Underpinning and structural repairs under The Party Wall Act 1996

This video looks at the issues when Building Owners wish to carry out underpinning and structural repairs close to neighbouring properties. The relevance of the Party Wall etc Act 1996 is explained.

Nick Huband, structural and forensic building engineer, of William J Marshall and Partners, is interviewed by Neil Thind, Managing Director of Arun Associates, Chartered Surveyors, Edgware, London.

Underpinning
Firstly, what is Underpinning ?

Underpinning is the lowering of foundations by constructing concrete or similar additional concrete support below existing, older foundations of a building or structure. It is a way of transferring stresses to the subsoil which is at a lower level, and more safely transferring loads to the subsoil from the structure above, reducing the risk of cracking and movement.

There are older, more traditional methods of underpinning, and more modern solutions, involving types of piles or columns of reinforced concrete, but the principle is the same. The loads are being transferred lower down, and this always means some disturbance of the soil and subsoil. Managing that disturbance is the key, to keep properties safe and structurally stable.

A neighbour/Building owner may wish to carry out underpinning of foundations and structural repairs, and in doing so, requires their builder to work close to an adjoining property under different ownership. In some cases, this can be within three metres or six metres away from a neighbour’s wall and foundation, to trigger the Party Wall Act. You don’t have to be disturbing the actual, shared party wall to trigger the Act, and those provisions would then fall within the ‘etc’ part of the title to the Act.

The reason for the underpinning of existing foundations can fall into different categories; For example, tree root damage and drying out, or desiccation of clay soil, causing movement and cracking of buildings; Installing foundations lower than an adjoining owner’s foundations and within certain close proximity of those footings; basement excavations which invariably go much deeper than a neighbour’s property; Adding an additional storey to an existing property, which then increases loads on the foundations as compared to previously.

But building owners need to take care before they start work as they may well have to serve notice on one or more adjoining owners, to comply with the legislation. A neighbour may be affected by vibration, undermining of their foundations, and excess noise as a result of the works, and their principle concern might be the loss of support to their foundation, when excavations are made.

What risks are potential then, with underpinning near an adjoining building?

Older buildings such as Victorian terraces are more vulnerable to structural movement than many modern buildings, where the foundations may be much deeper. The brickwork of older properties may vary in quality and type, with better bricks often saved for the front elevations and poorer quality bricks for the rest of the walling.

Some older foundations of buildings are in a very poor state, due to the materials they were constructed with, and size of foundations. Some were just several layers of brickwork stepped to widen the contact area, and then the ground consolidated underneath, without any concrete.

Others, had poor quality mixed concrete, barely sufficient to support the structural walls, floors, and roof above. Disturbance of the ground may result in cracking, structural movement, and distress. Some buildings will already have shown cracking and movement over many years, but older buildings were built with soft lime mortar, and can accommodate movement more easily than modern buildings, which used brittle, cement-mortar for their walls.

Monitoring
Monitoring of properties from a structural viewpoint may be advisable. This can be achieved with various systems, often called ‘tell-tales’, across existing cracks, steel stud pins and measuring callipers, and on bigger jobs the use of monitoring ‘targets’, such as was used in the Crossrail project, which used dynamic monitoring, using computers, and a 24/7 readout of any movement of the ground. Party wall surveyors can advise on the desirability of monitoring of the neighbouring property.

Metal pins and measuring with callipers also can be used for monitoring.

Surveyors sometimes talk of ‘the three-metre rule’, or the more complex ‘six-metre rule’, which is set out in the Act. It does not matter if your property is terraced, detached or semi-detached, the Act can still apply.

Sequence
The sequence of underpinning must be carefully controlled to avoid unnecessary structural movement and failure. Excavation pits under existing foundations can be carried out in bay widths of about a metre wide at a time. But on should never excavate adjacent to the pit, and Nick Huband explains in the video the need to spread the stresses during underpinning using his hand and fingers to give a guide as to the underpinning sequence of excavation pits.

Note the diagram above shows 1.5 metre wide pits, but a narrower width pit of one metre is better to control movement. The wider the pit, the less underpinning bays needed, but one has to take extreme care on older buildings. The diagram demonstrates the sequence which spreads the stresses in the underpinning process. Buildings have collapsed in the past where builders were inexperienced in underpinning techniques and took risks.

Once the pits are excavated, the concrete is cast below the foundation, leaving a gap of about 75mm thick or so, to allow ‘pinning-up’ to connect to the existing foundations. It is imperative the underside of the old foundation is cleaned of any loose material, soil, etc, to obtain a good structural bond. The ‘pinning-up’ ensures a good connection between the new and old concrete foundation.

Then, a dry pack of rich, cement mortar, with little water content is packed into the gap between new and old foundations. This is often done using a mechanical tool to ram the material tight and leaving no voids which would be a weakness. The blocks of concrete underpinning are sometimes connected together at the faces of the concrete where they abut with dovetail slots or ‘keys’, to ensure a more continuous structural unit.

Stepping of Underpinning
When working near adjacent properties, it is necessary to step the underpinning to reduce it down in a series of structural steps and avoid transferring higher stresses to adjacent, older, shallower, foundations. If one did not step the underpinning, then it would be possible to have a weak, shallow adjacent foundation next to a very strong underpinned foundation, and that would create a stress point at the junction. The movement could then occur at the junction and cause cracking of the masonry above.

One issue raised in the video, is who will pay for the stepped underpinning near the boundary? Insurers may not be willing always to do so. This is of concern to party wall surveyors, and each case will be different.

Piled underpinning can be a particular problem in causing vibration or movement to an adjoining property, so careful selection of the type of system is crucial. Piling involves removing soil and replacing it with reinforced concrete or sometimes steel, and the ground will need to adjust itself to cope with the disturbance. Depending upon proximity to adjacent foundations, there sometimes could be subsoil movement affecting foundations and cracks developing in walls and floors, often referred to as ‘movement’. This is due partly, to a loss of support to the foundations of an adjoining property. Hence, the movement. Some modern mini-piling rigs using screwed or bored piles for underpinning can work well if the contractor is experienced, and plans his work carefully to avoid unacceptable risks.

Structural engineers should be able to assess the likelihood, and degree of movement and predict what might occur in practice. It is important that Party Wall surveyors have a dialogue with engineers in the project on more complex schemes. A design engineer will arrive at a preferred underpinning/foundation design, taking into account the type and depth of excavation. Trial pits or bore samples of the subsoil might also be taken, and analysed, before work starts.

The Adjoining owner, or neighbour, will be concerned in potential loss of structural support if excavations or underpinning is carried out next door. Loss of support to subsoil near their foundations could have catastrophic results if not managed and controlled.

Temporary supports, propping and strutting of the excavation can become critical to ensure that movement is kept to an absolute minimum. Party wall surveyors may discuss these issues with the design engineer and obtain independent structural advice on complex projects, which is permitted under the Act. Nick Huband, the forensic engineer makes the point that maintaining support to foundations is crucial.

Neighbours affected by building works will naturally wish to have some comfort in the building owner and builder mitigating any risk in disturbance or movement of their walls and foundations, where there might otherwise be loss of structural support. The risk can be reduced by the engineer doing a risk-assessment and reducing the concentration of work in one area where possible.

Structural repairs can be accommodated to various areas of cracked or defective masonry walls. Modern methods include ‘Helibars’ or stainless steel rods which are cut into mortar joints and set in place using epoxy mortars. These can help to bridge across cracks and spread the loads more evenly and can be useful for some arches and openings too.

Reinforcement of brick corners can be achieved structurally with installing steel metal straps or sometimes concrete block elbows in the structure, positioned at heights of 25% or possibly at multiples of one-third points vertically up the wall.

Masonry repairs can involve cutting-out and replacing sections or patches of brickwork with matching bricks of the same colour, style and type of the existing, such as where bricks have eroded or fractured. The selection of appropriate mortars is important on older buildings in particular, as the tendency has been for builders to use too strong and rich a mortar mix where the wall was built in softer, lime-mortar.

Nick Huband explains in the video that one can apply a stainless steel mesh over larger repairs which have clay nodules on the mesh. This is screwed to the brickwork with the repairs behind, and then rendered over and the clay nodules help to form a good key to the background. However, rendering over facing brickwork should be considered as a last resort, where the repaired appearance is poor and concealment is the most appropriate option.

This may not be an option on listed buildings, or in some conservation areas, so specialist advice then needs to be obtained, before rendering a previously unrendered wall.

Note that any building works may require statutory consents and approvals, such as Building Control, Planning, Listed Building Consent where appropriate, and Party wall notices served in good time in accordance with the Party Wall etc., Act 1996.

If you are planning a structural repairs project or have any issues or concerns regarding your neighbour’s impending building works or have received Party Wall Notice you are welcome to contact us to discuss further on 020 8381 3910 or message us at Arun Associates by clicking here.

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This text is provided to amplify the video ‘Underpinning and Structural Repairs and the Party Wall Act’ and was produced on behalf of Arun Associates and William J Marshall & Partners LLP. Whilst every care has been taken in the information provided, no liability can be accepted by parties relying on the content in respect of loss, damage, or consequential loss. © 2019 Arun Associates.