The paper reviews the chief structural weaknesses of rocks and rock masses that is the departures from the properties of an ideal material, continuous, homogeneous, isotropic, including a neutral state of stress and the permanence of these properties along time, whatever the environment. Two simple cases of heterogeneity are focussed: the spherical inclusion, and the sandwich, that is superposition of parallel beds. More general cases are randomly distributed spherical inclusions and lenticular formations. Use is made of series and parallel assemblies. A better knowledge and understanding of the influence of structural defects on mechanical behaviours of soils and rocks raises the question of more naturalistic description of these defects. Here is the true place of geology in soil and rock mechanics.


Any rock or rock mass the engineer has to deal with can be seen as a weak rock, along the definition given in the announcement of the symposium. Some have high permeabilities and are dangerously water-bearing, some have a clay content leading to swelling and slaking behaviours with cyclic variations of moisture, some have purely structural defects or weaknesses. The paper only reviews structural features relevant to mechanical behaviour.

Rheological sciences apply to ideal mediums and the first ideal solid is defined as continuous, homogeneous, isotropic, including a neutral state of stress, and the permanence of these properties along time whatever the moisture and temperature changes.

At the crystal lattice scale, mineral matter is highly anisotropic; at the rock matrix scale heterogeneity appears from various minerals side by side, and the most where voids are present as void is the most significant heterogeneity; at the rock mass scale, surfaces of discontinuity are pervasive. The geological engineer is concerned by length scales from 10 -3 to 10 3 m, about a millimeter or less in comminution processes, a few meters in stability of underground works, and more in stability of high dams and slopes.

Fig. 1 - Classification of soils and rocks after their first heterogeneity that is porosity. The scale of porosity is logarithmic below 1.

Most of soils and rocks can be entered in the three columns, either detritic, or igneous or chemical

Below n = 0,02 any voids are flaky (of course this limit must not be understood as a sharp one).

(Figure in full paper)

The structural description of the rock mass through its surfaces of discontinuity was the first step of rock mechanics, and it does not deserve any more mention. On the contrary heterogeneity was not given as much interest up to now, in spite of some papers by the author (Duffaut 1970, Duffaut and others 1979) Of course most of anisotropies derive from geometrical disposition of heterogeneities and surfaces of discontinuity, and fortunately the number of such dispositions in Nature is limited.


Two classifications are proposed. The first one includes soils up to the loosest ones as they are ranked after porosity alonga logarithmic scale from 1 downward. other "chemical" rocks.

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