IAEG2006 Paper number 492 © The Geological Society of London 2006 1 Engineering geological classification of weak rocks MARION NICKMANN 1 , GEORG SPAUN 2 & KUROSCH THURO 3 1 Engineering Geology,Technical University of Munich. (e-mail: marion.nickmann@mytum.de) 2 Mueller-Hereth. (e-mail: office.freilassing@mueller-hereth.com) 3 Engineering Geology,Technical University of Munich. (e-mail: thuro@tum.de) Abstract: Although weak rocks (meaning the domain of hard soil/soft rock) occur frequently in construction projects all over the world, only few, and mostly unsatisfactory, durability testing methods exist. In various construction projects it became evident that especially slake durability of weak rocks varies strongly from spontaneous decay to slow degradation in months to years. The conducted investigations have shown, that the usually performed testing methods, as stipulated in national standards (e.g. DIN, Ö-Norm, ASTM) or suggested methods (e.g. IAEG, ISRM), are not suitable to cover the immediate response and long-term behaviour of weak rock in construction practice. A wrong assessment of the durability however can lead to significant problems with the stability, excavatability, transport or reassembly of the excavated material. In this study, 40 different rock types (sandstone, mudstone/clay-siltstone and marl) of 7 different locations were tested. Not only the behaviour of the rock in simple and cyclic slake tests, but also many other rock parameters as e.g. pore volume, carbonate content, grain size distribution and compressive strength were determined. A classification based on the rock behaviour in a cyclic slake test lead to the definition of five categories of durability. Thereby the variable durability is not dependent on a single rock parameter, but on a combination of several parameters as e.g. compressive strength (expression of the bond strength of the matrix), grain size distribution (content of clay minerals susceptible to water) and pore volume (degree of the water conductivity). This so-called "structural strength" makes the border established between weak rocks and solid rocks, on the one hand, and soil, on the other hand, more accurate. Résumé: Bienqu’on puisse trouver des roches faibles souvent dans de nombreux projets de construction, il y a seulement quelques procédés d’investigation concernant la durabilité, ceux-ci souvent n’étant pas satisfaisants. Plusieurs projets ont montré clairement que la durabilité des roches peut varier d’une désagregation spontanée à une désintégration lente pendant des mois à années. Les tests effectués ont montré que les procédés habituels inclus dans les normes internationales (DIN, Ö- Norm, ASTM) ou d’autres méthodes proposées ne sont pas complètement appropriés à décrire le comportement immédiat et à long terme des roches faibles en pratique. Une evaluation incorrecte peut causer des problèmes importants en ce qui concerne la stabilité, l’excavation, le transport et la réutilisation du matériel excavé. Par l’étude présente 40 types différents de roches (grès, argilolites, marne) de 7 locations ont été examinés. Ce n’est pas seulement le comportement des roches dans les tests simples et cycliques de la durabilite qui était défini mais aussi beaucoup de paramètres différents de roches comme par ex. le volume poreuxle contenu du carbonate, la distribution de la grandeur de la graine et la résistance compressive. Une classification dépendant du comportement dans un test modifié et cyclique de la durabilite a eu pour résultat la définition de 5 catégories de durabilité. Selon cette définition la durabilité variable ne dépend pas d’un seul paramètre, mais d’une combinaison de plusieurs paramètres, par ex. la résistance compressive (l’expression pour la résistance de la matrice), la distribution de la grandeur de la graine (contenu de minéral argileux susceptible de l’eau) et le volume poreux (degré de la conductibilité de l’eau). Cette résistance appelée «résistance structurelle» rend le limite entre les roches faibles et les roches résistantes d’une côté et le sol d’autre côté plus précis. Keywords: weak rocks, durability, laboratory tests, classification, compressive strength, porosity INTRODUCTION Weak rocks often form up a big part of the shallow stratums all over the world. Therefore they build up the foundation ground of many construction projects as e.g. deep construction pits, tunnel excavations and open cast mines. However, the extent for investigating the behaviour of these rock types in building projects rarely corresponds to their frequency and to the engineering geological problems they cause. One reason may be the difficulty to apply the testing methods from rock mechanic and soil mechanic on these rock types. Thus an assumed knowledge of the rock behaviour is more often the basis for estimation of the rock properties than the determination in laboratory tests. But although the classification of weak rocks and its separation from (cohesive) soils and hard rocks may be very difficult, they are very important in the phases of planning and construction and for the building contract. A wrong estimation of the rock behaviour often means an increase of time and costs during the construction phase as well as a decrease of the quality of the building. Therefore the behaviour of weak rocks was investigated in a Ph.D.-thesis in order to develop a practicable method for testing and classifying weak rocks and to make the separation of these rock types from the soft soils on the one hand and the hard rocks on the other hand more evident.