Movements and failure of cuts and natural slopes constitute an important geotechnical problem that involves a variety of geomaterials in a variety of geological and climatic contexts, and which has a major socio-economic impact in many countries. The paper reviews the state of knowledge in this domain, examining the basic aspects of soil mechanics that are relevant to the context of slopes, the importance of water on slope behaviour, and then the different stages of slope movements: pre-failure, failure, post-failure and reactivation. Finally it is shown how the geotechnical characterisation of slope movements can be used, in particular for assessing risk associated with such movements. Emphasis is put on for the brittleness of soils and its practical implications for the progressive failure developing at the pre-failure stage and on the characteristics of post-failure movements. The influence of other factors such as creep, fatigue, destructuration, partial saturation and infiltration is also considered.

https://www.icevirtuallibrary.com/doi/pdf/10.1680/geot.2001.51.3.197

Natural slopes and cuts: movement and failure mechanisms

This paper presents the results of a comprehensive experimental study of the desiccation of fine-grained soils. Air drying of initially saturated soil slabs in controlled conditions is investigated by performing three kinds of tests: free desiccation tests, constrained desiccation tests (prevented shrinkage), and crack pattern tests. Strains, suction, water con- tent, and crack geometry are investigated. Results reveal that unconstrained drying exhibits two stages: a domain with large, mostly irrecoverable deformations and degree of saturation close to 100%, followed by a domain with lower defor- mations at a decreasing degree of saturation. Homogeneous soil macroscopic cracking is possible only in the presence of boundary constraints and (or) moisture gradients, inducing the build-up of tensile stresses. Results also show that, for the initially saturated remoulded soils tested here, in the whole sample and near a crack initiation point, the degree of satura- tion remains very close to 100% until cracking, while cracking onset, the air-entry suction, and the shrinkage limit are close to each other. Cavitation nuclei and the formation of an irregular drying front at cracking initiation are commented upon in light of this observation. Finally, results suggest that the crack pattern geometry is the result of energy redistribu- tion. A quantification of the process is proposed. Resume´ : Cet article presente les resultats d'une etude experimentale complete de la dessiccation de sols fins. Le sechage de sols initialement satures est etudiedans des conditions controlees et atemperature ambiante. Trois types d'essais ont eteeffectues : essais de dessiccation libre, essais de dessiccation avec contraintes (le retrait est empechedans une direc- tion) et essais de plans de fissures. Les deformations, la succion, la teneur en eau et la geometrie des fissures ont eteinves- tiguees. Les resultats demontrent que le sechage sans contrainte presente deux etapes : un domaine durant lequel les deformations sont grandes et irreversibles et le degrede saturation reste pres de 100 %, suivi d'un domaine durant lequel les deformations sont plus faibles et le degrede saturation diminue. Pour un sol homogene, des fissures macroscopiques apparaissent seulement lorsqu'il y a des contraintes aux frontieres et (ou) des gradients d'humidite ´, ce qui induit des contraintes en tension. Les resultats montrent aussi que pour les silts et argiles remanies initialement satures utilises dans cette etude, le degrede saturation reste tres pres de 100 % jusqu'ala fissuration. De plus, la fissuration, l'entree d'air et la limite de retrait sont proches les unes des autres. Suite a ces observations, l'existence de germes de cavitation et la forma- tion d'un front de sechage irregulier lors de l'initiation des fissures sont discutees. Finalement, les resultats suggerent que la geometrie des fissures resulte d'une redistribution de l'energie a travers le corps. Une quantification du processus est proposee. Mots-cles: dessiccation, initiation de fissure, succion, succion a l'entree d'air, geometrie des fissures.

https://people.duke.edu/~hueckel/desiccationpapers/2009.can.fundam of cracking.pdf

Fundamentals of desiccation cracking of fine-grained soils: experimental characterisation and mechanisms identification

Experimental characterization of shrinkage and desiccation cracking in thin clay layer

https://hal.archives-ouvertes.fr/hal-00539624/document

Experiment evidence on the temperature dependence of desiccation cracking behavior of clayey soils

The authors wish to thank the discussers for their interest in the work on crack formation in clays during drying. The model presented by the authors holds for soils with a high clay content in which desiccation cracks form while the soil is still saturated. This was the case for the Saint-Alban clay for which cracks appeared for a suction of about 23 kPa, 18 hours after the soil surface was exposed to evaporation. Only the onset of cracking was modelled, and crack depth is approximated with linear elastic fracture mechanics in mode I, i.e., opening of the crack. Further crack propagation occurs with the ongoing evaporation leading eventually to the formation of secondary and even tertiary cracks if the soil is restrained in all directions. Furthermore, these cracks, depending on soil type, may be initiated in an unsaturated soil. Hence, we agree that the model must then also consider unsaturated soil mechanics. Owing to differential behaviour between a relatively dryer surface layer and soil at a higher moisture content with increasing depth, secondary crack propagation may also be controlled, in some cases, by shear failure at the crack tip. As stated by the discussers, the cracking problem is complex and the subject received little attention so far.

A idealized framework for the analysis of cohesive soils undergoing desiccation

An excavation with three different levels was exposed to continuous evaporation during 35 days in order to observe and document the formation of shrinkage cracks in an intact and weathered sensitive marine clay. Desiccation of the intact Saint-Alban clay under restrained conditions, at an average evaporation rate of 0.018 cm/h and an initial moisture content of 103%, produced visible primary cracks after 17 h with an average spacing of 20 to 24 cm. Secondary cracks within each original polygon appeared after about 70 h of evaporation. Subhorizontal cracks at a depth of 6 to 8 cm were also created owing to differential strains induced by differential shrinkage of the soil polygons. Further evaporation resulted in the formation of a new set of vertical cracks after about 150 h of evaporation in the soil below the subhorizontal shear plane. The shrinkage-induced volume change associated with these different sets of cracks produced polygons with a protuberance at their bottom part indicating that the depth of crack propagation below the subhorizontal shear plane was about 3 to 4 cm. In weathered clay at an initial moisture content of about 50%, the average spacing was 10 to 12 cm, indicating that spacing between primary cracks is related to soil type. In the cohesionless topsoil, no cracks were observed.

J M Konrad

Papers

Desiccation of a sensitive clay: field experimental observations