Underpinning is a method of construction that sees the depth of the foundations to a building being increased. The soil beneath the existing foundation is excavated and is replaced with foundation material, normally concrete, in phases.
Underpinning requires close attention to design, methodology and safety procedures. If not carried out properly, this kind of work poses very real risks and could see damage to or collapse of the existing home.
Reasons for underpinning are generally:
- The existing foundations of the building have moved – this is caused by poor soil or changes to the soil conditions (e.g. subsidence has occurred).
- There has been a decision to add another storey to the building, either above or below ground level, and the depth of the existing foundations is inadequate to support the modified building or load (weight) of it.
Method and inspections
The exact method to be employed for underpinning will depend on the many circumstances of the case. To avoid excessively undermining the existing foundations, causing further damage to the structure above, the excavations for the underpinning should be carried out to engineer’s instructions and details.
A typical method is for short sections of underpinning to be carried out one at a time. Depending on how much of the foundation is to be underpinned it may be possible for more than one section to be carried out simultaneously – subject to them being sufficiently remote from each other.
The excavation for each section of underpinning will normally be inspected by a design engineer and a building control surveyor before it is concreted. Filling the excavation with concrete will not guarantee that the underpinning will provide sound support to the existing foundation, because of the real possibility that voids between the two will remain. Therefore, it is usually necessary for a sand & cement packing to be rammed into the void to ensure the support. This may also be inspected by the engineer and building control surveyor.
The timing of each stage and the specification of the materials to be used will vary on a case by base basis and should normally be the subject of a structural engineer’s design.