Underpinning Foundations

Underpining Foundations

The term used in Construction & Building for Stabilising and strengthening the foundation of an existing structure

Unless you are building on solid rock all buildings need foundations. Most post 1920 houses are built using standard concrete strip foundations.

Prior to this simple spread foundations were often used.

First A bit about foundations.

Concrete strip foundations.  A trench typically 600mm wide by 1200mm deep      (or often deeper) is excavated and then filled with a minimum of 600mm of mass poured concrete.

Spread foundations pre 1920- a shallow trench was excavated in some cases this may be as little as 300mm deep, the Brickwork was started straight from the soil however is was often started double the thickness of the wall for a few courses allowing the pressure of the completed wall to spread over more area.

This is a mini piling rig

This is a mini piling rig

In many cases building constructed on poor sub soils like Clay have suffered from subsidence, as the clay is prone to high expansion and shrinkage especially in drought conditions.

Generally buildings suffering from subsidence will develop cracks.

There are loads of articles on this site that will help you understand more about foundations and soils etc. – look for the links at the bottom.

Traditional underpinning

Typically a series of pits are excavated 1m x 1m and deep enough to extend down past your existing foundations until a solid suitable base is found. The depth required, will usually be assessed by a structural engineer, who makes calculations based on drilled soil sampling or a series of trial holes.

The soil directly under the existing foundations is also removed.

Traditional underpinning excavation of sequental pits

Traditional underpinning excavation of sequental pits

This pit is then filled with concrete to within 50mm of the underside of the existing foundation. The small gap left is then dry packed with a dry mix of mortar that is rammed in under pressure once the concrete has set. If you’re wondering why- Concrete is poured wet and will shrink when cured. Dry packing doesn’t.

The pits are excavated & filled with in an approved series. Obviously if you excavate too many before filling the house will likely fall down.

When complete all the pits will form a complete new foundation under the existing foundation.

Piled and beam or knuckle underpinning

Piling – is the process of forming deep concrete piles with a DIA of 150mm -600mm deep into the ground.

In some cases where the soil condition is very poor or the underground water table is very high. Piled underpinning may be required.

This chart shows how foundations work and therefore is a part of underpinning

This chart shows how foundations work and therefore is a part of underpinning

Pile and Knuckle system for domestic dwellings

This system is only suitable in some circumstances.

First the holes are set out at a precise pre calculated spaces and depths are augured into the sub soil adjacent to the foundations.

Steel reinforcement is then placed into the hole.

The hole is then filled with high strength concrete.

A meter wide trench is then excavated to the expose the existing foundations.

Once the piles are formed. Holes are then broken out into the existing foundation.

A Concrete knuckle reinforced with steel is then cast into the hole -this connects the Knuckle to the pile head.

Once these are all completed some of the existing brickwork is removed thus transferring the pressure onto the new piles- (this brickwork is removed below the finished soil line so it is not visible.)

A trench like foundation is needed for underpinning.

A trench like foundation is needed for underpinning.

Pile and beam underpinning

Same as above with the exception, Piles are formed on both the inside and outside of the walls in pairs – the piles pairs are connected using a cast in-situ reinforced concrete beam. This is considered to be a more stable system as the weight of the existing walls is distributed in a more vertical plane.  (Note- massive disruption you have to move out).

Pipe and beam

Pipe and beam