build during drought? (poured concrete foundation)

kcinkc71October 29, 2012

I know the havoc the drought has caused here in the midwest on foundations. We sold our house and are looking for a tear down in an older neighborhood. My question is this. When is the best time to pour a new foundation? we are at the driest in 60 years, so the clay has sunk/shrunk. If we pour now, will it heave later? I intend to mitigate all I can (larger footings, piers under footings, gravel back fill) or am I just doomed to the joys of expansive clay soil? Thanks in advance

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worthy

Dry expansive soils have a greater propensity to move than wet expansive soils.

However, the specific mineral content of the soil on your lot--there are at least a half dozen types of clays, for instance--and other factors that will affect the moisture balance must be taken into account in designing and pouring your foundation. Not to mention, the type of foundation you're considering--slab, piers, piles, shallow.

You should have a soils engineer's report before undertaking any work.

    Bookmark   October 29, 2012 at 6:10PM
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kcinkc71

Thanks. Yes, fully intend to do soil analysis when we find a lot. So you are saying that current conditions really don't matter, more knowing what you are dealing with and building around it (or with it)? We are in KC area, poured concrete foundations, just trying to keep it stable for as long as possible, hence the piercing below footings as an option ( not done often here)

    Bookmark   October 29, 2012 at 6:25PM
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worthy

When I spent time in and around Winnipeg, I found they fondly call their clay-rich soils Manitoba Gumbo. Homes there are often as not built on piers 20 feet down to stable rock.

    Bookmark   October 29, 2012 at 8:33PM
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virgilcarter

Second Worthy's recommendation for a soils engineer and specific recommendations for foundations and other site work. It will be worth every penny and serve you well for the long term.

Good luck on your project!

    Bookmark   October 31, 2012 at 4:56PM
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virgilcarter

Depending on soils conditions and recommendations of the soils engineer there are at least two different foundation types common for expansive soils:

--Pier and grade beam;
--Mat slab

Your soils engineer will also have recommendations about any pre-wetting or drying prior to construction. Just hire an experienced professional and let him/her worry about the rest. You will be at rest a lot longer this way!

    Bookmark   October 31, 2012 at 9:06PM
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kcinkc71

Thanks, good advice. we are willing to spend more up front to keep things stabile for longer. Been recommended to also backfill around foundation w gravel vs. dirt to help relive some of the hydrostaic pressure on 9' basement walls. we continue to look for a lot to build, then start w soils testing.

    Bookmark   November 1, 2012 at 9:27AM
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galore2112

I don't think that the moisture level in the ground should determine when to build. The foundation should be designed to be solid for any moisture level.

My basement is 12ft deep/tall in very expansive clay soil (the worst). The 1ft thick poured concrete walls and footings were designed by a structural engineer, who used quite a bit of rebar. The rebar design consists of two parallel layers (6 inch separation) of a grid made out of #4 spaced 8 inches apart vertically and 12 inches apart horizontally. The footings are up to 6 foot wide where the concrete basement ceiling is open (staircase), making this part of the basement wall a cantilevered retaining wall.

It's backfilled with a 3ft wide section of free draining crushed concrete all the way up to grade, all around the basement.

The basement was built during the Texas drought in the summer of 2010 and has since experienced very wet conditions and right now another dry spell. No cracks whatsoever.

    Bookmark   November 1, 2012 at 10:38PM
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