The Incidence and Associated Geotechnical Issues of Swelling Clay in Stockholm

Abstract: The Incidence and Associated Geotechnical Issues of Swelling Clay in Stockholm  Viktoria Clarin & Anna Clark  Previous tunnel failures have shown that inadequate reinforcement in tunnels can lead to cave-ins, whereby swelling clay is one of several factors that can result in these damages. Swelling clay minerals possess the ability to absorb water molecules and cations leading to an increase in volume. Instability in tunnels is a consequence of the mobilised swelling pressure caused by lack of room to accommodate for the change in volume. Several tunnels are projected throughout central Stockholm in the near future, whereby numerous drill cores have been logged. This bachelor thesis will therefore focus on the swelling potential and swelling pressure of clay samples selected from a drill core traversing the Söderström fault system. The samples selected for further analysis were collected from a drill core from Slussen, Stockholm, with the aim of determining the reinforcement requirements for future tunnelling projects.  Several samples from the drill core were selected for analysis, whereby free swelling test was conducted to determine the swelling potential for each sample. One sample displayed more than 100% volume increase and was further tested to determine swelling pressure. Tests were performed using an oedometer, resulting in a measured swelling pressure of approximately 155kPa. To identify the clay type several X-ray diffraction tests were performed on the sample.  Similar swelling pressures have been measured in tunnels affected by cave-ins in Norway. Due to these previous events, the swelling pressure is of imminent importance when constructing new tunnels. Based on the obtained results, an adequate reinforcement can be estimated and used as a foundation for future tunnel constructions within the area. The tests show that the Söderström fault contains swelling clay and precautions will have to be taken when tunnels are constructed.