1. Introduction
Many lakes in Switzerland are situated in the foothills of the Alps. Since the last glacial period, several layers of fine glacial detritus have settled. Among these is a layer of grey coloured lacustrine chalk, which is very loose and has extremely poor mechanical properties. This layer results from the segregation of loose limestone (CaCO2) particles on the lake floor. Such layers are typical for lakes located at a confluence of limestone mountain catchments. During the 19th and 20th centuries, several lakeshore towns started land reclamation projects to provide better access to the lakes and create new urban spaces designed to meet the needs of a fast-growing population. In the context of these projects, shore fillings were undertaken to accommodate public squares, roads, railways and housing. Building a motorway net in Switzerland required fillings along lakeshores in some areas between 1965 and 1990.
Land reclamations on very soft soil, such as lacustrine chalk, led to several massive landslides claiming many casualties and causing severe damage. Therefore, there was an urgent need to understand better the behaviour of such soils. In his PhD, Prof. Jachen Huder analysed several case histories and performed large scale field tests. He also developed theories for a rheological model of lacustrine chalk, alongside solutions on how to build on such sensitive soil types.
2. Well-known shore landslides in Switzerland caused by fillings
Such landslides have occurred on the shores of several Swiss lakes:
- Lake Zurich: in the surroundings of the city of Zurich: filling for Quai bridge in 1883, Riesbach shore in 1885, and at Horgen railway station in 1875
- Lake Zug: landslides in the suburb of Zug in 1435, and again in 1594, well-documented on 5 July 1887
- Lake Geneva, on the Vevey shore in 1877, and again on 24 March 1933
- Lake Neuchatel, north-east of the city of Neuchatel, due to filling works needed for the construction of the A5 motorway between 1965 and 1980
- At several places on the shores of Lake Bienne when fillings were performed for the construction of the A5 motorway
The town of Zug experienced two shoreline collapses in the past:
- 4 March 1435, „Niedere Gasse“ area, approx. 60 dead, 26 buildings collapsed
- 5 July 1887, „Vorstadt“ area, 11 dead, 16 residential buildings collapsed
A detailed expert report describes and analyzes the shoreline collapse of 5 July 1887 (Prof. Dr. A. Heim, Chief Engineer R. Moser, Dr. A. Bürkli-Ziegler, [12]):
„The shoreline collapse is said to have been caused by a combination of various structural measures (a quay wall resting on 450 wooden piles using an unreinforced concrete beam, an unusual method for the conditions prevailing at the time) and meteorological factors. According to the experts, the shoreline failure was mainly due to the fact that a large part of the lakeshore had been in a permanent state of unstable equilibrium for a long time.
This results from shore fillings that were performed over a long period on top of a soft, thick layer of silty sand, partly mixed with lacustrine chalk. In today’s terms, the silty sand would be described as light clayey, silty fine sand, with a soft bedding at the top that becomes increasingly dense with depth. The sliding surface is thought to be on more densely bedded areas of the silty sand layer. Experts estimated the sliding body to be about 9 m thick and up to 1,000 m long. The sliding surface has a slope of approximately 4%. Even experts were astonished at the large size of the fractured body and the low inclination of the sliding surface.„
3. The problem with sensitive soils on lakeshores containing very sensitive silt
The issue of shore fillings in lakes has already been known for a long time. As the planning of the Swiss motorway net began, the Versuchsanstalt für Wasserbau und Erdbau (VAWE) at ETH Zurich started research on the behaviour of lacustrine chalk. Prof. Jachen Huder (16.08.1922 – 10.12.2008), who lectured at ETH Zurich, performed large scale field tests and several triaxial tests to characterise the properties of lacustrine chalk.
At the end of the 1950s, the results of laboratory tests performed on lacustrine chalk samples were under criticism because the recalculated shear strength values for lakeshore landslides of structurally sensitive soils, in general, did not correspond to earthworks lab results. The values obtained after recalculating were much lower than the lab values determined with the same type of soil.
Between 1957 and 1963 Prof. Jachen Huder – then a research assistant under Prof. Gerold Schnitter, head of VAWE (today’s Institute of Geotechnical Engineering – ETH) – addressed this issue. As the final result of his investigations and experiments, he was able to show that the representation of the course of triaxial tests using a vector curve as introduced by Casagrande (1953) could be developed into an evaluation method. This method made it possible to demonstrate that field and laboratory determination of shear strength could fully agree.
In the bulletin issued by Versuchsanstalt für Wasserbau und Erdbau (VAWE), No 58 (1963) [1], Prof. Huder provided a detailed description and a subsequent assessment of the findings and the procedure.
Large scale tests directed in Tüscherz on Lake Biel by Prof. R. Haefeli from ETH Zurich revealed that it was possible to significantly increase the undrained shear strength of lacustrine chalk by carefully pouring the filling material in stages [9].
The problem once again gained importance when filling works were being planned on the shore of Lake Bienne between Twann and Ligerz in 1970. The Swiss Railway Company SBB intends to build a new tunnel in 2023 on the shore of this lake. Fillings are necessary for the portal section on the east side of the planned tunnel.
4. References
[1] Huder, J. 1963. „Bestimmung der Scherfestigkeit strukturempfindlicher Böden, unter besonderer Berücksichtigung der Seekreide“, Mitteilungen der Versuchsanstalt für Wasserbau und Erdbau an der ETH Zürich, Nr. 58, überreicht durch die Schweizerische Gesellschaft für Bodenmechanik und Fundationstechnik, Mitteilung Nr. 41 (1963) (in German) [https://doi.org/10.3929/ethz-a-000103446]
[2] Schindler, C. and Gautschi, M.A. 1972. „Geotechnische Untersuchungen für die Erweiterung des Utoquai in Zürich; Probleme in einem labilen Ufergebiet und ihre Lösung“, Mitteilungen der Schw. Gesellschaft für Boden- und Felsmechanik, Nr. 85, Francis de Quervain zu seinem 70. Geburtstag, 24. August 1972, pp. 12-19 (in German)
[https://geotechnikschweiz.ch.vtxhosting.ch/wp-content/uploads/2017/04/Heft85.pdf]
[3] Schindler, C. 1976. „Eine geologische Karte des Zürichsees und ihre Deutung“, Eclogae Geologicae Helvetiae, Bd. 69 (1976), pp. 125-138 (in German)
[4] Jaeckli, H., 1989. „Geologie von Zürich; Von der Entstehung der Landschaft bis zum Eingriff des Menschen“, pp. 191-193, Haefeli (in German)
[5] Neue Zürcher Zeitung, 2012, „Vom See verschlungen“ In Horgen, am rechten Zürichseeufer: Abgleiten des Bahnhofgleisfeldes in den Zürichsee, 1875 (in German)
[6] Schindler, C. and Gyger, M., 1989. „The landslide of Zug seen 100 years after the analysis of Albert Heim“, A.A.BALKEMA/Rotterdam/Brookfield/1989; Editor Bonnard, C., Ecole Polytechnique Fédérale de Lausanne, Suisse, pp. 123-126
[https://geotechnikschweiz.ch.vtxhosting.ch/wp-content/uploads/2017/04/Heft117.pdf]
[7] Amrein, M., 2012. “Wenn eine Stadt im See versinkt“, Neue Zürcher Zeitung
[8] Feuille d’Avis de Lausanne, 25.03.2016, „Le 24 mars 1933: le quai de Vevey a disparu dans le lac“, (in French) [http://www.24heures.ch/vivre/histoire/24-mars-1933-quai-vevey-disparu-lac/story/20411826]
[9] Zeindler, H., 1967. „Zur Bestimmung und Verbesserung der Scherfestigkeit einiger Materialien“, Bielersee, Alfermé/Tüscherz, Mitteilungen der Schw. Gesellschaft für Boden- und Felsmechanik, Nr. 69, Frühjahrstagung 12. /13. Mai 1967 in Lausanne (in German)
[10] Zeindler, H. 1972. Remblai d’essai pour la N5 dans la baie d’Auvernier, Lac de Neuchatel, Separatdruck aus „Strasse und Verkehr“ No 4/1972, p. 5 (in French)
[https://geotechnikschweiz.ch.vtxhosting.ch/wp-content/uploads/2017/04/Heft83.pdf]
[11] Zeindler, H. and Jacobsen, H. J. A. „Die Seekreide am Bielersee“, Schweizerische Bauzeitung, 95. Jg., Heft 23, 09.06.1977 (in German)
[https://www.e-periodica.ch/cntmng?pid=sbz-002:1977:95::314]
[12] Heim, A., Moser,R., Bürkli-Ziegler, A., Die Catastrophe von Zug, 5. Juli 1887, Gutachten der Experten, Zürich, Verlag von Hofer & Burger, 1888