How we uncover the mysteries of expansive soils

Houses with a foundation on natural ground ('fundering op staal' in Dutch) that are at risk due to the swelling and shrinking of the clay in the subsoil are located in various places in the Netherlands, from Groningen to Limburg and from Utrecht to the river area. Since the extremely dry summer of 2018, the number of reports of foundation damage from these areas has increased by 25 per cent. The actual number of damage cases is probably much higher.

The risk assessment system Deltares works on clearly identifies areas where foundations are most vulnerable and which precautions can be implemented to prevent damage. These precautions include measures for homeowners. Additionally, by supporting the development of national or European standards, engineers and authorities can make informed decisions to prevent costly damage.

Enormous force

At Deltares, various experts have been researching the relationship between the subsoil and foundation damage for a long time. This year, expansive clay from the above-mentioned areas was subjected to a series of tests in the Deltares geotechnical laboratory. Experts want to determine which types of clay are susceptible to swelling, to what extent and to access how drying and wetting cycles impact the soil’s properties such as permeability and strength.

Laboratory tests showed that the clay can swell between eight and ten per cent and shrink between 25 and 30 per cent. These differences depend amongst others, on the thickness of the clay layer and the properties of the clay. Furthermore, swelling pressures as high as 1200 kPa are measured in the laboratory. "It is clear that the force of swelling clay can be enormous, the force we measured is comparable to the weight of a 24-storey apartment building," says Harry van Essen, physical geographer at Deltares.

The effect of shrinkage and swelling is also described as a slowly progressing earthquake

Harry van Essen, physical geographer at Deltares

More frequent droughts

Changes in (ground)water management or other human activities can influence the shrinkage and expansion of these types of clay. Also the presence of vegetation like trees is an important factor in the behaviour of sensitive clays. As a result of climate change, the effects of swelling and shrinking clay will increase further, according to Harry. "In the Netherlands, we have long lived in a temperate climate, which means that the properties of this clay soil went unnoticed. Now that we are experiencing increasingly extreme periods of drought and heavy rainfall due to climate change, that clay is suddenly reacting."

This has consequences for the built environment and the road and dyke network. Buildings in these areas not only show cracks in walls, but sometimes houses are even declared uninhabitable. Many recent damage reports come from homeowners in the river area (Duiven, Zevenaar), but damage cases are also known outside this area, such as in Rekken in Gelderland and Roden in Drenthe. Harry: "The independent geologist Peter van der Gaag already recognised in 2004 that swelling clay in Groningen was one of the causes of foundation damage, which we confirmed in a study financed by the NAM and Deltares."

Lab research

Deltares is conducting research into the clay soil at most of these locations. This includes taking clay samples, which are then examined in the laboratory to determine their properties. Deltares is also setting up measuring points to monitor the behaviour of the clay in response to changes in the groundwater level and soil moisture, which in turn are influenced by changes in climatic conditions.

The laboratory research consists of measuring the volume changes of drying clay samples, defining the relation between soil suction and water content, and performing different types of swelling tests.

This fieldwork is part of Deltares' self-funded research into expansive clays, that also is funded by the Ministry of Housing and Spatial Planning. "A fascinating material," says Harry, "about which we need to know much more. Which minerals cause the volume change, and which types of clay are most susceptible to it? How does shrinkage and swelling develop during successive periods of drought? Could future wet periods cause damage due to swelling? And could a rise in the groundwater level, for example due to soil subsidence, increase the risk of swelling damage?”

Investing in knowledge

The extent to which clay swells or shrinks varies from place to place. Since 2000, Harry and his colleagues Henk Kooi and Otto Levelt have been investigating the locations where foundation damage caused by clay has been reported , but already in 2019, they mapped out the situation for the first time for the report 'Shrinkage and swelling of clay and consequences for foundations' for the Kennis Centrum Aanpak Funderingsproblematiek (KCAF, Knowledge Centre for Foundation Problems).

Following the publication, attention to the problems associated with expansive soils in The Netherlands began to increase, leading to the allocation of the first internal research budget on this subject by Deltares in 2020. "On the KCAF site, a map is constantly updated with all the reported damages to buildings”, says Harry. “There are many similarities between our map of clay thicknesses and the KCAF's Damage Register on the site."

Shrinkage and expansion of clay beneath foundations on natural ground are increasingly causing damage to houses and buildings. More knowledge about this clay is needed in order to develop effective measures

Harry van Essen, physical geographer at Deltares

"What we want to achieve with our research is to reduce stress among homeowners and residents in the Netherlands as a result of the risk of foundation damage," says Harry. "This can be addressed, for example, by providing information and solutions to contractors to repair or prevent damage to foundations."

The research of Deltares focusses on three subjects:

• Laboratory research to gain insight into the relationship between soil characteristics – such as clay content and clay type - and the swelling and shrinking characteristics
• Developing a risk map to identify the location of expansive clays in The Netherlands
• Developing a computer model to calculate the relationship between environmental factors such as vegetation, rainfall and temperature, and soil response based on the results of laboratory experiments

“Identifying whether a house is built on expansive clay is not straightforward. While there are correlations between standard geotechnical laboratory tests and sensitivity to shrink and swell, a mineralogical analysis is required. If we can scale up the monitoring of shrink-swell clay, we will gain clarity more quickly about risk factors and the effectiveness of measures.”

How does clay behave?

Soils in nature are subjected to climate change and undergo periodic wetting and drying cycles, which can alter their hydromechanical behaviour and damage earth structures. These environmental impacts on soil behaviour were investigated in the Deltares laboratory by a team led by Maria Konstantinou, a senior geotechnical engineer.

“Expansive clay samples from the Zevenaar location in the Netherlands underwent controlled cycles of wetting and drying. Meanwhile, we performed tests in the laboratory to investigate how the successive drying and wetting cycles affect swelling potential, while we used a 3D scanner to observe changes in volume. We do this to investigate the soil behaviour under varying water content conditions to better predict field performance", says Maria.

To examine the strength profile of the clay under dynamic water content conditions, mini-CPT tests were also performed. Is dry clay stronger and how does repeated drying and wetting affect the shear strength? Micro-CT scans were taken at Delft University of Technology to examine changes on pore evolution and pore structure of the clay during wetting and drying. "How does the pore size distribution and the formation of cracks looks like? This can change as the clay dries out and swells more often," explains Maria.

During these laboratory tests, we worked with our 'feet in the clay'. It was very educational, because you can immediately see how the material behaves and reacts, which helps you identify problems earlier and interpret the results more accurately

Maria Konstantinou, senior geotechnical engineer at Deltares

Clays consist of plate-like minerals with a very large active surface, explains Harry. “In this context ‘active’ means that water from the pores is absorbed or drained between the plates based on the surrounding conditions and the characteristics of the clay minerals. This behaviour forms the basis for swelling or shrinkage of clay. For the most sensitive clay mineral (montmorillonite), the total surface area of the minerals in just one gram, if spread side by side, can be as large as a football field.”

Solutions

The results of the laboratory tests help us understand how the subsurface behaves, says Maria. This understanding is necessary to determine which measures against foundation damage are effective. "In a house in Rekken, a plastic screen has been placed around the house underground to isolate the soil beneath the house from the surrounding soil to stabilise the moisture balance under the house," adds Harry. "This is one of the possible solutions we are monitoring. Over the past two years, measurements have shown that the house is no longer moving up and down."

There is no miracle cure; foundation damage caused by swelling and shrinking clay is a difficult problem to combat. Harry: "This is because the groundwater under the house is not always evenly distributed. So the clay swells on one side of a building, but not or less on the other. This can cause severe damages to houses. In addition to cracks in walls and floors, the foundation can also be harmed."

Major impact

"Many homeowners are unaware that they are at risk," says Harry. Foundation damage is often invisible, but can manifest itself in wider cracks in the exterior walls, interior walls and floors, sticking windows and doors, sloping or crooked floors and differences in height between the house and the pavement. The effect of shrinkage and swelling is also described as a slowly progressing earthquake."

More and more Dutch homeowners are facing foundation problems. The impact is significant: in addition to high costs, people experience stress, uncertainty and sometimes even a feeling of insecurity in their own homes. Foundation problems are therefore a growing and urgent social challenge.

Expectation

In 2024, Deltares and TNO calculated that approximately 425,000 buildings will suffer foundation damage in the short term. This analysis was part of the Rli advisory report “Well-founded: advice on achieving a national approach to foundation problems”. Deltares contributed its knowledge of the subsoil and foundations, and TNO contributed its knowledge of the superstructure, i.e. the building on the foundation.

One of the recommendations from the joint analysis is that more specific information and knowledge about the subsoil is needed. Over the next few years, Deltares will focus on expanding the soil types investigated in the laboratory, setting up monitoring sites across the Netherlands in collaboration with TNO, and improving both the computer model and national risk-map. A key are of focus at present is understanding how water content varies beneath and around building foundations.