Rockfall protection embankment as reinforcement

Situation

A good kilometre north of the village of San Giacomo Filippo in the province of Sondrio in Lombardy (Italy), the pilgrimage church of Madonna di Gallivaggio is located between the steep rock faces of the San Giacomo Valley. For many years, isolated rockfalls, massive boulders and large masses of rock have posed a considerable safety risk to the pilgrimage church. In May 2018, there was a huge rockfall with an estimated total volume of around 7,000 m³. A considerable number of boulders, stones and rubble could not be held back by the existing passive protective structures and it severely damaged the pilgrimage church, surrounding buildings and the country road 36. The volume of traffic on country road 36 is high in both summer and winter, not only because of the cultural and religious significance of the pilgrimage church, but also because it runs between Milan and the Swiss border along the scenic Splügen Pass.

Solution

To minimise the future risk of damage from rockfall and landslides, the existing protective structures were replaced with an avalanche barrier reinforced with Fortrac T geogrids. The new soil wall is not only longer, which means that the protected areas are larger, but also higher to catch falling boulders that previously overcame the existing barriers. In addition, the position of the soil embankment has been moved downstream to an area with a lower slope gradient, resulting in a higher retention capacity. The new avalanche barrier consists of a back-anchored reinforced concrete retaining wall in the lower third and a geogrid-reinforced soil embankment, which was constructed with a slope inclination of 70° on both sides in the upper third (see illustration below).  The Fortrac geogrid-reinforced soil embankment was constructed using steel grid formwork. The maximum height of the avalanche protection wall at the front is a good 23 metres high. The stability of the reinforced avalanche protection wall was analysed in accordance with the applicable regulations, for example

• The internal and external stability, as well as the stability in compound failure mode (sliding surfaces that run both inside and outside the reinforced soil embankment) were verified using circular (Bishop) and polygonal (Janbu) sliding surfaces
• Tilting, sliding and ground failure of the soil embankment
• Impact of a single rock body with a diameter of 1.44 metres, a weight of 8,270 kilograms, moving at 98.11 metres/second and a kinetic energy of 39,800 kilojoules

All stability calculations were carried out under both static and seismic conditions.

Advantages

To ensure the safety of the Gallivaggio pilgrimage church and the national road 36, the planners decided to build an avalanche protection dam, which was reinforced with Fortrac T geogrids. Thanks to their long service life of up to 120 years (BBA-certified), they met the high structural requirements. The high flexibility of the Fortrac T geogrids also enabled quick and easy installation on site, allowing the construction company to complete the entire structure ahead of schedule.

• Service life of up to 120 years
• Extreme load-bearing capacity
• High composite flexibility
• Very good adaptability
• Low creep tendency