As part of the pre-feasibility study for the capacity expansion of the Basel transport capacities the railway tunnel "Herzstück" the AUG was commissioned by the Swiss Federal Railways (SBB) to provide expert advice and to develop the geological basis and to address hydrogeological issues. After the geological, geotechnical and hydrogeological foundations in the project area were researched and prepared in 2022, the focus in 2023 was placed on the presentation and analysis of the geological and hydrogeological conditions along the various planning variants. Based on the geological 3D-model, which was updated in the project perimeter, 43 longitudinal and transverse geological profiles were made available to the engineering consultants.

To assess the influence of the underground structures on the groundwater flow regime (groundwater flow direction, velocity and balances), various aspects were considered: (A) reduction of the cross-section for groundwater flow; (B) groundwater mounding and drawdown in the area of the underground structures; and (C) changes in the water balances in the aquifer and in the area of the underground structures. In addition to an initial assessment, which primarily involved a qualitative hydrogeological evaluation of the reduction of the flow cross-section in the context of the groundwater flow regime, a detailed investigation was carried out to integrate the underground structures into the 3D groundwater model (groundwater flow and heat transport) and detailed calculations were carried out in relation to the courses of the current railway variants and the exact location of the station structures. The evaluations allow a quantitative assessment of the hydraulic impact upstream and downstream of the underground structures and changes in the groundwater flow rates through various areas of the underground structures. In addition, initial proposals and the effectiveness of individual technical measures (large culverts and horizontal drainage systems Hori-Drains) have already been evaluated for individual sub-projects (Figure).

The current evaluations focused on hydraulic impacts. Thermal impacts have not yet been assessed, but will have to be considered at a later date, particularly with regard to compliance with legal boundary conditions and in connection with thermal groundwater utilization in the vicinity of the underground structures. It is to be expected that large-scale impact on the subsurface, such as that associated with the Herzstück, will lead to a non-negligible diffuse heat exchange with the groundwater body.

Commissioned by: SBB

Project duration: 2021 – 2024

Horst Dresmann, Michel Walde, Brian Steiner & Jannis Epting

Figure: Hydraulic impact (mean period 2010-2021) in the area of the Basel SBB underground station. A: Current situation compared to the situation with structure of the underground station without compensation measures; B: Current situation compared to the situation with structure of the underground station and uniform compensation measures. Bottom right: Diagram of large culvert systems.