One of the main soft rock characteristics are relatively rapid deterioration and degradation of strength when soft rock is exposed to atmospheric agents. In the investigation of strength degradation caused by weathering, wetting-drying process (as a part of weathering) is the factor that has most influence. Deterioration of shear strength due to weathering simulated by wetting-drying is tested on marl samples from Eocene flysch formation. Measurement is carried out with a portable shear apparatus, and a modified process of the sample preparation is implemented. Weathering of samples was simulated in laboratory conditions. Groups of five samples were subjected to 2 to 8 cycles of this process. The results provided the conclusion that the magnitude of the strength degradation depends on the strength of unweathered material and the number of simulated weathering cycles. However, with the increase of wetting-drying cycles, the changes become insignificant.
Weathering of soft rocks is well known phenomenon that causes many problems in geotechnical constructions. Marl from Eocene flysch strata found in Dalmatia region in Croatia, is just one example of these soft rocks. Excavations in these strata such as those for construction pits or cuttings for roads, have considerable problems with durability. Durability problem is caused with marl strength degradation induced by the weathering. Excavation in these materials can be performed only with use of heavy machinery (rock breaker) or explosives, as well as in any other rock. For an example, as a consequence of weathering, a cut that was stable after excavation even with almost vertical inclination, could become instable in the engineering time scale (i.e. a period ranging from a few years to a few decades, in which construction is in use). In general, weathering includes two dominant processes (Fookes 1988), physical and chemical weathering.