Underpinning is the process of strengthening or stabilising foundations of an existing building. There are several reasons why this may be necessary
- Original foundations of the building is simply not strong, stable enough.
- The supporting soil may have changed its bearing capacity. For example water may have penetrated the area.
- New construction may require excavation close to the existing footings, thereby reducing their structural strength as a supporting agent.
- It is simply more economical to fix the existing then to build a new structure. For example, the floor slab of the house may have sunk 50 mm. Rather than rip it up and replace it. One can just hydraulically raise by underpinning it with grout.
Diagrammatic video shows how Micro Piling one type of underpinning is achieved.
Above Typical stair crack from footing subsidence
Underpinning is achieved by strengthening the bearing capacity of the soil by injecting grout, or by increasing the area of the footing itself or putting a mass concrete footings or combination concrete, steel structures under the existing footing or Micro piling as shown in the previous video
Typical underpinning structure.
The following picture shows a basement being dug out to increase head height, so making the space user-friendly. As the floor slab needs to be lowered we have to underpin the existing footings of the surrounding walls. In this case the underpinning was performed by laying bricks rather than mass concrete. This is an example of point number four, “more economical”. In this case the underpin of footing depth varied between 200mm and 1200mm as original footing depth varied due to house being built on a slope. The previous DIYer’s had dug below grade of footings and had just concreted in front of them instead of doing the laborish job of underpinning. This no doubt accounts for some of the external wall cracks visible on this property.
Basement is excavated to grade here and it is easy to see how far the existing footings are above the new level. These walls need underpinning.
You can see here partially build new brick foundation serving as underpinning for the existing 230mm brick wall. No concrete or piling was required here.
Brick walls above supported with steel beams inserted, then the internal footings removed, subsequently excavated to level and then common property sandstone walls underpinned to rock.
Screw piles can be used before or after construction to provide solid foundation for new footings or underpinning. See samples in above video.
If your house is cracking up or you need to extend the basement, then you will need underpinning, you will need a specialist trades person. This is not a do-it-yourself project as the risk is far too high of causing damage to your valuable asset.
References: Underpinning information: http://en.wikipedia.org/wiki/Underpinning
Trustworthy Builder: http://all-builders.com.au
Confined Excavations: http://bestexcavations.com.au
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