As the construction industry works towards a ‘Net Carbon Zero’ future, we are asked to take a hard look at current practices to find areas where the carbon cost associated with our built environment can be reduced. The cement industry is widely recognised as being one of the main producers of carbon dioxide emissions, accounting for around 7% of global emissions. Low carbon and carbon neutral concrete products are already entering the market, but the use of timber as an alternative to concrete piles provides an opportunity to use a sustainable foundation material. This brief article considers the benefits, limitations and suitability of using timber piles in the UK.
In 2009 a technical paper by Reynolds and Bates was published in Ground Engineering magazine, titled “The Potential for Timber Piling in the UK”. It explored the current global locations and circumstances in which timber piles are used and discussed the potential for UK locations with comparable ground conditions where their use could be considered. The article states that timber piles are currently widely used in the Netherlands, the United States, New Zealand, Canada and Australia, and concludes that timber piles are “an entirely viable, low cost and sustainable alternative to steel and concrete, with the potential to be used for many projects”. A notable example of timber pile use given in the article is the Cargo Terminal at John F. Kennedy Airport in the USA. BRE Digest 479 (Timber Piles and Foundations) cites a number of historic examples where timber piles have been used, including Old London Bridge in the UK, the Royal Palace in Amsterdam and the Pont Notredame bridge in Paris.
Timber Pile Characteristics
A timber pile is essentially the trunk of a tree, stripped of branches and bark. They are currently used in the UK in maritime construction, such as groynes, piers, jetties and for sea defences, where timber is favoured due to its ease of handling in overwater environments, its ability to absorb impacts and low cost.
Since timber is a renewable material it has a significantly lower carbon cost associated with its production and use than its concrete and steel counterparts. The monetary costs to produce and transport timber piles are lower than those of concrete and steel (as described in Reynolds and Bates, 2009). The placement of timber in the ground is a form of carbon sequestration, which plays an important part in ensuring the overall Net Zero compliance of a project.
One of the main drawbacks to using timber is that it is biodegradable. Decay occurs fastest when the timber is exposed to the air, which allows fungal decay and rot to occur. As such, a typical life span for timber placed in an unsaturated environment is c. 25 years.
Timber placed below groundwater level, in a saturated, anoxic environment, will decay at a much slower rate and can have a design life of up to 100 years depending on the timber species used. The piles can be chemically treated to provide additional resistance to biodegradation, however, the chemicals used may have the potential to introduce contamination into the ground.
To minimise degradation, the top of the pile that extends above the water table (i.e. not in saturated ground) is typically formed of concrete. This can be a pre-cast concrete section connected to the timber or can comprise a concrete pile cap spanning several timber piles. Pre-boring the pile locations and setting the timber pile in concrete or grout can also be used to slow degradation, however this will increase the amount of concrete used and the arisings that require disposal.
Although timber, as a pile, will have a lower compressive strength compared to concrete or steel piles, it is often the case that the bearing capacity of the pile is governed by the properties of the surrounding soil rather than the strength of the pile itself. On this basis the use of timber piles may be equally suitable to that of concrete in certain settings.
Pile length is another design consideration that may impact the suitability of using timber piles. As timber piles are typically difficult to splice, the maximum pile length is restricted by the height to which the tree can grow, which is also dependant on the species. Conversely, timber piles can easily be cut down to size as needed and the off cuts are biodegradable.
Pile Construction and Suitable Locations
Installation of timber piles is predominantly carried out using driven techniques. The success of this technique will depend on the ground conditions, since the presence of dense layers, hard bands or variable strata could result in the refusal of the piles at unplanned depths. Driving through harder bands may be achieved with the inclusion of a driving shoe at the toe of the pile, therefore a good understanding of the ground conditions present is key to ensuring the successful installation of timber piles.
Ground conditions most amenable to the installation of driven timber piles comprise soft deposits such as soft clay or peat, over a denser material such as gravel or rock, with a high water table. Reynolds and Bates identify such ground conditions to exist in numerous estuarine and river valley locations around the UK, including the Severn estuary, the Fens in east Anglia and the marshes of Dartmouth.
The seasonal variation of groundwater levels at a site needs to be well understood if the development is to consider using timber piles, as the degradation, and consequently the design life, of a timber pile is primarily governed by whether the ground is saturated or not. This places a greater importance on groundwater monitoring and modelling to understand the existing groundwater regime as well as the impact that climate change will have on groundwater levels through the design life of a project. Understanding the groundwater regime of a site is already a key part of geotechnical design, and a requirement of Eurocodes. As such, any additional data gathering at ground investigation stage of a project may not extend significantly beyond the typical geotechnical needs of a project.
Ground conditions exist in the UK that may be amenable to the use of timber piles. As with any geotechnical design, the collection of relevant information at ground investigation stage is crucial, but wouldn’t necessarily require any particular measures beyond what would be expected in order to satisfy current standards. The use of timber for on-shore piling is not an impossible or unreasonable solution to reducing the carbon cost of construction and may, in due course, be driven by necessity to address the climate crisis.
Article provided by Katharine Barker, Associate, Campbell Reith