Foundation damage calls for better data and practical guidance

With foundation issues due to be discussed in the Dutch House of Representatives on 10 September, Deltares is underlining the importance of better data, a national risk picture and practical guidance. Deltares is also contributing this knowledge to the National Approach to Foundations.

Foundation damage is not a local problem

Foundation damage is often associated with peat meadow areas and historic city centres, but the problem affects a much larger part of the Netherlands. More and more buildings are experiencing foundation damage. Prolonged drought and falling groundwater levels are increasing the risks for buildings with timber pile foundations as well as buildings with shallow foundations.

According to Deltares estimates, around 425,000 buildings already have damage, such as cracks or sloping floors, or are at increased risk before 2035. These are mainly older buildings. Although only a small proportion currently have serious damage, buildings with limited damage are also expected to deteriorate in future, meaning that measures will eventually be needed.

The societal impact is significant. It affects not only the safety and liveability of homes, but also property values, investment decisions and future area development.

National foundation risk model

To provide better insight into where foundation problems may occur, Deltares is developing a new foundation risk model for the whole of the Netherlands together with TNO.

The model combines three types of information:

• hazards such as land subsidence, drought and fluctuating groundwater levels;
• data about buildings and their location;
• and the vulnerability of those buildings, such as foundation type, year of construction and structural characteristics.

Together, these components provide a better picture of the likelihood of foundation damage and its potential consequences.

The model is being developed in a modular way so that new data on soil, groundwater, climate, buildings and monitoring can be added step by step. At the same time, this approach shows where uncertainties remain, for example about foundation type or damage processes. In this way, the model can evolve alongside new data and practical experience.

The results help to interpret foundation risks consistently. Because different damage processes each have their own cause and uncertainty, the model is not about producing a single overall score, but about prioritisation and practical guidance: where monitoring is needed, where further investigation is advisable and where measures are required.

Shrink-swell behaviour in clay unravelled step by step

An important uncertainty in foundation problems is the influence of shrink-swell behaviour in clay soils. Since the dry summer of 2018, damage has been reported in various parts of the Netherlands where it had previously been rare.

Deltares is therefore investigating which types of clay are most sensitive, how ground movements relate to building damage and how factors such as groundwater fluctuations, vegetation, paving, seepage and infiltration affect foundation risks. Work is also underway on a national map that provides insight into areas where shrink-swell problems may occur.

Otto Levelt, land subsidence expert at Deltares, says: “For me, the key lies in carefully disentangling causes and effects. Shrink-swell behaviour in the subsurface is still a relatively unfamiliar phenomenon in the Netherlands, yet it can have major consequences for homes. By combining knowledge from different sources, we can develop information that helps residents, municipalities and professionals understand not only that there is a risk, but also what they can do.”

Data is the basis for the risk model

Although the causes of foundation problems are increasingly well understood and can be incorporated into the risk model, important information is still lacking for accurately determining risks. In particular, data on foundation type and the actual condition of foundations is often not yet available. As a result, it is still very uncertain at present to make reliable risk assessments for individual buildings and areas.

Better collection, standardisation and sharing of foundation data by the many parties in the sector are important steps towards addressing fragmented, incomplete or inconsistent information about foundation risks. Only then can a national and uniform picture of the risks emerge, enabling residents, municipalities, housing associations, financiers and policymakers to work from the same evidence base and gain better practical guidance.

From risk insight to practical guidance

Knowledge alone is not enough. Deltares is therefore also investigating which measures are effective in preventing or limiting damage. This includes looking at solutions such as moisture barriers, adapting vegetation around buildings and water management measures that help prevent major fluctuations in the subsurface.

The aim is to give residents, municipalities and other stakeholders more practical guidance. This will allow preventive measures to be taken earlier and unnecessary repair costs to be avoided.

Contribution to the National Approach to Foundations

Through the foundation risk model, research into shrink-swell behaviour in clay and the development of new knowledge products, Deltares is making an important contribution to the National Approach to Foundations. This approach focuses on improving insight into foundation risks, supporting authorities and homeowners, and developing effective strategies for a climate-resilient built environment.

More information

Would you like to learn more about the current state of foundation-related challenges in the Netherlands? Read the Progress Letter on the National Foundation Strategy (in Dutch only). You can also download the Deltares Position Paper on Foundation Issues (in Dutch only) via the Dutch House of Representatives website. This paper was prepared before the summer in response to the parliamentary debate on foundation-related issues in the House of Representatives.