Multidisciplinary Near-Surface Investigation of a Quick-Clay Landslide Prone Area in Southwest Sweden

Abstract: Quick-clay landslides are considered one of the most important geohazards in Sweden, Norway, and Canada. The deposits involved are glacial and postglacial clays and silty clays, which are very sensitive to increased stress that may collapse their structure and cause liquefaction.A multidisciplinary approach was adopted in this study of quick clays in an area of southwest Sweden that is prone to landslides. It was mainly based on geophysical methods, but was complemented and validated with geotechnical, geological, and hydrological data. Downhole geophysics, land and river reflection seismics, radio magnetotellurics, P-wave refraction tomography, magnetics, and multichannel analysis of surface waves comprised the main geophysical methods used in this research. Laboratory measurements of core samples, such as grain size analysis, mineral magnetic properties, fossil content, X-ray fluorescence, cation exchange capacity, X-ray powder diffraction, electrical conductivity, and pH were also taken. Hydrological modelling was used to obtain information on the properties of groundwater within a coarse-grained layer, given the importance of this information in the formation of quick clays.The evaluation of the physico-chemical properties of the coarse-grained layer revealed high values of magnetic susceptibility, probably as a result of fluvial sorting, which tends to accumulate denser minerals such as magnetite. Potential quick clays were visually observed above this layer, and their presence was also confirmed by geotechnical data acquired in previous studies. Marine fossils identified within the coarse-grained layer confirmed the glaciomarine origin of the clays. Geophysical results revealed the presence of large-scale structures, an undulating fractured bedrock and a coarse-grained layer sandwiched between clay deposits, with leached sediments on top and unleached sediments below. This layer, important for the development of quick clays in the area, was 3D modelled in a regional context. Magnetic data also revealed that the coarse-grained layer together with quick clays, have the potential to act as a sliding prone layer. Multichannel analysis of surface waves helped to geotechnically characterize the area. Although the results could not reach the deeper layers, it did yield information about the shallower layers of clay, silt and sand. The VS30 values indicated the presence of soft soils, as well as soft clays/silts with high plasticity index and high water content.This PhD is expected to improve the current knowledge of quick-clay landslides and how they are preconditioned. Climate change will probably affect the study area in the near future, most likely increasing landslide risk, therefore, research should continue and advance to new levels. Application of other geophysical methods such as borehole or geophysical monitoring, and induced polarization, could provide more information about the formation of quick clays and associated landslides.