Arana College retaining wall

This project involved removing and replacing a 30-metre retaining wall beside the Arana College halls of residence at the University of Otago.

Usually retaining walls are built from concrete block or timber in an environment with straight lines and fixed points to measure off. We built this wall to follow the natural contours of the landscape, using 36 ground anchors drilled into the retaining wall face, a double cage of reinforcing, shotcrete, and a timber fence along the top.

Once we’d removed the old wall and decades of vegetation, we discovered that the design for the replacement wall wasn’t tall enough. We installed a secondary wall to solve the problem, using extra ground anchors and footings. The rock behind the wall also turned out to be more unstable than anticipated. The solution we came up with was to stabilise it was a sacrificial layer of reinforcing mesh pinned to the ground, which we covered with polythene to keep the weather off over the Christmas break.

With most of the students off-site over the summer break, our main challenge on the confined site was to ensure that the shotcrete didn’t stray onto neighbouring properties. We managed this with a temporary polythene wall, which proved very effective at keeping the sprayed concrete out of everyone’s gutters!