Varangu kihistu savi kasutamine graptoliitargilliidi utiliseerimiseks [Magistritöö. Juhendaja: R. Hints]

Using open cast technology for phosphorite mining in the Toolse phosphorite deposit requires removal of a lot of overburden, including graptolite argillite of the Türisalu Formation and clays of the Varangu Formation. Graptolite argillite is known to be prone to spontaneous ignition when stored under atmospheric conditions. At the same time, clays from the mining wastes have been used as a low-cost material for build hydraulic barriers and covers for safe storage of black shales like graptolite argillite.

For this thesis, I tested the geotechnical properties of the clays from the Varangu Formation to assess their potential usage for graptolite argillite utilization. The material for the study was sampled from the Toolse PH014B drillcore. I conducted Atterberg limit tests of the material to study the plastic behaviour of the clays. The number of other geotechnical properties such as granulometric composition, the content of water, compaction properties (Proctor test), unconfined compressive strength, shear strength, and hydraulic conductivity were determined by laboratory testing. The geochemical and mineral composition of samples were analysed by X-ray fluorescence and X-ray diffraction methods. Additionally, the thesis gives a brief overview of existing technologies of black shale storage in mining to delineate potentially safe methods for handling graptolite argillite wastes.

According to the tests, deposits from the Varangu Formation could be classified as low plasticity clay (soils). Based on the granulometric analysis, a fine silty fraction dominates in their composition, and the material falls into the soil category known to be vulnerable to freeze/thaw damage. The measured hydraulic conductivity of the clay, compacted under optimum water content (14,5%), is rather low – 0,77×10-9 m/s. During the direct shear strength test, the failure of the sample was recorded under low confining stress (30 kPa). In the mineral composition of studied samples, clay minerals, potassium feldspar, and quartz dominate, while sulphur content stays below 1%.

Based on the results, the studied material has mostly adequate geotechnical and hydraulic properties to construct effective compacted clay barriers. The volume of the clays from the overburden is
insufficient for building a whole liner system if layered dumping of graptolite argillite is planned.