Switzerland is geographically divided into three geologically differentiated regions linked to the Alpine orogeny: the Alps, the Plateau and the Jura. The Plateau separates the other two entities and constitutes the glacial foreland. From a stratigraphic point of view, it is classically characterised by a sub-horizontal molasse overlaid by Quaternary glacial or periglacial deposits inherited from the Würmian glaciation. The quintessential morainic facies of the northern Alps is the ground moraine, which varies in thickness but is often metric to plurimetric. This moraine has low permeabilities (less than 10^-6 m/s) and generally separates the underlying aquifers (present in the discontinuities of the molasse) from the aquifers present in the superficial moraine and various periglacial formations.
In this article, we discuss an example in the northern region of Lausanne, where the subsurface was explored for the design of a shallow railway tunnel, at the interface between molasse and Quaternary deposits. Instead of the classic ground moraine, an atypical silty moraine was encountered. The granulometric characteristics of this moraine are discussed. In-situ permeability values measured, pumping tests conducted to understand the site's hydrogeology, and monitoring of piezometric surveys show that this atypical more permeable moraine does not guarantee a tight waterproofed separation between the Lausanne Aquitanian grey molasse and the Quaternary deposits. A continuous vertical aquifer connects the two stratigraphic units and had to be taken into account for the design of the structures, leading to the choice of a sealed (initially drained) tunnel. This experience highlights the extreme heterogeneity of glacial and periglacial formations and the need to carefully study them to properly design structures and control environmental impacts (groundwater lowering).