With the increase in housing developments on Brownfield sites gaining pace over the past 20 years, alongside the rising cost of excavated material disposal, the viability of piled foundations is becoming a key consideration for many housebuilders, contractors and architects, particularly when working on low- rise housing. Ted Chandler, project champion at the NHBC Foundation explains
IT is widely understood that piled foundations typically use less material to construct than conventional trench fill, which can have positive implications for cost, speed and sustainability. However the exact criteria for the use of piled foundations remain somewhat uncertain. This lack of information was the key issue that NHBC Foundation sought to address in the recently published Efficient Design of Piled Foundations for Low-Rise Housing:Design Guide. The report clarifies the design approach and selection of a piled foundation in relation to the construction of low-rise housing.
The Potential Benefits of Piled Foundations
A key finding highlighted in the report and contrary to some previous perceptions, is that separate design criteria is not necessary when piled foundations are considered for low rise housing. Instead, the governing factor that influences foundation design is the potential damage which could occur to the built structure. Naturally, fissures and cracks are unacceptable in construction and the focus of any foundation choice should be to alleviate the potential for differential movement, which leads to these issues. Designed in an efficient way, piled foundations create an accurate and sustainable substructure that prevents differential movement.
In addition to the avoidance of differential movement, the selection of piled foundations can bring other benefits to the housebuilder. As previously mentioned, typically less material is required for construction; while extensive excavation of ground material can be avoided, meaning a piled foundation solution can be constructed cost effectively and at speed. The report identifies these benefits as ‘direct’ savings, and there are also ‘indirect’ savings that must be taken into account.
One of the most significant indirect savings is the avoidance of deep excavation. Not only does this alleviate some potential health and safety fears, it also addresses one of the key concerns of the Environment Agency – the potential for contaminated groundwater to permeate underground water sources as a result of disruption caused by foundation building.
When building on brownfield sites, where a degree of contamination is a very real risk, this should represent an essential consideration for any construction professional. The correct pile foundation type could provide a real option to minimise any environmental risk.
In addition, when considering the vital question of sustainability, research identified in the report shows that the construction of piled foundations results in far less embodied carbon than alternative options. Certainly in terms of waste, less excavation results in less costly and carbon-generating disposal, complementing the intention of the 1996 Landfill Tax, which tends to penalise traditional trench fill foundations thus encouraging more environmentally friendly disposal methods.
Looking beyond the construction stage, piled foundations can go some way to ensuring new build houses meet the requirements of the Code for SustainableHomes. Relying on independent assessors, the Code for Sustainable Homes has been adopted by local authorities in the drive to provide a greener future for the UK. ‘Geothermal piles’ are one way that piled foundations can be utilised to make homes greener. A ground closed loop heat exchanger can be incorporated in to the design, utilising existing subterranean thermal differences to heat the buildings above. These represent a cost effective, and carbon neutral way of supplementing traditional heating systems and contribute to attaining the standards set by higher levels of the Code for Sustainable Homes.
Achieving Optimum Construction
To identify the optimum approach to construction, the report advised that it is important to carry out a site investigation.
Piled foundations are best suited to a weak soil and this should be identified by a geological survey. For brownfield sites in particular, site investigation is essential due to the potential for diverse historical use which, in turn, can lead to a myriad of challenges to construction. Such an investigation allows a well informed decision to be made on the suitability of piling as an appropriate and cost effective solution, which should go some way to guaranteeing that there are no unwelcome surprises at the construction stage.
In terms of in-situ performance, the report describes key characteristics of the foundations themselves, proposing that a reasonable approach for the design of piled foundations supporting low-rise housing would be to limit total pile settlements to the order of 10 mm under working loads, although it is conceded that there may be occasional exceptions to this rule.
In addition to Efficient design of piled foundations for low-rise housing: Design guide, and for further detailed and specific guidance on piling, developers would be advised to consult CIRIA Report PG1, Tomlinson and Woodward’s Pile Design and Construction Practice or Fleming et al’s Piling Engineering. These supplementary publications are identified in the NHBC Foundation report as useful aids for developers seeking to achieve the most well informed, effectively designed foundations.
The approach to any design should ultimately be governed by whether the pile settlements at working loads are within acceptable limits for the supported structure. Foundations must fundamentally guarantee an adequate factor against failure, not just the overall ability to carry the load, and this represents the true limiting factor in design. If this rule is followed correctly, a structurally sound and sustainable development can be achieved in which the occupants can enjoy a safe and long-lasting environment.