Even backfilled masonry foundation

[rothbard]

Howdy,

Just getting started in SE Arizona on a 20x30 based mostly on the 1 story countryplans design.

I was initially geared up for a pier foundation but convinced after readings (especially don_p) to abandon this idea and go with a full perimeter.  The soil is mostly clay and/or sandy loam.  Attempting to do this with least excavation possible while still maintaining a crawlspace.  My idea was to leave at least 18" above grade to joists and then evenly backfill a masonry foundation on footing below it (see picture).  Trenches would only be dug below grade for plumbing going directly into soil and through masonry foundation.



My question is whether this seems to be a better idea than the pier, and if anyone sees any obvious failure to this plan.  I'm especially interested also in others with experience in building in high desert climate.

Thanks!

[rothbard]

As a followup... the masonry between footing and grade is basically only there to allow passing plumbing through without interrupting the solid cement footing.  Would be interested if anyone has experience passing plumbing BELOW the footing, as this would allow to shorten the height of masonry considerably.

[MountainDon]

All of our plumbing lines pass below the perimeter foundation of our high desert home. Placement of the lines before any concrete is poured is key to having things in the right place after the concrete is done.

[rothbard]

All of our plumbing lines pass below the perimeter foundation of our high desert home. Placement of the lines before any concrete is poured is key to having things in the right place after the concrete is done.

Interesting, thanks for your response.  How far underneath the footing did your lines pass?  What kind of precautions were taken around the lines?

[MountainDon]

Our concrete footings are 18" below grade. The fresh water and waste lines are about 3 to 4 feet below grade. Power lines at 40" below grade. We use 18" as the frost depth. No unusual protections around the service lines.

[rothbard]

Very useful information thanks.  Based on what I'm reading I'm inclined to totally eliminate the extra below-grade CMU and run the pipes below the footing.  This makes excavation very simple.  

The last foundation challenge is the switch from piers to perimeter footing creates much bigger challenge in volume of concrete.  I imagine most people pick piers in the first place in part because it is suitable for hand mixing in remote locations.  I'm thinking of doing sections of footing tied together with rebar (not in seismic zone D) to relieve the stress of coming up with enough volume in a short period of time.  Does anyone have any reason to believe this could be worse than doing piers, or does this sound like a dumb idea?

[MountainDon]

If you have rebar embedded in the concrete and they extend from one pour area to the next adjacent it can only be better than separate piers. It possibly may not be as good as a continuous pour, but it would never be worse than piers.

[rothbard]

Thank you MountainDon for sharing your experiences! 

I went ahead and poured the first section of the foundation footer.  I used a hybrid approach.  The sewer (4") ran underneath the footing with a PVC sleeve 2 sizes larger to provide relieving arch, while the water and electric (2") went above the footer with through the stem wall. 

This gave the best of both worlds... I was able to place footer at minimum required depth while placing the large sewer pipe below minimum required depth.  Since water and electric pipes are not as large they could go above the footing and still be below 12" on the deep side of the trench (due to grade).

I think the only thing I would consider changing if I did this again is I would have used a stepped footing at 8" steps on grade (would only be one step where I live) rather than digging deeper trenches on the deep side, to dig to minimum depth and still be to code.  We have pretty brutal hard clay soil and it would reduce rental time on the excavator.  But then again I'm not sure if that would yield as solid of a continuous foundation and it would have been more difficult to rebar with the step.