Road maintenance in Kockengen
How do you make optimal use of InSAR to plan municipal road maintenance?
Kockengen in the Municipality of Stichtse Vecht has a problem with soft soil as old as the village itself: subsiding roads must be maintained periodically to keep them functional.
Under the name 'Kockengen Waterproof', the municipality, water board, and province are working on measures to raise the public areas of Kockengen and bring roads up to standard in a sustainable and economically viable manner. Roads in Kockengen “show a wide variation in the thickness and quality of the applied foundation and asphalt layers.” Partly due to the variation in the quality of maintenance, some roads continue to subside much faster than others, even after completion.
To assess the effectiveness of all those repair works on all those roads, the Municipality could use the Soil Subsidence Map data. How well will this work?
The maintenance of the roads in Kockengen has so much variation. Thanks to SkyGeo, we knew exactly what to expect and where the work was done well ... and the road has remained in good condition.
Jules Nieuwenbroek, Project Manager Kockengen Waterproof, Municipality of Stichtse Vecht
We did two things differently: 1. used higher resolution SAR images and 2. processed the images specifically to generate as many measurement points on the road surface as possible.
The first example in the T-shaped purple box north of the village center is clear: due to the generic processing of the SAR images, almost no reliable measurement points are visible here on the Subsidence Map.
On the right, in the SkyGeo version of the map with these two improvements, we were able to provide the Municipality with at least 20 points per 20-meter road section for all roads throughout the village. 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
In the subsidence map, we did not perform the calculations in the context of work on the road surface itself in the middle of the InSAR time series, which would cause a disturbance of the millimeter-sensitive signal. However, if you want to make a “before and after” comparison regarding the raising of the road surface, this is required.
On the left is the BDK 2.0 map for the calculation, which does not take into account the road surface modifications. 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. As a result, the measurements on the right are much more useful, because the quality of the road surface work throughout the entire village can be monitored in detail.
Separation of road and pile-supported buildings
In the second example in the purple rectangle for Kerkweg east of the center, there is another problem with the generic data. The localization of the measurement points in the Soil Subsidence Map has a standard deviation of +/- 5 meters in (x,y) – 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 road section” would therefore include a large number of measurement points for buildings supported by piles, which means that the Municipality receives an incorrect, namely too low, estimate of the subsidence rates. In fact, the number per road section is derived from how close the buildings are to the street, rather than the actual subsidence of the street alone… A measurement problem that thus occurs with BDK in most cities in the Netherlands.
The standard deviation in (x,y) localization of the measurement points in the SkyGeo map, on the other hand, is much smaller: +/- 1 meter. As a result, we can separate measurement points of the road much better from measurement points of buildings supported by piles.
The contrast between the street and the houses is clearly visually apparent in the purple polygon: for operational use, we only include the yellow-orange-red points of the road itself. Variations in subsidence are easy for experts to interpret and translate into a calculation per road section of the timeframe requiring maintenance.
Based on the accurate and reproducible SkyGeo measurements per road section, the Municipality can ultimately assign a simple pass/fail rating for repair work.
