New land subsidence maps for cost-conscious building on soft soil
Deltares and Sweco have developed land subsidence maps for new building projects. They provide project developers, municipal authorities and the national government with a clearer picture of where new building projects on soft soils are possible and which measures are required. The maps are a valuable building block as the Netherlands has plans to build 900,000 homes between now and 2030, and to increase the rate of construction to 100,000 new homes a year.
A significant proportion of Dutch plans for construction are located in areas that are susceptible, or very susceptible, to land subsidence. This raises the question of how to build responsibly and affordably. The new national subsidence maps from Deltares and Sweco show for the first time where traditional building methods suffice and where alternative technologies are needed to avoid high costs and damage in the future. They consider traditional building and planning methods, but also alternative approaches such as light raising materials, floating construction, and pile foundations for public spaces.
Construction costs like management and maintenance costs calculated for 100 years
For all these alternatives, the researchers calculated the net present value for a 100-year period. That work involved using requirements for residual settlement, reclamation and connections to the surrounding area. Applying this approach on a national scale results in a unique picture of where building is relatively straightforward, and where drastic measures are needed to build to cope with land subsidence.
The maps show that, in much of the Netherlands, using sand for integral raising is the most cost-effective approach. In other areas, alternative technologies and planning approaches – such as lightweight materials, partial raising and/or complete pile foundations – may be more cost-effective. The map also shows the locations in the Netherlands where the cost of construction, management and maintenance as a result of subsidence is high. In regions where costs are more than 25% higher than in areas that are not susceptible to subsidence, the land subsidence challenge is substantial.
These insights are relevant not only for municipal authorities with major housing agendas but also for the national government. The information from the maps helps municipal, provincial and water authorities, as well as housing corporations and property developers, to make a sound joint assessment of whether building is desirable, and if so, which measures will be required to mitigate risks. The Ministry of Infrastructure and Water Management is looking at how the results can be included in the Dutch ‘Spatial Assessment Framework for a Climate-Adaptive Built Environment’.
The fact that we have been able to map this for the country as a whole makes the maps unique: they combine land subsidence knowledge with information about the costs and benefits for new building locations.
Otto Levelt, Deltares land subsidence expert
Building to cope with land subsidence in order to reduce the cost burden for future residents or the living environment
This approach also provides a sound basis for a first step on the road to national residual-settlement standards. It is important for municipal authorities and owners of new buildings to know how much subsidence can be expected after the site has been raised and or pre-loaded. Building to cope with land subsidence is essential to reduce the cost burden for future residents or the living environment.
The maps distinguish between areas where subsidence plays a role, or not, in construction and what the costs are for the traditional construction of a new housing area (with integral raising). Where subsidence is limited to 8 cm or less when pre-loading has taken place with 1 metre of sand, land subsidence will not result in any problems. The remaining areas are split up into those where the land subsidence challenge results in a relatively minor increase in costs (up to a maximum of 10%) and those where costs increase substantially. Construction is possible on the former without any problems after integral raising (pre-loading). In areas where costs are up to 25% higher, it is desirable to investigate whether other technologies are cost-effective in addition to pre-loading.
If land subsidence is not properly taken into account in the construction phase, a municipal authority may easily have to spend 30,000 euros more for each home on management and maintenance. In general, the more has to be done, the higher the costs, and the more necessary it becomes to take other technologies and construction methods into consideration.
Arend van Woerden, climate adaptation consultant at Sweco
Building in subsidence-prone areas requires alternative technologies
If the building is planned in subsidence-prone areas, it will be necessary to look at alternative technologies and planning approaches in terms of cost-effectiveness. A tailored approach to planning, and other technologies such as floating buildings, light raising materials or integral piling will be necessary to limit or stop subsidence, and to reduce costs and problems in the future. There are many possibilities now but future-proof construction means making choices. The new maps provide government authorities and developers with a concrete and objective picture, and an action perspective, that will allow them to make better decisions and therefore to meet the housing challenge responsibly and in future-resilient ways.
