Digging monolithic slab foundation with drain
#1
01-05-16, 11:34 PM
Digging monolithic slab foundation with drain
I have a question about order of operations for my digging and formwork. I'm building a 15' x 23' detached MIL unit in my back yard. I've decided to use a reinforced monolithic slab.
There will be a foundation drain around the perimeter of the slab, made of perforated pipe buried in gravel and enclosed in fabric, which will drain to the side sewer. Lastly, there will be rigid foam insulation under the slab, as well as vertically along the exterior edge of the slab (including the above-grade portion, which will be 6"). The thickened edge of the slab will extend 18" below grade.
I'm still waiting for my final drawings from the engineer but it will basically be this:
So, how do I go about actually digging the trench for the thickened edge? It seems I have two options:
Are these both valid approaches? Is one clearly better?
There will be a foundation drain around the perimeter of the slab, made of perforated pipe buried in gravel and enclosed in fabric, which will drain to the side sewer. Lastly, there will be rigid foam insulation under the slab, as well as vertically along the exterior edge of the slab (including the above-grade portion, which will be 6"). The thickened edge of the slab will extend 18" below grade.
I'm still waiting for my final drawings from the engineer but it will basically be this:
So, how do I go about actually digging the trench for the thickened edge? It seems I have two options:
- dig a 14" wide trench, line the outsidewith a 2 ft. wall of vertical insulation, and build 6" high forms for the above-grade portion, then after the concrete has set, dig another trench and lay the drain; OR
- dig a trench that's 3+ ft. wide that will contain both the footing and the drain, which means I need to build 2" high forms inside the trench that will need to hold back a whole lot more concrete. Then I can remove the forms, lay my drain, and backfill everything.
Are these both valid approaches? Is one clearly better?
Last edited by dlukas; 01-05-16 at 11:36 PM. Reason: fixing image
#2
01-06-16, 04:01 AM
I think that you meant a dry well not a sewer. How is the land pitched? Will there be gutters & downspouts, on that side of the structure? All that work seems like over kill.
#3
01-06-16, 11:02 AM
I appreciate that you're trying to save me time, but we have over 50 architect and engineer hours into the design at this point. I'm looking for advice on implementation, not design--specifically on digging and formwork.
To answer your questions, the lot is basically flat, with very minor undulations (<1 ft.). And yes, the downspouts will tightline into the same drain system.
To answer your questions, the lot is basically flat, with very minor undulations (<1 ft.). And yes, the downspouts will tightline into the same drain system.
#4
01-06-16, 12:26 PM
In answer to your question it would be better and actually probably easier to dig the full width for the wall and the drain and use a form to pour the wall. Otherwise the outside of the wall isn't going to be smooth and get good contact with the insulation. You have to build a form anyway for the section above above grade anyway.
I do agree with Pulpo however that the drain system is a major overkill in this case. I've actually never seen a turned down slab with a trench drain system around it.
I do agree with Pulpo however that the drain system is a major overkill in this case. I've actually never seen a turned down slab with a trench drain system around it.
#5
01-06-16, 01:12 PM
I appreciate that you're trying to save me time, but we have over 50 architect and engineer hours into the design at this point.
#6
01-06-16, 01:27 PM
Thanks Msradell. I'm in Seattle, have poorly-drained soil, and this area in particular already has standing water for much of the winter, even without an additional 350 sq. ft. of impermeable surface added to it. And the drain is pretty small potatoes in comparison to the under-slab plumbing that will already need to be done. That's why we're going ahead with it.
