Ngaio Gorge container wall may grow
A shipping container wall that has become a permanent feature in Wellington since a slip in 2017 is about to get longer but its end is also in sight.
For Wellington City Council’s transport assets manager Deven Singh, the Ngaio Gorge Rd slip of July 2017 started out as just another one of hundreds of smaller slips the council deals with every year, albeit on a major road.
‘‘I’ll tell you what, there was a lot of pressure from the public to open the road,’’ he said.
‘‘I was talking to one of the police sergeants who was also here and I said ‘I still think there’s some risk’. He said, ‘if you’ve got any concerns let’s keep it closed’.
‘‘Sure enough, that night, the bloody thing came down and the whole road was blocked off.’’
Since then, the path of one of the city’s main roads has been protected from one of the capital’s largest slips by a 42m-long shipping container wall that is 18 containers strong.
The city council’s 10-year-plan allocated nearly $11 million to secure the road which carries 10,000 vehicles a day and is a major connection between the northern suburbs and the city.
On Monday, a tender was put out for geotechnical work that would bring the city closer to getting rid of the container wall.
Singh said the council had a set of preliminary designs for the road, post-container wall, but he preferred to call them ‘‘ideas’’ until geotechnical work and more detailed investigations could be done on the slope.
The first stage of that work would likely involve flying in drills by helicopter and perching them on the side of the slip to drill down and see how deep some of that loose material on the slope goes.
Singh said the investigations could dislodge more material, meaning more containers and a set of concrete barriers would be needed, extending the container wall by an additional 40m while work was being done.
‘‘We need to know what sort of ground conditions exist there; I mean, how deep are those loose materials. We don’t know.’’
Actual construction work on a permanent solution for the road will start in September.
Wellington City Council’s preferred construction option is a redesign that moves the road away from the slope and smooths the curve of the road so that it is less of a sharply winding chicane.
Further up the slope, attenuator fences would hang off anchors drilled into the rock at strategic points. These fences drape, sag and are designed to slow the speed of falling rocks.
Three retaining walls would secure the slope and a 2m-high steel mesh catch fence close to the road would act as a final backstop for any rockfall.
Construction work will be completed by October 2020.
‘‘We need to know what sort of ground conditions exist there.’’ Deven Singh