Pacific coast roadway in New Zealand saved with Redi-Rock anchor wall

The challenge was in constructing a retaining wall structure that could support the road, while keeping what was left of the existing road open for traffic.

Repairing the broken road with a new anchor wall.
Repairing the broken road with a new anchor wall.
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When a massive road slip sent 49 feet (15 metres) of roadway down a cliff into the Pacific Ocean in northern New Zealand, the saying "Ehara taku toa i te toa takitahi, engari he toa takitini" – which translates to "My strength is not mine alone, it is the strength of many" – became a motto for the large team tasked with creating a solution.

The challenge

The damage to Mahia East Coast Road in Wairoa, NZ occurred in April 2019 when shoreline erosion undermined an old wooden crib wall along the Pacific Ocean coast, causing part of the road above it to drop into the sea.

The main challenge was coming up with a tall retaining wall structure that could support the road with limited horizontal space for construction. The repair also needed to be done while keeping what was left of the existing road open for traffic.

The solution

When the road slip occurred, Wairoa District Council partnered with Land Development and Engineering (LDE) and Quality Roading and Services (QRS) to design and implement a practical and affordable solution.

“The local authority made a good decision in being quick to get an options assessment underway, which allowed for development of what became the final design,” explains Greg Bush, civil and geotechnical engineer with LDE.

A temporary prefabricated bridge was placed over the damaged roadway to keep it open, and LDE’s design team came up with a Redi-Rock wall that would be reinforced using rock anchors to address the space constraints.

By August 2020, LDE's design was approved and the QRS team began work on the enormous reconstruction project.

The design

Design of the retaining wall
Design of the retaining wall

The design proposed by LDE included the construction of a 24-foot (7.3-metre) high by 46-foot (14-metre) long retaining wall using Redi-Rock blocks.

A series of earth anchors and reinforced concrete waler beams would create a tie-back system for securing the 2,310-pound (1,050-kilogram) precast modular blocks to the rock face.

The new wall would be supported by an existing bench in the slope which supported the previous failed crib wall. The slope below the bench was reinforced with earth anchors as well, as the slope consists of weak sandstone and siltstone layers.

As the slope bench and base of the Redi-Rock walls sat 23 feet (7 metres) above sea level, neither the blocks nor the metal components of the tie-back system were at risk of saltwater corrosion.

To be thorough in the coastal environment, the design called for concrete to cover steel components and the earth anchors to be galvanised and wrapped prior to installation.

The Redi-Rock wall was further protected by shotcrete facing and large rock revetment at the cliff base.

The installation

onstruction of the retaining wall
Construction of the retaining wall

When construction began in September of 2020, Mike Wilson, QRS project manager, first called in the ground engineering specialists at Rock Control to place ground anchors, 6.5 to 26 feet (2 to 8 metres) in length, into the cliff wall around the slip. Reinforced mesh was then installed to prevent rockfall during construction.

Local Redi-Rock supplier Duracrete supplied the massive blocks to construct the new retaining wall, and Lattey Group assisted QRS with some of the project's heavy lifting. A skip bin was used to lower blocks to the base. An 8-ton excavator also had to be lowered down with heavy equipment atop the cliff.

After materials and equipment were delivered and the slope bench proved stable, the crew began installing 204 massive Redi-Rock precast modular blocks. The wall was composed of multiple courses of Redi-Rock blocks, including two rows of 60in (1520mm) blocks, 13 rows of 41in (1030mm) standard blocks, and one row of 28in (710mm) Redi-Rock blocks.

Every other row of Redi-Rock 41in (1030mm) blocks were precast with metal hooks that joined the blocks to the concrete waler beams, fortifying the wall strength.

The cliff walls beneath and flanking the Redi-Rock wall were sprayed with reinforced shotcrete. Finally, a 98-foot (30-metre) long by 4.9-foot (1.5-metre) high section of rip rap rock revetment was installed at the cliff base to protect against future erosion.

The outcome

As the project came to completion in February of 2021, all those involved in the colossal project had reason to celebrate. Built in just five months, the 1,104-square-foot (102.5-square meter) wall had to be carefully constructed in a volatile coastal environment nearly impossible to access.

“Working on a vertical rock face above the sea with a skinny road above and getting it done without incident has been a huge accomplishment,” says Wilson.

To read a more in-depth case study on this project, visit www.redi-rock.com.

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