Until now, whenever bridge building was being designed, the traditional approach for a bridge support was to consider a concrete abutment wall or a cast-in-situ bored piled foundation. A project in Ilsenburg, Germany, has shown there is a comparable option to the conventional form of construction, namely Geosynthetic Reinforced Soil (GRS).

The starting point was the tender issued by the Wernigerode local authority covering the construction of a replacement bridge over the River Ilse as part of the upgrading of the K1355 district road at Ilsenburg. The authority's proposal intended the bridge to be supported on bored piles. In addition, the abutment was designed as an anchored gabion structure incorporating the piled elements.

The proposal from consulting engineers IBH - Herold & Partner Ingenieure, Weimar, acting for contractor Matthäi, Magdeburg, was to construct the abutment as a GRS structure. The design would dispense with bored piles, making it Germany's first permanent bridge abutment to be constructed using GRS - built with HUESKER's Fortrac® geogrid; planned and designed by IBH - Herold & Partner Ingenieure.

In place of load transfer through the piles, this proposal involves load transfer onto the GRS structure, onto which the abutment bearing shelf is directly cast. This structure comprises layers of fill placed and compacted onto Fortrac® geogrid.

The front face is constructed from galvanised steel gabion baskets filled with crushed stone. The geogrid is double-looped around the steel facing and turned back. The reinforcement material - Fortrac® geogrid type R140/30-30MP made from PVA, is cost-effective, has excellent structural engineering properties and fulfils two important requirements. First of all, PVA geogrid has excellent long-term durability with a working life of 120 years, even when placed in a high pH fill, in this instance pH 10 -11. Secondly, the geogrid has an extension under working load of much less than 6 % and so is particularly suited to this situation, where only a very low deformation can be tolerated. A further advantage of Fortrac® geogrid is its flexibility in all weather conditions, making installation work simple and cost-effective.

The GRS construction method offers several advantages enabling the work to progress rapidly and above all with cost benefits. The sequence of operations in the construction of the GRS abutment is quite simple, leading to a substantial shortening of the construction period. In addition, there is no requirement for dewatering, sheet piling or shuttering and concrete curing time is eliminated.

Compared with the client's scheme, the GRS solution offered around 30 percent saving - and was half the price of a conventional reinforced concrete substructure. There are similar savings in construction time; 25 percent reduction opposite the client's scheme and 70 percent compared with the reinforced concrete version.

On the question of stability GRS is subject to structural calculations and certification. External and internal stabilities require a check according to EBGEO /3. The stability check on the Wernigerode bridge abutment proved to be excellent.

But that is not all. As this bridge was the first of its kind, a vertical inclinometer was installed at each abutment, which would allow the horizontal movements in the GRS structure to be accurately recorded. Settlement measurement points were set up on the bearing shelf to enable surveys of the vertical movement of the structure to be carried out.

DBut that is not all. As this bridge was the first of its kind, a vertical inclinometer was installed at each abutment, which would allow the horizontal movements in the GRS structure to be accurately recorded. Settlement measurement points were set up on the bearing shelf to enable surveys of the vertical movement of the structure to be carried out.

The abutment was completed in October 2000 and opened for traffic in December 2000. So far the measured vertical deformation is 4 - 8 mm and horizontally in the order of 1 - 2 mm.

The use of Fortrac® geogrid also makes structures more flexible than the comparable traditional construction method, which means that cracks, caused by deformation or movement in a stiff concrete structure, are less likely.

This project represents a true innovation in bridge construction in Germany; a time-saving and cost-reducing solution, offering recycling potential and environmental benefits. Thanks to Fortrac® geogrid from HUESKER - every bit as good as the traditional design.

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