The comparison of different stress measurements in determining the stress profile of a near-surface low-stress hard-rock mine

• Main Author: Hertogs, Max (author)
• Other Authors: Barnhoorn, A. (mentor), Soleymani Shishvan, M. (graduation committee), Schmitz, Robrecht (graduation committee), Rinne, Mikael (graduation committee), Feldmann, Yannick (graduation committee), Delft University of Technology (degree granting institution)
• Format: Master Thesis
• Language: English
• Published: 2023
• Subjects: Norway; Stjernøya; Northern Norway
• Online Access: http://resolver.tudelft.nl/uuid:934ca6d4-01f6-4e5f-ad3d-b0e62c6c73c7

Measuring rock stress is a difficult process, especially in the Stjernøya Nepheline-Syenite mine in Northern Norway. The complex nature and topography of the shallow deposit makes it difficult to conduct industry standard tests. Therefore, this thesis aims at answering how different stress measurement techniques and their results compare to each other in a shallow low-stress hard-rock mine. To do this, an elaborate laboratory work is done to find a relation between stress (UCS) and acoustic properties and velocities of the nepheline syenite. Using that information, a similar combination of tests is done in the field, namely a flat jack test and acoustic velocity measurements with a hammer and geophones. To give further insight in the gathered field-data, visual classification methods, like RMR are done to verify and aid the tests and the results. The laboratory tests gave more insight in the relation between stress and acoustic velocity. There exists a positive, somewhat logarithmic relation between confining pressure and velocity through the sample, until the end of the elastic domain. Depending on the sample, this becomes more apparent after an initial loading phase between 0 and 10-20 MPa. In this low-stress zone, the wave arrival times could not be accurately observed and tend to give very low results. This problem translated to the field. Despite the flat-jack provided an understandable vertical pressure of 9.8 MPa, the acoustic measurements in the field did not always coincide with this pressure, according to the laboratory relationship, generally giving lower velocities than expected. However, a relation between rock quality and wave-velocity could also be present, as the cracks in the heterogeneous rock could heavily influence acoustic velocity. Therefore, this research concludes that both the flat jack and the acoustic investigative methods have their use in understanding the stress-profile of the shallow deposit. Given the low stresses in the mine, the acoustical investigative method could not be used ...