Foundation repair
II
Last post we talked about why 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 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 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 foundations 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’m Kevin Smith, and I’ll see you out on the
property.
Kevin Smith
Forward Assist Inspections
(713)858-1330
Texas Real Estate License #3234