Unlike the UK, traditional underpinning is rarely used in Ireland to address subsidence problems. It is more often applied where a foundation needs extending downwards for basement construction or where other works are being completed close to shallow foundations. However, it can still be applied in buildings of any risk where suitable bearing soils can be found within a metre. Otherwise, the cost and risk of deep excavations may render the process non-viable, so ground improvement or pile and beam construction may be a more appropriate solution.
The Building Research Establishment defines underpinning as ‘the extension of the existing foundation to reach stiffer or more stable ground’. Homebond and Technical Guidance Document A offer simple field tests in different soils to determine the load bearing capacity. The process involves systematically excavating beneath the foundation at alternating locations until a good load-bearing stratum is obtained, usually less than 2-3 metres. Concrete is then poured to about 50 mm below the existing foundation, which is dry-packed the following day to ensure a tight joint between the underpin and the original foundation. The drypack could be a simple sand and cement mix, a non-shrink grout or a sand cement mix with an expanding and plasticising agent.
The most recent traditional underpinning project that Substruck completed was in 2018 where a shallow random rubble foundation needed to be extended downwards both to mitigate the risk of further movement and to accommodate a new insulated concrete floor. The key difference in this project when compared to normal underpinning works was that a 300 mm projection on the new underpin would not accommodate the new insulated concrete floor and services, both internally and externally. To overcome this, rising walls were installed between the new underpin and formation level. The final pinning up was completed by dry-packing with a semi-dry non-shrink grout rammed into place from both sides.