The materials we are going to speak about are typically stiff fissured clays. The behaviour of these materials, when they are intact, looks much more like one of a rock than of a soil. The mechanical characteristics of stiff clays to be considered in a calculation are difficult to define because of the heterogeneous fissuration which involves an important scattering of the results of lab tests.

This study is about the behaviour of a hard fissured clay during triaxial compression. It has been done on the intact material and during the residual stage. All the tests have been made under confining pressure of 250 kPa. The following points have been studied: the influence of strain rate, the influence of stress level and the behaviour under cyclic loading. The corresponding tests have been realized by means of a device allowing either to impose the strain rate of the samples during monotonous tests or to control the strength during cycling loading (amplitude and frequency being stated). We have recorded the load deformation curves.

A supplementary device allows to count the number of loading cycles. The frequency of the cycles is 0.1 Hz and their shapes is Practically sinusoidal.

First of all we are going to show the principal elements allowing to characterize the material from the physical and mechanical points of view.


The material used comes from Nancy surroundings and belongs to the Domerian geological stage (Lias). It is a silty clay highly overconsolidated with a weak void ratio and a great fissuration. It is difficult to cut the samples because of this fissuration; it is the reason why a particular sampling device has been concieved (Perrot &Vigouroux 1979).

A part of the fissuration is parallel to the stratification plans and consequently doesn't react in the same way according to the orientation of the samples in relation to this stratification. Among these fissures some are visible whereas others are potential; the majority of the tested samples shows visible fissures on the surface, only 10% of the samples look massive.

These clay stones are more or less silty (quartz and mica) and lightly calcareous (10 % CaCO3). The most important clay mineral is illite(Troalen 1978).

The average values of the index properties are as follows: - natural water content: 10 %, - dry specific weight: 20 kN/m 3, - liquid limit: 40 %, - plasticity index: 20 %.


The mechanical properties of the Domerian clay stones have been studied by Vigouroux (1977 & 1978) and Troalen (1978). The main results obtained by these authors are about the influence of the strain rate, the size and the orientation of the samples, and the state of alteration of the material on its shear strength. They show an important scattering.

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