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Slope failure closes part of State Highway 1

Wellington, the capital city of New Zealand is located at the south end of North Island.  The city was built around a sheltered harbour, however there is limited flat land and development has moved on to areas with challenging topography.  Excavations are commonly required to accommodate housing, and infrastructure.  As a result, we are interested in understanding the behaviour of the slightly metamorphosed sandstone and siltstone rocks (locally known as greywacke) that underlie the city.

Yesterday a small rock fall landed on the south bound lanes of State Highway 1, one of two major access roads into the city.  The road was closed while repairs were carried out, and traffic was severely disrupted across the highway network.

Two of our academic colleagues have commented on the rock fall:

https://www.stuff.co.nz/dominion-post/news/wellington/94651404/explainer-how-did-the-wellingtons-ngauranga-gorge-slip-which-closed-state-highway-1-happen

https://www.stuff.co.nz/national/94625277/dynamic-landscape-makes-nz-prone-to-slips

I believe that one of the important factors not discussed in either news item is the effect that lack of maintenance has on such slopes.  When designed and built, the benches had functioning drainage systems, and were clear of vegetation.  Now they are well vegetated with root systems that are penetrating rock fractures reducing their shear strength. Access to the benches is difficult, and this would limit any inspections or maintenance efforts in recent years. As a result, I think it unlikely that drains have continued to function as designed.

Where we can obtain access to such slope failures, we are using UAV derived data to investigate the structural controls of stability, with the detailed 3D data defining the composite failure surface.  Using TSLOPE we can then carry out a back analysis that accounts for the 3D effects, and gives us further data points that we can use to characterise the strength of greywacke at low confining stress.

If you are carrying out slope stability analysis, you do not need to be limited to a simple 2D representation of the slope.  As you can see from the recent greywacke slope failure that has disrupted highway traffic, it is structurally controlled and the mechanics of failure can only be analysed with a 3D program such as TSLOPE.

 

 

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Comments

  1. Lis, the photo you provided shows a slip in almost the same place on Ngauranga Gorge in the 1960’s. Great strides have been made in HSE since then, from 2 guys on the slope in hard-hats to the proliferation of hi-viz vests in 2017. TSLOPE, our 3D slope stability package shows similar improvements over software that is based on 1970’s legacy 2D packages.
    Ngauranga Gorge

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