Underpinning Existing Foundations

Do I need underpinning to my property?

The sinking of a property due to ground movements is called Subsidence. This in turn gives room for the existing foundations of a structure to move. Consider that your property, the floors, the walls and the roof all bear onto the foundation. Thus, if the foundation moves, so does the rest of the house, which often leads to cracks on your walls as the building experiences forces in unusual and unexpected directions.

The movement of the ground away from the structure usually occurs for the following reasons;

·??????Leaked drainage or rainwater pipes. This the ground becomes saturated with water and softens. This allows the ground to move as it weakens against the load that it had previously held up so well.

·??????Trees in close proximity. Trees (especially the high-water intake trees) can suck up most of the water in the ground and thus leave the ground dry and bare. This creates voids, as the total volume?of material reduces, and allows the ground to fall in. Thus in turn the foundation falls in also.

·??????The natural movement of the ground material. Such movement over time can be more significant.

·??????Sinkholes that occur naturally on the Earth's surface

·??????Excavation of ground material in proximity i.e. basements, and therefore this can have a knock-on effect on properties walls close-by as the ground moves slightly into the excavation.

Underpinning is a construction technique where the foundations of an existing building or structure require strengthening and stabilizing to rectify structural distress caused by differential movements. This is done by excavating soil from beneath existing foundations and replacing it with material, usually concrete, in a series of phases that increase the overall depth.

Now the question is - when does one know they require underpinning to their home?

Common reasons why underpinning may be necessary

·??????Subsidence (changes in the condition of the soil) or poor soil properties which may have caused the existing building’s foundations to move. This may be due to natural causes such as earthquakes, floods, droughts, and so on.

·??????Additional building storeys, either above or below ground level, are to be added, meaning that existing foundations need to be strengthened so as to adequately support the modified load.

·??????Structures have been built nearby that alter the soil stability.

·??????The original foundation is found to be inadequate in terms of its strength or stability.

·??????The usage of the structure has been modified.

If the underpinning works are to be carried out adjacent or in close proximity to another existing building, this must comply with the requirements of the Party Wall Act 1996 and party wall agreement with the adjoining owner. Where a building has been damaged by foundation movement and has required underpinning, there may also be a need to carry out structural repairs or strengthening work to the superstructure. Such works should be designed and specified by a structural engineer or a specialist structural repair company.

Types of underpinning techniques

There are a number of different types of underpinning, depending on the circumstances.

Mass concrete underpinning

Underpinning Procedure- Mass Concreting Underpinning; Image Courtesy: Alleure Associates

The most common method is traditional mass concrete underpinning - a simple technique that involves excavating a segment of the ground below the existing building foundation in controlled stages, to a depth where suitable bearing strata exist. The excavation is then filled with concrete and allowed to cure before the next 'pin' is excavated. To transfer the building load safely to the new pin, a dry sand cement packing mortar is rammed in between the new and old foundation. This method is low cost and suitable for shallow depth underpinning. Work can be carried out from one side of the wall and in areas of difficult and restricted access. It is suitable for heavy foundation loads and massive structures and for the formation of new cellars and basements beneath existing buildings.

The mass concrete technique cannot be adopted as a solution for all foundation failures. Other techniques will be required to overcome difficulties with groundwater, losing ground or fill. Ensure that the decision to use traditional mass concrete underpinning has been made by a competent person such as an experienced structural engineer. Where foundation movement has been caused by ground heave, (such as moisture recovery of clay soils following removal or decay of a tree) the underpinning scheme should incorporate appropriate anti-heave protection.

Beam and base underpinning

This is a more technically advanced adaptation. A reinforced concrete beam transfers the building load to mass concrete bases, the size and depth of which are dependent upon prevailing ground conditions and the applied loads of the building.

Mini-piled underpinning

This is most suited to sites with variable ground conditions, restrictive access, or environmental pollution. It is used when foundation loads need to be transferred to stable soils at considerable depths.

Expanding resin injection

This is a more recent development that is cleaner and less disruptive. It involves the injection into the ground of a structural resin and hardener mix, or strengthening grout, that chemically reacts to expand and compact weak soil, thereby raising and re-levelling a structure.

Did you know that underpinning of foundations is notifiable work under the Building Regulations? You must make an application to the building control authority in all cases, without exception. To ensure that structural risks are mitigated, there are design, methodology and safety procedures that must be adhered to.

Underpinning?works?must also comply with the?Construction,?Design?&?Management?Regulations?(CDM) 2015, and if the?building?is adjacent to, or in?close proximity?to other?buildings, then the requirements of the?Party Wall Act?1996 may need to be complied with.

Do you need a structural assessment of your building done? Contact Ora Labora for an Engineer to visit your property and inspect the damage, and advise on a way forward.