A practical solution using the Normet Self-Drilling Dynamic Bolt as an effective deep tendon support for high stress, squeezing ground and broken ground conditions.
Allan Punkkinen, Normet Canada
With the progression of mining to ever-increasing depth, degradation of the fracture zone deeper into the pillar of nearby development headings will occur as the stress front advances, the degree depending on the stresses and the material properties involved. While risk of rockburst activity can be mitigated with the use of dynamic support systems, there is often a practical limit as to the length of drill holes that can be drilled before installation becomes problematic. The degree of fracturing will result in significant effort to keep the borehole open long enough to be able to install the support component, particularly in walls and shoulders of a development heading. In the creation of intersections in high stress, brittle rock mass conditions, effective diameter of an opening may be compromised as it is slashed larger due to redistribution of stresses and subsequent seismic activity after blasting. This can result in a rockburst with depth of failure greater than the length of installed primary support itself. For long-tendon support, a practical solution has been available for years in the form of hollow-core bolts (self-drilling anchors), although not suitable as dynamic support adding only direct confinement to the rockmass. To maintain the integrity of the rockmass such that it can develop its own confinement, the design of stressed excavations requires yielding support. In response to demand from industry, Normet developed the SDDB® (Self-Drilling Dynamic Bolt), designed for squeezing ground and broken rock mass conditions where traditional bolts are difficult to install. Two variations of the bolt are available with a smooth seamless tube made from ductile steel and can be connected together with R32 thread on both ends providing deformable tendon support. The bolts are capable of >15% elongation and are a suitable grouting/resin injection and as a cable bolting alternative.
Self-Drilling Dynamic Bolt