A local government in New Zealand has used a Redi-Rock reinforced retaining wall system to protect coastal lands from flooding and erosion and preserve access to a popular recreation site.
Pāpāmoa, New Zealand, is the largest suburb of Tauranga, the country’s fifth-largest city and located 227 km (141 mi) southeast of Auckland.
Perched on the Pacific Ocean’s Bay of Plenty, the region is known for its picturesque golden sand beaches.
Pāpāmoa’s Harrisons Cut carpark and boat ramp provides access to a host of water and beach activities including swimming, surfing, kayaking, kitesurfing, and fishing.
The challenge
The area was battered by a major spring storm event in 2018 August.
This compromised Harrisons Cut’s drain and threatening the stability of nearby dunes and the adjacent access road, carpark, and pedestrian walkway.
The Tauranga City Council (TCC) was worried another significant storm event would cause the drain to fail, resulting in the roadway’s collapse and severing access to the car park and boat launch.
The municipal body immediately initiated planning to remedy the damage, upgrade the drain, and install protections against future storms.
The challenge of protecting coastal habitats and wildlife
Several challenges were identified during project planning including a requirement for significant site identification, testing, and surveying.
An ecological plan in place to protect coastal habitats required the TCC to obtain a Department of Conservation permit to pursue the project.
These key stipulations extended the planning process to more than 2.5 years before construction began in 2022 April.
The additional preparation identified the need to relocate at-risk wildlife during a critical stretch of the project.
The affected wildlife included skink and spiders, as well as eels, inanga, and native freshwater bullies.
Design challenges
The design of the retaining wall itself also proved demanding.
An initial proposal to utilise a cast-in-place retaining wall was abandoned when cast-in-place was deemed too costly and incompatible with the wet conditions of the site.
The Harrisons Cut Beach Accessway project was scheduled for the region’s winter season to limit its impact on site usage.
While this user-centric decision ensured access to the site during the prime summer season, winter storm weather necessitated dewatering and diverting of the live drain, a complex process that threatened the completion schedule.
“We undertook the works off-peak to ensure we limited disruption to the community,” saidTCC’s waters manager Radleigh Cairns.
Finally, the proximity of the roadway, pedestrian pathway, and carpark demanded consideration of load-bearing requirements as potential wall solutions were evaluated.
These surfaces also prompted the redesign of the stormwater handling network and its integration with the retaining wall system.
The solution
New Zealand-based Redi-Rock manufacturer Duracrete Products has supplied precast modular blocks (PMB) blocks for numerous high-profile projects in the region.
These efforts include several urgent infrastructure repairs made necessary by significant storm activity.
As a result, TCC and the project’s engineering firm, AECOM, were familiar with the Redi-Rock engineered retaining wall system and believed it could offer the versatility, straightforward installation, and visual appeal the project required.
“Aesthetically, Redi-Rock looks fantastic.”
“And once the contractor gets into a rhythm they’re easy to install.”
Gareth John, Tauranga City Council’s project manager for city waters.
Wet project sites like Harrisons Cut can be a dealbreaker for cast-in-place concrete walls, and corrosion created by exposure to moisture limits the lifespan of gabion baskets.
In contrast, Redi-Rock blocks’ durable wet-cast, air-entrained concrete stands up to waves, currents, and storm surges with ease, and does so for decades.
Redi-Rock’s dozens of precast block options work together seamlessly to create durable long-term solutions, not hasty, ineffective fixes.
Redi-Rock’s versatility allowed AECOM to create three interlocking reinforced wall designs to address the Harrisons Cut site’s varied conditions and load demands.
Depending on site conditions, Redi-Rock reinforced walls can achieve heights exceeding 15m (50 ft) with the installation of geogrid or earth anchors behind the wall.
How the retaining wall was installed
Installation commenced with the dewatering of tidal streams and pouring un-reinforced concrete wall footings.
Redi-Rock 720-mm (28-in) Positive Connection (PC) Middle blocks were selected to comprise the majority of the wall sections, which varied in height from 2.29m (7.5 ft) up to 4.57m (15 ft).
PC Middle blocks feature a patented knob and groove technology and include a centre core slot cast directly into the block.
Then, 300-mm (12-inch) wide strips of PVC-coated polyester yarns - or biaxial geogrid - wrap through the slot and extend into the reinforced fill zone.
The walls also included 720-mm (28-in) PC Top and Bottom blocks.
The Redi-Rock PC system with geogrid strips eliminates connection failures and simplifies installation compared to directional geogrid sheets.
Geogrid strips also allow for enhanced flexibility when navigating behind-the-wall utilities or stormwater and drainage infrastructure, as was the case with this project.
To ensure proper handling of water runoff from the roadway, carpark, and walking path, drainage backfill was used to a distance of 300 mm (12 in) directly behind the blocks.
Two manholes were also installed, connected to stormwater outlet pipes exiting the retaining wall at the second course, extending past the face, and resting on the block below.
Threaded rebar rods, steel plates, and nuts placed into the wedge between blocks anchored the manhole’s underground enclosure to the retaining wall.
More strength was added by the inclusion of hooked threaded rods specially cast into the Redi-Rock blocks and secured to the enclosure with steel flange nuts.
The versatility of the Redi-Rock system is on full display when something more than just a wall is required.
Conveying water or utilities through the wall with conduit can be accomplished with ease and minimal additional engineering in most cases.
The outcome
Harrisons Cut is a popular regional recreation destination that prompted the TCC to specify a retaining wall as attractive as it was functional.
It was paramount that the solution be visually appealing upon its completion.
This led to the selection of Redi-Rock’s deeply detailed Ledgestone texture, which finishes with a random, stacked stone appearance.
Ledgestone features up to 10 individual face moulds, offering up to 10.5 sq m (113 sq ft) of non-repeating pattern.
“It looks amazing,” TCC’s Gareth John observed. “We’ve had numerous positive compliments since the project started.”
To further enhance the appearance of the wall, two rows of 1030-mm (41-in) Redi-Rock planter blocks were incorporated to host native vegetation.
The project utilised 96 of these planter blocks, which offer setbacks up to 422mm (16.6 in) – the system’s largest batter option available – and can be integrated seamlessly with any Redi-Rock project to add both depth, detail, and design to the wall’s face.
The TCC was pleased with the finished project and the construction process.
“The team managed to deliver in time for summer as planned, while keeping under budget,” concluded the TCC’s Cairns.
The installation of the retaining wall and boat launch, and the upgrades to the access road, walking path, and carpark were budgeted for NZD$3.35 million, finishing with a savings of nearly NZD$700,000.
Excepting a few minor future enhancements like seating and an adjacent boardwalk, the Harrisons Cut Beach Accessway project was completed in December 2022.
Ultimately, the TCC’s John believes the solution was the right fit for the important project.
“Redi-Rock is an excellent product that was selected for its durability, constructability benefits, and overall aesthetics. I would recommend it to anyone and would use Redi-Rock again on future projects.”
To learn more about how you can use Redi-Rock to control water and solve earth retention problems, download the Water Application Guide.
Project details
- Project: Harrisons Cut Beach Accessway
- Owner: Tauranga City Council
- Manufacturer: Duracrete Products
- General Contractor: J Swap Contractors
- Designer: AECOM and Aurecon
- Location: Tauranga, New Zealand
- Year Built: 2022
Products featured in this case study
- Positive Connection (PC) Wall Blocks
- Planter Blocks
- Ledgestone Texture Blocks
Search category filters
- Batter: Standard
- Textures: Ledgestone
- Wall Type: Reinforced
- Industries: Government, Transportation, Water
- Solutions: Barriers & Parapet Walls, Seawalls, Channels, Shoreline Restoration, Boat Docks / Boat Launches
- Products: 28PC (710mm), Planter Block
