that is specified for a new concrete reservoir in that time period.

The key target for successful construction was recommissioning the reservoir before the summer peak demand. However, while the start date for the works was agreed to enable that timing, due to the unexpected issues highlighted above, the actual construction took longer than anticipated. Fortunately, there was a wet start to summer, and the reservoir was recommissioned before the dry period commenced.

Article based on Wither Hills reservoir upgrade by Fletcher Bruce, Simon Edmonds and Doug Stirrat presented at the 2019 Concrete NZ Conference, Dunedin, New Zealand with John Steele. 

Taken from Concrete Magazine.