Smart thinking off the beaten track

Every winter many local councils engage in a struggle that rarely makes headlines – finding money to repair roads damaged by flooding.

The roads may be in remote areas, but that doesn’t lessen their importance to the local community. A solution must be found quickly that involves lateral thinking to obtain maximum value for the limited funds available.

This then was the challenge facing Gisborne District Council when Whakarau Road was badly damaged in not one but two places by winter flooding from the adjacent Waikohu River. The road carries only light traffic, up to 150 vehicles a day – but it is an important alternative route to Gisborne if State Highway 2 is closed.

It is in the council’s western region, where Downer EDI Works maintains 850 kilometres of rural road network amid some of the most unstable country in New Zealand.

Only Whakarau Road’s first 6.5 kilometres of its near-40 kilometre road are sealed, and the long unsealed part is steep and winding, with a side cut approach to its construction.

The flooding at the two sites caused serious erosion to the batter slope and toe of the retaining walls and ultimately led to the walls’ failure.

At the first site there was scouring and undermining of the rock buttress. This loss of gravity support to the batter, coupled with a loss of shear strength to the sensitive batter material due to saturation, led to a slope failure, which resulted in a loss of foundation to the wall and the wall’s collapse.

At the second site the retaining wall was founded in papa mudstone. The failure at the upstream end of the wall was most likely caused by heavy rainfall reducing the cohesiveness of the mudstone material, leading to slope movement and failure. An exacerbating factor was a very active spring 150 metres above the road.

The failure of the slope at the downstream end of the wall was most likely due to eddies in the swollen river eroding the bank (mudstone) and hence the toe of the slope.

To achieve a cost-efficient and speedy repair at both sites, Gisborne District Council opted for a collaborative approach and design-build methodology.

“In a geologically unstable environment with limited road-maintenance budgets, the local-authority roading engineers have problems that they want rectified. They’re not interested in gold-plated solutions but practical solutions fit for purpose that represent value for money,” says Downer EDI Works design engineer Daniel Crichton.

“The methodology adopted led to construction efficiencies due to the good communication between designer and builder on material and machinery selection. It allowed the engineer to manage engineering risk by being involved in the construction process, with on-site design modifications made quickly.

“However, despite the same organisation undertaking the design and construction, the client expects a level of quality management and transparency in the design-build process.

“A common perception is that contractors can take shortcuts to save costs. The backbone of the entire process is that quality is paramount. If quality is not maintained, there is increased risk of design failure and loss of client confidence in the process.”

At the first site several inadequate design details were thought to have contributed to the retaining-wall failure. The original rock buttress was not thick enough and did not extend sufficiently deep into the riverbed to adequately protect against design scour forces. The batter slope material was of extremely poor engineering quality and had extremely low shear strength when saturated.

The proposed repair consisted of a suitably designed rock buttress and a batter slope constructed from imported granular fill and a tied-back wall. The new wall included components salvaged from the destroyed wall at the second site, resulting in significant cost savings.

An initial assessment at the second site indicated that the retaining wall could be reinstated. However, this proved to be impractical as the wall had insufficient embedment and a too-large rock-and-concrete buttress would have been needed to protect it from future floods.  

It was also realised that removing a small rise in the road meant a means had to be found to support unstable earth above the road. Further, any repair solution had to include the reinstatement of the true left-hand bank of the river and suitable protection from any of the scour forces associated with heavy rainfall.

Accordingly, a suitable buttress was designed to protect the reinstated left-hand bank and a suitable restraining structure to prevent activation of mobile earth and to support existing stable earth.

It was believed that the unstable material above the road was due to subsoil water. This water resulted in lower soil shear strength but also acted as a lubricant by migrating along the mudstone substrate. Therefore it was decided to install a drain at the spring at the top of the slope.

Crichton says the repair of the two flood-damaged sites on Whakarau Road is a good example of a partnering approach to solving road-maintenance issues.

“The partnering process, where the client and contractor work together as a team, can enhance whole-life value while reducing total cost, improving quality and bringing about innovation, and deliver a project more effectively than in a traditional fragmented approach.”

• This article was adapted from a paper Daniel Crichton delivered at this year’s annual conference of Ingenium, the organisation for professionals engaged in the management and engineering of public infrastructure.