Where the foundations of an existing building or structure require strengthening and stabilising, a process of underpinning may be necessary. Excavated soil from beneath existing foundations is replaced with material, usually concrete, in a series of phases that increase the overall depth. This forms a new foundation beneath the existing one.
To ensure that structural risks are mitigated against, there are design, methodology and safety procedures that must be adhered to.
Common reasons why underpinning may be necessary might include:
- Subsidence (changes in the condition of the soil) or poor soil properties which may have caused the existing building’s foundations to move. This may be due to natural causes such as earthquake, flood, drought and so on.
- Additional building storeys, either above or below ground level, are to be added, meaning that existing foundations need to be strengthened so as to adequately support the modified load.
- Structures have been built nearby that alter the soil stability.
- The original foundation is found to be inadequate in terms of its strength or stability.
- The usage of the structure has been modified.
 Types of underpinning
There are a number of different types of underpinning, depending on the circumstances.
Due to its low cost and suitability for shallow depth underpinning, and heavy foundation loads, this is the most common method of underpinning. The ground below the existing building foundation is excavated in controlled stages (or pins). When strata suitable for bearing the weight of the building has been reached, the excavation is filled with concrete and left to cure before the next pin is excavated.
Safe transfer of the building load to the new pin is achieved by ramming a dry sand cement packing mortar between new and old foundations. This technique cannot be adopted as solution for all situations, and to overcome difficulties such as groundwater, lose ground or fill, other techniques will be required.
This is a more technically advanced adaptation. A reinforced concrete beam transfers the building load to mass concrete bases, the size and depth of which are dependent upon prevailing ground conditions and the applied loads of the building.
 Mini-piled underpinning
This is most suited to sites with variable ground conditions, restrictive access, or environmental pollution. It is used when foundation loads need to be transferred to stable soils at considerable depths.
 Expanding resin injection
This is a more recent development which is cleaner and less disruptive. It involves the injection into the ground of a structural resin and hardener mix, or strengthening grout, that chemically reacts to expand and compact weak soil, thereby raising and re-leveling a structure.
 Method of underpinning
The Building Regulations definition of ‘building work’ includes: ‘(f) work involving the underpinning of a building’. This refers to all or part of the building foundations. Compliance with the building regulations will usually be dependent on the preparation of a structural design of the underpinning, and a description of the process involved.
Underpinning works must also comply with the Construction, Design & Management Regulations (CDM) 2015, and if the building is adjacent to, or in close proximity to other buildings, then the requirements of the Party Wall Act 1996 may need to be complied with.
Prior to commencement of the work, existing structures will be assessed and trial holes excavated next to existing footings for inspection. Particular attention will need to be given to any below grade facilities, such as sewers, drains, electricity cables, and so on.
It is essential that the structural engineer’s instructions should be followed to avoid excessive undermining of existing foundations or causing further damage to the structure itself. Short sections of excavation can be carried out individually, with the timing of each stage and materials specification will be dependent upon the structural engineer’s design. This work will generally be subject to inspection by the engineer, and the building control body.
 Related articles on Designing Buildings Wiki
- Cracking and building movement.
- Ground anchor.
- Ground heave.
- Grouting in civil engineering.
- Preventing wall collapse.
- Soil nailing.
- Why do buildings crack? (DG 361).
 External references
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