Charting the subsidence and rotting piles of houses
It is estimated that around 425,000 houses in the Netherlands have, or will have to, deal with foundation problems, such as rotting timber piles or shallow foundations in subsiding soils. Consequence: cracks, subsidence and even risk of collapse.
“I also had a house with rotting piles,” says Chris Geurts, structural engineer and researcher at TNO, “where, within a fairly short space of time, there were rather serious cracks in the walls. In an investigation, it appeared that timber piles gradually turn into “snot”. I actually wrote it off when it was sold.”
Geurts is not merely an expert by experience. His employer TNO, in collaboration with Deltares, provided the technical section of a report by the Council for the Environment and Infrastructure (Rli), which advocates a vigorous and nationwide approach to this problem.
Nationwide problem
The problem has been known for decades, according to geotechnical engineer Mandy Korff of Deltares, “but everyone now realises that it is no longer merely a problem of individual home-owners. We see that it is a nationwide problem, so we want to do something about it nationwide.”
This has resulted in the two organisations being ask by the Rli to combine their existing knowledge, to draw up an estimate of the scope of the problem, and to present proposals for further analysing and addressing the problem. And to do so rather quickly.
“The request was received in October 2023, and had to be ready to be sent to the new Dutch government in February”, says Korff. This made the choice obvious for the duo TNO and Deltares on account of their field of expertise: “In 2021, we had jointly already broached this subject in a knowledge agenda.”
We see that it is a nationwide problem, so we want to do something about it nationwide.
Mandy Korff
Rough-to-fine approach
“The parties began working at breakneck speed”, says Korff. “Most of the work depended on a good set-up: what are we actually talking about?” Specifically, the researchers substantiated the number of houses with foundation problems. “Until then, the number of one million houses had always been assumed. The Rli wanted to have that investigated in more detail.” To find out which 425,000 houses were involved, the researchers advocated a rough-to-fine approach.
“There are 6.5 million houses, which do not all need to be examined and cause stress for people. A more thorough examination was needed, especially for buildings of more than 50 years old, which were built on timber piles or have a shallow foundation. If there were already cracks in the building, there was a greater risk that the foundation was the cause.”
Archives
“Further research can then be done based on archives”, says Korff, “except, one of the major issues is that we do not have a centralised archive in which we document foundations. It is also one of our most important recommendations to the government: to create such an archive.
“The information is often there”, adds Geurts, “but it is poorly accessible. “Once you realise that you’re getting none the wiser, you look at the property itself.” Only in extreme cases will you proceed with a time-consuming and costly inspection of the foundation, which requires excavation down to the piles.
Any remedial work will have to be addressed. Sometimes the damage is mainly cosmetic: some cracks do not affect the safety or functioning of a building. But, sometimes the foundations also need to be restored, which is a far-reaching and expensive operation, although postponing it often leads to greater damage and repair costs. Sometimes demolition is the only solution, but usually the overall housing quality is unsatisfactory too. Also, if no action is taken, the number of houses with problems will continue to rise.
425 years
“Innovation, both in terms of inspection and repair work, is urgently needed”, says Korff. “We are now renewing the foundations of a thousand houses a year, so there will be work for the next 425 years. We have to move faster.”
The Deltares researcher sees that the urgent challenge based on existing knowledge, did stir things up. “We are now developing models to refine and improve which properties are most at risk, and we must also take into account the consequences of climate change.”
“Follow-up studies with the same partners are already in the pipeline”, says Geurts. “This will include even more specific scrutiny on where the problem manifests itself. Can we identify certain types of houses, and which action should we then tackle first? Because not everything be tackled at once.”
In addition to the technical aspects, the Rli also includes informative, administrative, and financial aspects, with an urgent recommendation to address this problem nationwide, rather than to wait and see. Otherwise, the number of problematic properties will increase and the volume of damage will only get bigger. Korff: “If you do nothing, the problem only gets bigger.”
