Underpinning in construction involves a strengthening in the process of building or renovation. There are a number of reason underpinning may be needed. One reason would be that the original foundation lacks sufficient stability or strength to withstand the construction process. Another reason to use underpinning would be if the intended future use of the structure differs and requires more strengthening than was required for the original use. There also could be material changes in the quality of the soils that support the foundation. Naturally occurring subsidence would be one example. Drought, floods and earthquakes are all more profound examples of natural factors changing soil quality. In urban settings, there are times when the proximity of adjacent or nearby building projects necessitates the strengthening of the foundation. Increased depth capacity and enhancing load factors of a foundation are required for projects such as second storey additions. Perhaps the most obvious need for underpinning is derived from the fact that it is less costly to improve an exiting foundation than to build one that’s entirely new.
Extending the breadth or depth of a foundation are means of strengthening support. Using micropiles and jet grouting can serve to accomplish this. An alternative to underpinning the foundation can be jet grout introduction directly into the soil supporting the structure. Urethane based engineered structural resins are another alternative strengthening agent along with grout. Underpinning, through common usage, has become synonymous with foundation work and whenever problem (P class) soils become problematic in construction.