With the rise in Brownfield housing developments over the last two decades, as well as the rising cost of excavated material disposal, the viability of piled foundations is becoming a key consideration for many homebuilders, contractors, and architects, particularly when working on low-rise housing. Ted Chandler, project champion for the NHBC Foundation, explains
Piled foundations are widely accepted to require less material to construct than conventional trench fill, which can have a positive impact on cost, speed, and sustainability. The exact criteria for using piled foundations, however, remain somewhat ambiguous. The NHBC Foundation addressed this lack of information as the primary issue in its recently published Efficient Design of Piled Foundations for Low-Rise Housing:Design Guide. In the context of low-rise housing construction, the report clarifies the design approach and selection of a piled foundation.
The Benefits of Piled Foundations
A key finding highlighted in the report, and contrary to popular belief, is that when considering piled foundations for low-rise housing, separate design criteria are not required. Instead, the governing factor that influences foundation design is the potential damage to the built structure. Naturally, fissures and cracks in construction are unacceptable, and the goal of any foundation selection should be to reduce the possibility of differential movement, which leads to these issues. When designed correctly, piled foundations create an accurate and long-lasting substructure that prevents differential movement.
Aside from avoiding differential movement, choosing piled foundations can provide the homebuilder with additional benefits. As previously stated, less material is typically required for construction, and extensive ground material excavation can be avoided, implying that a piled foundation solution can be built cost effectively and quickly. These benefits are classified as ‘direct’ savings in the report, but there are also ‘indirect’ savings to consider.
One of the most significant indirect savings is the avoidance of deep excavation. This not only addresses some potential health and safety concerns, but it also addresses one of the Environment Agency’s primary concerns: the possibility of contaminated groundwater permeating underground water sources as a result of foundation building disruption.
This should be a top priority for any construction professional when building on brownfield sites where contamination is a real possibility. The right pile foundation type could be a viable option for mitigating any environmental risk.
Furthermore, when it comes to the critical issue of sustainability, research cited in the report shows that piled foundation construction produces far less embodied carbon than alternative options. In terms of waste, less excavation means less expensive and carbon-generating disposal, which complements the 1996 Landfill Tax’s goal of penalising traditional trench fill foundations and encouraging more environmentally friendly disposal methods.
Looking beyond the construction stage, piled foundations can assist new construction houses in meeting the requirements of the Code for SustainableHomes. Local governments have adopted the Code for Sustainable Homes, which is based on independent assessors, in their efforts to provide a greener future for the UK. ‘Geothermal piles’ are one method of using piled foundations to make homes more environmentally friendly. By utilising existing subterranean thermal differences, a ground closed loop heat exchanger can be incorporated into the design to heat the buildings above. These are a low-cost, carbon-neutral way to supplement traditional heating systems, and they help to meet the higher levels of the Code for Sustainable Homes’ standards.
Getting the Best Building
The report suggested conducting a site investigation to determine the best construction strategy.
Piled foundations work best in weak soil, as determined by a geological survey. Site investigation is especially important for brownfield sites due to the potential for a wide range of historical uses, which can present a slew of construction challenges. An investigation of this nature allows for a well-informed decision on the suitability of piling as an appropriate and cost-effective solution, which should help to avoid unpleasant surprises during the construction stage.
The report describes key foundation characteristics in terms of in-situ performance, 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, though it is acknowledged that there may be exceptions to this rule.
For more detailed and specific piling guidance, developers should consult CIRIA Report PG1, Tomlinson and Woodward’s Pile Design and Construction Practice, or Fleming et al’s Piling Engineering, in addition to Efficient design of piled foundations for low-rise housing: Design guide. According to the NHBC Foundation report, these supplementary publications can help developers create the most informed, effectively designed foundations.
Any design approach should ultimately be guided by whether the pile settlements at working loads are within the supported structure’s acceptable limits. The true limiting factor in design is that foundations must fundamentally ensure an adequate factor against failure, not just overall load capacity. If this rule is followed correctly, it is possible to achieve a structurally sound and sustainable development with occupants who can enjoy a safe and long-lasting environment.