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/).

Earthquake-induced liquefaction is a relevant natural hazard due to the damages caused in numerous buildings, facilities and infrastructures worldwide. The damages caused to the infrastructure by this phenomenon are caused by the loss of stiffness and strength in granular soils, which leads to settlements and lateral spreading. Earthquake-induced liquefaction typically occurs in saturated deposits composed of non-plastic soils. Hence, the degree of saturation reduction is considered one of the most favourable and optimistic methods for liquefaction resistance mitigation. This paper explores the earthquake-induced liquefaction in saturated and gassy sands, varying their degree of saturation and state parameters. The state parameter was used to analyse the mechanical behaviour by combining the effects of relative density (or initial void ratio) with confinement pressure. Results show that liquefaction resistance improvement caused by the reduction in the degree of saturation is higher as the state parameter increases. This improvement can be described and quantified by multivariate models integrating the effects of degree of saturation and state parameter on liquefaction resistance. This provides a potential solution for improving the resilience of infrastructures susceptible to earthquake-induced liquefaction.

Earthquake-induced liquefaction is a geological disaster that caused extensive damage to buildings, railways, dams. Due to the construction techniques and economic conditions, the subsurface layers of some buildings must be reinforced to resist seismic loads. Microbial-induced desaturation is a development technique which can be used for existing buildings to mitigate liquefaction. Shaking table tests were conducted to survey the effect of microbial induced desaturation on liquefaction-prone foundations beneath buildings. The test results showed that, lower saturation degree delays the generation of excess pore pressure and reduces its magnitude. It appears that the resistance to excess pressure increases as saturation degree is reduced from 100% to 93.4% or 85.6%. Desaturation prevents the decay of the amplitude of acceleration oscillations, but increases the accelerations of the structure. The settlement of the sandy soil decreases as the saturation degree decreases. Resistance to liquefaction increased by more than twice than that in the saturated sample after induced desaturation to 93.4%. The weight of the building structure contributes to the anti-liquefaction capacity.

Electrolysis desaturation is an emerging ground improvement technique with significant potential for widespread application in liquefaction mitigation. This method reduces the saturation of foundation soils, thereby decreasing soil liquefaction potential during earthquake. To date, there is still a lack of systematic research on the microstructure evolution of silica sands during the electrolysis desaturation treatment. In this study, non-destructive low-field nuclear magnetic resonance (NMR) technology was employed to investigate the effects of electrolysis desaturation on the silica sands at the microscale. The results showed that the electrolysis desaturation treatment had negligible effects on the structures of micropores and mesopores. The macropores in fine sand expanded during electrolysis, and the increasing current amplified the extent of this expansion. Conversely, in coarse sand, the macropores contracted during electrolysis. Bubbles generated by high-current electrolysis tend to aggregate, causing cracks and surface uplift in the fine silica sand. For the coarse silica sand, the generated gas accumulates within the existing voids, resulting in an insignificant impact on the soil structure. The electrolysis desaturation treatment primarily facilitated the expulsion of free water in both fine and coarse silica sands. In fine silica sand, employing a high current can reduce saturation more effectively within the same duration, but it also allows for more gas bubbles to escape after resting. Coarse silica sand maintained a high desaturation efficiency due to its greater porosity. This study provides a rational explanation in microscale of the structural impacts of electrolysis desaturation treatment on foundation soils.