The effect of micro-fractures on the aggregate quality - limestone deposit Nielepice as a case study (Krakow-Wielun Upland, S Poland)

Rock mass divides naturally into blocks along parallel surfaces of discontinuity. It is provoked by different factors e.g.: sedimentation processes, thermal changes and tectonic stress. Zones of weakness manifest as visible fractures or remain as latent structures in the rock. It was assumed that variable stress conditions are responsible for forming of fractures. In practice this process can be initiated by blasting in quarry, crushing during aggregate production, atmospheric conditions and pressure impact on the aggregate grains used in road or railway tracks.
The phenomenon of appearance of originally invisible fractures has been analysed during the research on Jurassic limestone. The limestone deposit from quarry Nielepice known as Mlynka was investigated. It is located ca 20 km west of Krakow. Three facies of limestone can be observed in the quarry: dominating massive and bedded limestone and platy limestone recorded only in the lowest part of quarry. The investigations were conducted in two positions of bedded limestone (I and II level) and one position of platy limestone (III level).
Measurements of joints orientation have been made and samples of rocks with determined orientation have been taken in each of these places. Systems of fractures obtained for all levels are similar to each other indicating the presence of three orthogonal joint sets. Oriented blocks of rocks were cut into cuboids which were subjected to various stress conditions under the critical value or repeated freezing. Changes occurred in the samples were determined by comparison between results of water absorptivity, apparent density and velocity of longitudinal ultrasonic waves measured in different directions before and after making an impact on rock material. It was found, that the range of changes which occurs in limestone is connected with its lithological type.
The series of microscopic slides, which had been made perpendicularly to the direction of most intensive marked fractures, were used to describe the nature of dynamic fatigue in rocks. The method of slides impregnation with blue resin was applied. The research project provided the results of relation between joints orientation recorded in quarry and frequency of micro-fractures in analysed samples taking into consideration their fatigue degree. There was also investigated an impact of fractures on the shape of aggregate grains. The method of digital image processing was used to confirm above-mentioned relations.
The study was supported by the AGH - University of Science and Technology, internal grant 11.11.140.562.