Underpinning is a common term in foundation works. Underpinning in foundations becomes necessary when there is foundation failure or cracks that necessitate changing of foundations. It also becomes important when there is a need to increase the load on a building and the foundation needs to be strengthened.
Reasons for Underpinning of Foundations
- Uneven settlement of foundations – due to uneven loading; poor soil condition.
- Increase in loading – due to increase/addition in floor storey or change of use.
- Lowering of adjacent ground – usually required when constructing a basement adjacent to existing foundations.
Precautions to be taken during Underpinning Works
- Notify the adjacent property owner of the work.
- Carry out a detailed survey of the building to be underpinned including adjacent/adjoining buildings and note any defects or cracks.
- Put an indicator over existing cracks to monitor any other movement during the process.
- If the underpinning work is necessitated by settlement, ensure that the cause of the settlement is established and other measures taken before commencing the task.
- Reduce loads on buildings before starting any underpinning work by removing imposed loads from the floors.
- Also, protect any other services within the vicinity of the underpinning work.
Types of Underpinning
Underpinning to Walls
The work here should be carried out in short lengths called legs or bays. The length of the legs or bays depends on the total length of the wall to be underpinned, wall loading, general state of repair and stability of wall and foundation to be underpinned, nature of subsoil beneath the existing foundation, and estimated spanning ability to existing foundations. Suitable bay lengths are 1 to 1.5 m for mass concrete strip foundations; 1.5 to 3 m for reinforced concrete strip foundations. Generally, the total sum of unsupported lengths of walls should not exceed 25% of the total wall length.
Jack Pile Underpinning
This method is used when the depth of a suitable bearing capacity subsoil is too deep to make traditional underpinning uneconomic. This type is quiet, vibration-free, and flexible since the pile depth can be adjusted to suit the subsoil conditions encountered. The existing foundations must be in good condition since they will have to span over the heads of the pile caps which are cast onto the jack pile heads after the hydraulic jacks have been removed.
Needle and Pile Underpinning
This method is used where the condition of the existing foundations is unsuitable for traditional or jackpile underpinning techniques. The brickwork/blockwork above the existing foundation must be in sound condition because the method relies on the ‘arching effect’ of the brick bonding to transmit the wall loads onto the needles and ultimately to the piles. Small diameter bore piles are commonly used here.
Pynford Stool Method of Underpinning
This method can be used where the existing foundations are in a poor condition and it enables the walls to be underpinned in a continuous run without the need for needles or shoring. The reinforced concrete beam formed by this method may well be adequate to spread the load of the existing wall or it may be used in conjunction with other forms of underpinning such as traditional or jackpile underpinning.
Rootpile or Angle Piling
This is a much simpler alternative to traditional underpinning techniques, applying modern concrete drilling equipment to achieve cost benefits through time savings. The process is also considerably less disruptive as large excavation volumes are avoided. Where sound bearing strata can be located within a few metres of the surface, wall stability is achieved through lined reinforced concrete piles installed in pairs at opposing angles.