Underpinning is, by definition, modifying a foundation that exists so it is supported by either extending it or via additional supports extended down into firmer soils beneath the surface. The practice may be employed when the land has failed beneath the foundation or the soil, such as clay, is inadequate to the task of supporting a slab. Underpinning a garage slab that has cracked and no longer level can be bring the slab back to level again.
Drill down into the soil through the slab.
Insert brackets into the holes excavated.
Drive the auger still further until limestone or some solid strata is reached.
Insert levelling rams and lift up slab to its previous level position.
Allow the slab to settle onto the underpinning piers.