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Re: Enlarging a footing

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Gerard,

I have several points to make which you're completely free to disregard
(especially if you've already completed your drawings...)

1.  Soil bearing capacity:  1000 psf.   Concrete:  2000 psi.   Strength of
the weakest concrete you would specify is 288 times the "strength" of the
soil.  Shear of undercut ledges/corbels does not seem a pressing  issue.

2.  Moment developed in transverse direction (like cross-grain bending, only
in concrete) would be very minimal.   Coat-hanger wire at 24 inches probably
would work....

3.  The tool used to install dowels into the existing footing will probably
be a "rotary hammer".  I have one of these on my porch right now that is 18
inches long WITHOUT a bit.   You will need a 30-inch-plus wide trench in
order to drill 12-inch deep holes perpendicularly into the existing footing.
In your first post you imply that you are roughly doubling the width of the
footing.  This means only 12-inch wide trenches, and drilling holes at a
very steep angle.

4.   As for soil compaction at the bottom of the new footing, any loose soil
left after the excavating will get quite well trampled under the workers
boots as they pace back and forth installing dowels.  (This is assuming that
the trenches are not severely over-excavated, leaving lots of fluffed-up
soil in them.)

5.  Have you considered soil friction as resisting uplift?  This may not
appeal to you for seismic forces, but I often use the weight of a wedge of
soil when I know the soil has some cohesion.  Soil wedge with side slope of
2
vertical to 1 horiz. seems prudent.  I have pried many rocks, fenceposts and
other objects out of "granular" soils that were gripped quite well by the
soil.  Geotechs are free to chime in and point out the error of my ways,
if they see it that way.

6.  Surface roughness should not be a problem if you are pouring against
concrete that was originally cast against earth.

Most of these comments come from recent personal experience building an
addition on my house.   The world's best training ground for engineers....

Best wishes,

Thor Matteson

www.shearwalls.com





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