1. General

1.1 Task

The federal road B 21/E 641 is one of the most important traffic axes in the district of Berchtesgadener Land. Within the district, it connects the municipality of Schneizlreuth with the city of Bad Reichenhall and is the shortest connection for the city and state of Salzburg to the Austrian vacation regions in Tyrol and Pinzgau. Due to its location along the high and steep southern and eastern slopes of the Ristfeuchthorn and Rabensteinhorn as well as the north-western slopes of the Lattengebirge with its steep rock faces, accidents and closures due to avalanche, rockfall or mudslide events occur here time and again. After several extreme events with casualties and longer closures of the road, it was decided to develop an integral protection concept against gravitational natural hazards. The aim was to protect the road with technical and silvicultural measures in such a way that it could reliably meet regional and supraregional traffic requirements all year round despite the hazards from the adjacent slopes. In other words, the hazards had to be reduced as best as possible and the availability of the road had to be significantly increased. With all efforts, however, it was clear that 100% protection of the road from natural events could never be achieved.

1.2 Consideration of measures, impacts and load approaches

So far, there are no binding guidelines in Germany for a risk-based assessment and prevention of gravitational natural hazards. Likewise, there are no regulations for the design of protective structures to determine the impacts from rockfall, mudslides and avalanches and for the combination of these load cases. In order to comprehensively record the natural hazards for the federal highway, intensive surveys were carried out by the construction office. To this end, an event-oriented hazard assessment was carried out by means of expert opinions from the specialist departments of the Bavarian State Office for the Environment (geology, mudflows, avalanches). Based on these data, in-depth 2-D and 3-D simulations were subsequently prepared for the various types of processes and different event scenarios (e.g. different block sizes, different avalanche types) depending on the expected return probabilities. Based on these results, a risk assessment was performed and an urgency assessment was made. Furthermore, the simulations made it possible to derive the design and load approaches for a protective structure, as well as to precisely investigate the effectiveness of individual and bundles of measures. In close cooperation with the test engineer, a comprehensive load diagram with an impact report was developed. For the various avalanche scenarios, for example, snow weights of 3.00 kN/m3 (for flowing avalanches), 4.00 kN/m3 (for a naturally deposited snowpack) and 5.00 kN/m3 (for deposited avalanche snow) were considered. The governing debris load is composed of a vertical component of 4l kN/m2 and a horizontal component of 9 kN/m2. The largest rockfall load considered was an approx. 8.5 t fall block impacting at a maximum velocity of 34.4 m/s. The individual impacts defined in the load diagram from the process types debris flow, avalanche and rock or blockfall as well as the partial safety and combination coefficients to be applied formed the basis for the further design of the protective structure according to Eurocode. In a final step, all the resulting data was used to provide economic proof of the planned protective structure by means of a cost-benefit analysis.

1.3 Structure design

The design of the structure was largely determined by the structural, economic and traffic-related boundary conditions. Lighting, fire protection, traffic safety, construction technology, construction costs and, above all, the time factor influenced the choice of structural solution. The architectural design of the portals was carried out with the aim of making the originally very bulky-looking, massive ceiling construction appear more filigree when entering the building. For this purpose, the parapet elements were tapered from 1.50 m to 80 cm in the direction of the portals. In addition, the parapet element is offset in two stages at the end faces in accordance with the load-bearing construction; the lower section was beveled slightly inward. This design creates a graduated shadow effect that makes the parapet beam of the portals appear much lighter. The desired reduction in construction and blocking time ultimately led to an uncomplicated prefabricated solution at the portals.

2. Building design

2.1 General

The protective gallery is a 139 m long reinforced concrete structure which is used by two-way traffic. The gallery consists of 13 partially covered blocks with different lengths and standard cross-sections. At each end of the gallery, directly behind the portal walls, there is a guide wall. The two guide walls prevent torrents, mudflows and avalanches from breaking out towards the portals. They also provide the portal areas with additional protection against rockfall events. The plateau on the gallery must also be accessible in the future for any clearance work that may be required. For this purpose, an approx. 70 m long service road was constructed starting in front of the west portal. At the upper end, the service road crosses the western guide wall via a lead-through structure secured with a beam lock. On the uphill side, the gallery is completely filled in. The height of the embankment is generally approx. 3.00 m and a maximum of 4.75 m. The gallery is completely filled in on the uphill side. On the valley side, the 1.50 m high parapet ensures the minimum cover of the structure. Height, inclination and slope breaking edges of the embankment were planned according to the specifications of the Swiss guideline "Ein- Wirkungen infolge Lawinen auf Schutzgalerien (ASTRA 12 007)". In order to keep the forces on the structure low, the deflection path of the avalanches was placed as far as possible uphill of the gallery. The high height of the embankment is necessary, on the one hand, to reduce the localized energies from rockfall events for the structure and, on the other hand, to be able to safely divert debris flow and avalanche events over the structure. To ensure that debris and rock do not remain on the structure when the natural events occur, but are drained away, a minimum slope of the backfill of approx. 16° is required. The backfill consists of a protective layer on the structure, a damping layer and a top layer. The material used for the damping layer was the slope debris excavated during the construction project, after its suitability had been investigated by soil mechanics and confirmed by simulation calculations. The requirements for the geometry of the backfill led to a shift of the existing route of the federal highway towards the uphill side. The pre-cutting areas resulting from the route shift were secured on the uphill side with 50 m and 60 m long and 5.0 m high retaining walls. In contrast to the main area, the existing mud channel at the western end of the gallery was only covered with a slight slope of 2.5%. The channel is 4 m wide and filled with armourstones. From the parapet formed in this area with a channel cross-section, the water falls directly below the gallery onto a "stilling basin" constructed with armourstones. Afterwards, it flows downhill as in the existing structure.

2.2 Construction

Since there is good bearing slope debris in the area of the structure, the supporting structure could be founded flat on strip footings. The block lengths of the gallery are 10 m in the main area (RQ1) and 12.5 m for the four blocks in the area of the Murger channel (RQ2). Due to local conditions, the protective gallery is designed as a rectangular cross-section that is semi-open towards the valley side. The cross-section is composed of the closed back wall on the uphill side, the supports on the downhill side and a slab-beam ceiling. The slab consists of 75 cm high and 50 cm wide precast reinforced concrete girders spanned in the transverse direction of the gallery. On the valley side, they rest on the supports via a shear-bolt connection and cantilever out by approx. 1.20 m. The ceiling is made of prefabricated reinforced concrete beams. To improve the incidence of light, the cantilever is slanted upwards at an angle of approx. 42°. This ensured the required luminance in the gallery. Above the precast girders is a 38 cm thick in-situ concrete slab, which was concreted on 6 cm thick precast concrete elements as permanent formwork. The 1.50 m high parapet forms the end of the slab on the valley side. It consists of precast reinforced concrete elements that are connected to the concrete slab in a flexurally rigid manner. The thickness of the parapet is 45 cm. In order to be able to do without lighting, the column cross-sections also had to be reduced to a minimum. The slimming of the columns and the cross beams necessitated the use of a high-strength concrete of strength class C 60/75 and a very high degree of reinforcement. To reduce the impact loads on the columns, a 1.50 m high parapet was provided on the valley side. This parapet also prevents debris or snow from flowing back into the gallery in the event of natural hazards. To ensure increased structural fire protection, polypropylene fibers had to be added to the structural concrete of the cross beams, the floor slab, the columns and the walls.

Due to the integration of the gallery structure into the uphill slope and the high loads from backfill and natural events (especially the rockfall and rockslide events), the forces arising from the earth pressure could no longer be transferred via the frame structure itself. Therefore, the existing embankment had to be secured in advance by a nailed shotcrete wall. The structure, which was designed as a permanent support, has a maximum height of approx. 12m. In the backfill area between the uphill gallery wall and the shotcrete protection, a drainage mat made of geotextile and filter concrete were installed. These measures significantly reduced the earth pressure. Furthermore, it was ensured that any stratum water that may occur in the future is drained off safely and in a targeted manner and that no water pressure can build up behind the gallery wall.

2.3 Technical equipment

New approaches were taken with regard to the operational equipment of the gallery. Due to the remote location of the building, high costs would have been incurred for the installation of a power supply. For this reason, the gallery was planned in such a way that it was possible to dispense completely with electrical equipment. Nevertheless, the structure complies with the requirements of the RABT 2006, with the greatest challenge being compliance with the luminance values in the structure required by the RABT. In order to be able to do without lighting, the supports had to be slimmed down considerably in order to achieve as little shadowing as possible in the structure. A 3.0 m high reflective coating on the uphill gallery wall also improves the brightness in the structure.

To compensate for the lack of power supply, special solutions had to be found several times. For example, a vacuum pump was specially planned and implemented for emptying the retention basin for harmful liquids.

3. Construction work

Due to the difficult detour situation and the high traffic load on the federal highway, efforts were made to avoid or reduce closure times and, above all, full closures as far as possible. Construction of the main gallery works started on 04.08.2014. In the first two months, slope removal and securing works were carried out. With the gradual completion of partial sections of slope stabilization in the gallery area and the construction of a partial bypass, concreting work for the foundations, the uphill walls and the downhill parapet wall, including the supports for blocks 5-10, could begin in mid-September. For these blocks, the gallery slabs, the waterproofing and a temporary backfill had to be completed before the winter break so that any avalanche events that might occur during the winter season would not cause major damage to the structure. Due to weather conditions, no work was planned for the winter. Due to the local climate (low sub-zero temperatures and high precipitation, mostly in the form of snow), an economical and technically flawless construction of the structure could only have been guaranteed at high additional financial cost. After the winter break, work was resumed in March 2015 with the demolition of an existing run culvert. This was followed by the construction of the remaining uphill foundations and gallery walls of the western blocks 1-4. From this point on, the partial bypass for the central blocks, which had been constructed at the beginning of the project, also had to be dismantled and the traffic already routed through the gallery in one lane. After the traffic had been rerouted into the gallery, the still missing east-side foundations, columns and slope-side walls could be constructed. Within a full closure period of only three weeks in May 2015, all remaining cross beams, parapet elements and floor slabs were lifted into place and the necessary fixtures in the gallery (e.g. drainage pipes, slot channels, frost protection and base layers) were completed. All further concreting, sealing or earthworks (e.g. backfilling of the structure, construction of the guide walls) had to be carried out while maintaining and repeatedly rerouting traffic through the gallery. Subsequently, the final works on the roadway and the debris flow channel, the construction of the retention basin and the installation of the passive protection devices could take place. At the end of November 2015, the new protective gallery was opened to traffic in time for the second winter period. 

 

• Region: Bad Reichenhall, Bavaria
• Tunnel use: Road
• Client: Freistaat Bayer repr. by Staatliches Bauamt Traunstein
• Consultants: Gebauer Ingenieur GmbH, Traunstein; Höllige & Wind, Anger
• Contractors: HABAU Hoch- und Tiefbauges. mbH, Perg(A)
• Total length: 139 m
• Clear width: 9,80 m
• Contract value: 6,1 Mio. Euro
• Construction time: 08/2014 till 11/2015