Some topics, which difficult the effective use of RMR for dam foundations, are reviewed and a new geomechanics classification system, DMR (Dam Mass Rating), is proposed -as an adaptation of RMR- giving guidelines for several practical aspects in dam engineering and appraisal of dam foundation.
On revise quelques problems qui ont fait difficile l'application du RMR pour les fondations des barrages. On propose un nouveau classification système, DMR (Dam Mass Rating)-comme un desarroi du RMR-qui donne recommendations pour quelques problèmes pratiques dans le projet et construction des barrages, et de ses fondations.
Es werden einige die praxisgerechte Nutzung des RMR (Rock mass Rating) erschwerende Themen betrachtet und ein neues Geomechanisches Einteilungssystem, das DMR (Dam Mass Rating), eine Anpassung des RMR, vorgeschlagen. Daraus resultieren neue Richtlinien fuer verschiedene praktische Aspekte beim Dammbau und bei der Beurteilung von Dammunterbauten
The RMR geomechanics classification was originally proposed by Bieniawski (1973) for use in tunnels, slopes and foundations. In fact the use of RMR has been very diverse: extremely frequent in underground works, very scarce in slopes and almost nil in dam foundations. There is only a seminal paper (by Bieniawski and Orr, 1978), no chapter in the Bieniawski Jubilee Volume and very few application papers, except in only an important topic: estimation of the rock mass deformation modulus Em. Several authors have referenceal the use of RMR as a useful tool for the description of rock mass foundations (Di Salvo, 1982); Van Schalkwyk, 1982; Sanchez Sudon and Mañueco, 1991; Hemmen, 2002) and specification for excavation. Serafim (1988) states that "appropriate rock mass classifications can … be used to obtain a good estimate of (shear strength and deformability) parameters". Difficulties in RMR use for dam foundations derive from several points: consideration of the water pressure is very doubtful (the pore pressure ratio varies along the dam foundation, dams must operate with changing water levels…), there is no good rules for quantifying the adjusting factor for the joint orientation (which must allow for the safety against total failure by horizontal shear, for local failure, for water leakage through the joints…), there are changes in properties of both the rock and the joints induced by watering changes (saturation, desiccation, flow along the joints…).
General stability against horizontal sliding is not a very common problem (although there are failures as in Malpasset). The dam engineer needs, when comparing possible dam sites, rapid appraisals of several topics: general adequacy of site for type of dam, depth of excavation of altered rock (if needed), required amount of foundation treatment (grouting). So, there cannot be only an adjusting factor and a sole guideline. Besides conditions will be different according the type of dam.
An appraisal of the deformability of the rock mass is needed in order to calculate stresses, strains and deformations in dams. Hence, empirical correlations between geomechanical classifications and deformation modulus of the rock mass Em have always been very popular. The first one of these correlations was proposed by Bieniawski (1978).