Relation between hydrogeological setting and swelling potential of clay-sulfate rocks in tunneling

In this study, an approach to estimate the swelling potential of clay-sulfate rocks in tunneling is presented. Swelling of clay-sulfate rocks leads to damage in tunnels that is difficult and costly to repair. Swelling is caused by the transformation of the sulfate mineral anhydrite into gypsum, which involves an increase in rock volume in a system open to water flow. Knowledge of the hydrogeological situation and the groundwater flow systems at the tunnel is essential to better understand the swelling processes. The present study was conducted for the Chienberg tunnel in Switzerland. It investigates the hydrogeological situation of four zones in this tunnel crossing the Triassic Gipskeuper formation. In two of them, heavy swelling occurred after tunnel excavation, while in the other two no swelling occurred. In addition, the groundwater flow systems before and after tunnel excavation are investigated based on numerical flow modeling. The findings suggest that in certain situations after tunnel excavation, depending on geological and changing hydraulic conditions, the excavation damaged zone around the tunnel provides a “hydraulic short circuit” between the weathered Gipskeuper and the anhydrite-bearing strata of the unweathered Gipskeuper. As a result, water from the weathered Gipskeuper gets in contact with anhydrite, triggering its transformation into gypsum and, thus, rock swelling. The results of the study may also contribute to improved swelling experiments in the laboratory and a more reliable planning of restoration measures in tunnels that are damaged by rock swelling.

► Tunnels can provide a “hydraulic short circuit” between aquifers and clay-sulfate rocks that triggers rock swelling.
► Aquifer–clay-sulfate rock linkage by tunnels depends on the geological setting and hydraulic changes induced by tunneling.
► Geological analysis combined with groundwater modeling is able to assess the swelling risk of clay-sulfate rocks.