Extension fractures that appear as very rough and irregular and with or without infill material, are common in thick-bedded and karstified limestones for which present-day methods for shear strength estimations are inappropriate. With the aim of determining the shear strength of such discontinuities, field research was carried out, in which large samples of blocks with discontinuities, as well as samples of filling material were prepared, on which then detailed laboratory tests were performed. Based on the obtained results, a modification of Barton's JRC-JCS empirical model was made in such a way that the friction angle of the built-in filling material in the case of discontinuity with a filling was applied, instead of the residual friction angle. In addition, the joint contact coefficient (JCC) was introduced as the measure of the actual contact area between discontinuity walls by means of which the normal stress on the discontinuity surface can be corrected.
It is generally known that the process of karstification creates specific morphological forms of grounds such as: caverns, sinkholes, shafts, swallow-holes, karstic depressions, karstic fields, and plateaus. However, apart from the specific morphological forms of ground, the process of karstification can create specific morphological forms of discontinuity wall surfaces (Hrženjak et al. 2019). Through the process of karstification, a process of chemical weathering of carbonate rocks under the influence of water acting like a weak carbonic acid, the walls of certain discontinuities are dissolved, resulting in very irregular and rough surfaces with prominent irregularities on both a small and large scale. This is especially prominent in thick-bedded and karstified limestones with many extension fractures that can be very rough and irregular, with or without filling material, as is the case in dimension stone deposits. A typical example of a such situation is the "Kanfanar" deposit, in which underground exploitation of dimension stone was introduced by the room-and-pillar excavation method (Figure 1). "Kanfanar" is one of the biggest limestone quarries in Croatia located on the Istrian peninsula.
