Excavations and Party Wall Act

This video looks at the issues when neighbours wish to carry out alterations and excavations are close to neighbouring properties. The relevance of the Party Wall etc Act 1996 is explained, particularly those aspects which affect Section 6 of the Act.

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.

A neighbour or adjacent owner may wish to carry out improvements or repairs to their property, and in doing so, requires their builder to excavate close to a neighbouring property. 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.

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 and excess noise as a result of the works, and their concern might be the loss of support to their foundation, when excavations are made.

The reason for the excavation might be to repair a defective drain or cable, the creation of an extension to the original property, the forming of a basement, or some other reason. This is where a Party Wall surveyor, or party wall surveyors for both parties may have to become involved. There would have to be an inspection of the proposals in looking at detailed plans and drawings, checking measurements, and perhaps even recording the state of the adjoining property wall and finishes, including measuring position and size of existing cracks and defects.

What risks are potential then, with excavations near an adjoining building? This depends upon the existing structure, the depth of existing foundations, type of subsoil and proximity of the proposed excavation by the neighbour, or building owner. If the excavation can be a safe distance away from existing foundations, there is much less risk in undermining those foundations, and hence no damage should occur. Modern buildings will have deeper foundations than say, older terraced Victorian properties, so the age and construction is relevant.

Foundations can be set back from neighbouring foundations with, what is termed a ‘cantilever foundation’. This design will not impede so closely as a traditional foundation which is not cantilevered, and therefore the risk is much reduced. Excavations can be hand-dug, machine-dug with an excavator, or piled foundations. Hand dug excavations are usually down to a metre or so, and little risk of vibration. Machine-dug excavations are of greater risk due to use of machines and deeper excavation of soil. Even narrow strip foundations dug by machine, called ‘trench-fill’, can sometimes trigger the need to serve notice on a neighbour under the Party Wall Act, dependent upon distance and depth of the foundations. 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.

Piled foundations can be a particular problem in causing vibration or movement to an adjoining property, so careful election 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 revered to as ‘movement’.  This is due partly, to a loss of support to the foundations of an adjoining property. Hence, the movement.

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 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.

Deep excavations may involve concrete piling or steel-sheet piles to support the perimeter of the excavation. 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, forensic engineer explains a situation with an old church and special supports, which compensated for movement during the excavation works.

Drainage may accommodate movement better than old walls. A lot depends on the age of the building, the depth of old foundations and the quality of the masonry and mortar. Older lime mortar may be more accommodating of movement than modern, brittle, and harder, cement-mortar.

As regards piling, there are two main types. (a) Driven piles, with the analogy of hammering a nail into wood, and the pile is driven into the soil; (b) Augered piles, where the subsoil is removed carefully by an auger screw, (which looks very much like the drill bit of a carpenter’s brace and bit), and then the reinforcement and concrete is poured. The latter causes far less vibration than the driven pile, and is the preferred option for built-up areas, where piles must be used.

Steel sheet piles have to be driven in with a powerful mechanical hammer, and therefore vibration can be an issue in some cases, depending upon proximity to neighbouring foundations.

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. Driven piles should not be used in built-up areas due to the risk of damage, but are useful to open green-field sites, such as in rural areas.

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. Party wall surveyors can advise on the desirability of monitoring of neighbouring property.

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 notifiable adjacent excavation project or have any issues or concerns regarding your neighbour’s impending project or have received Adjacent Excavation 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 ‘Adjacent Excavations 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, consequential loss.