Impact of tunneling on regional groundwater flow and implications for swelling of clay–sulfate rocks

Tunnels play a key role in many transportation concepts. The swelling of clay–sulfate rocks leads to serious damage to many tunnels crossing such rock, producing great difficulties and high extra costs in tunnel engineering. The swelling is caused by the transformation of the sulfate mineral anhydrite into gypsum, entailing a 60% volume increase. The transformation involves anhydrite dissolution in water, transport of the solution with groundwater flow, and gypsum precipitation at a different location. Therefore, the knowledge of groundwater flow systems at the tunnel and adjacent areas is essential to better understand the swelling processes. The present study investigates the groundwater flow systems at the Chienberg tunnel in Switzerland before and after the tunnel excavation, based on numerical flow modeling. The models include faults and the hydrostratigraphic layering in the subsurface to assess the role of the hydrogeological setting. The results of this study indicate effects on groundwater flow caused by the tunneling, which may trigger rock swelling by favoring anhydrite dissolution and gypsum precipitation, including (1) increase of flow rates around the tunnel, (2) broadened, shifted and more distributed capture zones leading to a change in origin and age of groundwater, (3) access of groundwater from preferential flow paths (e.g. faults) due to the drainage effect of the tunnel, and (4) change in geochemical equilibrium conditions because of decreased pore water pressures in the tunnel area.

Research Highlights
► Increased flow rates after tunneling allow for enhanced water access, facilitating anhydrite-gypsum transformation.
► Tunneling broadens and shifts capture zones leading to changes in pore water chemistry, which may favor gypsum precipitation.
► The drainage effect of tunnels can attract water from nearby major flow paths, possibly facilitating anhydrite-gypsum transformation.
► Decreased pore water pressure after tunneling may disturb the thermodynamical equilibrium and cause gypsum precipitation.