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Property Surveying Blog
Written by Karim El Shenawi-Ali

Property Foundations are as old as the building itself, foundations are considered to be the most important aspect to property stability and safety. They carry the weight of the building, in the process, transferring the load to the soil below allowing the building to stand securely.

That is why when considering undertaking a rear, side or basement extension, it is important to review both your foundation conditions and also take the adjoining owner’s structure into account to help prevent future damage.

If you are considering adding additional storeys to your existing structure, if your property is subject to heavy structural defects i.e. subsidence, or if new structures have been built close by that may cause the condition of the surrounding soil to change, you may find that strengthening your existing foundations may be answer.

However, don’t be too hasty, you may need to have a structural engineer take a look at your foundations to assess their conditions, depth and strength before making any judgement or decisions on whether you may need to strengthen your foundations.

I recently visited a site where a basement extension was taking place. A trial hole was dug and the findings by the structural engineer concluded that strengthening to the adjoining owner’s structure was not required.

What is Underpinning?

In a nutshell, underpinning is undertaken when the existing foundations of a building need strengthening in order to facilitate the proposed extension, or when the foundations generally just need strengthening because of an internal or external factor i.e. persistent subsidence issues or surrounding soil conditions changing as a cause from frequent flooding in some cases.

As underpinning is considered to be increasingly structural risky to both your property and the adjoining owner’s property, method, design and safety has to be carefully considered.  Underpinning poses a significant threat to the building and therefore you will need to employ a structural engineer prior to the underpinning commencing.

You will commonly find that the risks of underpinning are generally increased by poor methodology, leaving the building/structure unsupported for too long, or failure in considering the behaviour of the existing structure.

Common forms of Underpinning

There are numerous different underpinning procedures, however the most common form of underpinning in residential properties is mass concrete.

Mass concrete underpinning is widely used as a method of underpinning when trying to prevent future subsidence issues, or for basement extensions.

It is both a simple and efficient way of performing underpinning, as it will involve excavating a section of the ground below the existing foundations in a controlled manner or steps/stages.

This method involves the builders increasing the existing footings to a further depth to a point where suitable soils conditions are found. Commonly it is undertaken in a sectioned manner, in other words short widths no more than 1.2 metres at a time.

Mass concrete underpinning is usually required when the existing load is too heavy or will be too heavy for the existing foundations. Therefore if you intend on adding an additional floor to your existing structure, structural engineers may seriously consider this method of underpinning.

The depth of the underpinning will vary dependant on the surrounding conditions of the ground that is being excavated. It is important to note that the Local Authority Inspectors will view the trial pit prior to the section being filled by concrete. This is ultimately to prevent any issues from occurring and to ensure the best possible underpinning solution is designed.

Once the inspection is undertaken and the underpinning is approved, the open trench/pit is then filled, the contractor then moves onto the next section or in common terms the next “pin”.

Once the concrete has cured and set, you will need to then transfer the load from the building safely onto the new pin, a dry sand cement packing mortar is then placed in below the new and old foundation in order to achieve this. This is referred to as the “dry pack”.

Generally speaking, mass concrete underpinning is relatively simple and is conducted with a limited amount of.

If you are considering on underpinning your foundations, feel free to give us a call and we will be more than happy to advise and assist you.