Foundation underpinning is structural support added to the foundation of an existing structure. Underpinning can be done for a number of different reasons, and is usually performed by a construction firm which specializes in foundations, and may specifically focus on doing underpinning work. While installing foundation underpinning can be costly and sometimes time consuming, it can be very important for structural integrity and safety, and may be required by law in some cases.
One common reason for foundation underpinning is damage to a foundation. Foundations can crack over time as a result of settling or poor construction. Foundations can also be damaged by events like severe weather. In these cases, it may not be necessary or possible to replace the foundation, but some extra support is needed, and underpinning is used to arrest the damage and stabilize the foundation so that the structure above will stay safe.
Underpinning is also done as a form of retrofitting when new information about a structure or the land it was built on emerges, or when the law changes. In earthquake-prone regions, for example, many older structures are not considered earthquake safe under the law, and they need to be retrofitted. Foundation underpinning might also be needed if soil conditions under a structure change or if the results of a soil test reveal that previous information about soil stability was wrong.
There are a number of different techniques which can be used for foundation underpinning. Usually, a structural engineer visits the site to examine the structure, and collects information about the surrounding soils and other topics. The structural engineer may recommend several different foundation underpinning options which can be used to stabilize and support the foundation. Often, underpinning involves deepening and extending the foundation, sometimes with the goal of reaching bedrock for additional stability.
In some cases, a foundation contractor can accomplish foundation underpinning without disturbing the occupants of the building. In other cases, the building may need to be vacated for all or part of the work for safety. If foundation underpinning is required because a previous contractor did not install a foundation properly or information about the conditions at the site was falsified, the contractor or person who lied on the records may be liable for the cost of the retrofit. It may be a good idea to retain a lawyer in these cases to receive the best advice about how to proceed.