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If you need to underpin all or part of the foundations of your building, building regulations apply. The regulations specifically define this as 'building work' and appropriate measures must be applied to ensure the underpinning stabilises the movement of the building.

Particular attention will need to be given to any sewers and drains near the work.

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.

The 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.

Underpinning work requires very careful planning and execution.  If you propose to underpin an existing foundation, approval under the building regulations will normally be required.  Gaining such approval will usually involve the preparation of a structural design of the underpinning, including the process to be carried out during construction.  An initial step, before substantial commencement of the work, will generally be for a trial hole to be dug next to the existing footings for a structural engineer or surveyor to make an assessment of the circumstances of the case.

Methods 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.

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.