Methods Of Underpinning - An Overview

Underpinning is the technique used to repair faulty foundations or increase the foundation depth.  If you notice cracking in your external walls, it could be the case that your home requires underpinning by an experienced buildings services firm.  There are several different methods of underpinning a property; here's a helpful overview of what's involved.

Mass pour

Mass pour is the most commonly used method of underpinning.  The technique entails segmentally excavating sections of the foundations to a pre-determined depth below the footings.  Concrete is then poured into each of the pits that have been created to reinforce the entire affected area.

Screw piles and brackets

Underpinning using screw piles and brackets is usually used in circumstances where it's not possible to use the mass pour method, for example where a building needs excavating to a great depth.

Screw piles and brackets can installed in foundations that can offer sufficient capacity in compression and tension to withstand lateral a vertical wind forces and vibration.  Once the system is in place, the structure can be lifted back onto the level and the foundation's weight transferred to the pier and bracket system.

Pile and beam

The pile and beam method involves installing a mini-pile on either side of the affected wall.  The brickwork can then be removed below the wall and reinforced needle beam used to reconnect the piles so that the wall is supported.  This enables the wall to take much higher loads, although the weight-bearing ability of the underlying ground will determine how many piles are needed, together with their depth, diameter, and spacing.

The advantages of using this form of underpinning include:

• speed of fitting
• high load-bearing capacity
• less spoil generation and disruption of surrounding ground
• useful for restricted access areas  

Piled raft

Piled raft must be used for underpinning when the whole building needs to be underpinned.  It is commonly used where the building's foundations are very deep and unsuitable for other methods of underpinning.

The piles are sited as determined by loading conditions.  Pockets are then formed below the piles and reinforced needle beams are inserted to take the wall's load.  The needles are then linked via a ring beam, and the entire structure is poured with concrete.

This system has the advantage of not requiring any external access and provides traverse and lateral ties across the whole structure.

In conclusion

If your property shows signs of subsidence, you may need to have it underpinned.  For more information about what's involved, have a chat with a buildings service, such as Jeffrey Hills and Associates.