The essential features of the mechanical behaviour of soft rocks are first presented. It is suggested that strain-hardening plasticity, such as the Cam-clay model for soils, is a convenient framework for the description of such a behaviour. In order to get a better match between theoretical predictions and experimental results, a more refined model is presented. Comparisons are shown for a wide range of materials and tests. The possibility of using the model for engineering purposes is shown next. It is further demonstrated that the theory can be used as a conceptual framework for modelling diagenesis, artificial cementation such as grouting and conversely weathering and weakening due to temperature increase.


Les caracteristiques fondamentales du comportement mecanique des roches tendres sont tout d'abord presentees. La theorie de la plasticite avec ecrouissage, de type Cam Clay, utilisee pour les sols, est envisagee comme un cadre de reference convenable, pour decrire leur comportement. Pour ameliorer l'accord entre previsions theoriques et resultats experimentaux, un modèle plus raffine est propose. Plusieurs comparaisons sont presentees pour une large classe de materiaux et d'essais. Les possibilites d'utilisation du modèle pour des problèmes pratiques sont ensuite demontrees. La theorie peut aussi être utilisee comme cadre conceptuel pour la modelisation de phenomènes comme la diagenèse, la cementation artificielle dûe aux injections de coulis mais aussi de phenomènes tel que l'alteration et la perte de rèsistance dûe à I'augmentation de la temperature.


Die wichtigsten Merkmale des mechanischen Verhaltens von Weichgesteinen werden zuerst prasentiert. Es wird vorgeschlagen, dass die Plastizitat der Dehnungsverfestigung, wie das Cam- Clay Modell fuer Boden, ein passendes Gefuege fuer die Beschreibung eines solchen Verhaltens darstellt. Um eine besseres Anpassung zwischen den theoretischen Voraussagen und den experimentalen Ergebnissen zu erhalten, wird ein verfeinertes Modell prasentiert. Vergleiche fuer einen umfangreichen Bereich von Materialien und Tests werden aufgezeigt. Die Möglichkeit das Modell fuer Zwecke des Ingenieurwesens zu verwenden, wird als nachstes demonstriert. Weiterhin wird aufgezeigt, dass die Theorie als ein Begriffsrahmen fuer die Diagenese, kuenstliche Zementierung wie Injektion und zum Gegensatz Verwitterung und Entfestigung aufgrund eines Tempertaturanstieges, verwendet werden kann.


With the term 'soft rocks' we usually refer to a broad class of geomaterials which lie in the grey area of transition between soils and rocks, as commonly perceived by geotechnical engineers: soils seen as granular media with little or no bonding between particles and rocks considered as hard and strong materials, whose engineering behaviour is governed more by the properties of the discontinuities pervading the rock mass than by the intact rock properties themselves. This transition zone is often designated also with the term 'weak rocks', Fig. 1, and a clear distinction is made between soils, weak rocks and rocks tout-court on the basis of the value of the (characteristic) uniaxial strength of the geomaterial (Dobereiner and De Freitas 1986). With the term soft rock, however, it is now come into fashion (see e.g. Anagnostopoulos, Schlosser, Kalteziotis and Frank (1993)) to refer to a less clearly defined class of geomaterials, mostly belonging to the set of weak rocks, but comprising bonded soils and, for special problems, even hard rocks weakened by weathering or temperature effects. The common feature of the class of soft rocks is that of being ‘intact’ materials, in the sense that they can be considered as continua, and that their behaviour is similar to that of hard rocks at low confining pressures and similar to that of soils for higher confining pressures. A rock which is weak because of intense fracturation or existence of cleavage planes may not be a soft rock in the sense above. To the class of soft rocks belong geomaterials, usually but not exclusively, of sedimentary origin, for instance marls, calcarenites, tuffs, shales, chalks. Limestone, slate or saprolitic rocks can also be considered as soft rocks even though their uniaxial strength is generally higher than that of the aforementioned materials. On the other hand grouted and naturally cemented soils also belong to this class, not withstanding that their uniaxial strength is considerably lower than that of a weak-rock.

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