Utilizing a novel fiber optic technology to capture the axial responses of fully grouted rock bolts

Rock bolts are one of the primary support systems utilized in underground excavations within the civil and mining engineering industries. Rock bolts support the weakened rock mass adjacent to the opening of an excavation by fastening to the more stable, undisturbed formations further from the excavation. The overall response of such a support element has been determined under varying loading conditions in the laboratory and in situ experiments in the past four decades; however, due to the limitations with conventional monitoring methods of capturing strain, there still exists a gap in knowledge associated with an understanding of the geomechanical responses of rock bolts at the microscale. In this paper, we try to address this current gap in scientific knowledge by utilizing a newly developed distributed optical strain sensing(DOS) technology that provides an exceptional spatial resolution of 0.65 mm to capture the strain along the rock bolt. This DOS technology utilizes Rayleigh optical frequency domain reflectometry(ROFDR) which provides unprecedented insight into various mechanisms associated with axially loaded rebar specimens of different embedment lengths, grouting materials, borehole annulus conditions, and borehole diameters. The embedment length of the specimens was found to be the factor that significantly affected the loading of the rebar. The critical embedment length for the fully grouted rock bolts(FGRBs) was systematically determined to be430 mm. The results herein highlight the effects of the variation of these individual parameters on the geomechanical responses FGRBs.

An in situ monitoring campaign of a hard rock pillar at great depth within a Canadian mine

A recent research campaign at a Canadian nickel-copper mine involved instrumenting a hard rock sill drift pillar with an array of multi-point rod extensometers,distributed optical fibre strain sensors,and borehole pressure cells(BHPCs).The instrumentation spanned across a 15.24 m lengthwise segment of the relatively massive granitic pillar situated at a depth of 2.44 km within the mine.Between May 2016 and March 2017,the pillar’s displacement and pressure response were measured and correlated with mining activities on the same level as the pillar,including:(1)mine-by of the pillar,(2)footwall drift development,and(3)ore body stoping operations.Regarding displacements of the pillar,the extensometers provided high temporal resolution(logged hourly)and the optical fibre strain sensors provide high spatial resolution(measured every 0.65 mm along the length of each sensor).The combination of sensing techniques allowed centimetre-scale rock mass bulking near the pillar sidewalls to be distinguished from microstrain-scale fracturing towards the core of the pillar.Additionally,the influence and extent of a mine-scale schistose shear zone transecting the pillar was identified.By converting measured rock mass displacement to velocity,a process was demonstrated which allowed mining activities inducing displacements to be categorised by time-duration and cumulative displacement.In over half of the analysed mining activities,displacements were determined to prolong for over an hour,predominately resulting in submillimetre cumulative displacements,but in some cases multi-centimetre cumulative displacements were observed.This time-dependent behaviour was more pronounced within the vicinity of the plumb shear zone.Displacement measurements were also used to assess selected support member load and elongation mobilisation per mining activity.It was found that a combined static load and elongation capacity of reinforcing members was essential to maintaining excavation stability,while permitting gradual shedding of stress through controlled pillar sidewall displacements.