Assessment of instability initiation in exposed salt rock roofs

Stability of rocks is the main requirement for the safe operation of mines. For this purpose, certain measures are applied for the protection and support of underground openings, including roof support design and roof arrangement in the most stable rocks. Stability assessment of underground excavations is largely related to their roof stability. Determination of possible instability conditions in mine roofs governs the choice of support system design and parameters of mine excavations. The Upper Kama Potash Salt Deposit represents a stratified layer of solid salt rocks. Roof instability develops as stratification and roof collapse. The Mohr–Coulomb criterion of coherent rocks is currently used to estimate parameters of a possible damage zone in the exposed roof. This criterion allows evaluating shear plane angles in roof rocks and, as a result, finding parameters of the possible collapse zone. The experience of mining operations in the Upper Kama deposit shows different failure conditions as against the Mohr–Coulomb criterion as the stress state is scarcely included in the criterion used. This study is an attempt to assess parameters of rock exposures by solving a Lame problem in terms of a single mine excavation. The analytical results were compared with the parameters obtained from the plane elasticity solutions by the finite element method. Based on the implemented studies, an engineering approach is developed for the assessment of anticipated instability parameters in exposed roofs in horizontal excavations driven in salt rocks.

Unit weight patterns of overlying rocks above commercial-value salt strata in the Upper Kama deposit

Stability of rocks is the main requirement for the safe operation of mines. For this purpose, certain measures are applied for the protection and support of underground openings, including roof support design and roof arrangement in the most stable rocks. Stability assessment of underground excavations is largely related to their roof stability. Determination of possible instability conditions in mine roofs governs the choice of support system design and parameters of mine excavations. The Upper Kama Potash Salt Deposit represents a stratified layer of solid salt rocks. Roof instability develops as stratification and roof collapse. The Mohr–Coulomb criterion of coherent rocks is currently used to estimate parameters of a possible damage zone in the exposed roof. This criterion allows evaluating shear plane angles in roof rocks and, as a result, finding parameters of the possible collapse zone. The experience of mining operations in the Upper Kama deposit shows different failure conditions as against the Mohr–Coulomb criterion as the stress state is scarcely included in the criterion used. This study is an attempt to assess parameters of rock exposures by solving a Lame problem in terms of a single mine excavation. The analytical results were compared with the parameters obtained from the plane elasticity solutions by the finite element method. Based on the implemented studies, an engineering approach is developed for the assessment of anticipated instability parameters in exposed roofs in horizontal excavations driven in salt rocks.

Determination and verification of the calculated model parameters of salt rocks taking into account softening and plastic flow

The article suggests using a combination of the modified Burgers model and the Mohr – Coulomb model with the degradation of the adhesion coefficient and the increase in the friction coefficient to determine the parameters of salt rocks. A comparative analysis of long-term laboratory tests and field observations in underground mine workings with the results obtained using a calculated model with certain parameters is carried out. The parameters of the Mohr – Coulomb model with the degradation of the adhesion coefficient and the increase in the friction coefficient were obtained from the statistically processed data of laboratory tests, and the parameters of the modified Burgers model were determined. Using numerical methods, virtual (computer) axisymmetric triaxial tests, both instantaneous and long-term, were performed on the basis of the proposed model with selected parameters. A model problem is solved for comparing the behavior of the model with the data of observation stations in underground mine workings obtained from borehole rod extensometers and contour deformation marks. The analytically obtained coefficients of the nonlinear viscous element of the modified Burgers model for all the analyzed salt rocks did not need to be corrected based on the monitoring results. At the same time, optimization was required for the viscoelastic element coefficients for all the considered rocks. The analysis of the model studies showed a satisfactory convergence with the data on the observation stations. The comparative analysis carried out on the models based on laboratory tests and observations in the workings indicates the correct determination of the parameters for salt rocks and the verification of the model in general.

Insight into a salt diapir: microstructural study of Praid (Transylvanian Basin, Romania) salt rocks

<p>Middle Miocene salinity crisis in the Central Paratethys resulted in significant amounts of marine evaporite deposits in the Transylvanian Basin (TB), Romania. The thickness of salt at Praid area is potentially suitable for underground storage of radioactive waste or gases. One of the main factors that determines the potential usage of this voluminous salt body for storage or disposal of various materials is the microstructural characteristics of the salt rock.</p><p>Praid is located at the eastern margin of the TB as part of the eastern diapir alignment. The underground salt mine at Praid has been operating there continuously for centuries.  It is an ideal place for sampling the internal part of a salt diapir body, where 20 representative samples were collected. The aim of this study is to extend our understanding of the deformation mechanism in the Praid salt rock.</p><p>Primary and secondary structural features were observed and distinguished through detailed petrographic observation. Two types of salt rock were identified: 1/ massive grey salt with large, elongated halite crystals, containing primary fluid inclusions (FI<sub>p</sub>), accompanied by submicrometer sized grains of halite and clay matrix, and 2/ layered salt with more uniform grainsize distribution showing alternation of greyish (clay rich) and white (clear halite) layers. The layered rock type has mosaic-like structure with a large number of secondary fluid inclusions (FI<sub>s</sub>). Beside halite, authigenic anhydrite and dolomite are present subordinately (~ 1 vol. %). Secondary fluid inclusions, composed of nitrogen and methane, are indicators of fluid migration pathways throughout the salt body.</p><p>Electron Backscatter Diffraction (EBSD) mapping was performed both in the massive and layered salt samples to shed light on the microstructure of the salt rocks. Gamma irradiation was carried as a complementary method of EBSD mapping. Comparing the subgrain diameters obtained from the two techniques, the values are fairly overlapping. The detailed microstructural observations allowed to recognize both dislocation creep and pressure solution processes, which acted concurrently in the Praid salt rock. The differential stress calculations on the salt rock samples indicate a maximum differential stress less than 2 MPa for the massive salt and less than 1.8 MPa for the layered salt. The strain rate calculations (total strain rate between 7.3*e-11 s<sup>-1</sup> and 1.8*e-10 s<sup>-1</sup>) are in good agreement with the observed features in the salt mine, where one of the ~260-year-old salt extraction chambers suffered at least 10 % compressional deformation.</p><p>The microstructural characters of the salt body reveal a complex deformation history where fluids have played an important role. The results of this project will be useful and comparable with the regional geological knowledge, to better understand the evolution of this Middle Miocene salt body.</p><p>The project is supported by the Cooperative Doctoral Programme of the Ministry for Innovation and Technology (ITM).</p>

Relationship between crack length, wedge-shaped cutter geometry, and salt rocks viscosity in a cross-cutting scheme

Introduction. Existing publications lack studies on the relationship between salt rocks physical properties, cutters geometry, cutting force, and intergranular fracture of rock under the cutter. By analyzing the system of cracks formed by the cutters between the cutting lines, it is possible to estimate the effi ciency of fracture and design the nature and conditions of cutting. Research aim is to obtain an analytical dependence that links cracks size, cutter geometry, and salt rocks crack resistance; calculate the cutting force; experimentally determine sylvine, halite and carnallite fracture toughness coeffi cients necessary for the calculation. And fi nally, based on the obtained data, the research aimed to build 3D graphs of crack length dependence on cutter geometry for a cross-cutting scheme. Methodology. Sylvine, halite and carnallite crack resistance coeffi cients were obtained by indentation. The coeffi cient values were used in the formula for calculating the size of cracks between the cutting lines in these rocks. The formula was corrected after D6.22 cutter indentation test in salt rocks. Light microscopy technique was used to study fl uid inclusions in salt rocks. Results. Analytical dependence, values of crack resistance coeffi cients were obtained. 3D graphs for halite, sylvine and carnallite were constructed for the cross-cutting scheme. The type, size and concentration of fl uid inclusions along the grain boundaries are given that accelerate intergranular fracture under the cutter. Conclusions. The resulting formula relates cutter geometry (cutting rim width and cutting angle) to the cutting force and viscosity of rocks. The formula was used to build the 3D graphs for sylvine, halite and carnallite for the cross-cutting scheme. The size of cracks under the cutter is connected with the presence of fl uid inclusions. The obtained analytical dependence allows to model the spatial distribution and size of microfractures in salt rocks under the action of the cutting tool. Excessive branching of cracks, energy intensity, and the number of small fractions decrease, when the trajectory of the cutter partially passes through the “technogenic” cracks of the previous bed. This is implemented in the cross-cutting scheme, where this group of cracks plays the role of “starting” ones. Their length in actual practice is important for justifying the optimal cutting parameters and estimating the cutter’s effi ciency.

The Lotsberg Salt Formation in Central Alberta (Canada)—Petrology, Geochemistry, and Fluid Inclusions

The Lotsberg Salt Formation (LSF) of the Lower Devonian age occupies a large area in Alberta (Canada). It has been used for brine production, disposal, and storage purposes since the 1950s. Its petrological and geochemical features remain poorly understood up to now. Previous studies showed that these salt rocks are large crystalline and distinguishable by a very low bromine content (2–5 ppm). Our studies reveal that the main impurity is dolomite with an addition of haematite. It showed, also, a lack of sulphate minerals (anhydrite). Manganite also occurs within the halite crystals. Microthermometric measurements of primary fluid inclusions in halite show a large range of homogenization temperatures from 32.4 °C to 357.0 °C with the highest temperature in the upper part of the salt profile. Geochemical analysis confirms the low bromine contents, which is between 0.67–12.74 ppm. Potassium contents (166–3651 ppm) seem to be in the normal range for salt rocks, but magnesium content (25–177 ppm) is much lower than potassium. Rubidium is, as well, within the normal range, with values between <0.01 ppm and 3.13 ppm, while caesium contents (5.07–211.22 ppm) are almost sixty times higher in comparison to those of rubidium. The high concentration of Cs, Mn, Rb, and the high homogenization temperatures of the host minerals suggest that the LSF underwent extensive ion exchange related to hydrothermal inflow. These hydrothermal solutions originated from the basement of the LSF.