Abstract
In order to protect residents in the Laugneri residential area from block slips and landslides, the municipality of Weggis has implemented a novel protective structure on steep terrain as part of its natural hazards master plan. Potential dynamic impacts are absorbed by the reinforced earth protective dam and transferred to the stable rock substrate together with the dam's own weight. A reinforced load-bearing cushion transfers the surface loads to the bored piles sunk into the underlying rock. The latest generation of geogrids made it possible to design this central structural element economically because, despite the wide pile grid, the spacing between the high-tensile longitudinal and transverse reinforcement layers could be kept to a reasonable level.
1. Introduction
In central Switzerland, between Lake Zug, Lake Lauerz and Lake Lucerne, rises the 1,798 m high Rigi, which lies in front of the Alps and consists of subalpine molasse layers. The municipality of Weggis lies between the lake shore and the steep south-western slope of the Rigi, in a region characterised by tectonic thrust faults and various fracture systems. The main Alpine thrust zone is located to the northeast. Due to the morphological and geological disposition, various settlement areas near the lake are exposed to a considerable risk from gravitational natural hazard processes. In accordance with the national concept of integrated risk management, various coordinated action plans were developed and evaluated for each endangered area in Weggis in order to provide a sound basis for decision-making.
In many cases, the hazard posed by falling debris originates from the typical rock bands known as "Nagelfluh". These coarse-grained, continental-fluvial conglomerates are of Palaeogene age and lie in alternating layers with
sandstone and weathering-prone marl of the Lower Freshwater Molasse (USM). In a late phase of Alpine formation, these sedimentary layers were pushed onto younger deposits and tilted from their original horizontal position. The resulting S-SE-directed dip of the subalpine molasse cover layer promotes sliding processes and rockfall, which has led to damage on several occasions. Weathering-prone marl layers, which are interbedded with the coarser-grained rock bands, often play a decisive role in this process.
The Laugneri settlement area was at high risk, primarily due to rockfalls, stone and block falls, but also due to potential landslides. Prevention through technical measures in the form of a protective dam with a rockfall protection net system on the crest was a sustainable and cost-effective solution for this area. The division of tasks in the planning phase made it possible to finalise the protection measures concept and to plan and implement the structures efficiently. The protective dam is part of the overall protection concept for the municipality of Weggis to protect against natural hazards.
2. Reinforced earth dam against rockfall and landslides
The dam is located on a slope with a gradient of 35° – 40°. As the project perimeter of the Laugneri II dam did not originally offer sufficient space to position the necessary fill material and retention area, it was necessary to adjust the terrain surface above and in the dam area. By choosing a slim dam reinforced with geogrids, it was possible to reduce the extent of the terrain incision. In addition, the excavation results in a lower jump height for the expected falling objects, as the newly created surface is relatively smooth. Thanks to the possibility of placing a rockfall protection net on the crest of the dam and anchoring it in the earth-reinforced support system using micro piles, rollovers and stone splinters can be efficiently retained.
3. Formation of the structure
Above the stable marl lies a medium to deep layer of hanging debris. Geotechnically, it is not possible to absorb additional static loads and dynamic effects in this loose rock layer on a steep slope without risking failure of the subsoil. The vertical and horizontal loads are therefore transferred to the rock subsoil at various depths via bored pile discs and a dispersed bored pile grid.
A load-bearing foundation pad is placed above the piles and pile discs. This transfers the surface load of the dam to the large bored piles. The load-bearing foundation pad is a layer of gravel sand reinforced with geogrids lengthwise and crosswise. Highly rigid SYTEC NN geogrids with tensile strengths of up to 1000 kN/m' were used for this purpose. The design was carried out in accordance with the "Recommendations for the design and calculation of earth structures reinforced with geosynthetics – EBGEO" using the model for reinforced earth structures on point or line-shaped load-bearing elements. In addition, horizontal loads, in particular from rockfall and landslides, are transferred via the load foundation pad. For this purpose, the transverse reinforcement of the load foundation pad is permanently anchored to the rock on the mountain side via a concrete bar and anchor system.
The 3.0 m high rockfall protection dam is founded directly on the load foundation cushion. It should have as low a dead weight as possible and still offer the necessary resistance to rockfall (up to 2,000 kJ) and landslides. These requirements can be optimally met with a reinforced earth support system. The SYTEC TerraMur earth-reinforced support system offers a slope angle of 70° on both the valley and mountain sides and is filled with locally sourced, sorted soil material. The design of the earth-reinforced support structure is based on Swiss standards and ONR 24810 "Technical rockfall protection".
4. Design with regard to geosynthetics
Geogrids are a central part of the supporting structure in the artificial structure. Accordingly, the use of geogrids must be planned in detail. In addition to the structurally correct design, great attention must be paid to the professional installation of the geogrids. Construction and installation plans were drawn up for the load-bearing foundation cushion and the earth-reinforced protective dam in close cooperation between the planner and the system supplier. Due to the dam length of 100 m and the slight curvature in the alignment of the protective dam, two joints in the direction of force were necessary in the load-bearing foundation cushion. A dimensioned overlap of the geogrids of several metres ensures a force-fit connection. Such important details must be shown in the installation plans. In addition, the contractor and site management were instructed on site by the system supplier in the specific features of the system design in order to ensure efficient and high-quality implementation on the construction site.
The entire protective structure, with the load-bearing pads described above and the earth-reinforced dam, was completed on schedule and within budget. Due to the durable and robust protective effect and the comparatively simple maintenance of a reinforced earth dam, both in terms of rockfall and landslide hazards, this protective structure, which is unique in Switzerland, proved to be an efficient and economical solution despite the costly foundation. The innovative protection system best met the technical and landscape requirements of the client.
Acknowledgements:
We would like to express our sincere thanks to C. Zahno, O. Fontana (Fellmann Geotechnik GmbH), A. Käslin (Schubiger AG Bauingenieure) and B. Lottenbach (Weggis municipal council) for their constructive co-report.
Project participants:
| Client: | Municipality of Weggis, Municipal Council |
| Senior site management: | Canton of Lucerne, Natural Hazards Department, Kriens |
| Overall planning | Schubiger AG Civil Engineers, |
| and construction management: | Hergiswil Marty Ingenieure AG, Schwändi |
| Geology and | Fellmann Geotechnik GmbH, Lucerne |
| Geotechnics: | Louis Ingenieurgeologie GmbH, Weggis |
| Specialist planner for geosynthetics: | SYTEC Bausysteme AG, NeueneggBBG Bauberatung Geokunststoffe GmbH & Co. KG, Espelkamp |
| Building work: Supplier of support systems | ARGE Gasser Felstechnik AG, Lungern Josef Küttel AG, Weggis
Brun AG, Emmenbrücke |
| and greening:5. Sources | SYTEC Bausysteme AG, Neuenegg |
GRAF, R. et al. (2016): Risk Management in the municipality of Weggis, 13th Congress INTERPRAEVENT 2016 – Lucerne, Switzerland, Excursion guide Weggis,
EX1 National Platform Natural Hazards PLANAT; www.planat.ch
Illustrations of the project:
Fig. 1: Top view of the load-bearing foundation cushion above the concrete bar system and the bored pile heads during installation (source: SYTEC Bausysteme AG)
Fig. 2: View from the mountain side during the construction of the reinforced dam body
(Source: SYTEC Bausysteme AG)
Fig. 3: Completed Laugneri II protective dam with secured mountain-side terrain cut, October 2018;
In the background, the bedload collector with outlet structure and the Laugneri I protective dam (source: SYTEC Bausysteme AG)
Fig. 4: Normal section of the Laugneri II protective dam (Source: SYTEC Bausysteme AG)
This article was first published in 2019 at the Naue Colloquium.
Geotechnical classification
- Mountain community Weggis between Lake Lucerne and the steep south-western slope of the Rigi, where residential areas at the foot of the slope are exposed to gravitational natural hazards such as rockfall and landslides.
- Unfavourable geological conditions with alternating sandstone and weathered marlstone layers, tilted and weakened by Alpine tectonics, promoting block detachments and sliding along discontinuities.
- Limited space between existing development and the steep hillside, so conventional wide rockfall embankments and large catch areas cannot be realised without major earthworks and land take.
- Innovative earthworks solution using geogrid-reinforced soil structures and a load transfer platform on piles to form a slim rockfall protection embankment that can safely carry vertical and horizontal impact loads.
- Design objectives: reliable impact energy absorption, long-term slope and structure stability, controlled deformation behaviour and good integration into the landscape while meeting local hazard mitigation requirements.
Typical project questions
- How can settlements and infrastructure at the toe of steep Alpine slopes be protected against rockfall and landslides where space is restricted and conventional embankments or galleries are not feasible?
- Which design approaches and reinforcement layouts are required for geogrid-reinforced embankments and load transfer platforms to safely transfer both vertical loads and dynamic impact forces into deeper, stable rock?
- How can geosynthetic earthworks be combined with structural rockfall barriers, anchors and long-term monitoring to achieve a robust, low-maintenance risk reduction concept for gravitational natural hazards?
Suitable Naue products & system solutions
A project-specific system selection enables protective structures to be implemented efficiently, durably, and tailored to local conditions.
Reinforced soil embankments for rockfall protection
- Secugrid® Geogrids – primary tensile reinforcement in steep, geogrid-reinforced rockfall embankments, increasing shear resistance, limiting deformations and contributing to the energy absorption capacity of the soil structure.
- Secugrid® m3 systems (reinforced soil) – system solution for reinforced soil walls and berms that allows slim rockfall protection structures to be constructed with defined facing and internal stability, even on steep existing slopes.
Load transfer platforms on loose or fractured rock
- Secugrid® Geogrids – high-strength geogrids arranged longitudinally and transversely in gravelly sand layers to form load transfer platforms above piles or weak rock, distributing stresses and bridging heterogeneous foundation conditions.
- Secutex® nonwoven geotextile – separation and filtration layer between foundation soils and granular LTP or embankment fills, maintaining bearing capacity by preventing fines migration and ensuring durable drainage behaviour.
Drainage and pore water pressure control in earthworks
- Secudrain® drainage mats – planar geocomposites that collect and drain seepage water within or behind reinforced soil structures, reducing pore water pressures and improving stability under changing groundwater and rainfall conditions.
- Secutex® nonwoven geotextile – filter component in drainage cross-sections, ensuring long-term filter stability between soils and drainage cores and preventing clogging of the drainage system.
Surface protection and erosion control on steep slopes
- Secumat® erosion control systems – erosion control mats installed on the face of reinforced embankments and slopes to protect against rainfall and surface runoff erosion while supporting vegetation establishment.
- Combigrid® Geocomposite – reinforcement, separation and filtration in thin cover soils on steep slopes or access benches, improving stability and bearing capacity while reducing the risk of surface instabilities.
More Naue solutions and geosynthetics
Secugrid® Geogrids
Secugrid® m3 systems (reinforced soil)
Combigrid® Geocomposite
Secutex® nonwoven geotextile
Secudrain® drainage mats
Secumat® erosion control systems