What happens when the ground beneath your home moves?

At first glance, the monumental house in Zoelen appears to be in excellent condition. Look more closely, however, and cracks become visible in the façade. These cracks are caused by the ground beneath the building. Changes in weather conditions cause clay soils to shrink and swell, leading parts of the house to move as well.

This phenomenon is causing increasing damage to foundations and buildings across the Netherlands. Together with a range of partners, including TNO, Deltares is working to improve understanding of subsurface conditions and the vulnerability of buildings.

Today, a team of researchers from Deltares and TNO is installing dozens of monitoring instruments in and around a nineteenth-century house in Zoelen. Between rain showers, they place sensors on walls and beneath the ground and set up a camera system. The equipment will collect data on movements in both the building and the subsurface as part of a broader study into the causes and consequences of foundation damage in the Netherlands.

The Deltares team includes physical geographers and subsidence experts Otto Levelt and Harry van Essen. While Deltares focuses on processes in the subsurface, TNO examines the condition and behaviour of the building itself. Together, the organisations aim to build a complete picture of the interaction between soil and structure.

Locations for long-term monitoring

Houses with shallow foundations that are vulnerable to expansive soils can be found across the Netherlands, from Groningen in the north to Limburg in the south and from Utrecht to the riverine regions. Since the exceptionally dry summer of 2018, the Netherlands Cultural Heritage Agency (RCE) and the Knowledge Centre for Tackling Foundation Problems (KCAF) have received an increasing number of reports from residents about cracks and foundation damage.

Together with Deltares and, later, TNO, these organisations identified locations suitable for long-term monitoring. The RCE proposed a listed farmhouse dating from 1850 in Rossum, where monitoring began in 2024, while KCAF identified the property in Zoelen.

“This is extremely important for us,” says Otto Levelt. “Long-term monitoring locations give us the opportunity to understand how processes in the subsurface develop and how buildings respond to them. These houses were selected not only because they are founded shallowly on clay soils, but also because they provide sufficient space for installing monitoring equipment. And, of course, because the residents are willing to participate.”

From crack to sensor

“What we want to know is: what happens in the subsurface, and how does that become visible at the surface? How quickly does the ground respond to different weather conditions? How strong and how rapid is the interaction between movement in the soil and movement in the building? And when does damage start to develop?” Otto explains.

To answer these questions, Deltares and TNO are installing an extensive monitoring system around and on the house. One of the most visible instruments is the inclinometer, which measures how much the building tilts and in which direction; other instruments measure how existing cracks develop over time. Are they widening or closing?

In Zoelen, the researchers have also installed a fixed camera that takes photographs of the same section of façade every day. Using image analysis and artificial intelligence, they can detect minute changes in joints and cracks. “We want to see whether those joints move over time,” says Otto.

Moisture and drought

Otto and Harry focus on instruments that provide information about groundwater levels, soil moisture contentd and the behaviour of different soil layers. At various depths, they install extensometers that reveal how individual soil layers move relative to one another. They also place tensiometers in the ground, which indicate how strongly water is being retained within the soil.

According to Otto, this information is an important addition to the soil moisture sensors. “Tensiometers show how much water is being drawn from the soil by evaporation and by plants and tree roots. If roots have to work hard to access moisture, the soil is very dry. Tensiometers give us a much better picture of drought stress within the ground.”

Meanwhile, a weather station records rainfall, temperature, humidity, wind conditions and solar radiation. The measurements make it possible to link changes in both the soil and the building to weather conditions. The researchers also take into account environmental factors that may influence soil moisture levels, such as paving and terraces.

“Our assumption is that paved surfaces help keep soil moisture levels relatively stable because less water evaporates and less water infiltrates the ground. Whether that is actually the case is something we are measuring here and at other monitoring locations as well. If our assumption is correct this would help us finding simple solutions to confine the effects of shrinking and swelling of the soil.”

Residents as citizen scientists

The residents of the monitoring sites are highly engaged. The homeowner in Zoelen not only made his house and garden available for research, but also helps collect data. “We ask him once a week to use a spirit level to measure the angle between two posts driven into the ground at different depths in his garden,” Otto explains. These simple measurements provide valuable information about small movements in the subsurface.

Most data are transmitted automatically through data loggers, but Deltares and TNO maintain close contact with residents. “What they observe gives us additional insight into what is happening on site and can help confirm our measurements,” says Otto. “Many residents also consider this research very important and help us keep the issue of foundation damage on the public agenda.”

A long-term commitment

Although the monitoring equipment collects data continuously, Deltares and TNO are not expecting immediate answers. “Processes in the subsurface are slow and vary considerably from location to location,” Otto explains. “That means this type of monitoring requires patience. After one year you learn something, but after five years you start seeing recurring patterns and trends. We also continue to improve our monitoring methods as we go.”

The researchers are keen to expand the number of monitoring locations. In addition to Rossum and Zoelen, Deltares is conducting measurements at other sites across the Netherlands where shrink-swell clay occurs. “In Arnhem, we have the opportunity to monitor a house that is due to be demolished. This allows us to test the theory that there is less ground movement beneath a house than alongside it. We are investigating differences in moisture content beneath the building and in the surrounding soil.”

At all locations, Deltares collects soil samples for analysis in its own laboratory. “We have hundreds of samples and assess, among other things, their plasticity index, which provides an indication of their potential to swell.”

Towards practical guidance

By combining all collected data, researchers are gradually building a nationwide picture of where shrink-swell-sensitive clay occurs and how buildings respond to it. Ultimately, the measurements are intended to contribute to practical guidance for homeowners, municipalities and policymakers, and to support the National Foundation Strategy, which aims to ensure that people can live safely in homes that are resilient to future challenges.

Which locations are most at risk of foundation damage? Which signals indicate that damage may occur in the future? And which measures can help prevent foundation problems? Answering these questions will require much more research and long-term monitoring, Otto emphasises.

“And that means more fieldwork as well. Field observations are essential because reality is always more complex than what is described on paper. I am continually surprised by how much the subsurface can vary over short distances. You can arrive at a location expecting to find clay and discover that conditions are quite different. You only find that out when you are in the field and put a borehole into the ground. Fieldwork helps us improve our models and expand our knowledge base. And, honestly, it is enjoyable to step away from the desk and work with your hands for a change, even when it is pouring with rain, as it was in Zoelen.”