Not only the area where natural gas is extracted in Groningen is colored red on the new subsidence map, but also the Green Heart region and along the Betuweroute railway line. From the latest measurement data produced by the Netherlands Geodetic Commission (NCG) it appears that the Netherlands’ land surface is sinking faster than expected. A worrying observation, believes Rijk Blok. As assistant professor at the Innovative Structural Design group, he conducts research on foundation techniques and teaches soil mechanics.

“In addition, the loads exerted in urban areas by more intensive land use and new buildings are still increasing. In areas rich in peat there’s the added problem of oxidation: the low groundwater table is causing vegetal fibers to die off and the top part of an entire soil layer is being lost. Add to that the massively dry period in the summer, due to which the stable clay and sand layers in the river delta are also settling increasingly. It is thus not surprising that in places other than Groningen foundation piles are at risk of rotting and walls are showing cracks due to settling occurring in an imbalanced way,” says the assistant professor.

Enhance stability

Now that there’s widespread clarity that the subsidence problem is more complex than had been thought, we need to tackle it seriously. “Good public information, looking building by building at the restoration of foundations, and more research on new materials and construction techniques,” says Blok.

“All kinds of techniques are now available for making small-scale reinforcements of the ground or foundations. For example, lime or cement can be added to clay to enhance stability and polyurethane foam can push sagging floors back into the right position. We ourselves are working to develop a foundation floor combined with shock absorbers and thermal insulation. Initially to build earthquake-proof houses in Groningen, but it may be possible to extend this to new builds in other ‘sinking’ regions.”

“For future infrastructure too, such as new roads, we may be able to use lighter materials, exploiting the capacity for buoyancy. Anything to reduce increasing the load - and thus the likelihood of subsidence.”

Biggest issue

“But the biggest issue concerns the groundwater level. Should we reduce this further to maintain the characteristic polder landscape, or will we decide to raise it so that massive damage to historic city centers is avoided where possible? One option is to use drainage pipes in wet seasons to withdraw water and, conversely, in dry periods to add water wherever it is needed. At any rate, we will have to reach a better balance if we want to avoid heading for a landscape of marshes or the deterioration of our monuments.”

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