According to Arie-Jan van Renswoude, geotechnical engineer at ABT, foundations account for 30% to 60% of a building's environmental impact. Half of a building's impact is thus buried under ground. While the building industry is gradually transitioning to the use of more biobased materials, thus decreasing its environmental impact, concrete is still the go-to material for making foundations. This is a problem for three key reasons:

[1] As the Dutch government is planning to build a 100.000 new homes a year, the use of concrete for foundations will have a significant impact on CO2 emissions, and climate change as a consequence. The concrete industry is accountable for roughly 8% of global CO2 emissions.

[2] Concrete requires valuable resources such as water, sand, gravel, limestone, and steel. These resources all come from somewhere, and their processing needs a lot of energy. The pile foundations we drive in the ground are not only invisible for the naked eye, they also leave traces in the ground somewhere else.

[3] Foundation piles penetrate different soil layers. Were we to take these piles out of the ground, the leftover hole connects ground water from different layers. To prevent this hydraulic short circuit, foundation piles are generally left in the ground forever. Its concrete can thus not be recycled, which makes it a waste of valuable CO2.

Architects tend to think that a well designed building will last a long time. In reality, however, it is impossible to predict what requirements future generations will have. Concrete was once considered an affordable and effective material to build with. Its rigidity, however, makes post-war buildings difficult to renovate and insulate. It seems easier to demolish a building and start with a clean slate, but this is wasteful and unnecessary. Assuming the foundations are still in a good state, there are two simple solutions we should consider before driving even more concrete in the ground:

[4] The most sensible path is to not demolish a building. We can take better care of the buildings we use. Just like trees, buildings are connected to the ground. The modernist dream of a tabula rasa ignores what's going on beneath that table. It's not an easy task to renovate buildings, but since when do architects shy away from a good challenge?

[5] If a building really does need to be demolished, for whatever reason, we can at least try to keep its concrete foundations. A few new piles may have to be added where existing piles are damaged, but 95% of piles can probably be reused. A new building will have to be designed "bottom-up", that is: starting from the load bearing capacity of the existing foundation.

An existing foundation is not always present. The very least we can do is to look for suitable soil to build on. In the Netherlands we don't typically build on bedrock but on sandy soil, whether that layer reaches the surface or hides deep under ground. In places like Rotterdam, for example, this layer may be as deep as twenty meters. A lot of people live in the Dutch delta, that's where the work is at, but we may have to reconsider whether building there is actually viable in the long run.

[6] An architecture that is "soil-driven" requires planners to look for suitable soil first. Before we question how to build, we should first question where to build. This works on all scales: even smaller plots may offer various types of soil to build on. Map the soil more extensively before designing a building, and see how a suitable soil location can dictate a design.

[7] Shallow foundations can be removed and thus be recycled. But do consider alternatives to concrete for shallow foundations as well. Can a building be placed on natural stone footings instead? A ground floor does not have to be made of concrete. Sandy soil is a great base for a rammed earth floor, which requires far less energy and is easily reusable.

It's a modern character trait to dwell where living is tough. We have made cities on peat and clay, and may have to build on weaker soil again. Only recently, however, have we fully transitioned to using concrete piles for foundations. Wooden foundation piles were typically used until World War II. Not only can we still build on wooden piles today, it is also a suitable way to store CO2. Wood is of course susceptible to decay. We need to take great care in making wooden foundations. Under the right conditions and permanently submerged under ground water, wooden piles can last for hundreds of years.

[8] A concrete extender or oplanger can be placed on top of the submerged foundation pile. The concrete extender bridges the danger zone where groundwater fluctuates.

[9] Reduce the weight of the building where possible. By raising the ground floor, for example, it is possible to build a light-weight wooden floor. Cross laminated timber (CLT) is a tempting alternative to concrete, but massive wooden structures are still heavy. Timber framing is a much lighter alternative.

[10] Reduce the snow load of a building by using a gabled roof. Snow will slide from a roof that has a 60º angle, while it will pile up on a flat roof. Because snow is heavy, an engineer will have to take into account the possible load in winter.

[11] Reduce the wind load on a building by using a hipped roof, or avoid placing facades perpendicular to dominant wind directions. Heavy winds may cause additional loads on the piles on the other side of the building.

[12] If we want to avoid using concrete, it is critical to minimize also the amount of wooden foundation piles as each pile requires a concrete extender. Wooden foundations may even require more concrete in extenders than in fully concrete piles otherwise. This may seem counter-productive, but concrete extenders can be removed and recycled while concrete foundation piles cannot.

[13] Make sure that the wooden foundation piles are continuously submerged. Take into account climate change and the resulting periods of drought that may expose the top of the foundation pile in summer. Keep a solid margin of error in determining the depth of the piles.

[14] Where possible, raise the ground water level. The deeper the ground water level, the longer the required concrete extender or oplanger. Additionally, it is easier to repair or replace foundation piles when ground water is close to the surface.

Historically, wooden planks connected the wooden foundation piles in strips. Directly on top of these wooden planks, masons would build brick footings for the building. Brick buildings were heavy, and consequently many wooden foundation piles had to be driven into the ground. For Het Witte Huis, an early skyscraper built in 1897 in Rotterdam, a thousand piles were used. While this is possible, the excessive amount of foundation piles caused the soil to move about. A neighboring building collapsed as a consequence.

These days, it is more common to use concrete extenders or oplangers on top of the wooden foundation piles instead of brick. The wooden foundation pile is first driven partly into the ground. The concrete extender is placed on top of it. A steel pin makes it possible to connect the foundation pile to the extender. The combined pile is then driven to the required depth, so that the wooden pile reaches the load-bearing soil while being permanently waterlogged. A wooden structure can be placed on top of the foundation using concrete footings. This is a proven strategy and can be used today. Connecting a wooden structure on a concrete extender is, however, a bit of a tedious process. Wouldn't it be possible to use a fully wooden foundation pile, and connect the wooden structure directly to its foundation?

Keep in mind that weak soil conditions aren't ideal for heavy machinery. While foundation piles are specifically necessary in peat and clay, these soil types make it more difficult for heavy machinery to enter the building site. Machines for driving concrete piles in the ground are typically much heavier than machines for driving wooden piles. This is an added benefit. Typically, a layer of sand is added to make concrete foundations in such soil types possible. In other words: we make a foundation to make a foundation.

[15] Wooden piles require lighter machinery than concrete piles. Let the soil quality determine which machines are sensible to use. In extreme situations, a manual or electric post hammer may even be sufficient to make shallow foundations.

[16] As wooden piles are lighter, it is possible to transport more piles per freight, reducing emissions. Where a truck could carry 150 wooden piles, it may only be able to carry 40 concrete ones.

Touching the earth lightly has a positive cascading effect. It is possible to use completely wooden piles for a foundation, but it has to be clear to everyone involved that wood will rot where groundwater fluctuates. The speed of decay depends on the type of wood used, the particular soil conditions, and their exposure to weather. Occasionally, permanent foundations simply aren't allowed according to local regulations. Eucalyptus Cloeziana is occasionally used for foundations of beach houses, for example. It can withstand at least 25 years in direct contact with the ground, but not much longer. Is this simply a ticking time bomb, or is there a way to embrace the temporary nature of foundations?

[17] As part of a project brief, determine how long a building needs to last at minimum. Many buildings only last around 50 years in practice, but still we build them for eternity. Perhaps we should accept that buildings are often temporary in nature. If a building only needs to last around 25 years, using hardwood foundation piles is sufficient.

[18] If buildings need to last a little longer than 25 years, design the foundation in such a way that it is possible to inspect and repair the foundation. Design for an elevated ground floor, and make sure the foundation is accessible in a non-destructive manner. Add sensors to monitor the decay from a distance.

[19] When a building rests on top of a foundation, it becomes very expensive to add more piles later. We could use a sacrificial pile on top of a traditional wooden foundation pile to overcome this problem. Typically, a concrete extender is around 250 cm long. Were we to replace this extender with a wooden sacrificial pile, then the typical ceiling height allows for replacement from within the building. Repairing the foundation can extend the life of the foundation with another 25 years, as long as the building is designed for it. We accept such maintenance for a thatched roof, so why not for the foundation as well?

[20] Prevent excessive amounts of rainwater from reaching wooden foundation piles. The more stable the soil condition, the longer the pile should last. Direct downspouts away from foundations and do use gutters.

Which types of trees are typically used for foundation piles? Not all species grow as tall, and not all species are as resistant to decay. Historically, an abundance of short alder trees were used to compact weaker soil types. They were around three meters long and did not necessarily reach sandy layers. All they did was distribute the loads more evenly. Venice has similar foundations, and according to the likes of Vitruvius and Scamozzi these alder trees were ideal for such foundations. Buildings could still settle unevenly, and thus builders used longer piles. In cities like Haarlem, local pine trees could be used to reach sandy layers around eight meters underground. More recently did we learn, however, that pine is not an ideal wood for making foundations as it is still susceptible to bacteria under ground water level. In regions where longer foundations piles were needed, like in Rotterdam, spruce trees were imported from Scandinavia. Spruce is still used today, although the bark beetle is threatening spruce forests in Europe. Nowadays, foundation piles often come from the Ardennes in Belgium.

It may take a lifetime for a tree to grow tall enough to be used as a foundation pile. Concrete has mostly replaced wood in the last decades, which means that the supply of wooden foundation piles has plummeted accordingly. Forest management relies on a demand from society: do we emphasize the ecological qualities of a forest, its recreational value, or its capacity for wood production?

[20] If we want to use more wooden foundation piles in the Netherlands, we need to make connections between the supplier and foresters: the more we use wooden foundation piles, the larger the market, and the more forests can be maintained for this purpose.

[21] In society, we need to accept that trees are harvested as part of a healthy building industry. We should be willing to see a harvester in action when we go for a stroll in a forest.

The problem of hardwoods like Eycalyptus Cloeziana is that it needs to be transported all the way from South Africa. The CO2 emissions of naval transportation may be relatively low, it still makes a lot of sense to look for local alternatives. Robinia Pseudoacacia, for example, is a durable hardwood that can be produced locally. Experiments are done in Flevoland, for example. Due to its irregular shape piles are constrained to about three meters in length. Its heartwood can last 20 to 25 years in the ground, which makes it suitable to be used as short sacrificial piles for certain conditions. But is that enough? What more can we do to prevent decay?

[22] Acetylation of local wood may be a more sustainable alternative to importing trees from afar. Alder wood functions a little like a large straw. Under water it will become completely waterlogged. This permeability makes it possible to acetylate the wood. After treatment, the alder wood could last more than 25 years in the ground, and would thus be an ideal candidate to be used as sacrificial pile.

How would such a sacrificial pile as replacement of a concrete top work in practice? Any maintenance to your foundation is typically intrusive and costly. Can we design a building in such a way that it becomes cost-effective to replace the sacrificial pile every 25 years? There are no clear-cut answers to this question, but we'll discuss two proposals. Keep in mind that both are still unpolished ideas that need further development:

[A] The first proposal is intended for peat soil where using heavy machinery is impossible. Think of recreational homes or other building sites that are difficult to reach. A large amount of alder trees can be used to compact the soil. On top of the alder trees, planks are placed under ground water to create an underground terrace for building on. Acetylated alder can be used as sacrificial piles on top of these underground foundations. Whenever problems arise, new piles can be added, and older piles can be lifted to even out irregular settlements. As ground water levels are generally higher in such soil conditions, and the soil itself is loose, it is relatively easy to wiggle new sacrificial piles in the ground. The more piles are added, the more the weight is distributed.

[B] When foundation piles have to reach deeper sandy soils we can make use of a combined pile: a sacrificial pile connected on top of the wooden foundation pile. The trick with this option is to lift and replace the top while preventing the soil from caving in. For this solution, pairs of piles are eccentrically placed below over-dimensioned floor beams. One-by-one the sacrificial piles can be removed through a removable access-plug in the ground floor. A steel tube can be hammered in the ground to prevent soil from caving in, allowing for a solid connection between the sacrificial pile and the deeper foundation pile.

In conclusion: we think the use of wooden sacrificial piles has potential in reducing concrete use for foundations. Not for every use-case, mind you, but for more than we may think. There certainly are downsides: wood will rot, and thankfully it does so. We may have to resist the urge to design buildings for eternity as conditions change in society. Well-designed structuralist buildings that aren't even fifty years old are demolished as we speak: let us make the mortality of buildings discussable. Burying concrete under ground forever is a societal risk we are taking. The amount of emissions that go along with that are directly responsible for climate change. It is certainly understandable to be risk-averse when designing foundations, but in doing so we are pushing our problems to others. In other words:

Risk-averse building for yourself is a risk to society at large!