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Weak rock mass strength estimation is a long-lasting challenge associated with geotechnical engineering due to its complex nature and limited definition. Weak rock masses normally refer to low strength, highly fractured decomposed and tectonically disturbed rocks which have properties intermediate from brittle rocks to ductile soils. Since the behavior of weak rock mass has not been fully understood, it is a common practice to apply existing empirical approaches, which are developed for competent rock masses influenced by joints, to determine their mechanical properties. This paper reviewed the current empirical approaches, and detailed weak rock mass strength calculations based on rock matrix, joint layout, joint condition and external factors. The limitations associated with these methods are discussed, and suggestions are provided for the selection of suitable methods.

The presence of iron sulphides in roofing slates affects their resistance to weathering. X-ray computed micro-tomography (CT) has been used on samples of Ordovician Luarca slate from Spain and Lower Carboniferous Culm slate from the Czech Republic. Iron sulphides were present as xenomorphic grains and grain aggregates of pyrrhotite in the Luarca slate and as very fine isolated grains of pyrite in the Culm slate. Results from the X-ray CT study were compared with those obtained from optical microscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses. Results showed that X-ray CT analysis could be a rapid and effective alternative technique for iron sulphide analysis, and can also show the spatial distribution of possible iron sulphides. The results were comparable to those of other methods for the Luarca slates, but the Culm slate was found to be on the very edge of applicability for X-ray CT analysis due to the similar size and densities of all the minerals present., The presence of iron sulphides in roofing slates affects their resistance to weathering. X-ray computed micro-tomography (CT) has been used on samples of Ordovician Luarca slate from Spain and Lower Carboniferous Culm slate from the Czech Republic. Iron sulphides were present as xenomorphic grains and grain aggregates of pyrrhotite in the Luarca slate and as very fine isolated grains of pyrite in the Culm slate. Results from the X-ray CT study were compared with those obtained from optical microscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses. Results showed that X-ray CT analysis could be a rapid and effective alternative technique for iron sulphide analysis, and can also show the spatial distribution of possible iron sulphides. The results were comparable to those of other methods for the Luarca slates, but the Culm slate was found to be on the very edge of applicability for X-ray CT analysis due to the similar size and densities of all the minerals present.

Results are presented from the Institute of Geonics in Ostrava, Czech Republic. Laboratory research into chemical grouts and geocomposites often requires different approaches from the standard laboratory tests used in the rock mechanics and building industries. The viscosity of several widely used two-component grouts has been tested for dependence on age and temperature. The physical and mechanical properties of cured grouts are tested by pouring them into cylindrical forms, using closed pressure forms for foaming grouts. There are two groups of tests for the grouting process: tests in pressure vessels filled with crushed material or tests of fissures in hard rock. The results from different methods used on the same materials are compared., Results are presented from the Institute of Geonics in Ostrava, Czech Republic. Laboratory research into chemical grouts and geocomposites often requires different approaches from the standard laboratory tests used in the rock mechanics and building industries. The viscosity of several widely used two-component grouts has been tested for dependence on age and temperature. The physical and mechanical properties of cured grouts are tested by pouring them into cylindrical forms, using closed pressure forms for foaming grouts. There are two groups of tests for the grouting process: tests in pressure vessels filled with crushed material or tests of fissures in hard rock. The results from different methods used on the same materials are compared.

Modern longwall mining technology was selected for the extraction of seam 504 at the Lazy colliery in the Ostrava-Karvina coalfield, Upper Silesia coal basin, Czech Republic. The seam is located at a cover depth of about 700 m and is 3.1-5 m thick. Two overlying seams were mined at average heights of 58 and 75 m from the planned working horizon of seam 504. The proposed longwall panel was adversely located below the goaf edges of the workings in the overlying extracted seams. Analysis of the inter-burden rock mass in the coal seams showed the presence of strong, massive strata consisting of sandstones and conglomerates with uniaxial compressive strength values of 70-120 MPa. Laboratory tests on a coal sample showed a high elastic deformation/total deformation ratio, indicating that the seam was prone to bursting/bumping. A de-stress design was adopted involving longhole drilling and blasting to fracture in advance the competent strata from both gateroads. The efficiency of the de-stress blasting at different mining stages was evaluated in relation to the seismic effect, calculated using seismic monitoring data and the weight of the charged explosive. Systematic adoption of de-stress blasting enabled the total 300 m length of the panel to be extracted without the occurrence of bumps or outbursts., Modern longwall mining technology was selected for the extraction of seam 504 at the Lazy colliery in the Ostrava-Karvina coalfield, Upper Silesia coal basin, Czech Republic. The seam is located at a cover depth of about 700 m and is 3.1-5 m thick. Two overlying seams were mined at average heights of 58 and 75 m from the planned working horizon of seam 504. The proposed longwall panel was adversely located below the goaf edges of the workings in the overlying extracted seams. Analysis of the inter-burden rock mass in the coal seams showed the presence of strong, massive strata consisting of sandstones and conglomerates with uniaxial compressive strength values of 70-120 MPa. Laboratory tests on a coal sample showed a high elastic deformation/total deformation ratio, indicating that the seam was prone to bursting/bumping. A de-stress design was adopted involving longhole drilling and blasting to fracture in advance the competent strata from both gateroads. The efficiency of the de-stress blasting at different mining stages was evaluated in relation to the seismic effect, calculated using seismic monitoring data and the weight of the charged explosive. Systematic adoption of de-stress blasting enabled the total 300 m length of the panel to be extracted without the occurrence of bumps or outbursts.