Manawatu¯ Gorge’s rocky history

It was branded ‘‘one of the most difficult roads in New Zealand to work on’’ by an engineering firm called in to deal with a major slip in the Manawatu¯ Gorge in 2011.

Now safety concerns have resulted in the road closing indefinitely, it’s fair to say it’s also one of the most dangerous.

State Highway 3 through the gorge was carved out of a steep and brittle greywacke rock chasm about 20 metres above the oftenfierce Manawatu¯ River by brute force – dynamite, manpower and heavy machinery.

The narrow, but vital link between Manawatu¯ and Hawke’s Bay winds tightly around the southern side of the gorge, giving passage to about 7600 vehicles a day, according to the NZ Transport Agency.

At least, it did, until large slips fell in April and engineers finally declared the unstable rock face too great a threat to the safety of travellers and contractors and closed it indefinitely in July.

Since its completion in 1872, a variety of local newspaper reports and records show the gorge has always had a turbulent past, marred by slips, stray rocks and overzealous drivers shooting off the edge.

There’s been real doubt about the future of the gorge since August 2011, when a massive slip closed the road for 14 months and cost $21 million to repair.

MWH engineering, which worked on the project, said at 40,000 cubic metres, it was the largest road landslide in New Zealand’s history.

At the time, Palmerston North MP Iain Lees-galloway said estimates of the cost to the Manawatu¯ economy were up to $8000 a day, and called for other routes to be explored. Transport agency officials at the time said any alternative would be too expensive and logistically difficult.

The transport agency’s report into the road’s weaknesses in late 2012 highlighted areas of potential risk of rock falls, including at least one point vulnerable to a large slip, particularly after heavy rain or strong earthquakes.

In April 2015, the road was closed for nine days after a slip and $250,000 of rockfall netting was installed. By September, a $2.2m upgrade of the gorge road began.

April 2017 could prove to be the final straw for the gorge when, after a month of storms, two slips fell. Geological surveys found a large section of the hillside above the road to be extremely instable and constantly shifting. Contractors clearing the slips were pulled out and calls for a permanent alternative route rang out once more.

Manawatu¯ Standard records show such cries for a route rethink have been made since as far back as 1971.

On Monday, 145 years after the gorge road was opened, the transport agency unveiled 13 possible alternative routes and asked for public feedback.

Options include building a stronger route through the gorge, through to building a tunnel, and range from 6 kilometres to 19km in length.

A shortlist will be revealed in October, and the preferred route is expected to be announced by December.