Durability of soft rocks in Eocene flysch formation (Dalmatia, Croatia)

The mechanical weathering of marl in the form of disintegration of intact material caused by repeated drying and wetting, also known as “slaking”, is an important factor affecting the durability and stability of cuts in soft rocks and Eocene flysch formations (Dalmatia, Croatia). It is often valued and used in rock classification as a slake durability index (SDI), an index property recommended by the International Society for Rock Mechanics and American Society for Testing and Materials. It was observed that the results of conventional laboratory tests on marl samples using two slaking cycles are not always in accordance with the appearance of the samples after testing (highly fragmented rock material, in spite of the high resistance/durability result based on the retained sample mass). Depending on factors such as porosity, hydraulic conductivity, and mineralogical composition, the effects of weathering might sometimes not be obvious within two cycles, which suggested that durability classification is not suitable for all soft rock types. In order to determine the additional criteria that would reduce the uncertainty of standardised durability testing in soft rocks, both the standard and modified tests of durability as well as a series of additional index tests (carbonate content, dry density, water absorption, porosity, and point load strength index) were conducted on 70 samples of marl. Several variants of the SDI were analysed with various numbers of cycles, cycle times, and combinations thereof. To emphasise the differences between various types of marl, a variant of loss slake index was introduced and determined using 30 samples. Based on the results obtained, according to the degradation type observed after the slaking, the samples were grouped into three groups. Based on the presented results, a classification with the defined criteria for high, medium, and low durability has been suggested for the purpose of a durability analysis of the cuts in marls.