A traditional method for stablising roadside slips is being put to good use as part of the Tararua District’s roading recovery. Twenty-four brush willow retaining walls are being built over the coming weeks, with 14 on Mangahei Road and 10 on Maunga Road.

Brush willow walls are a ‘soft engineering’ method that goes back to ancient times. They were commonly used in New Zealand from the 1940s to 1960s and are having a resurgence thanks to their low cost and environmentally sustainable benefits. These ‘living walls’ are used to shore up roadside slips and to stabilise riverbanks.

They are built by scraping off loose slip material, then laying brush willow horizontally, layered up with earth. In spring, the shoots take off and the willows grow vertically to shrub height. Beneath the surface, the roots intertwine to stabilise the soil.

Tararua Alliance Recovery Project Manager Blake Hedley is a proponent of sustainable construction methods and is overseeing the brush willow walls being built this winter.

“We’ve been using them here for the last six years. They’re a cost effective and environmentally sound option for repairing dropouts on rural roads with low traffic volumes. They’re also a strong candidate for this area because in earthquakes they move with the land and can be easily repaired compared to rigid retaining structures.”

Blake says there are a range of retaining wall construction methods underway in the district, and the solution for each emergency works site depends on several factors. “Arterial routes with high traffic volumes are more suited to mechanically stablished earth walls and timber retaining walls but these also carry a much higher construction cost. The brush willow walls are a great option for building resilience into lower complexity drop-outs on roads with lower traffic volumes.”

The window for constructing brush willow walls is from Easter until September as brush willow must be laid inert, with growth starting in spring. “We are using a shrub willow variety that was developed by Massey University – it’s a nonspreading willow.” Cost wise, a brush willow wall can cost a third of the cost of a timber retaining wall.