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.
If not carried out properly, this kind of work poses very real risks and could see damage to or collapse of the existing home. You are therefore advised to employ experienced people for the design (for example, an experienced designer and structural engineer) and construction (for example, somebody with experience of underpinning and general building work) to carry out the project.
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.