Rock protection on the B173: Around 340 m of fence, 1,000 rock nails and 3,000 m² of netting - LGA Institut für Umweltgeologie und Altlasten GmbH

Rock protection on the B173: Around 340 m of fence, 1,000 rock nails and 3,000 m² of netting.

Posted on January 13, 2022.

With more than 5,000 motor vehicles/24 h (census from 2015), the B173 federal road is one of the most important supraregional connecting axes in Upper Franconia. It connects the A 73 freeway near Lichtenfels with the A 9 freeway near Hof, which is accompanied by an intensive traffic volume with an above-average share of heavy traffic..

In order to ensure the safety of road users against geohazards such as rockfall and rock slides, the Bayreuth State Building Authority commissioned LGA IUA GmbH to carry out an assessment of potential danger areas along the B173 southwest of Schwarzenbach am Wald (Frankenwald). The reason for this had been the slipping of a rock package from an embankment in spring 2017. The rock mass came to a standstill behind the guard rail. As a result, the slope was examined and assessed for its rock and boulder fall potential. In addition, the rock-strewn steep slopes above the B173 as well as all rock slopes directly at the roadside in an approx. 2.5 km long road section southwest of Schwarzenbach am Wald were investigated up to and including August 2021, which, on the basis of an orienting site inspection, apparently harbored a high hazard potential.

Strongly loosened rock slope on the B173

Close-up of a fissured block in the steep slope above B173

After detailed terrain investigations, drone flights, evaluations of the digital terrain model and selected rockfall simulations, a total of 27 rock objects were assigned a hazard level > 1, which required short-, medium- or long-term technical protection of the objects, depending on the level.

Following the investigation, the second phase of the project involved the execution planning and tendering of the safety measures. The planned measures include the removal of vegetation whose roots could destabilize the rock slope or tip onto the road due to lack of vitality, the clearing of individual rock objects at risk of falling, and the installation of rockfall protection fences of various energy absorption classes with a total length of approx. 340 meters. In addition, four road embankments will be netted using high-strength mesh (total net area of over 3,000 m²). Over 1,000 rock nails will be installed to secure the nets.

Installation of netting

The contractor for the rock support work with a total contract volume of around €840,000 is Jähnig GmbH from Dorfhain (Osterzgebirge). Holz Prell GmbH Gundlitz from Stammbach was commissioned with the preparation of the total of four construction sites (clearing and clearance work). The local construction supervision is carried out by the LGA expert Simone Pflaum, who was also the project manager during the exploration and planning phase.

When work began on October 25, 2021, the B173 was closed on one side in the active construction area; for a short time, tree felling work led to a full closure of the B173. Snowfall and snow cover in December made the work increasingly difficult. After a 3.5-week winter break, the construction site is expected to restart on January 17, 2022.