ABSTRACT
Underground constructions are increasingly important for efficiently utilizing limited space in urban areas of Stockholm, Sweden. A combination of refraction seismics and Electrical Resistivity Tomography (ERT) was performed in central Stockholm city, Sweden where a new subway has been designed. When planning a tunnel track, it is important to have a thorough understanding of the depth to bedrock as well as the existence of weakness zones in the bedrock of the area for optimizing a design of the tunnel alignment and avoid unexpected challenges and costs during construction. To get an overview of the area, ten geophysical lines were surveyed in two sites with combination of bathymetry measurements. Results of the investigations showed a deeper bedrock level compared with what was expected, and indicated existence of weakness zones in the bedrock. In addition, several geotechnical soundings have been performed for calibrating results and to create a 3D model of the bedrock level. The results of this investigation were the main reason to change the tunnel alignment direction as well as to design the level of the tunnel. With the aim of completing interpretations of the weakness zones and to get a better knowledge of bedrock quality, some core drillings have been carried out in the area. This process illustrates the critical need for such geophysical studies in optimizing construction design and avoiding unforeseen costs in urban underground construction projects.
