The Port Hills rockfall problem

The Port Hills area on the edge of Christchurch was very seriously affected by the Christchurch earthquake sequence.  The Canterbury Earthquake Recovery Authority will today start the process of demolishing the rockfall affected houses in the Port Hills area.  Yesterday they released a statement describing the challenges of this work; this statement is well-covered in an article in The Press, which also includes a nice video taken by a drone of some of the sites.  A couple of years ago I visited many of these sites, and I have an old post that presents some of the images that I collected.  As a reminder, this is typical of the state of some of the buildings at the top of the slopes on the Port Hills:

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And this is the state of some of them at the cliff toe:

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Hazards from demolishing rockfall affected houses

Demolition of such houses is extremely difficult and can be dangerous.  At the foot of the slope the workers will be exposed to further rockfalls, although this is a manageable risk with care.  CERA have been using shipping containers to provide protection to vulnerable infrastructure, and the statement says that they will do so again

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A greater challenge may lie in the buildings at the top of the slope.  Many of these are precarious, one building may be supporting another, and in some cases the buildings may even be supporting the cliff:

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For this reason, CERA will proceed with this work very cautiously and will use a range of techniques, including blasts and water blasters.  I have no doubt that they will complete the work safely, but the challenges that they face are severe.

All of the background information to the hazard from rockfalls in the Port Hills area is available online on the CERA and CCC websites.