CPTU in soft post-flotation sediments

The history of copper ore mining in the Lower Silesia, a region in south-west Poland, goes back to the XIX century when the first shafts were bricked off. Since that time the industry has been continuously developed up to this day. One of the most important event in its history, i.e. establishing of KGHM Polska Miedź S.A., the managing company being now ranked as the seventh largest copper supplier and the second of silver supplier in the world took place in 1961. Unfourtunately, only 5% of mineral product includes copper ore; the remaining part is from the technological point of view completely useless. Mineral wastes – this is a proper name – are very specific kind of soil. Although at first sight they strongly resemble natural soil, they are in fact a man-made material being a by-product of technological process in which the metal-bearing rock is crushed to small fractions, varying from fine sand to clay particles. This posed a serious problem of mineral waste deposition, the solution of which turned out to be post-flotation sediment dumps. Of six dumps in the region, the newest is the reservoir of Żelazny Most, located between Lubin and Głogów. Started in 1977, today of the height exceeding locally even 45 m, the diameter of approximately 5 km, the length of dams over 14 km and accumulation capacity of 350 million m3 it is the biggest hydrotechnical construction in the world. Taking advantage of water, the crushed rock is sent to the reservoir through a wide net of pipelines. This transport method allowing dumping of the waste material is responsible for a significant variability of not only its grain-size distribution, but also of its physical parameters which affects the drainage conditions. Because of the complexity of the Żelazny Most project, several scientific centers and institutes both from all over the Poland and Europe (e.g. Norwegian Geotechnical Instuitute) have been involved in its supervision; moreover, International Experts' Board - an advisory body - was also established. Due to their efforts, the CPTU test was decided to be a reliable method of assessing current effective stress conditions. However, it soon became clear that interpretation of the data on artificial soils such as mine tailings had to be improved to some extent. Because of their special character, local correlations between penetration characteristics and shear strength parameters or deformations characteristics of post-flotation deposits should be found.