What is Underpinning?
Underpinning is a method of strengthening the existing foundation of a structure, in which the load carried by an existing foundation is transferred to a new level at a lower depth. It is usually required when the existing foundation deteriorate due to a range of factors like aging, environmental conditions, and changes in load patterns. Let’s discuss these factors to understand its necessity:
- Uneven Settlement – this could be caused by uneven loading of the building, unequal resistance of the soil action of tree roots or cohesive soil settlement.
- Increase in Loading – this could be due to the addition of an extra storey or an increase in imposed loadings such as that which may occur with a change of use.
- Lowering of Adjacent Ground – usually required when constructing a basement adjacent to existing foundations.
General Precautions Before Underpinning
Before any form of underpinning work is commenced the following precautions should be taken:
- Notify adjoining owners of proposed works giving full details and temporary shoring or tying.
- Carry out a detailed survey of the site, the building to be underpinned and of any other adjoining or adjacent building or structures. A careful record of any defects found should be made and where possible agreed with the adjoining owner(s) before being lodged in a safe place.
- Indicators or `tell-tales’ should be fixed over existing cracks so that any subsequent movements can be noted and monitored.
- If settlement is the reason for the underpinning works a thorough investigation should be carried out to establish the cause and any necessary remedial work put in hand before any underpinning works are started.
- Before any underpinning work is started the loads on the building to be underpinned should be reduced as much as possible by removing the imposed loads from the floors and installing any props and/or shoring which is required.
- Any services which are in the vicinity of the proposed underpinning works should be identified, traced, carefully exposed, supported and protected as necessary.
Methods of Underpinning
1. Underpinning to Walls
To prevent fracture, damage or settlement of the wall(s) being underpinned the work should always be carried out in short lengths called legs or bays. The length of these bays will depend upon the following factors:
- Total length of wall to be underpinned.
- Wall loading.
- General state of repair and stability of wall and foundation to be underpinned.
- Nature of subsoil beneath existing foundation.
- Estimated spanning ability of existing foundation.
Generally suitable bay lengths are:
1.000 to 1.500 for mass concrete strip foundations supporting walls of traditional construction.
1.500 to 3.000 for reinforced concrete strip foundations supporting walls of moderate loading.
In all cases the total sum of the unsupported lengths of wall should not exceed 25% of the total wall length. The sequence of bays should be arranged so that working in adjoining bays is avoided until one leg of underpinning has been completed, pinned and cured sufficiently to support the wall above.
2. Jack Pile Underpinning
This method can be used when the depth of a suitable bearing capacity subsoil is too deep to make traditional underpinning uneconomic. Jack pile underpinning is quiet, vibration free and flexible since the pile depth can be adjusted to suit subsoil conditions encountered. The existing foundations must be in a 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.
3. Needle and Pile Underpinning
This method of underpinning can be used where the condition of the existing foundation is unsuitable for traditional or jack pile underpinning techniques. The brickwork above the existing foundation must be in a sound condition since this method relies on the `arching effect’ of the brick bonding to transmit the wall loads onto the needles and ultimately to the piles. The piles used with this method are usually small diameter bored piles.
4. Pynford Stool Method of Underpinning
This method can be used where the existing foundations are in a poor condition and it enables the wall 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 and jack pile.
5. Root Pile or Angle Piling
This is a much simpler alternative to the traditional techniques, applying modern concrete drilling equipment to achieve cost benefits through time saving. The process is also considerably less disruptive, as large volumes of excavation are avoided. Where sound bearing strata can be located within a few meters of the surface, wall stability is achieved through lined reinforced concrete piles installed in pairs, at opposing angles. The existing floor, wall and foundation are predrilled with air-flushed percussion auger, giving access for a steel lining to be driven through the low grade/clay subsoil until it impacts with firm strata. The lining is cut to terminate at the underside of the foundation and the void steel reinforced prior to concreting.
In many situations it is impractical to apply angle piling to both sides of a wall. Subject to subsoil conditions being adequate, it may be acceptable to apply remedial treatment from one side only. The piles will need to be relatively close spaced.
6. Underpinning Columns
Columns can be underpinned in the same manner as walls using traditional or jack pile methods after the columns have been relieved of their loadings. The beam loads can usually be transferred from the columns by means of dead shores and the actual load of the column can be transferred by means of a pair of beams acting against a collar attached to the base of the column shaft.
Read More: Building Construction Handbook – Chudley
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