Posts: 75
Threads: 22
Joined: Sep 2013
Hello guys, I was curious about the ground prep out there to pour a slab. Can you scrap away the little dirt and pour right on the hard rock, or do you need to rip the hard rock out of there and then pour to be compliant? A building I have planned has no pipe work running under the slab except in one section which will be inches from the edge. I was hoping just the dirt could be scraped away and forms set up on the hard rock but wanted to ask. Thanks in advance everyone.
Posts: 8,465
Threads: 1,032
Joined: May 2003
What is the purpose of the slab?
Assume the best and ask questions.
Punaweb moderator
Posts: 14,116
Threads: 424
Joined: Aug 2012
I've been wondering exactly this for a while now; I have a project where it would be ideal to simply "skin" the pahoehoe to make a flat surface. Maybe with a thin layer of basalt and a vapor barrier?
All the projects I've seen are conventional packed gravel base w/forms.
A friend who recently had a pour (garage-type slab) opted for the "thread" (forget what it's called), they mix long fibers into the concrete, same strength as "conventional" wire reinforcement but without the work of laying all the screen in your form....
Posts: 297
Threads: 12
Joined: Jan 2012
I just watched them build a new house next door.
Dozed a path and large clearing. Broke up the whole space, pushed the rocks aside, and burried all the slash, trees included.
Covered back up with same rock/lava. Brought in gravel, piles a little, trenched a little, no compacting, set up forms. Set plumbing and poured.
Striped forms same day, start framing the very next day. Amazing but makes me wonder how it will all settle out
Posts: 75
Threads: 22
Joined: Sep 2013
Rob, the slab would be used for single story residential. We plan to build a small guest house/cottage prior to building our main home.
Posts: 8,465
Threads: 1,032
Joined: May 2003
Then you will need a footing in addition to the slab.
Assume the best and ask questions.
Punaweb moderator
Posts: 75
Threads: 22
Joined: Sep 2013
How deep do footings go in Hawaii?
Posts: 8,465
Threads: 1,032
Joined: May 2003
Depends on the structure. I would suggest that a single story would have a minimum of 10" in the footing depth.
Assume the best and ask questions.
Punaweb moderator
Posts: 1,839
Threads: 48
Joined: May 2007
On my garage slab, base course gravel was imported and compacted to 8" thick under the slab portion, leaving 12" perimeter for the footing. Concrete was placed mono-pour 12" thick from grade, which left a 4" slab over the graveled area and 12"x12" continuous footings around the perimeter. Footings were not excavated below grade. #3 re-bar mat 24" o.c. over slab and 2 #4 bars in perimeter footing.
Posts: 75
Threads: 22
Joined: Sep 2013
Thanks DanielP This is what I was looking for. This will save a lot of hassle not having to rip up the rock.
