Sound footings for new and relocated buildings
It is important to ensure there is an adequate level of construction monitoring to cover footing construction for piled foundations associated with both new and relocated buildings.
While the 1999 and 2005 BRANZ house condition studies identified inadequate subfloor bracing as the main subfloor non-moisture-related defect, there were also a significant number of instances of insufficient footing depth, structural cracks and subsidence recorded in the subfloor area.
Poor construction practice can result in undersize footings and site manufactured concrete not having the required strength.
Design issues
Foundations are either subject to a specific engineering design or to a non-specific design such as NZS 3604.
For typical domestic pile footings, the minimum design details cover the strength of the concrete and its dimensions. Dimensions include the base area and the depth of the footing. The details will depend on what type of pile it is. Footings which conform to NZS 3604 can be for an ordinary pile, an anchor pile or a braced pile.
The commonly used building standard for piled foundations of residential buildings is NZS 3604. This sets out details and rules relating to the size of piles, dimensions of concrete footings, thickness of the concrete surround, required ground-bearing capacity, and connection between piles and the subfloor system and bracing.
Construction issues
It is essential that foundations are constructed in accordance with the requirements of specific or non-specific engineering designs (for example, NZS 3604). In particular, the size and shape of prepared holes for pile footings must conform to the design details and site manufactured concrete must be subject to adequate quality control and meet design requirements.
Footings that do not conform to the minimum design requirements can break apart, settle (which may result in sloping floors), fail to provide adequate bracing or, in extreme cases, cause soil foundation failure.
The quality of concrete mixed on the site can be verified by on-site slump testing and off-site compression testing. Footings designed and constructed to NZS 3604 require a minimum strength of 17.5 MPa, and the concrete must not have a slump exceeding 60 mm. It is important that the concrete is thoroughly mixed to ensure uniform, dense concrete with the same physical properties throughout.
The requirements for the production of 17.5 MPa concrete are given in NZS 3104: 2003 Specification for Concrete Production. If concrete manufactured on site cannot be supported by adequate quality control, it should be supplied from a graded plant to ensure the design properties are met.
Compliance issues
A building consent is required to construct a foundation for a new or relocated building. The design details must be submitted to the building control authority as part of the consent application.
Specific construction issues, such as the quality of site manufactured concrete and undersized footings, can be controlled by the project specific inspection/monitoring regime that will involve building control authority inspections or professional engineering inspection/monitoring, or both, and be a condition of the building consent.
It is important that the design engineer be involved in monitoring foundation conditions and their construction where foundations are subject to a specific design. Professional engineering inspection/monitoring could be undertaken as part of an independent construction review, reported to the building control authority by using the IPENZ Producer Statement form PS4.
Building control officers should encourage owners to employ a suitably qualified design professional to ensure foundations meet the design specifications for foundations that are designed in accordance with NZS 3604.
The Licensed Building Practitioner (LBP) scheme is being phased-in from November this year. A person who is licensed will have been assessed as competent to supervise or undertake the work covered by the scope of their licence. It is important to ask contractors if their staff are licensed before selecting a contractor to do foundation work, including house relocation or repiling. The licence classes relevant to residential foundation work are the Site licences and the Carpentry licence. Site licences are for people who oversee, coordinate and/or manage building projects.
Becoming licensed will be a personal choice for people in the industry. From November 2010, supervising or undertaking work on core elements of a building ('restricted building work') will be limited to licensed building practitioners. The policy to define restricted building work has not yet been developed. Current proposals for restricted building work do not include re-piling/house relocation work, although detailed policy work is still to be done. The Department, however, will encourage house or building relocation contractors to employ licensed builders for this type of work.