2023 Top Projects: Retaining wall protects against future storms

In 2018, a major spring storm event compromised the live drain at Harrisons Cut, a popular recreation destination in Moa, New Zealand. The damage threatened the stability of the site’s bluffs, adjacent access road, carpark, and pedestrian walkway. Another storm event could have caused the drain to fail, resulting in the access road’s collapse.

The Harrisons Cut Beach Accessway project sought to remedy the damage, upgrade the drain and install protections against future storms. In addition to upgrading the drain, the project would reinforce the adjacent transportation and pedestrian infrastructure with a retaining wall. The wet conditions of the Harrisons Cut project site proved a challenge for a cast-in-place concrete wall or gabion baskets. Instead, the project team employed Redi-Rock precast modular blocks. The project designer used the
modular blocks to create three interlock- ing reinforced wall designs, varying in height from 7.5 feet to 15 feet.

Drainage backfill was used to a distance of 12 inches directly behind the blocks to handle runoff from the roadway, carpark and walking path. Two manholes were also connected to stormwater outlet pipes exiting the retaining wall. Threaded rebar rods, steel plates, and hooked threaded rods further reinforced the blocks throughout the structure.

For a popular recreation destination, an attractive appearance was essential for the wall. The blocks used Redi-Rock’s Ledgestone texture, and the wall incorporated 96 planter blocks to host
native vegetation. The project began construction in April 2022 and finished in December 2022. The project was initially budgeted for $2 million USD, but it finished more than 15% below budget at $1.68 million USD.