Underpinning methods do not treat the soil, but instead treat an existing foundation, either by modifying or repairing the foundation or by making additions or connections to a foundation. It is done in order to improve the foundation’s load carrying capacity, and transmitting foundation loads to an adequately strong soil layer. So, for existing structures, underpinning is an alternative to ground improvement methods, because soil is not treated here.
Planned additions or modifications to a structure, nearby excavations, foundation damage, or existing problems with settlement of the structure, may lead to this. Cracked walls (especially diagonally), cracked floor slabs may be indication of excessive foundation settlements, when a part of the structure settles more than the rest (differential settlement). Ability to access tight locations is important in underpinning, as most existing structures can be in highly developed areas of cities, where access may be very limited.
We also listed some of these methods under Pile Foundations section.
There are different methods for underpinning:
- Concrete Underpinning (Mass Pour Method): This is the most ordinary way of underpinning, where, the soil below the foundation is dug at small steps, replacing the excavated & removed volume with concrete and proceeding until the whole soil under the foundation is replaced with concrete, which means that the foundation depth is increased. Mechanical and hydraulic jacks can also be used to temporarily support the undermined areas. This is used for shallow foundations and in situations when increasing the footing depth is sufficient, where there is sufficiently strong soil right below the structure. This is a method that doesn’t involve any pile type underpinning.
- Helical Piers: See our post about Pile Foundations.
- Pushed (Jacked) Piers: See our post about Pile Foundations.
- Pit Underpinning: This is a method used to stabilize an existing structure foundation, before an adjacent excavation for a future neighboring structure takes place. Firstly, the underside of existing footings, which will be adjacent to the future excavation are partially excavated. Then this is filled by a cement based material such as concrete or mortar, to solidify and serve as a firm support layer under the existing footings. After this is complete, the new adjacent excavation can proceed, without harming the existing structure.
- Pile and Beam: Mini piles are first installed on both sides of a continuous footing of a wall. After the piles are installed, they are connected by needle beams, and the wall is supported on these needle beams, which run under perpendicularly to the wall at few feet intervals, to carry the wall.
- Piled Raft: Relatively complex technique which involves construction of a whole new raft under the structure, supported on piles. It is a good solution where the whole structure needs to be underpinned and when existing foundations are deep that small equipment for other methods can not access. One benefit is that it also provides a complete structural connection from below the structure among different structural members. This can be done totally from inside the structure.
In the next post of this series, we will discuss “Soil Stablization”