A.Foundation repair
contractor Dave Cunningham responds: That
house needs a deep pier system to support the
footings from below. It's a job for a specialty
contractor. Your choices on the market are concrete
pilings, push piers, or helical piers; which one
you choose will depend on what the local
contractors in your area have available, and on
which contractor you feel most confident about. I'd
get at least three proposals before you decide.
Some systems can be installed from inside the
basement if there are exterior elements you don't
want to disturb. But, generally, you end up
excavating around the exterior down to the footing
and attaching your piers to the footing from the
outside.
Concrete pilings are probably the costliest way
to go. That involves auguring a 24-inch hole
wherever you need a pier, placing a rebar cage in
the hole, and pouring a concrete pier. To raise the
house, you have to terminate the pier 2 feet below
the footing, with a shoulder that extends under the
footing where you can place your hydraulic jacks.
If you're just trying to stabilize the foundation
but not lift it, you bring the shoulder or shelf
right up to the bottom of the existing footing.
Push piers, your second choice, are steel tubes
that are pushed down into the soil with a hydraulic
rig. The depth is determined by the resistance that
builds up as the pier is pushed deeper. The piers
attach to the footings with metal brackets. Again,
if you're lifting, you'll have to provide for the
jacks.
The option I prefer is helical screw piers.
These are screwed or augured into the soil rather
than driven. If you have bedrock a reasonable
distance down, I'd use just one helix per pier and
take it down to rock. But if your rock is too deep,
you have to rely on the soil to carry the load. In
that case, you add more helixes. With two or three
helixes per pier, you may have to go down only 8 or
10 feet below the footing, depending on the
soil.
The soil is a critical factor -- its bearing
capacity will determine pier spacing and depth.
It's well worth the money to have a soils engineer
evaluate the job, do a soils test, and place the
piers for you. Here in Kansas City I pay $225 for
that service.
The job itself would cost on the order of $1,000
to $1,200 per pier in my area, and you're likely to
need a pier every 4 to 6 feet wherever the footing
needs support.