Although the grouted bolts are generally suitable for use in most types of rock mass,expansion bolting still accounts for a significant share of underground mine workings,this is mainly due to tha simplicity of installation and the resultant higher bolting performance.
The stability of workings secured wirh rock bolting depends to a large extent on the proper selection of bolting components,as well as the technology adopted and care exercised during the installation.The design of rock bolting requires a comprehensive analysis of a number of elements,including the geological structure of the rock mass, geotechnical parameters of the rocks around the working and others.The methods for the selection of rock bolting and its design are always subject to uncertainty due to the simplification of the actual conditions. The threat of cave-in remains one of the major hazards in the underground mining industry.It depends dierctly on the local geology,the presence of horizontal strains and the type of mining method and performance of the bolting employed.
Testing interactions of expansion bolts of different designs with rock mass of varying properties.
The studies included following:
selection of 6 distinctive expansion bolt (bolts from a single production batch were tested so as to minimise potential production related variations)
selection of 7 underground test sites with varying roof conditions(in different parts of the mine)
installing 3 times 6 of different designs at each site
cyclic testing of the load-bearing capacity of rock bolting in underground conditions at the previously selected sites.
laboratory testing of the load bearing capacity of rock bolting for the bolt vs. rock relationship identical to the one used at the underground test sites(bolt heads installed in cores from the underground test,from the depth they are embedded in the roof)
strength tests on rock samples from the cores
strength tests of the rock bolting material
The presented underground test of interactions of various expansion bolt designs with roof rock of varying properties acknowledge that,for expasion bolts(with point installation),the proper restraint of the bolt head in the hole is of key importance,which is mainly affected by the design,geometry and material of the bolt head,geo-mechanical properties of the roof rock where the bolt head is installed,and the quality of bolt installation(in particular its proper pre-tensioning)
It is possible identify expansion bolt designs for specific roof conditions in order to achieve the most beneficial bolt vs.rock mass interaction conditions;
It is possible to identify expansion bolt designs that work correctly with most of the roof rock types and those whose design does not guarantee the proper bolting performace(roof reinforcement);
It is the quality of the bolt installation,particularly applying the correct bolt rod pre-tensioning,that is of key importance for the correct interaction of the bolts with the rock mass;
For rock mass with low strength,it is not advisable to use a mechanically fixed bolting,and the grouted bolting along the entire length of the bolt rod should be used as a proper reinforcement method.