Mont Terri Project: Cyclic deformations in the Opalinus Clay

Shrinkage joints parallel to the bedding planes are frequently observed in clay rocks after excavation of tunnels and niches. Their long term behaviour as well as the interconnectedness of such joints with other structures related to the excavation disturbed zone are investigated in this study for a better understanding of the near field of a future repository. Shrinkage joints were also observed in the Opalinus Clay at the Mont Terri Rock Laboratory and are studied in the frame of the Cyclic Deformation experiment (CD). Tunnel climate and joint displacement have been monitored for more than four years and revealed deformation cycles of the joints with joint opening in winter, when relative humidity in the tunnel decreases to 65% and closing in summer at 100% saturation of tunnel air. Statistical analysis of the climate and displacement time series revealed annual and daily fluctuations of temperature and humidity. The same fluctuations can be seen in the displacement data with a delay of only several hours to the climate changes. Shrinkage joints build an interconnected network with the unloading joints of the stress induced excavation disturbed zone and link the latter to the tunnel surface. The article presents the field data and statistical analysis and provides a first hypothesis and conceptual models for joint formation and propagation. (authors)