The concept you need to keep in mind if you use either a perimeter footing
with interior piers or a SOG with thickened edges is that you need to keep
the moisture content relatively stable under the structure. This can be
done with perimeter trench drains (assuming you have a way to gravity outlet
the drain line) extending below the depth of seasonal moisture change and
below the bottoms of the footings. There are no guarantees however that you
won't have initial differential settlements with the slab in the interior
moving differently from the thickened edges.
A better system, if a firm bearing level is not too deep, would be to go
with drilled piers and grade beams. If you keep the details simple (i.e.,
use spiral reinforcement instead of ties, keep the dowels at the top of the
pier to grade beam connection free for adjustment, use void form or some
other means to minimize swell pressures on the grade beams, do not allow
mushroom tops at the piers, etc.) the cost penalty for producing a superior
design is not great. I don't know where you are located, but this is a
common type of foundation here in Northern California where expansive soil
conditions exist and a number of contractors are familiar with the process.
Bill Cain, S.E.
From: Gary Gill [mailto:Gary_Gill(--nospam--at)BKITECH.COM]
Sent: Tuesday, February 13, 2001 10:26 AM
Subject: Foundation on clay
I'm in the early design phase of developing a foundation plan for a
rectangular shaped one-story house. My initial design concept was pouring a
continuos perimeter footing with interior piers ( the house will have a
crawl space - floor joist). However, a preliminary soil report indicates a 4
ft thick layer of pipe clay about 6 ft below grade. I'm re-thinking my
initial foundation concept. A monolithic slab with turndowns might be more
appropriate. That way the entire house can float. Any other suggestions?