High strength wovens void over a karst area
Location of work/project:
Bad Wünnenberg - Germany
A section of the B480n road in Germany crosses a wide karst area where cavities can rise to the ground surface and endanger the infrastructure. A basal reinforcement geotextile was applied to secure the road against possible subsidence.
Bad Wünnenberg is a town situated, approximately 20 km south of Paderborn, in the state of North Rhine-Westphalia, Germany. The new section of the road B480n links to the Aftetal bridge is the central feature of the Bad Wünnenberg bypass.
To withstand high trafﬁc loads, Federal German roads require good quality foundations. This requirement was not achieved on a section of the B480n where the road should pass over a wide karst area. The underground cavities can rise to the ground surface, especially in cuttings, resulting in local deformations and subsidence.
Based on the results of the ground survey and geotechnical analysis, it was found that initial void diameters of 1.5 m could be expected in the fissured foundation stratum beneath the roadway alignment.The analysis showed that the expected subsidence diameter at pavement surface would be 1.7 m In the cutting sections where 1.1 m of earth fill was used and up to 3 m in the embankment sections.
The required basal reinforcement properties were determined in accordance with EBGEO (2011) and using the RAFAEL subsidence method. This resulted in a design tensile strength of at least 300 kN/m over a 100-years design life at the required strain levels. TenCate Geolon®PET800 geotextile reinforcement, with an initial tensile strength of 800kN/m, fulfilled the long-term design requirements and was subsequently used for the basal reinforcement.
The subgrade surface was first prepared by smoothing and compacting the ground surface. Next, the layer of TenCate Geolon® PET800 geotextile reinforcement was installed in the direction along the length of the roadway. On top of the geotextile reinforcement granular fill material, having good dilatancy properties, was placed and compacted. Further lifts of granular fill were placed and compacted until the level for the pavement subgrade was achieved. Finally, the roadway pavements were constructed on top.