Underpinning is a century old technique for strengthening the foundation of a structure. The method entails digging box-like holes below the foundation, and strategically pouring concrete into them. Thus, by increasing the depth of the foundation, it’s able to manage load more properly.
Underpinning is generally applicable when repairing shallow foundations, but it can still serve its function at 40-50 feet subgrade. Since this method doesn’t usually require have heavy mechanical machinery, it tends to be relatively cost-effective, as well.