Verformungsverhalten und Strukturfestigkeit norddeutscher Geschiebemergel
Results of studies on north German till indicate, that its strength is not a consequence of glacial pre-consolidation as was previously assumed. Furthermore, the often discussed compaction of grain structure via newlyformed minerals during various weathering phases, though obviously present, appears to be inadequate to account for the observed mechanical resistance. Arguments from soil mechanics and engineering geology favouring these critical observations are presented in the article.
Till exhibits dilatancy, manifested by an increase in volume or a negative, i.e. the onset of external wall deformation directed against the normal strain occurs just before failure strength.
A typical distribution pattern of the partial deformation is seen both within the samples and in the ground, which is dependent on the materials and the deformation strategy. This deformation- and densitydistribution is called here the shear structure. Next to the shear plane a loosening of the grain structure could be observed, which, strictly speaking, may be termed dilatancy s. str. The resulting negative deformations are removed in two mutual zones of compression following into a zone of extension. In other words, deformations are conducted, only to a very small extent, towards the external surface wall of the sample. The remaining, proportionally larger part of the sample, which is also subjected to stress, behaves rather intact; it participates only subordinately in the deformation of the external wall.
To simulate the deformation sequences under such complex conditions a new computer supported method was developed. The method takes into account not only the material characteristics of the sample (e.g. width and number of shear zones) but also its stress-dependent characteristics.
This complicated deformation behaviour is limited not only to till. It was also observed in other loose sediments and rocks. The reason appears to lie in the balancing of deformation within natural dispersion-systems. For approximate quantification, a model plane was constructed, which consists of the sum of contact planes between the individual grains. In its size and geometry, this plane represents the mechanically active internal surface of the sample. On this model plane, the external stress on the system is monitored and partly resolved into two shear components. The strength (friction and cohesion) of the contact planes determines the extent of supporting (below critical) or almost no resistance (above critical) parts of the inner surface.
The possibilities of further experimental and theoretical work for better understanding the mechanisms of shear deformation on the basis of a few phenomenological models is discussed.
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