Civil Engineering April 2021 | Vol 29 No 3

Civil Engineering April 2021 33 It was also found that the maximum bolt forces occurred in or around the relatively weak coal layers. Figure 9 pro- vides a typical output showing the total expected displacement and the mobilised bolt forces. Conventionally an “open” face protection system comprising a suitable draped and pinned mesh would have been appropriate to catch and hold loose rock fragments. However, as non-durable carbonaceous shale layers are susceptible to rapid weathering and deterioration, a decision was taken to fix drainage to the rock face and apply 25 MPa shotcrete/ sprayed concrete onto the meshed face. This served to protect the sensitive materials from the elements and to retain finer fragments that would otherwise have exited through the apertures of the mesh. Secondly, and after exposure and inspection of the final highwall face, localised measures were designed on a case-specific basis to safeguard against larger kinematic instabilities wherever these present. These localised measures are installed over and above the nominal stabilisation measures. Upon completion of excavation, the slopes were laser scanned by Pinpoint 3D in order to update discontinuity data and kinematic analyses. A shear zone, containing unstable wedges on the north of the highwall was noted. The shear strength of joints contained within the shear zone were estimated through inspecting joint roughness and infill with the aid of a telescopic handler. Joint surfaces were found to be mostly smooth- undulating to smooth-planar with little to no infill. Photos from the joint inspection process are shown in Figure 10. Wedge sliding and support analyses were subsequently undertaken for various localised and larger potential instabilities utilising Midas’ Soilworks deterministic limit equilibrium software. Joint param- eters were validated by seeking a factor of safety slightly above unity under currently prevailing conditions – good correlation was found. Additional support, in the form of a series of anchors, was designed for the localised potential instabilities such that adequate stability is maintained when less favourable load conditions manifest (increased water conditions on joints, etc.). A typical analysis conducted for a large local instability is presented in Figure 11. At the time of publication, rockfall net- ting, rock bolts and face drainage had been installed, while the installation of cable anchors and shotcrete were in progress. CONCLUSION The success of the project may be at- tributed to: Q Q Thorough geotechnical investigation including combining terrestrial laser Figure 9 FEM output from generalised slope protection analysis 0.0000 0.0002 0.0004 0.0007 0.0009 0.0011 0.0013 0.0015 0.0018 0.0020 0.0022 TotalDisplacement min (stage):0.0000m max (stage):0.0021m 0kNAxialForce 88.138kNAxialForce Figure 10 (a) and (b) Inspection of joint roughness and infill Figure 11 LE output with cable anchors installed (a) (b)