Stopping a 40 tonne boulder

January 25th, 2013 at 10:00 am by David Farrar

Rachel Young at Stuff reports:

A 40-tonne boulder has been turned into a political football after it smashed into an unoccupied house in ’s Port Hills.

Earthquake Minister Gerry Brownlee said the rockfall, which may have been caused by the recent dry weather, vindicated the Government’s decision to take no risks when it came to red-zoning some Port Hill properties.

But some residents forced out of their homes by the Government’s zoning decisions still believe rock protection work is possible.

I would have thought a 40 tonne rock would convince people that safety should be a real concern, but it seems not.

But Sumner resident Phil Elmey, who has vowed to fight the red-zoning of his land, said the house in Finnsarby Place was in a “bowling alley”. He said most of the red-stickered houses could be saved if money was spent on rock protection work.

“Even a rock that size could be stopped by the right protection . . . We think it’s disgraceful that it hasn’t happened.”

I am not an engineer, and I suspect neither is Mr Elmey. But if anyone out there is, maybe you can give us some idea of what sort of protection will stop a 40 tonne rock from ploughing through a house? And what if it was 100 tonnes?

UPDATE: Mr Elmey is an engineer, so I am happy for him to assess his own risk. So long as he is willing to pay for the rock protection himself, and also recuse himself from cover by ACC, health and welfare in case any rocks fall – then he should be free to stay in his house at his own risk.

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18 Responses to “Stopping a 40 tonne boulder”

  1. peterwn (3,271 comments) says:

    40 tonnes v 100 tonnes. Not much difference. The consequences are similar for example if you are zapped by 11,000 volts or 220,000 volts.

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  2. RRM (9,917 comments) says:

    In some situations you can drill a hole right through the loose rock you are worried about, and extending into competent (solid) ground behind the rock. Then place a rock bolt or length of steel reinforcing rod into the hole, and fill up the remaining air space between the rod and the walls of the hole with either epoxy adhesive or cement grout. The loose rock is then effectively anchored back to the hillside, a bit like a butterfly pinned to a collection board. The engineer earns his fees by correctly determining how many bars are required, what diameter they should be, and how far they should penetrate into the ground behind the rock to do the job. The contractor earns his fees by standing under a big loose boulder and drilling a hole through it!

    (Sorry this is not my area of expertise at all… am trying to recall a lecture I heard 13/14 years ago!)

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  3. Kleva Kiwi (289 comments) says:

    I am an engineer, and I will say that although it “is” possible, it is definitely not feasible.
    The problem is not really the weight of the rock, but the mass of the rock falling, which gains energy on its way down (in simple terms).
    To try to stop it falling would require a lot of anchoring/rock ties drilled into the rock face, which in itself is expensive abd dangerous work.
    The residents are in ‘away with the fairies’ if they think something can be done at a reasonable cost. Gerry received good advice on this one. The risk is to high and the cost to rectify would outweigh the properties 4:1.
    And who do these residents think will pay for this work if it where to happen? The tax payer? Why should I pay for their land to be stabilised so that they can live in their high end housing…

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  4. Kea (12,806 comments) says:

    The sort of people complaining about the zoning, are the very same sort of people who would be the first to demand money and heads to roll, if a rock caused damage. They want to best of both worlds, at the expense of others.

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  5. Fentex (971 comments) says:

    I’m sure it’s possible, I’m also fairly sure I don’t want to pay the price in my rates.

    If these people are so sure it’s so viable let them group together and pay the price of retaining these boulders to protect their houses.

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  6. barry (1,317 comments) says:

    So whats next – we all have to live underground in case on a meteor hitting earth or a gama ray storm from space.

    Why cant people be left along to take their own risks – Is it really justifiable to remove everyomne from their houses and wreck the houses just because there is a risk of someboulder rolling down the hill. Its stupid.

    [DPF: I'd let them live there if they sign a waiver for removal from the ACC, welfare and health systems in case of injury]

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  7. Richard Hurst (855 comments) says:

    Anything is possible if you have unlimited funds. We don’t have unlimited funds.

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  8. tvb (4,418 comments) says:

    Anything is possible if you are prepared to throw enough money at it. But economics tells us to spend scarce resources wisely. These people will not accept they have to leave. The Government’s resources are finite.

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  9. Peter (1,712 comments) says:

    I’m pretty sure putting LabGreenFirst in between the house and the boulder would stop it.

    Okay, not entirely sure, but it’s certainly worth a shot!

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  10. pq (728 comments) says:

    this is our risk, we will make our own decisions, we have the authority of our own lives,
    we will live where we want to

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  11. iMP (2,384 comments) says:

    The bolting or caging of large rocks like this, stops them starting a roll-on (like trying to push a car with a handbrake on, on a hill). The mitigation is designed to prevent the initial motion, that starts the rock on its tumble. We looked at some property in Chch that is below large cliffs, did not like the idea of living down hill from potential house-size rock tumbles.

    Short of netting an entire cliff or hillside, there is not a lot that can be done. Slicing a road across the path can stop the rock, as it hits flat ground and thuds to a stop. The engineers have predominantly exploded large dangerous ricks into smaller pieces on slopes.

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  12. Paulus (2,626 comments) says:

    Am I to assume that these houses are still insured ?
    Or, is the taxpayer/ratepayer expected to pick up any non insured damage costs and remediation.

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  13. scrubone (3,099 comments) says:

    What Kleva Kiwi said.

    There’s a time when you just have to take the best advice from the experts. This is one.

    Fact is, in a disaster of this magnitude bad stuff happens and not just on the day. That’s why it’s a disaster.

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  14. Nookin (3,341 comments) says:

    “this is our risk, we will make our own decisions, we have the authority of our own lives,
    we will live where we want to”

    Not if you expect everyone else to pay for the costs of so doing you won’t!

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  15. PaulL (5,981 comments) says:

    I think there are two different questions here:
    1. Could you build a wall or other device that stops boulders before they hit the houses
    2. Could you carry out prevention work that stopped boulders from falling at all.

    For the first one, mass matters. Stopping a 40 tonne boulder that’s travelling fast takes a big/strong wall. Stopping a 100 tonne boulder is substantially harder. There is clearly a weight (and I’d guess at less than 40 tonnes) at which it’s not plausible given reasonable expenditure of money.

    For the second one, we do lots of this. Drive through the Manawatu Gorge sometime. But it’s not foolproof – the Manawatu Gorge is often closed by rock fall. I think this could be done for a plausible amount of money. But I don’t think it would be a good spend of money.

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  16. djp (59 comments) says:

    >I would have thought a 40 tonne rock would convince people that safety should be a real concern, but it seems not.

    strawman…. Come one DPF you are smarter then that

    I recently saw a video of a truck smashing into a house, does that justify red stickering all houses in the vicinity of a road?

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  17. UrbanNeocolonialist (288 comments) says:

    (I are an engineer). Not really feasible to build a barrier (how do you stop rocks bouncing over), far easier to identify and destroy, bury, remove or otherwise prevent the rocks from rolling in the first place. Eg clear out the downhill properties for a day and blow them all up and clear away any resulting dangerous lumps. Job done.

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  18. Azeraph (604 comments) says:

    No one can afford battering potential sliders. declare these areas unsafe. Try shifting a forty ton boulder up hill usually takes two machines. Once the areas is clear of human settlement invite private developers to see if they want the land.

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