The Geological Survey of Estonia conducted various drillings in 2020-2021, and Estonian experts are currently analysing the valuable rock material obtained in the course of these. As of now, a part of the survey has been completed, with its overall aim to use new drill cores to assess the size of stable mine workings that could be geotechnically established in the phosphorite layer.
The completed work is based on ‘Temporal and spatial planning of a virtual mine’, which was prepared by OÜ IPT Projektijuhtimine in 2017 as part of a project of Tartu University entitled ‘The environmental impact of the possible extraction of mineral resources in the Virumaa region on groundwater and surface water and the landscape, analysed with environmental geological models, together with alternative alleviation measures’. At that time, the researchers emphasised that the reliability of the data used as input in the calculations was a problem in performing the analyses. The material drilled in the course of historical surveys was largely so fragmented that it could not be tested in order to determine its physical-mechanical properties. The quality of the cores freshly drilled by the Geological Survey of Estonia, however, was sufficiently good and allowed to determine the respective indicators of the rock samples at a laboratory, including unconfined compression strength (USC), volumetric weight, specifying gravity, etc. On the basis of the obtained lab results and the geological and geotechnical descriptions, OÜ IPT Projektijuhtimine in cooperation with the Geological Survey of Estonia updated and specified the geotechnical model prepared in an earlier project. The main conclusions of the survey are:
- Both laboratory tests and drill core descriptions indicate that the strength of phosphorite decreases as the overburden thickness increases.
- In the sample area of the Rakvere deposit, an excavation tunnel with a width of 1 metre at a depth of about 100 metres would entail a risk of collapse without a support system.
- In the sample area of the Toolse deposit, excavation tunnels with a width of up to 2 metres can be established at a depth of about 40 metres in a manner that ensures their stability.
It is important to note here that this survey is part of an on-going complex survey of critical resources in Estonia, and its results will be used as underlying materials for the subsequent stages. Above all, the information obtained from the survey is useful in providing assessments on the technology and cost of mining, which in turn is necessary in order to assess the overall reasonability of utilising the resource. The conclusions presented in the survey report should be construed as a generalisation, and they should not be used as an alternative to detailed modelling in the course of designing a possible mine.
The completed report can be found in the Geological Archive.
