Three-Dimensional Quick-Clay Modeling of the Gothenburg Region, Sweden
[chapter]
M. A. Persson
2013
Advances in Natural and Technological Hazards Research
Clay sediments are associated with a wide variety of engineering problems, of which landslides, together with settlement, are the most investigated due to the large associated costs. Quick-clay deposits, which if disturbed can transform into a liquid, pose a serious threat to society in southwestern Sweden and have been involved in several large landslides, sometimes with fatal consequences. Even though the theories that explain quick-clay formation are well advanced, no modeling that combine
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... ologic information and reasoning with hard geotechnical data to predict its distribution has previously been done. The stepwise multi-criteria evaluation technique suggested here involves identification of quick-clay preconditions from the literature. Then to derive criteria priorities, an expert group consisting mostly of geologists and geotechnical engineers carried out pairwise comparisons using matrices from which weights were calculated. The same group also participated in the development of the utility functions used to standardize the criteria to allow direct criteria comparisons. To populate the model, all criteria were quantified using empirical geotechnical data, existing geological documentation and/or environmental proxy data. The model results were later cross-checked at selected sites with geophysical methods. Finally, a rather large geotechnical data set was divided and used to add a depth dimension to the model results and to test the predictive powers of 2D and 3D models. Quick-clay type settings were separately defined to facilitate clear communication of quick-clay predictions to non-specialists and to provide a structure for comparisons to the depositionary and post-depositionary conditions in wellstudied east-Canadian and Norwegian quick-clay areas. These settings were derived from trends observed in geotechnical, geologic, geophysical and modeling records. Results of the predictive modeling were subsequently applied to landslide hazard zonation in SW Sweden. However, the framework could, with slight regional adaptation, also be applied in other areas (e.g. eastern Canada and coastal mid-Norway) or even to other issues, wherever groundwater fluxes and ground conditions are of interest (e.g. in contaminant transport, geological process studies and groundwater resource exploration). Language: English with a Swedish summary. Popular science summary Quick clay is initially stable but may after disturbances lose nearly all of its shear strength (i.e. sediment particle's resistance to move relative to each other) and thanks to its high water content transform into a liquid. The remoulded shear strength is low and less than one fiftieth of the undisturbed strength (i.e. the "sensitivity" is 50). The characteristic bottleneck-shaped landslide scars in Sweden remind us of the occurrence, behavior and consequences of quick clay. The presence of these highly sensitive clays does not degrade the initial stability but is strongly decisive for the final size and damage of landslides. In southwestern Sweden, major quick-clay landslides have repeatedly destroyed infrastructure, property and caused fatalities, from the major landslide at Intagan in 1648 continuing until today (e.g. 2006 in the Småröd area, Bohuslän). Although many landslides during the last 100 years have been at least partly triggered by man it is important to recognize that these events are linked to chemical and physical changes in the clays that have occurred over thousands of years. These changes in mechanical properties depend on local and regional geological conditions. Quick-clay developments, with few exceptions, occur in clay originally deposited in former marine environments next to the continental ice sheets. Subsequently, these sediments were lifted over sea level and exposed to groundwater leaching and other post-depositional processes affecting the sediment and its porewater. The distribution of cations, whose positive charge originally kept the negatively charged clay particles from repelling each other, is hereby removed. Many of the basic conditions that contribute to producing quick clays exist in southwestern Sweden and no significant difference between the quick and the non-quick clay deposits can be seen in grain-size distribution, mineralogy or sediment age in this area. It is mainly the depositional environment and post-depositional processes (mainly leaching) that explain local differences in clay strength. The quick-clay forming processes occur at different rates depending on the stratigraphic architecture, surrounding geology and geomorphology and groundwater conditions. The important changes in porewater chemistry are most effective where groundwater flow in sandy or gravelly sediments occur close to the clay. The conditions described above were parameterized to form the basis of the model designed to predict the conditions that can change the mechanical properties of the clay, allowing quick clay to develop in parts of the landscape. By using mainly geomorphological and geological factors, it was possible to build a computer model that can help predict the conditions for quick-clay formation over large areas. To facilitate this, a group containing geologic, geotechnical and chemical expertise weighted all of the contributing criteria relative to each other. The next step, which was also done in the expert group, was to standardize the effects of each criterion so that they could be compared with each other and added together. Data and information regarding all involved criteria were combined to produce maps and 3D models showing the expected leaching probability at each site, called the "quick-clay susceptibility index" (QCSI). Finally, the predicted results were tested against existing geotechnical measurement results, mainly from road and railway feasibility investigations for and areas of known landslide problems. Just over 80 % of the study area consists of exposed bedrock or coarse glacial sediments (sand and till which only rarely have clays beneath). The model suggests that leaching conditions that affect the remaining areas (approximately 20 %), where clay is present, vary, but that about 4% of SW Sweden is likely to contain quick clay at some depth below the ground surface. If the quick clay was formed very deep there is no threat since normal landslides only reach down to about 35 m at most. Other quick-clay areas are located far from river and stream banks (where many slides start), but these could still be hazardous if the slopes are steepened or overloaded. Generally the preconditions for quickclay developments are best fulfilled in central Bohuslän and along the Göta älv River valley (including many of its tributaries). This is reflected both in high QCSI values from the model, geotechnically documented highly sensitive clays and in a landslide scar geomorphology typical of quick clays. Quick-clay predictions can often not replace other more traditionally used geotechnical field and laboratory measurements but can help anticipate clay strength in areas without measurements, suggesting where the need for more information is the greatest. Although the model predicts areas with very weak clay strength (quick clay) and areas with quite high clay strength reasonably well, it does not predict the clay with intermediate clay strength (sensitivity values of 30-50), which may nevertheless be of geotechnical concern. This may be because most marine clay deposits originally had sensitivity values close to this intermediate strength, and these occur throughout SW Sweden, even in areas where some clay has been altered to be quick. Preface This dissertation consists of a background description (Part I) and five attached papers (Part 2) listed below.
doi:10.1007/978-94-007-7079-9_4
fatcat:5ub5pngbcvgvhh7uuugyhz25di