Fortrac® geogrids allow road construction in alpine terrain

The B114 Hohentauer road was built in the 1960s and passes close to the township of Trieben in Styria, Austria. Over the years, the unstable hillsides caused the heavily trafficked link between Trieben and Sunk to develop serious defects eventually leading it to be beyond repair.

That it was at all possible to build a new road on this site with its steep hillsides, is a testament to the commitment of everyone involved with the project.

The regional roads department for the State of Styria, Fachabteilung 18B Straßeninfrastruktur-Bau, working with Graz-based consultants, Dipl.-Ing. Dr. techn. Karl Lackner, and Dipl.-Ing. Johann Birner, and main contractor Alpine GmbH, Salzburg investigated a new solution, which, in view of the geological conditions of this location, would have to overcome some real challenges.

Built on geosynthetic reinforced earth (GRE)

After decades of use, while clinging on to an unstable hillside, the old road was now showing obvious signs of damage, a cost-effective and reliable alternative had to be found and be in place quickly. After considering numerous possible solutions, the client decided in favour of a new alignment on the opposite side of the valley, more or less parallel to the route of the old road. The almost impassable terrain with its extremely steep slopes and the marginal stability of the valley side made the construction of a road using conventional supporting works out of the question. Different options were investigated and feasibility studies produced. For reasons of engineering (flexibility), economic viability and compatibility with the existing landscape, all the public and private bodies involved with the planning of the new road agreed on constructing a large number of sections of the new route using geosynthetic reinforced earth (GRE).

Extreme demands on the construction materials

The steep embankments and supporting works constructed from GRE, required for the new road, consist of layers of compacted fill and reinforcing geogrids, with the latter being the key loadbearing members responsible for the stability of the whole system. The GRE structures, up to 28 metres high, imposed some specific technical requirements on the geogrids. In addition to the engineering design calculations (stability), the short- and long-term strain properties of the geogrid are also important (low deformations) and must last for the planned design life of at least 120 years.

The flexibility of the geogrid is very important to the precise and proper construction of the GRE embankments. Flexible geogrids ensure that the specified full compaction of the fill is achieved, even at the outer face of the embankment, and allow the geogrid to be precisely wrapped back at the front face, and on this project, also at the rear. The higher, steeper and more slender a GRE structure is (as in this case), the more important it is to achieve a strong bond between the fill and the geogrid. The outstanding bond associated with Fortrac^® geogrids was further improved for this application by using a project-specific geogrid with a large mesh size of 70 mm x 70 mm to suit the coarsely graded fill.

In the light of the highly complex technical requirements and after discussions and comparisons by the geotechnical engineers and road designers, HUESKER's Fortrac^® geogrid was selected as the most suitable product for this project.

The geogrids from HUESKER are particularly well suited for such challenging tasks, because they are very flexible and - as is necessary for this project - can be wrapped back at the rear; while at the same time, they have a high tensile stiffness and are therefore perfect for applications where only the smallest deformations are permissible. This combination also permitted the successful construction of complex curved geometries, such as the two 180° hairpin bends. The external stability of the system and steep mountain sides required additional measures such as rock dowels, anchors and soil nails. Finally, the construction of the complex composite earthworks structures: on the one hand flexible and adaptive, on the other resistant and compatible with the existing landscape.

We are seeing more and more uses of our products in alpine surroundings with difficult substrates, steep and sometimes unstable slopes and impassable terrain. Experience and the courage to try something new are essential for successful construction projects in these regions. Increasingly frequently clients are turning to GRE construction solutions – just as here in Trieben, Austria. Crucial for the lasting and safe use of GRE are an expert geotechnical design and the selection of the right materials. The technically challenging new alignment of the B114 was able to be successfully built with the selected products manufactured by HUESKER in Gescher, Germany.