The deep geological repository for radioactive waste in Switzerland will be embedded in an approximately 100 m thick layer of Opalinus Clay. The emplacement drifts for high-level waste (approximately 3.5 m diameter) are planned to be excavated with a shielded tunnel boring machine (TBM) and supported by a segmental lining. At the repository depth of 900 m in the designated siting region Nordlich Lagern, squeezing conditions may be encountered due to the rock strength and the high hydrostatic pressure (90 bar). This paper presents a detailed assessment of the shield jamming and lining overstressing hazards, considering a stiff lining (resistance principle) and a deformable lining (yielding principle), and proposes conceptual design solutions. The assessment is based on three-dimensional transient hydromechanical simulations, which additionally consider the effects of ground anisotropy and the desaturation that may occur under negative pore pressures generated during the drift excavation. By addressing these design issues, the paper takes the opportunity to analyse some more fundamental aspects related to the influences of anisotropy and desaturation on the development of rock convergences and pressures over time, and their markedly different effects on the two lining systems. The results demonstrate that, regardless of these effects, shield jamming can be avoided with a moderate TBM overcut, however overstressing of a stiff lining may be critical depending on whether the ground desaturates. This uncertainty is eliminated using a deformable system with reasonable dimensions of yielding elements, which can also accommodate thermal strains generated due to the high temperature of the disposal canisters. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Layers of volcanic ash and Andosol soils derived from the ash may play an important role in preserving snow and ice as well as in the development of permafrost conditions in (a) the immediate vicinity of volcanoes at high elevations or at high latitudes and (b) land areas that are often distant from volcanic activity and are either prone to permafrost or covered by snow and ice, but have been affected by subaerial ash deposition. The special properties of volcanic ash are critically reviewed, particularly in relation to recent research in Kamchatka in the Far East of Russia. Of special importance are the thermal properties, the unfrozen water contents of ash layers, and the rate of volcanic glass weathering. Weathering of volcanic glass results in the development of amorphous clay minerals (e.g. allophane, opal, palagonite), but occurs at a much slower rate under cold compared to warm climate conditions. Existing data reveal (1) a strong correlation between the thermal conductivity, the water/ice content, and the mineralogy of the weathered part of the volcanic ash, (2) that an increase in the amounts of amorphous clay minerals (allophane, palagonite) increases the proportion of unfrozen water and decreases the thermal conductivity, and (3) that amorphous silica does not alter to halloysite or other clay minerals, even in the Early Pleistocene age (Kamchatka) volcanic ashes or in the Miocene and Pliocene deposits of Antarctica due to the cold temperatures. The significance of these findings are discussed in relation to past climate reconstruction and the influence of volcanic ash on permafrost aggradation and degradation, snow and ice ablation, and the development of glaciers.