engleski

Durability of soft rocks (Dalmatian experiences)

The durability of a rock is defined as the ability to maintain initial mechanical and physical properties in a required period of time with respect to set of engineering serviceability requirements, whether it comes to its impact on construction, other rocks in the geological vicinity or usability as building material. Soft rocks, when taken in account as building material, are subject to intensive change of engineering properties. These changes can be caused by external influences and can manifest within a few months to several years, from the initial rock like properties at excavation to silty-clay residues at the end of the process. This durability feature has determined the common concept of soft rock to be a material that behaves like a rock in its intact condition, however when subjected to weathering process, it ultimately degrades to fine grained material with distinct change of physical and mechanical properties compared to its initial state. [1] Repeated wetting and drying is the dominant process that causes degradation of marl in Dalmatia, it is a major cause of physical weathering combined with chemical weathering on the surface of material as well as on the crack walls inside the material, i.e. all surfaces of material that can be in the contact with water. Although formally separated, both processes usually occur at the same time and complement each other [2]. Development of cracks during physical weathering increases the surface on which chemical processes may occur. On the other hand, chemical weathering replaces solid minerals with weak clays or pores, which make the material more susceptible to physical weathering. In mechanical terms, the degradation of marl manifests itself through reduction of strength. Since the causes of weathering can vary, there is currently no single physical property which can solely describe durability of either one type of rock, or a generalization on all types of rocks. Therefore, many authors have introduced a variety of experiments that can generally be divided into static and dynamic tests. An example is the so-called static experiment is "Jar slake test" [3]. The main disadvantage of static experiments is that the results are mostly descriptive and they don't give a numeric value of susceptibility to weathering. For this reason geotechnical engineers prefer dynamic tests such as the so- called "Slake durability test" [4]. According to Gamble [4] the best way to define durability for most rocks is durability index calculated after two cycles of Slake durability test - Id2. It is possible to notice that generally smaller index values indicate softer - less durable rock, while higher index values indicate harder - more durable rock. However, current research questions if this proposed classification is suitable for all types of soft rocks and whether there is need for changes or additional criteria [5, 6]. One way to deal with this problem is to increase the number of cycles to four or more, taking one or more values of durability index as relevant, and/or calculating some form of “Decay index”. The second approach usually extends the duration while maintaining a single cycle of rotation, i.e. to select 10, 25, 60 and 120 minutes. One way of presenting results can be determined by plotting the ratio of test results SDI10/SDI(time) a time scale, consequently the result would be the degree of regression curvature (LSI) which is different for each material. Moreover, it increases as the durability of a rock decreases [7]. Therefore, by using a regression analysis of the data, the relationship between aforementioned can be described in a common logarithmic function: When plotted on a semilog scale it represents the slope of the line going through (10, 0). Unlike SDI which is expressed on a scale of 0 to 100, where low-resistant materials take on lower values and high-resistant higher ones, in the case of highly resistant materials LSI values are lower (approaching to zero), while the low- resistant materials have higher LSI values. Consequently, results are presented in different orders of magnitude (from 0.001 to 10) and are therefore more understandable and easier to interpret. Several variants of the slake durability index were analysed on marl samples from Dalmatian region with different number of cycles, different cycle time, and combinations thereof. Based on the results obtained, according to the type of degradation after slaking, the samples are grouped into three groups. Considering the results, classification with the defined criteria for high, medium and low durability has been suggested for the purpose of durability analysis of cuts in marls [8 - 10].

Soft rock, Marl, Weathering, SDI, LSI ; Durability classification

http://mernokgeologia.bme.hu/ocs/public/conferences/2/s chedConfs/12/program-hu_HU.pdf