Shotcrete rock support exposed to varying load conditions

Sammanfattning: Field and laboratory tests and analyses This Licentiate thesis deals with the function of shotcrete as rock support. An extensive failure mapping of shotcrete in the Kiirunavaara mine has been conducted to improve the understanding of the performance of shotcrete. Furthermore, the adhesion strength obtained for different types of scaling/cleaning methods and the relationship between adhesion and compressive strength were investigated in field tests in the mine. The failure mapping showed that most of the observed failures of shotcrete are in areas with a thin shotcrete layer (<2 cm) together with a low adhesion strength. Because of that, it is very important to have a well- cleaned surface and to get sufficient thickness all over the shotcreted area i.e. to avoid areas with thin shotcrete. Furthermore, the growth of the adhesion strength of shotcrete on a sandblasted concrete wall and the growth of the compressive strength of shotcrete showed a clear correlation. The results from the field tests showed that the adhesion strength was significantly higher on rock surfaces that had been water-jet scaled (with 22 MPa water pressure) than on those treated by mechanical scaling followed by cleaning of the rock surface (water pressure 0.7 MPa). Higher rock stresses in the Kiirunavaara mine will probably require a more ductile surface support, which will increase the use of reinforced shotcrete. Panels and beams of reinforced shotcrete have been tested. The following issues were addressed, identification of failure modes of the shotcrete panels, comparison of the bearing capacity of shotcrete panels reinforced with two types of steel fibres and two dosages of steel fibres and comparison of the bearing capacity of steel fibre reinforced shotcrete with shotcrete reinforced with welded steel mesh. The test results indicate that the typical failure mechanism was a bending failure followed by punching failure. Furthermore, the tests showed that panels reinforced with steel fibres were more ductile and had higher punching capacity than panels reinforced with steel mesh. The effect of dome action was apparent in the tests and the measured bearing capacity was much higher than that calculated using yield line theory. Vibration measurements and dynamic analyses Rock support is mostly designed for static loads. In many mines, however, the openings are subjected to dynamic loads caused by blasting. The behaviour of the dynamic load from the production blasting has been investigated by vibration measurements. To study the behaviour of shotcrete exposed to the load induced by stress waves generated by the production blasting a single degree of freedom model (SDOF) was developed. The model consisted of a shotcrete layer and a symmetric rock prism. The recorded vibration was used as the external disturbance in the analysis. Observations in the field (failure mapping) have been done to identify failure mechanisms and the validity of the model. The vibration measurements showed a wide scatter of magnitudes of the particle acceleration, which was expected. The maximum particle velocity was 1.1 m/s at a horizontal distance of 4.5 m from the blast holes. The results from the linear elastic analyses, presented as a displacement response spectra, showed that a prism can be ejected from the roof as a result of production blasting, since the displacement was larger than the displacement at failure of the joints of the prism. This was also observed by the failure mapping of the roof of the cross cuts close to the drawpoint. Furthermore, the non-linear analyses showed that the reinforced shotcrete had the necessary bearing capacity to support actual prisms for load histories measured at a horizontal distance of 4.5 m away from the blasted fan. The failure mapping showed that failures in the reinforced shotcrete were observed at a horizontal distance of 1- 4 m from the drawpoint. The analysis also showed that plain shotcrete was too brittle. Field observations confirmed that the area or zone of failed shotcrete was much larger in areas with plain shotcrete compared to areas with reinforced shotcrete.