75. Hydrogeological Controls on the Swelling of Clay-Sulfate Rocks in Tunneling

The swelling of clay-sulfate rocks often poses a severe threat to tunnels. It causes serious damage and produces high additional costs during construction and operation. The swelling of clay-sulfate rocks is triggered by water access to anhydrite-bearing layers. Therefore, we propose that groundwater flow is a key factor controlling the swelling process. A case study from the Jura Mountains in Switzerland is presented that uses numerical groundwater models to calculate flow rates at the anhydrite level in different tunnel sections. The approach assumes that an increase of groundwater flow rates into anhydrite-bearing layers after tunnel excavation corresponds to an increase in swelling. A sensitivity study analyzes the impact of hydraulic parameters on calculated flow rates. Analyzed parameters include the hydraulic conductivity of geological units, properties of the excavation-damaged zone and the hydraulic potential in aquifers near the tunnel. Implications for site investigation and potential measures to counteract the swelling problem are suggested.