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Aloha,
When pouring piers into forms on a solid rock slab, before pouring...
how deep would you want to drill, to anchor the rebar into the rock?
I was thinking ~1 ft.
would that be sufficient? do I need to sink it in deeper? would 1 ft be overkill?
Mahalo
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Depends on the solidity of the rock substrate. Weak stuff = deeper. Strong stuff maybe 4".
There is no one answer. It all depends on conditions.
Frankly, this is where the average owner/builder should hire an experienced concrete crew and listen to them.
Assume the best and ask questions.
Punaweb moderator
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I was hoping you wouldn't say that!
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It should be on the blueprint,
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This is actually something I will need to know at some point.
The rule of thumb can probably be expressed as a function of the drill, as in "until it won't go any deeper", then size the rebar as appropriate...
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6" for lateral loads, 4" epoxied, for vertical loads
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Rainyjim,
I would assume this is a basic residential application. Use of a back hoe to test the stability of the rock is a wise first step.
Second, because of the random fracture/layering nature of the lava rock and because you've no way to determine the density and integrity, the best rule of thumb is to sink as deeply as possible and use epoxy to set the rebar. 1/2" bar - 5/8" hole etc. The epoxy should billow out of the hole somewhat when the rebar is fully sunk.
Use of a 16" roto-hammer bit should suffice in most cases for establishing depth.
Hope that makes sense and helps out.
I know it sounds extreme but ground acceleration and a house load are equally extreme.
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This is great info! So put it at least 6 inches and you are safe?
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If you were making a rebar ladder on a cliff, 6" would be deep enough.
6"-8" is a common depth for rebar into concrete, not lava.
Lava is prone to easy fracture when under heavy stress.
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When pinning to rock (any rock) for foundation purposes, it is common to anchor at a 24" depth.
The reason for this is stress fracturing when under seismic load, which when occurring with a shallow set anchor leaves the home with very little mass under foot. The 16" recommendation is a compromise to the 24" rule.
