Geotechnical challenges in the face of climate change

1. Introduction

Switzerland is a country of modest size but with very diverse geography and a high population density on a plateau alternating between cities, lakes and countryside, caught between the Jura and the Alps. From the beginning, it has had to face many natural risks, increased by the development of its built heritage. In the future, climate change will only reinforce the existing risks by multiplying the frequency and severity of extreme events. Geotechnicians will be on the front line of risk analysis and management in the natural or built environment.

2. Geotechnical developments and land use planning

Since its creation in 1955, the Swiss Society for Soil Mechanics and Foundation Works, renamed “Suisse Geotechnique” in 2010, has promoted the advancement of knowledge in its field through meetings bringing together theorists and practitioners in a biannual framework of conferences associated with visits to construction sites and works. The analysis of the 180 publications resulting from its activities shows an excellent adequacy between the progress of knowledge and the major stages of construction of transportation infrastructure works, energy exploitation facilities, as well as buildings in urban areas. It can also be observed that environmental concerns have always been present, directly, or at least implicitly, in the projects and the execution of works impacting the soils on the surface and at depth, or directly affecting the underground space.

2.1 Period from 1950 to 1970

For Switzerland, this period can be characterized by the diversification of the techniques of investigation and of soil and rock improvement in relation with two major axes of development: (i) hydroelectric schemes in the Alps and (ii) the creation of the national road network throughout the country. Geotechnical investigation campaigns for dams have progressed thanks to new collaborations between civil engineers and geologists, hydrogeologists and geophysicists. One of the pioneers was Professor Maurice Lugeon of the University of Lausanne, whose contribution to the creation of the new discipline of engineering geology is still recognized worldwide. Permeability tests in rock drilling are still used with their criterion in Lugeon Units defining the necessity of injection of cracked rocks to limit the leakage of the reservoir under the foundations of dams.

Figure 1 Maurice Lugeon (in Barrages et géologie, 1933)

For earth and rockfill dams, as well as for large highway embankments, extensive laboratory and in situ research, in conjunction with specialized firms, has led to the development of compaction, stabilization, injection, and freezing techniques under a wide variety of soil conditions.

The construction of shallow and deep tunnels has expanded considerably. The technology of anchoring in soft ground and in rock, with and without prestressing, has become essential in foundation and underground works as well as for large excavations in order to limit the deformations of structures which is often unavoidable.

2.2. Period from 1970 to 1990

The period from the 70’s to the 90’s corresponds in particular to the development of analytical or numerical methods of calculation to check the stability of the structures and the induced deformations during their execution in a built environment, with new characterization techniques such as that of auscultation and of other in situ controls. The classical methods of dimensioning foundations and retaining structures by verifying separately the stability at failure and the deformations by simplified approaches of elastic or pseudo-linear bidimensional models, set the basis for the design and the execution of good quality projects. However, the requirements for the control during construction and for the protection of existing structures in the vicinity of the construction sites have increased continuously since then. At the same time, computer-enabled numerical calculation methods offered new possibilities to study the global soil-structure behavior in more complex geometrical configurations close to reality. The condition is to find the right characteristic parameters of the soils and to be able to follow the behavior of the structures during the construction phase.

The mechanics of soils and discontinuous rock masses have progressed by improving the laws of mechanical behavior, the coupling with underground flows, the effects of time, temperature and dynamic stresses. New instruments for borehole measurements have been developed and used in real time thanks to the revolutions in microtechnology and real-time and continuous data acquisition.

Figure 2 With its length of 3150 m, the Beckenried Viaduct is one of the longest bridges in the Swiss road network.

2.3. Period from 1990-2010

The period around the turn of the century is about taking the natural and built environment more into account in the planning, design and execution of structures, the recycling of materials, risk analysis and their control over time.

New developments are taking place in the field of geothermal energy, frost effects, cliff stabilization and protection against falling rocks. It is interesting to note that the geotechnical achievements of the previous periods have allowed a better understanding and analysis of the geological and climatic events that have marked the recent evolution of the Swiss territory, in particular in the Jura and the Alps:

– slow, creeping landslides (La Frasse VD, Lumnez GR)

– rapid landslides with sudden onset (Belmont VD, Randa VS)

– cliff instabilities and rockfalls (Rodel NE, Les Créteaux VS)

– torrential debacles (Illgraben VS)

– Liquefaction due to melting of permafrost

– Accidents caused by open-air or underground excavation works, in case of unpredictable or unforeseen natural situations (fault zones, reactivation of old landslides, extreme groundwater flow, etc.)

Figure 3 La Frasse 3D geological model. View from S-E. (Commend, S., Geiser, F. and Tacher, L., 2004, August. 3D numerical modeling of a landslide in Switzerland. In Proceedings of the International Symposium on Numerical Models in Geomechanics NUMOG IX, Ottawa (pp. 595-601).

2.4. Current period

The current period attests to the progress made in risk analysis, interdisciplinary studies and probabilistic methods of decision support, to improve the design of structures under complex or unfavorable geotechnical conditions.

The topics addressed in the last twenty years concern areas with high environmental risks:

– swelling problems in geotechnics (tunnels in clay and gypsum rocks)

– construction on polluted sites

– creep phenomena in slopes

– design and dimensioning of structures for earthquakes

– flood protection and geotechnical works

– karst problems

The active use of geothermal resources by heat transfer between geostructures and the ground is a promising source of energy savings. Pile foundations, widely used in low bearing capacity soils, are the best example. Tests of energy piles, completed by thermomechanical studies and the development of a specific computer tool (Thermo-Pile) allow the dimensioning of exchange piles.

Figure 4 EPFL-Soil mechanics laboratory. Study of the stresses in pile systems (with respect to the pile structural resistance) and the deformations, for given thermal loadings

Finally, the current management of the territory requires the consideration of the whole life cycle of the infrastructures: planning, realization, operation-maintenance and deconstruction. This last point will become more important in the future.

3. Conclusion and challenges for the future

The environmental challenges of global warming will concern geotechnical engineers, continuing the interdisciplinary activities that have guided them over the past decades. Among climate, hydrological, and environmental scientists, it may well be up to civil engineers, and geotechnical engineers in particular, to find concrete constructive solutions to safeguard the climate and biodiversity of our planet.