Slab-on-ground foundations are not designed to be lifted
When a slab-on-ground foundation is designed and constructed, it is done under the presumption that the slab foundation will never be lifted. There are situations where some small portion of the foundation may become elevated. As long as the elevated portion is small, the foundation should continue to perform in a satisfactory manner.
They are designed to be ground-supported
The model used to design slab-on-ground foundations assumes that the foundation is fully ground-supported. That model, like all engineering models, is an idealized representation of reality. The model works okay as long as the slab is substantially in contact with the ground.
Consider the foundation in the photo at the top of this page. The entire foundation is elevated above the ground surface. This was done to make the house less susceptible to flooding.
Slab-on-ground foundations should be assessed for the changed loading
Take a look at the house at the top of the page. The exterior grade beam is clearly unsupported between the short columns made of concrete masonry units. There is no support between the short columns.
There was once continuous, uniform support from the ground; now there is no support between the columns and the load at the columns are point loads, not uniform loads.
The codes – and common sense – dictate that when the loading is altered, no permit will be issued unless it can be shown by calculations or experience that the altered foundation is structurally adequate. That is almost certainly no way this can be done. There are no drawings, no repair history, no design data such as beam locations and beam depth.
The Texas Board of Professional Engineers requires that the engineer who designed the repair consult the original design engineer. I find it hard to believe that any original engineer would encourage converting a slab-on-ground foundation into an elevated slab.
If the underlying cause of substandard foundation performance is not identified and corrected, foundation repair may not be effective
It is common for homeowners to decide they need and will benefit from foundation repair even though the underlying cause of poor foundation performance has never been identified. Foundation repair is largely a roll of dice. The cause of poor foundation performance is rarely a lack of piers or piles.
There is always a risk of the repair process damaging the house and the foundation
When a slab foundation is lifted, there is always a risk of structural damage to the slab or the house or both. I cannot overstate this: the slab was not designed to be lifted. In the typical situation, there are no drawings, no information concerning the presence or condition of any reinforcing steel, no information concerning any previous repair, the depth and location of stiffening beams, etc.
Foundation repair must be disclosed to buyers
If you repair the foundation, you will be required to disclose that on the Seller’s Disclosure Notice. Once the repair is disclosed there is a risk that some segment of the market will have no interest in your home. I am told that this is not true in every market and every subdivision, but, as a general rule, homes that have been underpinned are more likely to be discounted by the market.
If, say, a large oak appears to be a significant source of instability, the tree is removed and within 2-years the slab foundation movement has attenuated to the extent that foundation repair is no longer a cost-effective solution and there is no requirement I know of that the removal of the tree must be disclosed.