Interactions Between Tunnels and Unstable Slopes: Role of Excavation

International audienceBack 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

New methodology for estimating the shear strength of layering in slate by using the Brazilian test

A new method is proposed in order to estimate the shear strength of schistosity planes in slate in terms of Mohr-Coulomb cohesion and internal friction angle. The procedure consists in carrying out the Brazilian method under different loading-foliation angles, for which experimental tests were achieved in slates from the northwest of the Iberian Peninsula (Spain). The experimental fracture patterns were analytically studied and justified by simulating the stress field in the discontinuity planes contained in the whole sample, taking into account the first failure registered in the tests. By combining experimental and analytical studies and a procedure based on the representation of the threshold state of stressesin the elastic regimein the failure plane, it is possible to estimate the foliation's strength envelope through a lineal adjustment according to the Mohr-Coulomb criterion and, thus, to characterize the layering. Finally, the proposed procedure was validated by the direct shear test. The cohesion and the internal friction angle obtained with this convenctional test were very close to that calculated by the proposed method, verifying the methodology developed by the authors. This procedure may be interesting in various engineering applications, either in the study of the properties of cleavage in slate, which is commonly used as an industrial rock, or in dam foundations, underground excavations and slope engineering, since one of the main failures in civil engineering is due to sliding along weak planes.The authors of this paper would like to acknowledge the financial support of the PhD fellowship Severo Ochoa Program of the Government of the Principality of Asturias (PA-14-PF-BP14-067). Also, the authors are grateful to editors and reviewers for their suggestions and help us to improve this manuscript