Underground » Strata Control and Windblasts
In underground coal mining, the resin bond between the rock bolt and the strata is one of the critical elements of a roof bolting system, yet the Australian coal industry does not have an agreed standard for bolting system evaluation. A program of field and laboratory studies was undertaken to examine various factors influencing the load transfer mechanism between the bolt, resin and rock. The entire study used M24, 21.7 mm diameter X-grade Jennmar JBX bolts and the standard Orica fast setting resin. A series of Short Encapsulation Pull Tests were carried out in three mines. These mines were Baal Bone, Tahmoor and Gujarat NRE No.1. Additional studies included the evaluation of the anchorage performance along sections of bolts installed in steel tubes and variations in the strength properties of resin depending on sample dimensions. Factors of importance considered to affect bolt installation in strata include; borehole diameter, resin annulus thickness, installation time (including bolt spin to back and spin at back), the effect of gloving and its impact on installation quality and load transfer variation along the length of the installed bolt. 24 bolts were installed at each of Baal Bone and Tahmoor mines, and 16 bolts installed at Gujarat NRE No.1 mine. Installation of bolts in steel tubes was carried out at Springvale Colliery and subsequently tested in the Wollongong University Laboratory. The summary of the field studies found that:
· Bolts installed in holes over-drilled by 50 mm resulted in relatively higher load transfer capacity for the given installation time;
· Bolts installed in 27 mm diameter holes performed better than those installed in 28 mm holes;
· In some cases over-spinning was detrimental to the load transfer capacity of the installed bolt;
· The influence of gloving was reduced with over-drilling;
· Strength properties of resin tested at different length to diameter ratios did not vary considerably. In general, the length to diameter ratio of one was found to be a convenient dimension; and
· Consistency of the strength values obtained from testing resin samples was dependent on the methodology of resin mixing and casting.
Various laboratory procedures and testing resin and grout properties tests were evaluated as suggested by British and South African standards. Special emphasis was directed to determination of the following resin and grout properties:
· Uniaxial Compressive Strength;
· Young's Modulus of elasticity;
· Shear strength;
· Creep characteristics.
The results from this evaluation revealed that some aspects of the British standards have shortcomings in sample preparation, testing and presentation of the results. Therefore, a new sampling and testing procedure has been developed as part of this study. Laboratory tests indicate that the proposed testing method is reliable, repeatable, easy to conduct and produces meaningful results when compared to underground tests. The new testing procedure is considered to be acceptable for testing resins used in Australia.
An e-newsletter has also been published for this project, highlighting its significance for the industry.