5.1.2 More details on shrinking and swelling consequences (e.g. in France)

Shrinking and swelling of clayey soils constitutes a natural hazard, which is indeed less spectacular than sudden and rapid landslides. However, prolonged dry conditions have caused an intense desiccation of sensitive clayey soils. The desiccation cracks reached depths of up to two meters. Subsequent soil compression was induced by an increase in soil moisture during the wet seasons, and finally swelling of the underground. In many regions, these vertical movements have generated high damage to buildings, in particular to small single, individual houses. In France, in the period between 1989 and 1992, the amount of damages was more than 2 billions of Euros. At the end of the year 2002, the total amount of refundings carried out was evaluated to approximately 3,3 billion euros, which corresponds to several hundreds of thousands of damaged houses on the whole of France since 1989.

Cracks caused by shrinking of clayey soils in the Centre of France (Orléans region). In this specific case, the amplitude of the cracks is increased due to the presence of trees inducing root suction (differential sinking) (Photo: Maquaire, CERG)

Factors enforcing shrinking and swelling consequences

Differential compression can be amplified locally by tree or bush suction or by soil heterogeneity (in example, house partially located on sensitive clayey soils and on insensitive formation, e.g. limestone). This causes cracks and damage on bulding structures, and sometimes even the complete destruction of houses.

Shrinking-swelling process

Is there a specific house type which is especially vulnerable to this hazard?
The damage has often pinpointed certain deficiencies in constructions: insufficient depths of the foundations, and a high rigidity of the foundation structures and therefore inability to compensate the deformations of the underground.

A national analysis of approximately 800 cases has indicated the “typical” vulnerable construction: an individual house with a simple ground floor, a non- or inadequately steeled strip foundation, low anchoring depths (< 0.8m) in clayey soils or clayey-sandy soils, and masonry without any horizontal chaining. The susceptible soils are classed sensitive (15<Ip<30) to very sensitive (Ip>30) according to soil humidity variations (with Ip: Plastic index).

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