How the concrete foundation repair is done.
Find out if the slab needs to be repaired and how much early on.
How to tell a bad slab from a good one, here’s what you need to know about pier and beam foundations. This is important stuff, my friends, these repairs will easily run into 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 bought the property, then the foundation repair is going to come out of your refrigerator as you watch your profit margin swirl down the drain.
Diagramming for the foundation.
When you decide to repair a foundation, the first thing you do is make a diagram of what needs 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.
Two kinds of columns used for the foundation support.
The basic repair for concrete 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 (bell bottom piers) are dug into the ground and poured.
Drilled piers start as a hole that is dug into the ground to a 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 are also known as bell bottom piers because of the under-reaming at the base of the excavation. They 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.
Driven piers (driven pilings) are cylinders driven into the ground.
Driven piers are the other most common method of concrete foundation repair, or slab repair. This 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 much easier to level.
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.
Driven pilings are the fastest and common method of foundation repair.
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.
By Kevin Smith, with final editing and contributions by Joshua Berg
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