Fortrac® geogrid – sinkhole protection for the new BAB A 143 motorway

Germany's federal transport infrastructure plan agreed by ministers in 1992 identified 17 transportation projects under the umbrella of the Verkehrsprojekte Deutsche Einheit (VDE) to assist in the unification of the former separate eastern and western parts of the country. VDE road project No. 13 comprised the A38 motorway connection Göttingen-Leipzig, which included the A143 Western Relief Road at Halle. The proposal involved the construction of interchanges on the A7 at Göttingen and the A14 as well as the western peripheral road around Halle between the A38 and A14 motorways. As it approaches the residential suburb of Halle Neustadt, the route crosses an area of former brown coal workings prone to the formation of sinkholes. An alternative line was examined and rejected.

From the range of different options for ensuring the safety of the road, the designers chose a value-for-money Fortrac^® geogrid solution. Protection against subsidence due to the formation of sinkholes was provided by a bridging system formed using a single layer of high strength geosynthetic reinforcement made from Fortrac^® – AM (Aramid / PVA). When a sinkhole forms, the Fortrac^® synthetic geogrid bridges the resulting void and ensures the safety of the traffic above. The system for bridging sinkholes was designed assuming a diameter 4.00 m – this project called for dynamic compaction of the ground by lifting and dropping heavy weights prior to installation.

The Fortrac^® R 1200/100-10 AM geogrid was made-to-measure in accordance with the placing drawings to fit the dimensions of the site by manufacturer HUESKER Synthetic GmbH. A continuous chain of quality assurance measures involving in-house testing of the yarn and geogrid during manufacture by the accredited HUESKER laboratory was backed up with third party quality monitoring in accordance with DIN 18 200 and check inspections by the client, DEGES Berlin. Only after all this was the material approved for installation.

DEGES Berlin commissioned the Aachen consulting engineers WBI (Prof. R. Ing. W. Wittke, Beratende Ingenieure für Grundbau und Felsbau GmbH) with the planning, design and supervision of construction. The geogrid placing drawings were created by "DAS BAUGRUNDINSTITUT" Dipl.-Ing. Knierim, Leipzig with the help of the engineers in HUESKER's Application Technology Department. At the commencement of installation, additional specialist advice was provided by the road construction company, WALTER-HEILIT – Verkehrswegebau. The geogrid was placed in one layer longitudinally. The transfer of force was ensured by overlapping the geogrid longitudinally and transversely by the specified amounts. In order to mobilise the load capacity of the geogrid as quickly as possible in the event of a sinkhole forming, the geogrid was given a predetermined level of prestress during installation . To do this, HUESKER has designed a special placing beam, which applies a known prestress related to the setting of its unrolling resistance. Fill was then placed over each geogrid working forward progressively to fix the tensioned geogrid in position. This was followed by compaction the gravel bedding layer.

In total about 160,000 m² of geogrid were supplied. This was sufficient to protect 2.60 km of the A143 road in the Teutschenthal valley. The area where the all-purpose road L164 crosses the A143 motorway is also protected against the formation of sinkholes over a length of about 280 m; here this is in the form of geosynthetic reinforcement with Fortrac^® R 1200/100-10 AM.