Risk analysis shows costs of damage to foundations

Published: 17 January 2020

The dry spells in recent summers have raised awareness of land subsidence and damage to foundations. Some local authorities knew about this problem already but it was a surprise for others. Many authorities know very little about the risks associated with foundation problems. Deltares has developed a systematic risk analysis method for pile rot and the differential settlement of buildings on foothing foundations.

This approach, in combination with the available data on land subsidence, the subsurface structure, water tables, data about buildings and expert opinions, serves as the basis for a new quick scan for local authorities to identify the risk of foundation problems.

Problems with foundations in the Netherlands

Until the 1970s, the Netherlands used shallow foothing foundations and wooden pile foundations in clay and peat soils. Those structures are susceptible to damage as a result of subsidence and low water tables. Processes involved are generally slow but damage can occur faster if there is a sudden lowering of the water table or subsidence caused by building work. A drier climate can also trigger or accelerate foundation problems because of faster subsidence and lower water tables.

Spatial spread and order of magnitude of the costs of damage

The quick scan provides a picture of the spatial distribution and order of magnitude of the costs of damage in the period 2018 – 2050 at the municipality level. Sien Kok, an economist with Deltares: ‘We have never before had a picture of the risks at the municipality level. These results provide a concrete overview of the added value of policies at the local level.’ The results have been published on the klimaatschadeschatter website (in Dutch). The quick scan was conducted with KCAF and Stichting CAS as a part of the NKWK Climate-Resilient City project. The risk analysis focuses exclusively on pile rot and the differential settlement of buildings on foothing foundations in the Netherlands. Other damage mechanisms such as negative skin friction and bacterial damage have not yet been included.

Map of klimaatschadeschatter.nl

Screenshot of the map in klimaatschadeschatter. The colors on the map on the right show the differences in damage costs per inhabitant of a municipality. It concerns the damage costs for the period 2018-2050 according to the WH scenario.

The results provide an initial rough estimate of the expected costs of damage in individual municipalities between now and 2050, with and without climate change, Although the scan provides an overview of possible damage to foundations at the national level, it can also be used to identify problems at the municipal level in good time.

 

Results of quick scan

In addition to problems with wooden pile foundations, areas on sea clay such as Zeeland, Friesland and Groningen may also be affected by problems with the differential settlement of buildings on foothing foundations. Areas with river clay, including the rivers area in central Netherlands and areas near the Meuse in Limburg face similar difficulties. A drier climate makes problems more likely in places where the water table can fall substantially and in soils on clay that can undergo more shrinkage and swelling behaviour during droughts (Bronswijk and Evers-Vermeer, 1987; de Lange et al., 2009). The level of additional damage that climate change can cause varies according to the municipality: the analysis indicates that the western part of the country, Friesland and the municipalities on the IJssel seem to be particularly sensitive to climate change. The spatial spread of expected damage is broadly in line with the overview of municipalities for which the KCAF (the Dutch Knowledge Centre for Tackling Foundation Problems) has currently received reports.  The approach that has been developed serves as a basis for working together with other parties to develop a more detailed or local picture.