Roger Cojean

Interactions Between Tunnels and Unstable Slopes: Role of Excavation

Back analysis of old tunnels in an unstable slope context leads to consider several situations of tunnel entering a slope: cross-cutting tunnel to slope, sloped tunnel, or shallow tunnel following the slope. Identified pathologies of structures vary depending on these situations. This research focuses on different slope mechanisms of instability, especially on creep processes and on damages affecting the structure of tunnels parallel to the slopes. The analysis of the Geotechnical Influenced Zone of these tunnel structures in slope context is a significant point of this study. Through numerical simulations, the different aging and damage processes related to structures have been taken into account. Factors associated with geological and geomorphogical conditions of the slope along with their changes over time can be distinguished from factors corresponding to tunnel’s excavations and construction methods. The influence of the first factors on the kinetics of movements and on the emergence of pathologies on structures can be revealed through the study of different geological, geomorphogical, hydrological and seismic contexts. This study focuses on the impact of building methods’ parameters (excavation, reinforcement structures, supports and linings). The excavation of a tunnel leads to a slope destabilization due to a more or less important deconfinement of the surrounding massif. Construction techniques influence, on one hand, slope destabilization with the development of shear strips and, on the other hand, structural damages in the tunnel lining. The relative influence of the deconfinement level when setting out the lining, the morphology of the transverse section of the tunnel, its dimensions and the characteristics of the lining structure are studied through several parametric studies. As a conclusion, lessons are drawn about analysis methodologies of tunnel and the corresponding monitoring.

The 2014 Hans Cloos lecture: Engineering Geology—some feedback regarding the practice of a scientific and technical discipline

This Hans Cloos lecture paper develops general reflections inspired by Ian McHarg’s (1969) book, “Design with Nature” and concerning engineering geology. Three case studies are illustrated: landslides in reservoirs, gallery excavation in a shield volcano, large rockslides and rock avalanches. For these three case studies, the potential and limits of conceptual and numerical models are discussed.RésuméCet article reprenant la Conférence Hans Cloos développe des réflexions générales sur la géologie de l’ingénieur, inspirées par le livre de Ian McHarg «Design with Nature». Trois cas d’étude sont illustrés: glissements dans les retenues de barrage, creusement d’une galerie dans un volcan bouclier, grands éboulements rocheux et avalanches rocheuses. Pour ces trois cas d’étude, les potentialités et les limites des modèles conceptuels et numériques sont discutées.