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Underpinning is a method of creating support below the ground for an existing structure. This is required when a structuris either sinking or is showing signs of subsidence. These can appear as cracks in the masonry or concrete of the structure. Underpinning can also be used to create a void below an existing building, or to reinforce existing buildings.
Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on a stronger soil stratum or distributes its load across a greater area. Mini piling methods can also be used in conjunction with underpinning.
A small area below the existing foundation or wall base is excavated, then filled with concrete. We dry pack with non-shrink grout, before excavation of the next section is commenced. This can be done in several horizontal layers, one below the other. It is like building an underground concrete wall, from the top down, whilst supporting the structure above.
When is it needed?
Underpinning may be necessary for a variety of reasons:
- The original foundation is simply not strong or stable enough
- The usage of the structure has changed
- The properties of the soil supporting the foundation may have changed
- The construction of nearby structures necessitates the excavation of soil supporting existing foundations
- It is more economical, due to land price or otherwise, to work on the present structure’s foundation than to build a new one