Engineering geological classification of weak rocks

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, O-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 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 (expression of the bond 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.