Darcy flux as hydrological indicator for the swelling potential of clay-sulfate rocks in tunneling

Swelling of clay-sulfate rocks often poses a severe threat in tunneling. It causes serious damage and produces high additional costs during tunnel construction and operation. The swelling of clay-sulfate rocks requires groundwater inflow into anhydrite-bearing layers. Therefore, the Darcy flux into anhydrite-bearing layers surrounding the tunnel is suggested as a hydrological indicator for the swelling potential. A case study from Switzerland is presented that uses numerical groundwater models to calculate the Darcy flux at the anhydrite level in different tunnel sections after tunnel excavation. The approach, which assumes that a high Darcy flux at the anhydrite level after excavation indicates a high swelling potential, is tested at the study site. The results suggest that the Darcy flux can serve as a hydrological indicator for the swelling potential in tunneling. Equally important, however, is the fact that the Darcy flux depends on many parameters, all of which are uncertain. Hence, a sensitivity study is conducted to analyze the influence of these parameters. The sensitivity study shows that the significant parameters in the case study are the hydraulic conductivity of the aquifer above the tunnel, the groundwater level in this aquifer, as well as the hydraulic properties of the excavation damaged zone, of the tunnel lining and of faults. Based on the results of the sensitivity study, implications for site investigation and measures to counteract the swelling problem are discussed. The major contributions of this paper are therefore the development of an approach to estimate the swelling potential of clay-sulfate rocks based on a hydrological indicator, and the derivation of the parameters that have the strongest impact on the swelling potential of such rock.