With the availability of different types of bolts with different surface profiles, the present selection of bolts as primary supports in coal mines is mostly dictated by cost factors. Few mines assess the ability of different bolts to act effectively in different ground conditions that range from strong and competent formation to weak stratification.
Questions are often asked as to why some bolts have higher and wider spaced profiles while others have shallow and narrow spaced profiles, and how each type reacts in different ground conditions.
The common method of pull-out testing is unlikely to provide answers to the above questions as it is not dealing realistically with the causes of the level of shear stresses generated and load transfer mechanisms at the rock/resin/bolt interface, said associate professor Naj Aziz from the Faculty of Engineering, University of Wollongong.
Aziz said the recent research work involved the study of the load transfer mechanism of bolts by shearing the surfaces of a bolt against the encapsulation resin surface under constant normal stiffness condition, rather than under constant normal load.
Testing under constant normal stiffness conditions permits the rubbing surfaces to move apart during the sliding of the bolt against the resin surface. If the surrounding rock mass is unable to deform sufficiently, then the inevitable increase in the normal stresses occur due to shearing. Hence, testing under constant normal stiffness conditions represents a realistic approach to the shearing test (hence load transfer mechanism) as it accommodates the changes in normal stress due to shearing of the bolt surface against the resin surface.
According to Aziz, the research work initially consisted of a comprehensive programme of laboratory experimentation using a specially constructed constant stiffness testing machine. The findings from the laboratory work were supported by theoretical analysis as well as field studies.
The field studies were conducted at headings and cut throughs serving longwall faces in two mines in the southern coal fields of NSW. The details of the study are available on request.
The most important finding from the study was that bolts with wider spacing and deeper profiles accommodated more ground deformation and displacement in soft and yielding ground without losing their grip. Bolts with shallow and closer spaced profiles suited stronger and competent ground conditions with less deformation.
Thus, in headings adjacent to longwall faces the installation of bolts with higher and wider spaced profiles provide better load transfer characteristics than the bolts with lower and closer spaced bolts.
Aziz indicated that this is ground breaking research being undertaken at the University of Wollongong into rock bolt load transfer characteristics, and forms part of an overall research program into rock bolt performance.
“The results of this research will be released over the next 18 months and will enable the mining and tunnelling industries to select the most appropriate roof support based on independent scientific facts,” Aziz said.
“The facilities on rock bolting research is now recognised as being one of the most advanced and comprehensive facilities available in Australia and is widely used by consultants, manufacturers, mine operators and research organisations.”
