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Lacustrine clay is an extremely difficult and problematic soil concerning construction on or in it. A combination of high compressibility and very low strength lead to significant challenges in terms of design for serviceability and ultimate limit states to limit settlements and ground movements within tolerable boundaries and to ensure that failure does not lead to collapse of infrastructure. Since lacustrine clays can be found throughout the densely populated Swiss Mittelland, establishing appropriate tools for reliable characterisation of these soft sediments is a problem of national concern. Numerous costly failures during past construction activities have emphasised the urgency of the problem. In the framework of this project, we will develop new means for accurate and inexpensive testing of lacustrine clays. In particular, we will determine its spatial distribution and delineate key material properties, such as the shear modulus as a function of strain. Our approach will exploit the complementary nature of geotechnical and geophysical methods. Geotechnical measurements provide pointwise information from some insitu tests and laboratory experiments with individual samples and line information along boreholes and penetration profiles. In contrast, geophysical methods have the capability to extrapolate material properties away from the boreholes and thus provide 2D or even 3D images of the subsurface. Benefits and possible limitations of our investigation strategy will be tested with an extensive field experiment in the area of the Wauwiler Moos. This site has been identified to be most suitable for this project and will be later used as a facility for teaching, testing and calibration purposes. Our field programme will be initiated with a surface-based geophysical survey, with which suitable locations for boreholes and geotechnical probing will be identified. Geotechnical investigation techniques will include standard laboratory testing and self-boring pressuremeter and piezocone tests. This will be the first application in Switzerland of a self-boring pressuremeter. Moreover, a novel small-scale piezocone device will be employed. For the latter, we will benefit from the expertise of Dr. C. Hird, who will be on sabbatical at ETH Zürich during summer 2004. Detailed geophysical surveys will be performed with seismic and geoelectrical borehole-to-borehole and borehole-to-surface measurements. Hereby, a unique shear-wave borehole source will be employed, which will be key for determining the shear modulus at small-strains. Considering all the results of our experiments, we will be able to propose reliable and cost-optimized investigation strategies, which will be highly beneficial for the practitioners in the industry. This project is an important pilot study between the Geotechnics and Geophysics Institutes at ETH Zürich. It is an essential part of the preparation prior to setting up a multidisciplinary project at a much larger scale, in which combined analyses of geotechnical and geophysical data will be brought onto a new level.
Authors:Tanner, Simon and Springman, Sarah M. and Messerklinger, Sophie
Index Terms:Seebodenlehm; lacustrine clay; Wauwil; selfboring pressuremeter; cone penetration tests; geophysics; seismic; geoelectric; SoilGroup; Messerklinger, Sophie; Springman, Sarah M.; Tanner, Simon