MAHIA DROPOUT

Mahia Peninsula

QRS Ltd

Year

2020 - 2021

Known as the Rangitahi dropout, the slump occurred in August 2019 when erosion undermined an existing wooden crib wall, causing the wall, as well as a large section of the road to drop into the sea. To keep traffic moving, the road was reduced to a single lane traveling over a bailey bridge, this caused delays for the community and inconvenienced Rocket Lab who rely on this road. The coastline in this area is prone to erosion due to both its geology and the strong wave action, a resilient solution was required. The design called for a precast concrete RediRock wall to be constructed in place of the timber crib wall. The underside of the new wall needed to be protected from being undermined by installing anchors, rockfall mesh, and shotcrete at the cliff face either side. The RediRock wall is also held in place with rock anchors extending underneath the road.

The site is a vertical cliff face with limited access due to the position of the bailey bridge, the anchors were drilled with a limited access drill rig. Where possible this was suspended from a crane truck, in other areas the teams had to muscle it around on hand winches. The remote location of the site meant that shotcrete had to be sourced from batching plants two hours away, Rock Control worked closely with the concrete plants to get the mix design right. The lower part of the site was sprayed first, this was 20m below road level. We decided to lift in an excavator for this section so that we could use our shotcrete robot, this let us apply large amounts concrete each day and reduced the strain on the crew. A crane truck and man cage were used to spray the parts that the robot could not reach.

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*Some images supplied by QRS