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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
develop in the middle of the area subjected to tensile stresses.
But as previously mentioned, these tertiary cracks are only
formed when initial water content of the samples is high,
because under these conditions the soil continued to shrink after
secondary crack. However the base of the mould constrains that
shrinkage producing the new level of cracking.
The model illustrated in Figure 8 is limited to one
dimensional shrinkage, this condition is imposed by the shape
of the mould where soil desiccation occurs. The shape may be
considered by the shape factor (SF) that relates the major to the
minor dimensions. For a square or a circle, SF is equals to 1 but
for a rectangle SF is greater than 1. Shape Factors greater than
about 1.5 tend to impose conditions of one-dimensional
shrinkage and cracks appear in a more or less systematic way.
For FS lower than 1.5 more than one degree of freedom are
present in the shrinkage and cracking sequence, consequently
the orientation of those cracks are much more complex to
predict (Ávila, 2004).
4 CONCLUSIONS
The prediction of the initiation points and orientation of cracks
produced by a desiccation process is in general very complex
because many degrees of freedom are present and tensile
stresses are mobilized in multiple directions. However under
one-dimensional shrinkage that may be imposed to a soil,
systematic cracking patterns tend to occur and they may be
predicted. The experimental program on small samples prepared
under similar conditions and subjected to a common drying
atmosphere, showed the repeatability of cracking patterns for
primary, secondary and in some cases tertiary cracks.
For laboratory desiccation experiments systematic cracking
are commonly observed for moulds that have shape factor
(relation length to width) equal or greater than 1.5, for lower
values of shape factor the cracking is more complex and
difficult to predict.
A simple conceptual model is here proposed to explain why
the cracks appear in specific locations and following a particular
sequence under the described conditions. This is important for
the better understanding of the cracking phenomena in clayey
soils, and particularly for the development and validation of
numerical analysis of the hydro-mechanical problem of
cracking applied to more complex scenarios.
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