Abstract
Abstract. Quick-clay landslides are common geohazards in Nordic countries and Canada. The presence of potential quick clays is confirmed using geotechnical investigations, but near-surface geophysical methods, such as seismic and resistivity surveys, can also help identify coarse-grained materials associated with the development of quick clays. We present the results of reflection seismic investigations on land and in part of the Göta River in Sweden, along which many quick-clay landslide scars exist. This is the first time that such a large-scale reflection seismic investigation has been carried out to study the subsurface structures associated with quick-clay landslides. The results also show a reasonable correlation with radio magnetotelluric and travel-time tomography models of the subsurface. Other ground geophysical data, such as high magnetic values, suggest a positive correlation with an increased thickness of the coarse-grained layer and shallower depths to the top of the bedrock and the top of the coarse-grained layer. The morphology of the river bottom and riverbanks, e.g. subaquatic landslide deposits, is shown by side-scan sonar and bathymetric data. Undulating bedrock, covered by subhorizontal sedimentary glacial and postglacial deposits, is clearly revealed. An extensive coarse-grained layer (P-wave velocity mostly between 1500 and 2500 m s−1 and resistivity from approximately 80 to 100 Ωm) exists within the sediments and is interpreted and modelled in a regional context. Several fracture zones are identified within the bedrock. Hydrological modelling of the coarse-grained layer confirms its potential for transporting fresh water infiltrated in fractures and nearby outcrops located in the central part of the study area. The modelled groundwater flow in this layer promotes the leaching of marine salts from the overlying clays by seasonal inflow–outflow cycles and/or diffusion, which contributes to the formation of potential quick clays.
Highlights
Quick clays are sensitive glacial and postglacial sediments, mostly deposited in a shallow-water marine environment, whose structure can collapse and liquefy if disturbed (Osterman, 1963; Torrance, 2012)
Through an extensive reflection seismic investigation, which includes four new land seismic profiles as well as the incorporation of existing profiles, river lines, and their combination with other geophysical, geotechnical, and borehole data, this study allows for the large-scale delineation of a coarsegrained layer, underlying bedrock, and fracture zones in an area prone to quick-clay landslides in southwest Sweden
Hydrological modelling of the coarse-grained layer suggests that the dominant leaching processes are diffusion and/or seasonal inflow–outflow cycles of the groundwater flow between the coarse-grained layer and the overlying clays
Summary
Quick clays are sensitive glacial and postglacial sediments, mostly deposited in a shallow-water marine environment, whose structure can collapse and liquefy if disturbed (Osterman, 1963; Torrance, 2012). These sediments were deposited in the last deglaciation and early postglacial and subsequently isostatically raised above sea level (Torrance, 2012). In Sweden, where this work is focused, quick clays are defined as clays with a sensitivity higher than 50 and a remoulded shear strength of less than 0.4 kPa (Karlsson and Hansbo, 1989)
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