Underpinning is a method to create support below the ground for an existing structure. This is required when a structure is either sinking or is showing signs of subsidence which can appear as cracks in the masonry and concrete of the structure. It can also be used to create a void such as room or cellar below an existing building.
It can also be used to reinforce existing building and/or structures for additional storeys, extensions, the added weight requires strengthening the existing footings
It is a time consuming task as a small area below the existing foundation or wall base is excavated, then filled with concrete and allowed to cure, then dry packed with non-shrink grout before excavation of the next section is undertaken and the process repeated. This can sometimes be done in several horizontal layers, one below the other to allow excavation to a depth that allows full height rooms to be created. It is basically the building of underground concrete walls from the top down whilst supporting the structure above.