Foundation Extension Sydney Home
To achieve head height to make the basement habitable the walls are extended down after excavation of floor to new levels
It is not your typical house underpinning which generally occurs after cracks start to appear in the walls and floor of the family home or one notices the floors are bouncy, brick piers start to fall over or some doors are starting to jam.
Strategy for the damaged housw is generally to stop any further movement and so stop any further damage. This can be achieved by screw piers or placing solid pins of concrete under the moving walls/piers, possibly Jacking walls back into its previous position, and then repairing the damage that has been done to walls, floors and ceilings.
There are a few different methods to best achieve the desired result dependent upon the causes of the subsidence, the supporting soil type and Depth to rock..metal or concrete Piers can be used to prop off solid bearing or footing sizes can be increased to reduce the load on the supporting soil.
Hand dug hole under sandstone block party wall
Care needs to be taken to find correct depth pins needed to be set at, to best accommodate the existing situation. This house was a heavy structure on shallow footing with walls showing numerous movement cracks. We don’t want to create new problems with differential settlement.
Underpinning saves cracks developing in your house.
Cracks may have occurred due to differing reasons.
- 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.
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. Micro screw piling is quite typical if there is a continuous footing in reasonable condition.
Typical basement 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 labourish job of underpinning. This no doubt accounts for some of the external wall cracks visible on this property.
We removed sandstone footings and replaced with metal to create useable space then underpinned the common sandstone foundations of adjoining properties with a full concrete mass staggered footing of about 800mm depth. Screw piles not practical in this situation.
If your house is cracking up or you need to extend the basement, then you will need foundation 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
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