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mtnrdredux_gw

Misc. Structural Engineering Question

mtnrdredux_gw
10 years ago

We are installing an antique millstone as a fountain in the center of a koi pond and I want advice on the necessary support.

We are using a koi pond contractor and our landscape contractor, both of which seem knowledgable ... but no one has done this before.

The millstone is 62' in diameter, approx 8' thick, and weighs an estimated 3,000 pounds. We will probably move it in place by crane.

The fountain is 5' deep and 14'5 in interior diameter. We have dug down and extra two feet in the center to provide additional support for the millstone.

Question --- assuming we use sonotube (right?). What diameter does the sonotube need to be to provide the right support?

thanks in advance!

Comments (8)

  • virgilcarter
    10 years ago

    Surely you've made a typo--62-feet wide and 8-feet thick?

    I'm not a structural engineer, but a single poured footing under the center of the millstone (whatever the width may be) using a sonotube may not be the best solution. I'd think you'd need a footing approximating the diameter of the millstone for proper full bearing support. Any footing should go below frost depth.

    Alternatively you might be able to use 4 sonotubes at the 12, 3, 6 and 9 o'clock positions, of perhaps a minimum of 1-foot diameter going below frost depth.

    Good luck with your project.

  • brickeyee
    10 years ago

    A lot depend on the soil type you are putting the support into.

    A local geotechnical engineer can verify the weight bearing capacity of your soil and likely provide some advice on how large a support will be required to prevent sinking of the millstone.

    You may need a larger than stone footing type design to spread the load over a larger area depending on soil conditions.

    Columns transfer weight to a footer in many cases to reduce the per square foot soil loading.

  • mtnrdredux_gw
    Original Author
    10 years ago

    Thanks, both of you.

    Yes, Virgil sorry for the typo ... the millstone is 62" by 8", the pond is 14.5' interior diameter, 5' deep overall and 7' deep in the center.

    We are in CT,and based on what I have been told for other projects, 5' is well below the frost line. As for soil type, well there is a reason we have all those stone walls in CT! We had to sandblast several boulders to dig it out. It is pretty rocky.

    I was hoping we could support the millstone with something of a narrower dimension ... maybe it can widen out at the base to help distribute the load.

    Thanks again for your help.

  • mtnrdredux_gw
    Original Author
    10 years ago

    Thanks, both of you.

    Yes, Virgil sorry for the typo ... the millstone is 62" by 8", the pond is 14.5' interior diameter, 5' deep overall and 7' deep in the center.

    We are in CT,and based on what I have been told for other projects, 5' is well below the frost line. As for soil type, well there is a reason we have all those stone walls in CT! We had to sandblast several boulders to dig it out. It is pretty rocky.

    I was hoping we could support the millstone with something of a narrower dimension ... maybe it can widen out at the base to help distribute the load.

    Thanks again for your help.

  • renovator8
    10 years ago

    The minimum depth for habitable structure foundations in CT is 42".

    The usual way of supporting a heavy site element is to dig a hole the diameter of the object and fill it with compacted crushed stone. This avoids the use of multiple concrete piers and spread footing.

    Since it is not a habitable structure and the pond is larger than the stone, the depth of the support can probably be much less especially if the crushed stone is placed under the full diameter of the pond but it is important that it be coarse and compacted.

  • GreenDesigns
    10 years ago

    I think the bigger issue will be the pond liner and how it interacts with the support and the millstone. I wouldn't want the millstone resting directly on the pond line over a support. Nor, would I want it under the support for the stone. I suppose you could dig a big wide hole 54" deep and do a tamped gravel base as support, then lay the pond liner and about 6 layers of geo cloth, then some sand, then the millstone. That might give it the support it needs without it abrading the pond liner. But, I'm only a pond dabbler, not an expert.

  • live_wire_oak
    10 years ago

    What happens when you needs to replace the liner? That has to happen every 5-7 years.

  • mtnrdredux_gw
    Original Author
    10 years ago

    Thanks, Renovator.

    Hi Green and LiveWire,

    The millstone and its support do not rest on the liner. That would be a no-go. Without boring you with a handrawn diagram, its best described as being similar to lining a bundt pan. We are not doing this ourselves.

    In fact, we thought we should do gunite, like a pool, but everyone says that won't be necessary. Go figure.

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