InSite | Underpinning of existing foundations

Underpinning of existing foundations


When developing established properties, you may be required to underpin existing foundations to allow for improvement works to be carried out or to fix any problems such as subsidence. This is done by increasing the depth of the existing foundation in order to spread the load of the building.


Why underpin existing foundations?


  • To stabilise the foundations due to either;
    • Weak soil causing settlement
    • They have moved or slipped
    • Erosion has caused movement
    • There are tree roots causing subsidence
    • Mining
    • Vibration damage (caused by trains, machinery)
  • To Increase their strength due to a potential increased load
  • To be able to accommodate a new adjoining structure, basement storey or deep sewer.




A Structural Engineer should carry out an appraisal of the existing building and its foundations. If there is structural damage, the following should be included;


  • How significant the damage is.
  • What is causing it – if its movement then what is causing this? This will allow them to assess whether it can be resolved immediately or whether they need to investigate further in order to find the correct solution.
  • In some occasions, the cause will need to be monitored for a period of time to evaluate if it is progressive or not. This can cause time delays for developers.


In the most cases where movement is the cause, it is important to investigate the depth of the existing foundations. This will involve testing the soil types below in order to establish an adequate stabilising depth of underpinning and should be carried out by an industry expert or engineer. 




The operatives who complete the underpinning work should be competent; this will ensure that the workmanship is to the required level. The Association of Specialist Underpinning Contractors (ASUC) is a specialist contractor association who provide useful information on their website for underpinning works


For more information see chapter 12 of our Technical Manual