A rock slope classification scheme leading to slope stability assessment is in development in which factors are introduced to compensate for weathering and excavation disturbance and produce a rating for an imaginary unweathered and undisturbed ‘reference’ rock mass. The classification thence allows assessment of the stability of the existing or any new slope in the reference rock mass, with allowance for any influence of excavation method or (future) weathering.


Un système de classification pour des pentes (naturels ou artificiels) d'un massif rocheux est developpe et resulte en une determination de la stabilite des pentes. Le système comprend des facteurs pour compenser pour l'alteration et l'endommagement par le methode de l'excavation. Le resultat est une appreciation numerique pour un massif rocheux qui n'est pas altere ni creuse - le massif rocheux de 'reference'. La stabilite de la pente nouvelle ou la pente existante est classe en le massif rocheux de ‘reference’ avec des facteurs compensatoires pour l'alteration (future) et l'excavation.


Ein Klassifikationssystem fuer die Stabilitat von Felshangen ist in Entwicklung. Das System enthalt Faktoren fuer Verwitterung und Ausbruchsart. Das Resultat ist ein Wert fuer ein Fels der nicht verwittert und nicht ausgebrochen ist - der ‘Referenz’-Fels. Die Stabilitat des neuen oder existierenden Hanges ist dann kiassifiziert im ‘Referenz’-Fels mit Faktoren fuer Ausbruch und (zukuenftige) Verwitterung.


Rock mass classification schemes developed for underground works (Bieniawski, 1989, Barton, 1976, 1988, Laubscher, 1990) result in recommendations for support; some systems also apply to rock slope stability (Bieniawski, 1989, Romana, 1985). However, the use of rock mass classifications developed for underground works leads often to unsatisfactory results when applied to near-surface applications such as rock slope stability, and a new rock mass classification system for slopes has been developed based on the Laubscher system. The rock slope classification system was developed during three years of research in the Falset area in the north-east of Spain. Here new roads have recently been built through a mountainous terrain, necessitating a large number of new road cuts. Rocks in the Falset area vary from Tertiary conglomerates to Carboniferous slates and include rocks containing gypsum, shales, granite (fresh to completely weathered), limestone and sandstone, thus giving the opportunity to assess slopes in different materials. Different methods of excavation were used for the old and the new road cuts, allowing comparison of different excavation methods. Road cuts made for old roads some 40 to 60 years ago could be compared to road cuts not more than 4 years old. Also local variations in weathering, the influence of weathering, and the susceptibility of the rocks to weathering as a factor in slope stability could be studied in detail in the area. Existing old and new slopes have been classified and assessed on stability by the staff and students of ITC and the Technical University, Delft. Nearly all slopes have been classified and their stability assessed by more than one person to avoid observer bias.


Slope failure mechanisms such as plane sliding, wedge failure, toppling and, to some extent, buckling are discontinuity related. Also non-discontinuity related agencies such as deterioration of rock material, progressive weathering, intact rock creep, erosion due to (surface-) water and internal water (flow and pressure) can cause slope failures. In a relatively simple rock mass with clearly defined discontinuity sets failure mechanisms related to discontinuities can be analyzed and the stability can be calculated provided the shear strength along the discontinuities can be established. However, such an analytical approach might not be feasible for slopes containing multiple discontinuity sets with large variations of mechanical characteristics.

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