Foundation inspection in the municipality of Gouda
The Municipality of Gouda has a problem with soft soil as old as the city itself: the city center is vulnerable to subsidence. Under the name 'GOUDA STEVIGE STAD' (Gouda Steady City), the municipality is working with knowledge organizations on the Framework Plan for Soil Subsidence in the City Center.
Buildings in Gouda have different types and qualities of existing foundations. These show various trends in the Soil Subsidence Map 2.0. The groundwater level has been continuously lowered over the past decades, but now certain foundations are becoming exposed and less stable. Partly due to variations in the groundwater level, some houses are sinking faster than others. For privacy reasons, we have removed specific data from a number of screenshots. However, specific information regarding vulnerability to subsidence is required for each property.
To estimate the effects of this per property, including for groundwater level decisions, the Municipality could use our Soil Subsidence Map 2.0 data. How well would this support policy?
Due to our generic processing of the SAR images, measurement points are not visible everywhere in the Soil Subsidence Map 2.0. We subsequently reconfigured two things for Gouda: 1. used higher resolution SAR images and 2. specifically processed the images to generate as many measurement points as possible on each building.
On the right, in the SkyGeo version of the map with these two improvements, we were able to provide the Municipality with information for all buildings throughout the entire city center (and for roads, provide at least 20 points per 20-meter road section). Even in places where the Soil Subsidence Map shows almost nothing, we have a very high density of subsidence measurements; this is only possible by optimizing the processing locally, with knowledge of the policy issues we wish to support.
Contextual processing
The Municipality can also use InSAR data to support decisions regarding maintenance of infrastructure other than buildings: roads, sewers, pipelines, and the like. In the subsidence map, calculations are not placed in the context of construction work and other disturbances in the middle of the InSAR time series itself, which constitute an interruption of the millimeter-sensitive signal. However, if you want to make a “before-and-after” comparison regarding construction work, that step is required.
On the left is our BDK 2.0 map, for the calculation of which we did not take into account the road surface work carried out. On the right, you see the InSAR estimates, where we do take into account the disturbance in the InSAR signal caused by the elevation itself. The measurements are more reliable and therefore more useful.
The effects of raising the road surface have been included in our InSAR processing, allowing us to measure the quality of the repair work even after completion.
The properties are ranked according to local inspection urgency.
Separation of road and pile-supported buildings
In the second purple rectangle, we highlight another characteristic of the generic data, apart from the point density. The localization of the measurement points in our Soil Subsidence Map has a standard deviation of up to +/- 5 meters in (x,y) and +/- 10 meters in (z) – see the Knowledge & Explanation of the Soil Subsidence Map. As a result, measurements from high measurement points, such as houses, lie across the measurement points at the lower street level on the map. A representation of “measurements per house” would therefore include a large number of measurement points from surrounding buildings, which means that the Municipality receives an incorrect estimate of the subsidence rates. In fact, the number per house is derived from how close the buildings are to each other and to the street, rather than the actual subsidence of the building alone… A measurement problem that therefore occurs with the Soil Subsidence Map in most cities in the Netherlands.
The standard deviation in (x,y,z) localization of the measurement points in the SkyGeo map, on the other hand, is much smaller: +/- 1 meter. As a result, we can much better separate measurement points of individual properties from the street and surrounding houses. Variations in subsidence are easy for experts to interpret and translate into a calculation of the timeframe within which inspections must take place.
Ultimately, the Municipality receives comprehensive advice regarding the prioritization of local building inspections.
If that damage actually exists, it may be exacerbated when the municipality proceeds with the sewerage works. It is our duty of care to inform our residents about this .
— Spokesperson, Municipality of Gouda
