Last article we talked about why concrete foundations fail and what
happens to the house when they do. We talked about why it is important to
fix them from an investment point of view, and how to tell if the foundation
is going to need work.
How to tell a bad slab from a good one, what you need to know about pier and
beam foundations. Important stuff, my friends, these repairs can easily run
to several thousands of dollars. If you have provided for that in your
repair estimate before you buy the property, then you are in good shape. If
you do not find out that the slab has to be repaired until after you have
taken the property, then the foundation repair is going to come out of your
refrigerator as you watch your profit margin swirl down the drain.
When you decide to repair a foundation, the first thing you do is make a
diagram of what need to come back up. This is done by evaluating the
foundation with a spirit level (a bubble level), or with a Compu-level,
which is a computerized piece of equipment that has a base station and a
long cord with a sensor/sender unit. The base station sits in the middle of
the house, and the sender/sensor unit. The operator takes the sensor unit
around to the different parts of the house and he gets a reading as to
whether that particular part of the foundation is higher or lower than the
base unit. From this he is able to determine what has to be raised and how
much. He makes his diagram and then the leveling begins. This is all true
whether the foundation is slab or pier and beam.
The basic repair for slab concrete foundations is to dig holes under the slab at the
edge of the foundation, and install concrete columns under the slab that are
strong enough to support the weight of the foundation they will support.
These columns are of two kinds: drilled piers, and driven piers. Drilled
piers start as a hole that is dug into the ground to depth of about 12 feet.
Steel reinforcing bar cages are placed in the hole, and concrete is poured
in to fill the hole to about 18” from the bottom of the slab.
These drilled piers, also known as bell bottom piers because of the
under-reaming at the base of the excavation, must then sit for a week to ten
days for the concrete to cure hard enough to proceed with the next step.
When the concrete is cured, then the crew comes back to the house and places
jacks on the top of the concrete and literally jacks the house up until it
is level. The foundation is then blocked up in the level position, and the
holes are back filled.
Some engineers prefer the steel reinforced bell bottom piers because they
are not subject to lateral movement (side to side in the hole), and will not
allow for the deflection that is sometimes experienced with driven piers
when the pilings hit a rock and begin to go off at an angle. On the other
hand, just as many engineers like the driven piers because they are faster
and easier.
Let’s look at the other most common method of concrete foundation repair, or slab repair. The other type of
pier is called driven pilings. Driven pilings are concrete cylinders about 6
– 7” in diameter and about 13” tall. You start this repair by digging a hole
at the edge of the foundation. This hole will go under the edge and by about
24” deep under the slab, with a seat at the outside for the worker to sit on
while he drives the piers. It looks like a hole with a shelf at 12” and a
place to put your feet that goes to 24”.
When all of these holes are dug under the foundation the workers will begin
to push the concrete cylinders into the ground under the edge of the slab
with a 100-ton hydraulic jack. They push one cylinder down to into the earth
as far as it will go, when they relieve the pressure on the jack and put
another cylinder on top of the one they just pushed into the ground, and
begin to push them into the ground one after another until the slab begins
to raise up from the jacking.
This raising up is called the “point of refusal”, and means that the column
of concrete cylinders has reached the point where the sub-soil refuses to
accept further penetration. At this point a large concrete block called a
capstone is placed on top of the cylinder column, and the pier is ready for
the final leveling.
When all the columns have been driven to the point of refusal, then the
jacking begins. With either kind of pier, driven or bell-bottom, the jacking
part must be done all at once. That means that the whole side of the house
must be jacked at the same time or the slab will crack like a sugar cookie
as the wave of stress passes along the line of piers while it is jacked one
pier at a time.
Typical jacking involves having one man in each hole with a hand hydraulic
jack and a jacking foreman supervising the whole operation. The jacking
foreman tells the men in the holes how many times to pump the jacks. The
place where the foundation has settled the deepest may get 15 pumps on the
jacks to start with, and the ones on either side where the foundation is not
as effected may require only 100 or 5 pumps to start. The process of jacking
continues with the jacking foreman checking inside and out with a large
level or a Compu-level until the house is completely level. At this point
cement blocks and steel shims are placed at the top of the capstone to
support the foundation, the jacks are released and removed, and the holes
are backfilled.
Pier and beam houses are a lot simpler. First of all, pier and beam houses
are houses that sit up on blocks. Pier and beam is the name commonly applied
to this type of foundation in much the same way as soft drinks are commonly
referred to by the name Coke. Pier and beam means that piers were put into
the ground before the house was built and masonry blocks were placed on top
of them to form the foundation to build the house on. Some houses have this
system, most have a block and base system, which is a block of cement about
20” square and 4” thick that has concrete blocks or brocks built up on top
of it. Either way, the sills (4” X 6” timbers or beams) are put on top of
the blocks, and floor joists on top of these. Next comes a layer of ship lap
or plywood that covers all of this structural material, and that forms the
sub-floor of the house. This sub-floor is what we nail wood floors to, or
put padding and carpet or vinyl over. With the slab foundation we just put
flooring over the concrete.
The process of leveling pier and beam foundations also begins with a map of
what needs to be done, and how much you are going to lift the foundation at
each point on the map. Wood blocks and hydraulic jacks are used to jack the
sills back into position, nd again they are blocked into place using treated
wood and metal shims.
Pier and beam foundations are much easier to repair because they are more
accessible and do not require holes to be dug in order to do the work. Costs
for pier and beam repair normally run from several hundred dollars to about
$2,500 for the whole job. Driven piers run anywhere from $140 on up to $350
per pier for exterior piers, with interior piers being about another $75
each to break out the interior concrete floor and repair it when the work is
finished.
Which kind of piers do I like? I [prefer the driven pilings because they can
be done in one to two days start to finish. Drilled piers (bell bottom
piers) take longer to dig, and have to sit for ten days to cure before the
jacking can begin. Most investors I know of use the driven piers.
If you have questions about concrete foundation repair call me, or catch me at a meeting.
I’ll be happy to help. If you need an answer right away, you can call me and
I will tell you what to do. I
Kevin Smith
Forward Assist Inspections
(713)858-1330
Texas Real Estate License #3234
