These days, the building industry uses a lot of concrete. It’s a material that was great for the modern age: it lasts a long time, is sturdy, and is easy to calculate with. It is proven and comes with certificates. In other words: great for structures! (If you can live with the mining efforts elsewhere in the world, that is). Engineers typically design foundations in such a way that they may last forever. While that is reassuring for the client, this does come with a catch: we ram huge amounts of concrete piles in the ground for our future generations to deal with. We think that’s not quite right. Many architects argue that good architecture lasts forever. On paper, that is indeed a sustainable ambition. Hardly ever, however, does life go as planned. Societies change, and so do our spatial needs. For future generations, the rigidity and durability of concrete can quickly become a burden. We think we can do better. Wouldn’t it be a sensible requirement to make the owner of a building responsible for the potential removal of all building materials that do not naturally degrade? If so, concrete buildings would significantly degrade in value, making biobased alternatives more effective for some use cases.
Surprisingly, wooden foundations were the norm for thousands of years. In the Netherlands, archeologists have found traces of wooden piles used for farmhouses dating back to around 5000 BCE. Until the Middle Ages, variations of this technique were used. That’s 6000 years of timber foundations versus 1000 years of brick and concrete. Archeologists find holes where wooden piles used to be placed or rammed in sandy soil. Based on the shape of these holes they have also determined that wooden columns were occasionally removed, indicating that the timber structure may have outlived the required lifecycle of a farmhouse. The discolorations in the sand are the only traces left, while future archeologists would find a maze of concrete when investigating the modern era. Using new innovations and techniques, we’d love to go back to a building culture where we build according to the soil quality, and embrace a shorter lifecycle. If most buildings only last fifty years in reality, wouldn’t it be enough if foundation piles would last just that long? If we can include the negative effects of concrete in its price, and if financial models allow for more temporary buildings, then how we improve on the design of wooden foundations?
There are three main aspects to look at: (1) there are the trees to source the timber from; (2) there are the techniques to extend the longevity of timber around the ground water table; (3) and finally there are the connectors that allow wooden foundation piles to be effectively used in construction. Under the ground water table, the use of European spruce (Picea abies) would generally suffice. These trees grow tall and straight, but won’t last long unless fully submerged under water. Other trees, like European larch (Larix decidua) may be a little more durable but are difficult to treat for longevity as their cell structure closes in the process. There are hard woods that can grow locally, such as Robinia (Robinia pseudoacacia). Their stems are shorter and jagged, but could be used for the two to three meters above the ground water table. While not ideal, European ash (Fraxinus excelsior) has been used for pile foundations and is ideal to be treated. Due to illness, however, not many ashes are left in the Netherlands. This calls for planting more trees to be used in construction, and doing so in a way that the right trees are planted for a specific function. How we design our buildings is therefore intertwined with the ecosystem that we shape around us.
An older technique of increasing the durability of timber is the use of creosote oil, a coal product. This is called creosoting. For uses like electricity poles it is still used as wood preservative. Due to its potential negative effects on the immediate environment its use is restricted. Other techniques make use of heat and/or steam to make wood more durable: thermal and hydrothermal treatments. This does affect the wood. It becomes a more brittle, and may therefore be too sensitive to be hammered into the soil during the piling process. There are two potential treatment methods that can turn softer wood into hard wood, which may be used for foundation piles. The acetylation process can modify wood to such a degree that it can be used partly submerged as well. It is often used for decking and facades, but structural bridges have been made with modified wood as well. Another modification technique is based on biopolymers, which also makes wood stable and durable. Typically, Radiata Pine (Pinus radiata) is used for acetylation and polymerization, but woods like beech and ash can grow locally and should allow modification too.
It is not effective, however, to modify the entire tree, as the modification cannot easily penetrate into the heart of the wood. For the longevity of the foundation pile itself this may not necessarily be critical. It may, however, cause cracking and irregularities, which isn’t ideal considering the stress on the pile when being hammered in the ground. There are roughly two solutions to this problem: (a) to saw shallow grooves radially along the length of the pealed pile before it undergoes treatment, or (b) to join smaller beams together into a square column after treatment. The first option is a more experimental, the second requires a technique of glueing or nailing with at least the same durability. In both cases, there are no certifications in place that can guarantee its durability for longer than 25 years. The wood may last longer, but there are no tests in place to offer better indications. It is critical to get a better understanding of the longevity of modified wooden foundation piles, as this is would allow clients to decide whether or not the solution is feasible for their use.
The next step is to design the connectors between the submerged raw wooden foundation piles and the tops. There are three different connectors that could be used, depending on the underground. In delta areas with clay soil, wooden foundation piles reach all the way to the highest load bearing sand layer. The piles may need to be fifteen meters long in total. If modified wood is glued together into a square column, this top needs to be connected with the round pile with a steel connector. First, the foundation pile is rammed in the soil. The top is then placed in the connector, after which the composite pile is driven to the required depth. Alternatively, a mortise and tenon joint could be CNC milled to create a fully biobased connector. A second option is to add a steel screw top on a shorter wooden pile that is modified. Depending on the required load, the wooden pile could be screwed into the soil. This is currently done with steel screws, but their load bearing capacity is limited due to the smaller radius, and the costs can be high when longer screws are required. And finally, in weaker soil types, a propellor-shaped steel top could be added in a similar manner to maximize the surface area underground. These techniques were used hundred years ago for temporary structures. We see a lot of possibilities to continue on those experiments.
If we were to plant the right trees to make foundation piles and modified wooden tops, with connectors that make them useful in construction, then what affect will this have on architectural design? It is important to stress that wooden foundation piles cannot take as much load as concrete and steel piles. Theoretically, you can drive in as many wooden piles to compensate for the lack of load bearing capacity. This has been done in the past, but isn’t very effective. Rather, it makes much more sense to build as light as possible, to minimize the foundation works. This is also the key issue at the moment: if we were to use wooden foundation piles with concrete tops for multi-storied buildings, then we’d have to use more concrete than if were to use fully concrete foundation piles. That would only make sense if the concrete tops were made removable. One advantage of using modified wooden tops, is that wooden structures can be directly connected to them. Designing the ground floor slightly above the soil would allow for a fully timber structure, without the use of any concrete: a building that is literally rooted in the ground and leaves very few traces for future generations.
