Geotechnical characterization of mining rock waste dumps in central Evia, Greece

Open pit mining using drill and blast has been taking place in central Evia, Greece for many decades. This is so, due to the presence of ferronickel ores in the area. As a consequence of the mining activity, large volumes of overburden Upper Cretaceous limestone are being annually extracted and deposited at predefined sites, creating rock waste dumps or embankments. In the context of optimum land surface use and better stability conditions, a geotechnical characterization of these waste dumps has taken place and is presented in the current paper. First, image analyses are performed on fragmented limestone muckpiles and the degree and intensity of rock fragmentation due to blasting is assessed. Results compare reasonably well with computations based on the Kuz-Ram empirical model for fragmentation analysis. Overall, muckpiles on excavation faces consist of boulders and cobbles by 80–90% per weight. Following, a laboratory investigation program is executed on samples retrieved from the waste dumps. Based on the results, the dumps consist of heavily coarse grain material, mostly classified as GC. According to modified Proctor tests and in-situ density tests using the Troxler device, compaction on the crest of the dumps is characterized as very high, while permeability is empirically considered as medium to high. As far as the friction angle of the waste dump material is concerned, this is estimated based on observations (i.e. recorded inclinations of existing dumps), combined with an empirical model from the literature. Generally, it is considered that the friction angle varies considerably with the overburden pressure within the dump’s body. Nonetheless, if a single-value approach is necessary for simplicity purposes and stability considerations, then a friction angle of about 40° (characteristic value) is suggested as a reasonable assumption.