The present Master thesis is written in connection with the E4 Stockholm bypass, which is a future motorway linking northern and southern Stockholm, and will mostly run through tunnels. The bypass will cross the Lake Mälaren in the Lambarfjärden area, where a fault is indicated on the geological maps, which would create risks for tunneling. Several geological and geophysical studies and measurements have been carried out in connection with the bypass project. These include surface mapping, drillings (core logging and water-loss measurements) and geophysical measurements (reflection and refraction seismics). The objectives of this thesis are to interpret the available geological and geophysical data, integrate the reports and the results of these studies and finally to create a structural model of the Lambarfjärden area affected by the tunnel. The available data provided a great opportunity to examine the area. The core logging and geophysical measurements supported the existence of the fault indicated on the geological maps. The core logging revealed that this pre-existing deformation zone was reactivated, possibly as dextral strike-slip, and the orientations of the subsurface fractures corresponded well with the Riedel structures occurring in such shear zone. The field work has found evidence for ductile sinistral movement which, together with the results of the core logging, could indicate a conjugate deformation zone in the area. The water-loss measurements showed several intervals along the boreholes where significant water-loss took place. Furthermore, the thesis calls the attention to both opening and closure of differently oriented weaknesses, fractures. As the thesis highlights possible risks for the tunneling, its findings could be very useful during the construction of the bypass. / Reviewer: Karin Högdahl, Senior lecturer in Solid Earth Geology at the Department of Earth Sciences, Uppsala University.