In this contribution the hydro-mechanically coupled behaviour of a sand is experimentally investigated with the focus on settlements induced by changes in degree of saturation and suction. This phenomenon, referred to as collapse behaviour, is attributed to rearrangements of the grain skeleton due to changing capillary effects on wetting of the soil. For the experimental investigation of the collapse behaviour of a medium coarse sand cyclic dryingwetting tests are performed under oedometric conditions. In the test setup a sand specimen, subjected to a constant small vertical stress, is cyclically dried and wetted by volume control of the pore water, whereas matric suction is measured using a tensiometer implemented to the specimen loading plate (topcap tensiometer). The test procedure, originally designed to investigate the hysteretic nature of the soil-water characteristic curve of the sand, allows to measure the one dimensional volume change of the specimen as evoked by the applied hydraulic paths under constant net stress. By varying the specimen void ratio the impact of density on the collapse behaviour can be assessed. The test data is important for the development of a mechanical constitutive model which can take the volume change behaviour due to suction changes into account.
Im Rahmen der derzeit stattfindenden Variantenuntersuchung zur Erneuerung der Fehmarnsundquerung wurde in verschiedenen Tiefen Tarraston angetroffen. Wie schon bei den Baumaßnahmen vor 50 Jahren stellt dieser eine ernste Herausforderung dar. Insbesondere das Quell‐ und Schwellverhalten des Tons sowie seine geringe Scherfestigkeit im Bereich von Harnischflächen könnten beim Bau der neuen Brücke bzw. des Tunnels Probleme bereiten. Dieser Artikel vergleicht verschiedene als Tarraston eingestufte Proben im Hinblick auf ihre Zusammensetzung, ihre Mikrostruktur und ihr Quellverhalten und zeigt dabei Zusammenhänge auf. So konnte gezeigt werden, dass die Kornverteilung der untersuchten Proben des Tarrastons keine sichere Aussage über deren Quellverhalten zulässt, während Plastizität oder Wasseraufnahmevermögen zuverlässige Anhaltspunkte bieten. Zwei Tone, die sich in granulometrischer und tonmineralogischer Zusammensetzung sowie Setzungsverhalten kaum unterscheiden, wiesen beim Quellverhalten deutliche Unterschiede auf. Der Artikel diskutiert die Ursachen und geht dabei auch auf die Mikrostruktur des Tarrastons ein. Ferner werden die Versuchsergebnisse mit Literaturwerten des besser erforschten London Clay verglichen.
Spatial variations of soil properties occur even within nominally homogeneous soil layers. Therefore, to achieve a reliable geotechnical design, it is necessary to consider local variations of soil properties. Varying soil stiffness leads to spatially variable settlements, implying increasing dynamic forces on passing vehicles. Higher dynamic forces result in additional settlements and simultaneously reduce driving comfort. This process continues steadily until repair of the asphalt pavement is required. The aim of this study was to develop a new mechanically reasoned method for deriving the permissible spatial variation of continuous compaction control (CCC) data from driving comfort requirements. A numerical model was developed that allows modelling of the long-term evenness of asphalt pavements due to passing vehicles, taking into account spatial variation of the subsoil stiffness. The numerical model is based on the subgrade reaction method, modelling the surface and subjacent base courses by a series of spring elements. Thereby, a depth-dependent stress distribution and interactions of the base courses are considered. The initial stiffness of the spring elements is derived from oedometer tests. Depending on the traffic load and utilisation period, the long-term evenness of an asphalt pavement and the effects of a spatially varying subsoil stiffness can be investigated.
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