Factors Controlling Stope Wall Behavior Part 2

Reinforcement by cablebolting provided at selected locations, usually constrained by the distance between drilling sublevels, can be used to reduce the deformations experienced at the stope boundaries (crowns, walls, and rib pillars). Stope walls are pre-reinforced prior to any stope firings and, in most cases, cablebolts are installed from rings drilled within the stope access drives. Thus, stope wall reinforcement tends to be localized in continuous bands that are separated by the distance between the sublevel intervals. The function of such an arrangement is to divide the stope walls into a number of stable stope wall spans as well as arresting up-dip hangingwall failures.

Support from fill can also be used to minimize the deformations experienced by the stope walls while providing a restraint to any adjacent rock masses. In general, cemented fill is needed to recover ore from secondary stopes where stable fill exposures are required to minimize dilution. Cemented fill is essential in chequerboard extraction patterns within massive orebodies (Bloss, 1992), while uncemented fill is normally used in conjunction with bench stoping operations (Villaescusa and Kuganathan,1998). An example of a bench stoping extraction strategy linked to fill is shown in Figure 1.17. Here, the exposed wall length is usually limited to a critical value, defined by the distance between the fill and the advancing bench brow.

Blast Damage

Blast damage to a blasted rock mass refers to any strength deterioration of the remaining rock due to the presence of blast-induced cracks and to the opening, shearing, and extension of a preexisting or newly generated planes of weakness (Figure 1.18). It is generally accepted that the damage is caused by expanding gases through the geological discontinuities and the vibrations experienced from the blasting process. However, it is not easy to establish the approximate contribution to damage caused by the expanding gases, as it is difficult to measure their path within a rock mass discontinuity network. Nevertheless, significant backbreak may be regularly observed when the explosive gases are well confined within a volume of rock, and in some cases the gases can travel well beyond the location of the explosive charges.

Damage by the shock energy from an explosive charge close to a blast can be related to the level of vibrations measured around the blasted volume. Repetitive blastings also impose a dynamic loading to the exposed stope walls away from a blasted volume, and may trigger structurally controlled falloff and ultimately overbreak. Conventional blast monitoring and simple geophysical techniques can be used to measure the effects of blasting in the near field. Vibrations and frequency levels from the shock wave can be measured reasonably accurately (Fleetwood, 2010). These data can be related to damage provided the contribution (to the total damage) from the shock energy can be estimated. Vibration and frequency levels at the mid-spans of instrumented stope walls can be used to characterize the dynamic response to blasting at the stope boundaries.

Drill Drive Layout

Additional factors such as poorly located or preexisting drives, which undercut the stope walls, also contribute to dilution or falloff at the stope boundaries. In general, the number and location of drilling drifts in open stoping are usually functions of the width of the orebody. In wide orebodies,

hangingwall and footwall drill drives are used to provide cablebolt reinforcement and to minimize the impact of blasting at the stope boundaries (Figure 1.19). In such cases, drilling and blasting can be carried out in a plane parallel to the final stope walls or to any exposed backfill masses. Suitable values of standoff distance for the perimeter holes parallel to a stope boundary can be determined depending upon the rock type and the hole size being used.

Excessive wall damage, dilution, and ore loss may be experienced in cases where stoping requires drilling holes at an angle to a planned fill exposure or a stope boundary. Furthermore, hole deviation at the toes may create an uneven stope surface, thereby preventing effective rilling of the broken material to the stope drawpoints. In addition, hole deviation may cause excessive confinement at the hole toes, thus causing breakage beyond the orebody boundaries.

Share this post