The contribution deals with the development of a new monitoring device for the measurement of the whole stress tensor in rock masses. The stress field is one of the principal factors which in a decisive way together with mechanical and deformation characteristics of rocks affect behaviour of rock masses. This issue has been investigated at the Institute of Geonics for a number of years, particularly in the context of assessing the impact of changes of the original stress field caused by anthropogenic interventions in masses (mainly in connection with dynamically changing the geomechanical situation as a result of mining). The device is based on the measurement of local deformations at the conical bottom of the well (Compact Conical-ended Borehole Overcoring and Monitoring techniques), from which it can be calculated the principal components of the stress field, or changes to it, and their orientation in space. The previously developed device can participate in repeated measuring. This significantly affects the frequency of read data especially in areas with a rapidly changing geomechanical situation. The new equipment is developed for use in an environment with a CH4 explosion hazard too. It is based on the already proven cited techniques in an ordinary environment. The devices allow monitoring and recording data about changes in stress in several places at the same time. In the case of system failure, each of the probes goes into autonomous mode data reading. These data will then be transferred from the backup memory of probes into the central data repository after resuming the system.
Knowledge, as accurate as possible, of the stress-strain state and its changes in rock mass is the determiningfactor for the proper planning of roadway support and for the correct design of underground mining as well as rockburst measures.
Stresses as well as stress changes cannot be measured directly. Stress determination is made indirectly, e.g. by the measurement of strain (e.g. Zang and Stephansson, 2010). A wide overview of these stress measurement methods has been presented (e.g. Zang and Stephansson, 2010). Deformation values obtained from an unbalanced body approaching equilibrium in combination with theoretical knowledge about constitutive behaviour (stress-strain relationship) allows us to evaluate the state of stress existing in any deformable body. Cells with strain gauges belong to physical methods for determining stress and stress changes in deformable materials with application to rock mass (e.g. Zang and Stephansson, 2010). Compact Conical-ended Borehole Overcoring (CCBO) methods are one possible method for in-situ stress measurement, which was established by Sugawara and Obara (1999). A modified version of the CCBO method called the Compact Conical-ended Borehole Monitoring (CCBM) method has been developed in the Czech Academy of Sciences, Institute of Geonics by Stas (Stas et al. 2004, 2005, 2007, 2011) and was tested for in-situ stress changes measurement in different rock masses (e.g. Konicek et al. 2012, 2013; Ptacek et al., 2015; Soucek et al., 2017).