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How to do post footings into sandstone

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  1. #1
    boo
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    Default How to do post footings into sandstone

    Hi Guys,

    I'm stuck for how to do Lysaght galvanised posts for an awning into sandstone that's only 300mm below the surface.

    I've come up with options in the picture attached.

    1. Use chemical anchors
    2. Use a core drill for just enough rock removal
    3. Use a core drill multiple times and remove lots of rock

    Your thoughts on these or any other option?

    awning_footings.jpg

    Thx heaps.

  2. #2
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    Number one with a base plate is the go, no need for concrete around the post, the anchors will have to be a lot deeper than if they were to be in concrete & may need to be tested for pullout resistance, the other examples would have to a lot of mass & size to resist uplift / overturning forces so that adds costs in concrete & excavation.
    regards inter

  3. #3
    boo
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    Quote Originally Posted by intertd6 View Post
    Number one with a base plate is the go
    Like this ?

    awning_footings_v4.jpg

    I'm assuming the concrete pad only 300mm deep needs the chem anchors to prevent uplift. Any tips on the type would be welcome too.

  4. #4
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    not sure how well chemset works in sandstone but I had a similar problem couple years ago. Looks like you only need another 300 into the sandstone. I broke it up with a jackhammer and dug it out with a 2 handed shovel to get to depth. Jackhammer broke up the sandstone pretty easy for me. If you don't get quite to depth you could bell out the bottom to assist with hold down (and concrete volume)

  5. #5
    boo
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    Quote Originally Posted by stevoh741 View Post
    you could bell out the bottom to assist with hold down
    Thx Stevoh,

    Option 5:

    awning_footings_v5.jpg

    Suffice it to say that I'm totally over digging/breaking up sandstone, and might prefer to not have to do this for each post.

    Any other thoughts?

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    I would do a test drill into the sandstone about 300mm deep or more if possible ,See what comes up with the bit ,if its dry good if its not then you may be on floating rock . But being realistic its only an awning, unless its massive or you are in a high wind area . I would be happy with a plate and stainless dynabolts

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    Quote Originally Posted by boo View Post
    Like this ?

    Click image for larger version. 

Name:	Awning_Footings_V4.jpg 
Views:	296 
Size:	18.9 KB 
ID:	84306

    I'm assuming the concrete pad only 300mm deep needs the chem anchors to prevent uplift. Any tips on the type would be welcome too.
    forget the concrete, thats an added cost you don't need, along with the extra dyna bolts, just space the baseplate off the sandstone with a stack of shims to about 20mm & fill the space under it with non shrink grout, anchors could be 12mm threaded rod into a 16mm X 300mm deep hole, filled with flowable non shrink grout, from memory 450mm deep using this method was good for about 20mpa pullout resistance. Engineers like digging footings in sandstone in situations like this for some unknown reason, when it really isn't needed
    regards inter

  8. #8
    boo
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    Quote Originally Posted by jiggy View Post
    See what comes up with the bit ,if its dry good if its not then you may be on floating rock
    Well, the drill pulled up a grey and somewhat tacky sandstone (a little bit like bluetack), so I'm guessing that it's a floater, at least in one place. Mind you, it was solid right throughout the drill.

    Quote Originally Posted by intertd6 View Post
    forget the concrete, thats an added cost you don't need
    Unfortunately the post foot must be above ground, since the post shouldn't come in contact with the soil (Lysaght recommendation).

    Anyway, I still can't be buggered jack hammering out all that rock, so I'm going to pin it using a concrete pad, epoxy and galv booker rod, something like this:

    awning_footings_v6.jpg

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