House underpinning in Sydney 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 is generally to stop any further movement and so stop any further dama. This can be achieved by 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.
Care needs to be taken to find correct depth pins need 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.
Cracks may have occurred due to differing reasons.
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 piling is quite typical.
Typical underpinning structure.
The above pictures shows a basement being dug out in Annandale Sydney and house walls underpinned 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 laborious 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.
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: https://en.wikipedia.org/wiki/Underpinning