Fast or slow – identification and description of hydrogeological pattern (structures) transmitting pressure and flow A case study of the Varberg tunnel

Abstract

Groundwater can reside and flow in sediment and bedrock yet they exhibit different hydrological behaviors (Eveborn et al., 2017). Understanding hydraulic properties (e.g. porosity, transmissivity, hydraulic conductivity, connectivity), groundwater recharge, water-bearing structures and flow dimensions is essential for assessing groundwater movement. In addition, these factors must be evaluated to ensure that groundwater responses do not result in slope stability issues or settlement hazards (Gustafson, 2012). Underground construction projects, e.g. tunneling is likely to influence groundwater and have impact on the surrounding environment for rock and sediment (Gustafson, 2012). Groundwater inflow into tunnels may result in lowered water levels above the structure. Other hazards include dry wells, reduced groundwater availability and settlement risks to buildings in the nearby area. In cities such as Gothenburg (Sweden) where old buildings are built on clay-filled depressions, the drawdown of groundwater beneath these layers can lead to subsidence and structural instability (Gustafson, 2012). The objective of this master’s thesis is to identify and describe flow and pressure-bearing hydrogeological structures based on geological maps and data from the Varberg tunnel project. This is consistent with a study by Freer et al. (2002), but the emphasis here is on the depressions in bedrock where higher hydraulic conductivity is assumed. The methodology is a combination of literature study and simplified conceptual model to provide an overview of the area using published geological data. Comparison of the model with hydrological test results from WSP was used to confirm and/or reject the accuracy of the model. Data on fluctuations in groundwater level due to disturbances were incorporated in the discussion of groundwater connectivity in the area. The results of this study suggest that hydraulic conductivity (K-value) is higher in and along bedrock depressions, based on K-values derived from single-hole tests around Varberg and for sediment within bedrock depressions. Disturbances in groundwater levels due to infiltration and tunnelling were also observed. The connectivity among the groundwater wells in terms of flow and pressure can possibly be the result of Nye-channels, bedrock-incised channels active during the last glacial period, or to till that has been subjected to erosion resulting in higher hydraulic conductivity.