During the night of 31 May to 1 June 2018, several parts of the Grand Duchy of Luxembourg were severely affected by violent thunderstorms, which were sometimes accompanied by floods. This was particularly the case for the Mullerthal region, nicknamed « Luxembourg’s Little Switzerland », which is a popular tourist and hiking destination. The road linking Echternach to Berdorf and Beaufort (CR364), which runs along the Aesbach Valley on the south side, was particularly affected (Figure 1).

Due to heavy rains, with rainfall of up to 90 litres per square metre, several land slides occurred. In addition, the Aesbach flood caused considerable damage to the embankments and protective structures of the roadway, consisting of retaining walls, gabion walls and backfills. Many buildings were also severely affected by the floods, and several bridges and walkways were completely destroyed. Given the geology and the topography of the area, the occurrence of small landslides is not uncommon in the region, but never with the magnitude of these which occurred due to the bad weather, so the road had to be closed immediately.

The damage that occurred and the corrective measures implemented are described below for two areas in particular, but many other areas required emergency measures to be put into place. In addition to the measures described below, a large number of renovation measures such as retaining supports, installation of drainage systems, etc. were carried out on the entire road, but they are not the subject of this article.

The geotechnical studies and the planning of the measures allowing the rehabilitation of a portion of approximately 1.1 km of the CR364 road were carried out by the design office Géoconseils S.A. on behalf of the “Administration des Ponts & Chaussées du Luxembourg”.

Geological situation

According to the geological maps of Luxembourg (Figure 2), the section of the road concerned is located on sandstones from the Lower Liassic (Jurassic), indicated as “li2” on the maps. Due to the presence of the river, this geological layer has been eroded in places, giving outcrop or approaching underlying geological layers such as marls and limestones (Lower Liassic: li1), clay and clayey marls (Upper Keuper: ko2), sandstones and argillites (Upper Keuper: ko1) and variegated marls (Middle Keuper: km3).

• The li2 (more commonly known as « Luxembourg Sandstone ») is composed of calcareous sandstones of whitish color alternating with sandstones, of which the calcareous cement is less present or totally absent, of yellowish color.
• The li1 (« Elvange Marls »), is made up of alternating dark gray marls and partially sandy limestone beds.
• The ko2 (« Rhaetian: Argile de Levallois – Levallois Clay ») are clays and red clayey marls, the presence of which strongly influences the risk of landslides (by acting as a soap layer).
• The ko1 (« Rhaetian: Mortinsart Sandstone ») is composed of sandstones, conglomerates and black laminated argilites.
• Finally, the km3 (“Keuper with compact marnolites”) is made up of motley marls with thin dolomitic beds that may contain gypsum.

Due to the topography and the steep slopes, all these geological formations are covered by non-cohesive slope screes, composed of sandstone boulders in a sandy-silty matrix. These screes are the result of the weathering of the sandstone and the placement by gravity of the weathering materials along the slopes.

Zone 1

Damage and its causes

Damaged area 1 is located in the eastern part of the affected site. At this point, the Aesbach River is in the immediate vicinity of the CR364 road.

This area suffered a significant slip of the slope under the road (Figure 3a), causing it to crack. In addition, a bridge on the hiking trail over the Aesbach was completely destroyed. The lower part of the slope also shows strong traces of scour and erosion due to the force of the stream.

West of the site the slope was stabilised by gabions. Because of the speed and increased density of the stream due to sediment, floating debris and boulders that were transported, the gabion wall present over part of the area was undermined and considerably damaged (Figure 3b). The lower layer, which forms the basis of the entire retaining structure, was particularly affected. The road was affected by numerous longitudinal cracks due to the loss of wall integrity and landslides.

Geological and geotechnical surveys

For the geotechnical survey, three reconnaissance boreholes of 11 m, 12 m and 15 m depth, respectively, were carried out. To determine the geotechnical characteristics of the various layers required for geotechnical calculations and sizing, laboratory tests were carried out on representative core samples: classification tests (water content, Atterberg, particle size analysis) and uniaxial compressive strength tests.

The geotechnical investigations carried out encountered stony and gravelly backfills under the road over thicknesses of up to 3 m. Below there is slope scree consisting of sand and gravel with encrusted and partially altered layers of the Upper Triassic (Rhaetian) and the layers of the Middle Keuper. Rhaetian layers have high consistency in the dry state, but they are very sensitive to water. They soften very quickly on contact with water, thus playing the role of a « soap layer » which, in combination with the inclination of the layer, can constitute a potential sliding surface for overlying materials that are not very cohesive.

The water masses resulting from the heavy rains coming from the hill above the site reached the road, ran off on the embankment and caused significant erosion on the surface. The water which penetrated under the road quickly crossed the permeable layers (backfills and scree) and mobilised the sliding surface constituted by the Rhaetian layers, so that all the embankments and scree slipped, which led to the appearance of cracks in the roadway. Reconnaissance boreholes allowed us to estimate that around 10,000 m3 of underground materials had slipped. Figure 4 shows the cause of the damages, interpreted using the results of the exploration boreholes and the position of the cracks (GEOCONSEILS S.A., 27/09/2018; 2019).

Corrective actions undertaken

In order to find a sustainable solution and to protect the road against future heavy rains, the damaged existing embankment, which already had very steep slopes, was replaced by a steep embankment made of natural materials reinforced by geogrids that fits perfectly into the landscape.

Extensive geotechnical studies and calculations have shown that landslides can reoccur in affected areas if only limited corrective measures are put in place near the surface. To avoid long-term instabilities, it was necessary to excavate the entire body of the slip over the entire width of the roadway and down to the layers of Rhaetian at risk of slipping (Figure 5). The curved path of the Aesbach created additional requirements by limiting the space available for the basis of the reinforced earth construction

From a sustainability standpoint, most of the excavated material could be reused for backfilling in a way that saved resources. In this context, the excavated material, which consisted of stones, boulders, sand and gravel, was prepared on site with a crusher to produce backfill material with a grain size of 0/45 mm, which was temporarily stored on the road outside the damaged area. In this way, about 70% of the required backfill material could be obtained, which also had a positive effect on the ecological balance.

To provide the support structure for the earth retaining system fill sections, the steep slope system TensarTech GreenSlope was chosen, which allows slopes between 45° and 70° to be created. This system chosen for reinforcement is an adaptable, settlement insensitive securing system made up of steel grid elements in the front and horizontally laid geogrids, so that the backfill material results in a composite load-bearing body with a high degree of ductility.