Concrete Slab Pinning / Pea Fill
My garage is attached to my house. I have a full basement. When I mean full, I mean full, as the foundation walls are nine feet four inches tall.
When you excavate the hole to place the foundation, you need to over-dig 30 inches beyond the outer face of the foundation. This gives the foundation people the necessary room to work on the concrete forms. But, when the pour is over and the forms are removed, you have a 30 inch trench to fill.
In my garage, I used a product called pea fill. This is a mixture of coarse sand and pea gravel. It is a processed material that you purchase from a sand or gravel pit.
Because of the small granular nature of the product, it is practically self compacting. I simply dumped this material along side my foundation in the garage. Along the nine foot deep wall (the other three walls were only three feet deep) I put in three foot deep layers of fill material.
After installing each three foot lift I would thoroughly saturate the pea fill with water. The water carried small pieces of sand down into the fill which in turn removed any small quantities of air. When the entire garage area was filled and brought to the level I desired, I watered all the fill for several hours.
Needless to say, my garage floor has not settled, tilted or cracked. I know, most of the other builders are unwilling to invest this time. Well, if you are getting ready to build, put it into the contract. So what if it costs $50 to $100 more. You will never regret it.
I highly recommend using this type of fill material in any outdoor location which will be covered with some type of paving material (sidewalks, concrete, blacktop, paving brick, etc.) .
Patios, Porch Stoops & Pins
OK, your builder dumped dirt along side your foundation. Now what do you do? You can still avoid a tilted slab with just 30 minutes of work, a hammer drill and some 16 inch long pieces of 5/8 inch reinforcing rod.
All you (or your builder) needs to do is pin your slab to your foundation. It is so easy to do!
All you need to do is chalk a line along the foundation indicating the top of your finished slab.The locations of the pins need to be marked on the foundation wall. These holes need to be spaced two feet apart and placed below the chalk line so that equal amounts of concrete will be above and below the pin (if the slab is five inches thick, you will drill the hole 2 1/2 inches below your chalk line). The hole diameter is the exact same size as the reinforcing rod pins you intend to use (#4 pins require a 1/2 inch hole, #5 pins require a 5/8 inch hole). Using a heavy (four pound) mini-sledge hammer, pound the pins into the holes. Believe me, if you have drilled the right sized holes it will be a snug fit. If you have the time and inclination, paint the pins with a rust inhibiting paint prior to pounding.
Now all you need to do is install additional reinforcing steel which is tied to the pins. The purpose of this steel is to make miniature beams. The concrete will be supported by the pins at the house and then hopefully span to soil or dirt which was never excavated. This way, when the fill dirt settles beneath the slab, the slab stays in place supported by the pins and steel. If you rely only on the pins for support, the slab will very likely crack at the end of the pins.
WARNING: Do NOT use pins if the slab is structural. That is, if the slab supports the weight of any other thing other than itself, the pins are not sufficient support. You will need to have a structural engineer tell you what to do.
If you have a shallow foundation, say four feet or less, there is a better way to support a slab that is in contact with the foundation. Before the dirt backfill is placed, simply install some four inch concrete blocks on top of the footer along side the foundation. This little ledge is a great support for the slab.