European Geologist Journal 61
Multiple landslides in a natural touristic area (Mullerthal, Luxembourg): causes and safety works
by Steve Gruslin1* and Tiffany Hennebaut2
Abstract
The touristic region of Mullerthal is an area at particular risk of landslides. In recent years, the combination of complex geology, steep topography, and heavy rainfall, led to numerous landslides, causing extensive damage. The area is also a nature protection area which limits the options for securing the sites and means that innovative solutions had to be found. The area is characterized by complex geological formations: sandstones, marls, limestones, and clays whose presence strongly influences the risk of landslides by acting as a triggering layer. The article examines the underlying causes of a number of landslides and describes the measures adopted to mitigate them effectively and cost‑efficiently. It highlights the use of both established and innovative stabilization methods, implemented with due consideration for environmental requirements and aimed at ensuring the long‑term stability and resilience of the affected areas.
Keywords
Landslides; touristic region; protected natural area; complex geology; Luxembourg
Cite as: Gruslin, S.& Hennebaut, T. (2026). Multiple landslides in a natural touristic area (Mullerthal, Luxembourg): causes and safety works. European Geologist, (61). https://doi.org/10.5281/zenodo.21875219
Note:
Papers published in this special issue of the European Geologist journal have undergone a thorough peer-review process but have not been copy-edited. Authors bear full responsibility for the linguistic accuracy of their contributions.
This work is licensed under a Creative Commons Attribution 4.0 International License.
1. Introduction
Situated in the east of the Grand Duchy of Luxembourg, the touristic region of Mullerthal, also known as “Luxembourg’s Little Switzerland”, is an area at particular risk of landslides. In recent years, the interplay of challenging geological formations, steep slopes, and heavy rainfall has led to a significant increase in landslide events. These failures have caused substantial damage to infrastructure and, in many cases, left key road links obstructed for months. The area is also designated as a nature protection area (Natura 2000 and drinking water catchment areas), which limits the options for securing the sites and means that innovative solutions had to be foundThe article examines the underlying causes of a number of landslides and describes the measures adopted to mitigate them effectively and cost‑efficiently. It highlights the use of both established and innovative stabilization methods, implemented with due consideration for environmental requirements and aimed at ensuring the long‑term stability and resilience of the affected areas. The document offers five instances, four of which are situated on the CR364 route and one on the CR356 route (see Figure 1).
Figure 1: Mullerthal landscape location.
2. Geological situation
In the projects area, complex geological formations from Triassic and Jurassic periods are present, as shown on Figure 2 [1]. Li2, more commonly known as “Luxembourg Sandstone”, is composed of calcareous sandstones of whitish color alternating with sandstones, of which the calcareous cement is partialy or totally absent, resulting in a characteristic yellowish hue.
Li1 (called “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 clay-rich marls, which exert a significant influence on landslide risk due to their tendency to act as slip surfaces (by acting as a triggering layer). The ko1 (“Rhaetian: Mortinsart Sandstone”) is composed of sandstone, conglomerates and black laminated argilites. Finally, km3 (named “Keuper with compact marnolites”) is formed by a motley marl with thin dolomitic banks that may contain gypsum.
Figure 2: Mullerthal landscape’s geology.
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 or colluvial deposits originate from sandstone weathering and the gravitational displacement of the resulting material downslope
3. Landslides
This chapter examines the underlying causes of the landslides that have occurred in the Mullerthal and outlines the measures implemented to address and stabilise them.
3.1. CR364
The first landslide occurred on the road CR364 between Echternach and Berdorf.
During the investigations, stony–gravelly backfills up to 3 m thick were identified beneath the road as part of the embankment. Below these deposits lies slope scree composed of sand and gravel, underlain by partially weathered layers of the Upper Triassic (Rhaetian), and finally the formations of the Middle Keuper. The Rhaetian layers exhibit high consistency when dry but are extremely sensitive to water. They soften rapidly upon contact with moisture, effectively acting as a “detachment” which, in combination with their dip, can form a potential sliding surface for the overlying, weakly cohesive materials.
Heavy rainfall contributed significant runoff from the hillside above the site, with water reaching the road, flowing across the embankment, and causing substantial surface erosion. Water infiltrating beneath the road rapidly percolated through the permeable soils (backfills and scree) and activated the sliding surface formed by the Rhaetian layers. As a result, the entire embankment and scree mass mobilised, leading to the appearance of tension cracks on the roadway. Borehole investigations estimated that approximately 10,000 m³ of subsurface materials had displaced.
The slope damaged by the landslide was replaced by embankment (“reinforced earth” wall) between 3 and 9 m high constructed with in site won soil and polymeric geogrids [2,3]. Extensive geotechnical studies and calculations have shown that landslides can reoccur in affected areas if only limited near-surface stabilisation measures are taken. To ensure long‑term stability, the complete slip body had to be removed over the full width of the roadway, extending down to the Rhaetian strata in which the original slip surface was located.
From a sustainability perspective, a large proportion of the excavated material was suitable for reuse as backfill, contributing to resource conservation. The excavated stones, boulders, sand, and gravel were processed on site with a crusher to produce a 0/45 mm backfill material, which was subsequently stored temporarily on the roadway outside the failure zone. Through this approach, around 70% of the necessary backfill material could be supplied from recycled site materials, thereby improving the project’s environmental performance.
For the support of the reinforced earth fill sections, a naturally vegetated, steep-slope earth retaining system was adopted. This system enables the construction of steep reinforced slopes ranging from 45° to 70° and offers a robust, settlement‑resistant solution. It comprises steel grid facing elements at the front and horizontally installed geogrids within the backfill, forming a ductile, composite reinforced structure capable of accommodating differential movements.
Based on stability calculations using geotechnical software, the inclination of the elements made of steel grid is 70° and the distance between the layers of the horizontal geogrids is 66 cm. The reinforcement consists of expandable, uniaxially stretched, node-rigid and dimensionally stable geogrids in order to ensure optimal nesting of the backfill material and highlong-term strength. To ensure good grip and sufficient stability of the backfill vis-à-vis the existing slope, steps were made in the natural land, with a slope of 5° outwards to allow good drainage of infiltration water.
The lengths of the geogrid were adapted to the geometry of the supporting body and to the constraints of the road and the land and were between 5 m and 8 m. In total, circa 8,000 m2 of geogrid surface were installed in this section. Quality control of the backfill compaction was verified through plate loading tests carried out on each placed layer.
Figure 3: Stability calculations (left) and Principe_of_stabilisation system (right).
To protect the structure from erosion, a non‑woven protection mat was placed immediately behind the face of the steel grid elements. This layer also facilitates rapid vegetation growth on the exposed surface. As an additional protective measure, natural rock blocks were placed along the Aesbach to provide erosion protection during periods of elevated flow or flooding.
To account for the irregular topography of the site, the reinforced soil structure was constructed to heights varying between 3 m and 9 m. To achieve a visually uniform landscape, berms approximately 2 m wide were incorporated at vertical intervals of about 3 m. Owing to the use of a vegetated reinforced-soil system, the road alignment has blended harmoniously into the surrounding landscape since completion of the works in summer 2019. Figure 4 illustrates the principle and the site conditions before and after the execution of the reinforced soil works in April 2019.
Figure 4: Situation before and after.
The second landslide, which was also stabilised using a reinforced soil structure, is located at the area known as the “Priedegtstull” (“Preacher’s Chair”). The geological conditions in this zone differ from those of the first site, with slope deposits reaching thicknesses of up to 7 m overlying Luxembourg Sandstone, which is initially weathered and becomes intact at greater depth. In this area, intense rainfall significantly eroded the road embankment. Infiltrating water entered the embankment materials, reducing their stability and triggering multiple localised landslides.
Similarly to the Zone 1 works described above, the entire slip mass was excavated, and a reinforced soil structure using a naturally vegetated, steep-slope earth retaining system was constructed.
As in Zone 1, the material excavated from the failure zone was processed on site and reused as backfill. The variable geometry of the valley required adjustments to the construction height, resulting in locally differing wall elevations and the incorporation of 2 m‑wide berms.
In addition to the topography of the valley, the highly variable geometry of the rock that outcropped after the excavation of the landslide zone created an additional constraint. In several places, rock points protruded into the retaining structure, similar to those visible above road level at the “Preacher’s Chair”. Figure 5 shows the state of construction in April 2019. A rock outcrop integrated into the exterior face of the backfill is visible.
Figure 5: Rock outcrop integrated into the stabilization system.
Here too, the retaining structure blends harmoniously into the surrounding landscape, as illustrated in Figure 6.
Figure 6: Situation before and after.
The repair works had to address the underlying causes of the damage. In this case, intense rainfall and the resulting infiltration of water into the embankment—combined with the particular geological and topographical conditions—led to the landslides. It was therefore essential to capture and control surface and subsurface water through appropriate drainage measures and to safely discharge it away from the structure. Two key issues had to be considered: the risk of erosion caused by the river and the risk of run-off and infiltration into the backfill materials.
To address the risk of erosion, the lower sections of the structure were protected with a stone facing to prevent direct contact with flowing water. In addition, riprap was placed within the riverbed to dissipate hydraulic energy and reduce flow velocity, thereby limiting further scouring and erosion.
To manage surface water, drainage measures were implemented to prevent infiltration into the backfills, thereby avoiding softening of sensitive geological layers and reducing the risk of erosion. A drainage ditch was excavated at the base of the retaining structure to allow surface water passing through or around the structure to be safely conveyed away. Additionally, the berms were constructed with a 5% outward slope to ensure effective runoff and prevent water accumulation.
To drain the water originating from the slope, a drainage layer at least 30 cm thick, consisting of 8/16 mm gravel, was installed between the excavation face and the supporting structure. This layer is separated from the natural ground by a geotextile to prevent clogging by fine particles. The collected water is conveyed through a DN 200 drainage system, with discharge provided at two locations via solid pipes running beneath the structure. From there, the water is directed toward the receiving watercourse via constructed drainage ditches (see also Figure 10). Additional reinforced drainage and overflow ditches were installed in other areas where high flow rates are expected during periods of intense rainfall.
The third site is located near the village of Berdorf. In this area, backfill materials and slope scree, with thicknesses of up to approximately 6.4 m, overlie Luxembourg Sandstone. The embankment and scree deposits experienced instability due to significant infiltration of water, which triggered the slope movement.
As the area affected by the landslide was relatively small and bordered by dense woodland, the road was stabilised using 68cm piles installed along the downslope edge and spaced 1 m apart. In addition, the entire drainage pipe system has been replaced and new drainage channels have been created to ensure that water flows into the stream below and to prevent any seepage that could destabilise the embankment in the future.
Figure 7: Situation before and after.
The fourth landslide occurred along the road between Berdorf and Vugelsmillen. This section is located within an older landslide area that had previously been backfilled. The recent failure was triggered by water infiltrating the backfill materials, which reach thicknesses of up to 12 m and overlie either Rhaetian Sandstone or km3 marl, depending on the location. Geotechnical site investigations indicate the presence of a geological fault that brings the Rhaetian Sandstones and the Middle Keuper marls into contact at the same elevation over a very short distance. This fault is likely to influence groundwater pathways and may have contributed to the instability.
A reinforced earth retaining system has been proposed as the preferred stabilisation measure, by excavating the unstable layers down to below the level of the slip surface and, here too, reusing as much of the material from the earthworks as possible. Particular attention was also paid to surface water management to prevent the seepage that causes landslides, by creating drainage trenches on the upstream side of the embankment and ensuring that the water collected in this way is drained away beneath the road via outlet structures that direct the water towards the valley.
Figure 8: Tension cracks on top of the road and the embankment.
3.2. CR356
The landslide occurred on the downstream side slope of road CR356, between the villages of Mullerthal and Waldbillig. The area is characterised by steep slope scree deposits with inclinations of up to approximately 50°, predominantly overlying Luxembourg Sandstone and, in the lower part of the project area, the Li1 Marl Formation. Field observations identified a brittle shear fault as well as several normal faults, which explain the variable depth at which the sandstone bedrock was encountered and the occurrence of marl at elevations where sandstone is present in adjacent zones. The entire area is designated as a protected zone due to the presence of important water sources.
Figure 9: Geological interpretation based on the results of exploratory boreholes.
Multiple stabilisation measures were implemented in this project. The safety factor of the downstream slopes was assessed using the limit‑equilibrium method. To stabilise road CR356, a series of micropiles were installed at alternating inclination angles, connected by a reinforced‑concrete capping beam. In addition, a shotcrete facing with self‑drilling anchors was constructed to stabilise the lower portion of the slope. Approximately 1,000 m³ of material was required to stabilise the slope during the shotcreting phase, following a predefined sequence of works. Sub‑horizontal drains were also installed to improve subsurface drainage.
To meet the stability requirements for natural slopes and to match the existing masonry wall along the road, a system of steel mesh and soil nails was designed to further increase the safety factor. Moreover, naturalistic engineering measures—such as wooden meshes, palisades, and the planting of local pioneer species—were introduced to stabilise the upper metre of the steep slopes by enhancing near‑surface soil cohesion. Figure 7 illustrates the various phases of the stabilisation works in progress.
Regular site supervision was carried out throughout the various phases of the works. Particular attention was given to the installation of the micropiles, the construction of access ramps made from crushed aggregate, and the securing of the slope using steel mesh, among other activities.
During these supervision activities, we also provided guidance on the reuse of excavated sandstone. A transverse fault was identified during the inspections, offering additional insight into the overall landslide mechanism and explaining the presence of marl at higher elevations than initially anticipated.
Figure 9: CR356 safety works in progress. a: vegetation and surface material cleared from the slope to allow access for investigations and subsequent stabilisation works, b: micropiling ongoing works, b: micropiling ongoing works, c: soil nailing, d: tying beam rebar reinforcement connecting heads of micropiles, e: Reinforcement of the slope toe using rock blocks, combined with a soil‑nailed and shotcreted upper section. A crest‑level drainage ditch is provided to intercept and control surface runoff, f: Detail of drainage tube in a soil-nailed and meshed slope, g: general view of the stabilized slope.
4. Conclusions
The roads and steep embankment slopes of the Mullerthal require particular attention due to their distinctive geological and topographical characteristics, as well as the resulting demands on stability and drainage. Several examples have been presented to illustrate how landslides can be addressed safely and cost‑effectively using both conventional and innovative methods. These include reinforced earth systems and bio‑engineering techniques, all implemented with sensitivity to environmental constraints and with the objective of ensuring long‑term slope stability.
Author Contributions: Each author contributed to the preparation of this manuscript through writing, reviewing, and editing. All authors have read and approved the final version.
Acknowledgments: The authors would like to thank Mr. Claude Peschon of the City of Luxembourg and Mr. Marc Ries and Mr. Michel Simon of the Diekirch Roads Division of the “Administration des Ponts & Chaussées” for their trust, as well as Dr Romain Meyer, formerly of the Geological Survey of Luxembourg, now of the Geological Survey of Greenland for his opinions and advice.
Conflicts of Interest: The authors declare no conflict of interest
References
- Service Géologique de l’Etat. Carte géologique détaillé harmonisée aux échelles 1:25.000 et 1 :50.000, 2018.
- Gruslin, S.; Nola, D. Slope protection with an earth retaining system: respectful of natural resources and the landscape. In European Geologist, 2021; Volume 51, pp. 154–196.
- Gruslin, S,; Nola, D. Slope protection respectful of natural resources and landscapes thanks to earth retaining system. Proceedings of the 3rd European Regional Conference of IAEG, Athens, Greece, 6-10 October 2021
This article has been published in European Geologist journal 61 – 5th IPGC Special Edition 2
