Interaction between shotcrete and rock experimental and numerical study

Sammanfattning: At LKAB’s underground mines in Malmberget and Kiruna in northern Sweden shotcrete and rock bolts are the main rock support elements. A safe working environment for the miners and high accessibility to the mine are important issues. To address these issues and to improve the use of the support, LKAB initiated a doctoral thesis study on the interaction between its rock support system and the rock. The objectives of this thesis were: to identify important parameters involved in the interaction between shotcrete and rock and if necessary investigate important quantities of these parameters, to improve the understanding of the interaction of shotcrete and rock. To achieve the first objective a number of laboratory and field tests were carried out. The second objective was achived by numerical analyses. The main conclusions are: The analyses showed that the rock strength and the unevenness of the surface had a large impact on the number of failures at the interface and in the lining. Furthermore, the behaviour of the lining was sensitive to small amplitudes of the surface roughness. A high bond strength was favourable according to the analyses. The results from the field tests showed the importance of the bond at the interface. If bond failure occurs the ability to distribute and control the crack width ceases. Failure mapping in the mine showed that that a thin shotcrete layer (<20 mm) did not perform well. It is therefore, important that the designed thickness is achieved when shotcreting. The extent of the excavated damage zone had a minor effect on the behaviour of the shotcrete lining if the surface was uneven. For an even surface, the influence of the extent of the EDZ on shotcrete was obvious but not serious. The rock support is generally designed for static loading conditions. In many cases, however, the openings are also subjected to dynamic loads. By field tests, vibration measurements and analyses the influence from blasting induced stress waves on the performance of shotcrete was investigated. The main conclusions are: Failure mapping showed that the fibre reinforced shotcrete has the ability to give stable conditions close to the drawpoint. The non-linear analyses showed fair agreement with the failure mapping. The single-degree-of-freedom (SDOF) model, though it is relatively simple, can be used to predict the response of an arbitrarily shaped rock wedge supported by shotcrete.