Engineering 101
In building, as in life, anything worth doing starts with great foundations.
Arguably the most important aspect of any construction project, foundations transfer the huge forces exerted by the structure safely into the ground below.
It’s critically important to get them right – both for safety and economics.
Foundations
For residential homes set on ‘good ground’, New Zealand Standard NZS 3604:2011 provides generic foundation details which can be constructed by a Licensed Building Practitioner (“The Builder”) without needing input from an engineer. For smaller, simple projects this helps streamline the process and keep costs low, however councils often require a geotechnical engineer to determine if the soil is ‘good ground’ as there are specific requirements the ground must meet to be classified as such.
For projects falling outside the narrow scope covered by NZS 3604:2011, site specific engineering design is required. This applies to multi-unit apartments as well as commercial and industrial buildings. Residential foundations on soils failing to meet ‘good ground’ criteria also need to be designed by a Chartered Professional Engineer (CPEng).
Foundation design is, more often than not, a team effort. A structural engineer will typically work in collaboration with a geotechnical engineer, who advises on critical design parameters such as soil strength, slope stability or bearing depth.
Due to the unique requirements specific to a project there are endless variations of foundation techniques, though in general they all arise from one of four classifications.
Piles
Figure 1: Concrete piles
Piled foundations consist of a series of posts or poles embedded into the ground, constructed from treated timber, concrete or steel.
Piles are installed by excavation, driving or drilling depending on requirements, and are categorised as either ‘deep’ or ‘shallow’.
Deep piles often involve highly specialised expertise and equipment. Shallow piles can be less complex and several standardised designs are provided within NZS 3604:2011. Piled foundations are good solutions for sloping sites or where there is weak ground near the surface.
Figure 1: Concrete piles
Figure 2: Slab
Slab
Figure 2: Slab
Slab foundations are a flat plate of reinforced concrete poured over a compacted base. Typically, there are thickenings around the edges of the building and under internal loadbearing walls.
The Christchurch earthquakes exposed several deficiencies in how slab foundations were traditionally built. Today, slabs are required to be well reinforced with ductile, seismic grade steel mesh and adequately tied into the thickenings with steel bars.
Slab foundations are a great option for sites with preexisting ‘good ground’ but can also be enhanced to cope with the requirements of less ideal ground conditions.
Improving ground conditions prior to constructing foundations are almost universally technically complex and can carry considerable cost implications.
Rafts
Figure 3 (Left): Waffle/Slab Raft showing the polystyrene blocks and the ‘ribs’ between that will be fitted with concrete. Figure 4 (Right): Gravel Raft showing geotextile
A reinforced gravel raft is a foundation composed of layers of gravel and special fabric called geotextile and/or geogrid. Raft-based foundations are an obvious choice for sites vulnerable to liquefaction and lateral spreading, such as areas zoned as ‘TC3’ in Christchurch, or for sites on deep deposits of soft ground such as peat.
A number of foundation types can be constructed on top of a gravel raft, accommodating both concrete and timber floor options. A modified concrete slab set on top of the gravel raft is usually some sort of waffle or ribbed slab.
These systems are specifically designed to accommodate some degree of loss of support while optimising the overall weight of the foundation. There are several proprietary products available to the New Zealand market which often makes these solutions very cost effective. Incorporating polystyrene blocks will considerably improves the thermal insulation properties as well.
Figure 3 (Left): Waffle/Slab Raft showing the polystyrene blocks and the ‘ribs’ between that will be fitted with concrete. Figure 4 (Right): Gravel Raft showing geotextile
Figure 5: Strip footings
Footings
Figure 5: Strip footings
Placed directly under loadbearing elements, footings are easily adapted to different applications and may be isolated (‘pad footings’), continuous (‘strip footings’), or integrated with other types of foundations (‘combined footings’). As they are targeted to support specific structural elements, footings can be a cost-effective solution in many circumstances.
Each foundation option comes with advantages, limitations and associated cost implications. Engineers must carefully consider these factors to select, adapt and design the right foundation type for the particular project requirements and specific ground conditions. Foundation design is, more often than not, a team effort. A structural engineer will typically work in collaboration with a geotechnical engineer, who advises on critical design parameters such as soil strength, slope stability or bearing depth.
Geotechnical and structural engineers at EDC pride themselves on providing their clients with smart, cost-effective engineering solutions.
