Shoreline erosion impaired an old wooden crib wall along the Mahia East Coast Road, causing part of the road above it to drop into the sea. There were challenges with the repair, including coming up with a significantly tall retaining wall structure that could support the road with limited horizontal space for the construction. The restoration also needed to be done while keeping the remains of the existing road open for traffic. Additionally, because of burial grounds nearby, relocation of the roadway was not an option.

LDE was engaged as part of the project team to design and carry out a practical and affordable solution. The initial challenge of keeping the road open during the project was to use a temporary prefabricated structure to bridge the damaged roadway.

Our design was to construct a reinforced 7.3 m high by 14 m long retaining wall using Redi-Rock blocks. A reinforced waler beam and earth anchors would create a tie-back system for securing the modular blocks to the rock face. Anchor walls optimize load capacity in retaining wall applications where excavation isn't an option, such as a cliff face with space constraints. Once the anchors or 'tie-backs' are secured into the earth/rock, they can attach to Redi-Rock anchor blocks directly or connect to blocks precast with metal hooks through a cast-in-place concrete waler beam.

The slope bench and base of the Redi-Rock walls sat 7 meters above sea level. The design included concrete to cover steel components and the earth anchors to be galvanized and wrapped before installation to address coastal environment specifications. The Redi-Rock wall is further supported by shotcrete facing and a large rock revetment at the cliff base. It was built in 5 months in a hostile coastal environment with difficult access. The innovative use of the precast blocks and design schemes has produced a lasting solution that integrates with the environment.

Client: Wairoa District Council
Location: East Coast Road, Mahia