Civil Engineering April 2021 | Vol 29 No 3

30 April 2021 Civil Engineering Two slope designs were evaluated: 1. A three-tier design consisting of 15 m high vertical bench faces, and horizontal bench widths of 10 m on the upper bench, and 20 m to 30 m on the lower bench. 2. A steeper three-tier design, consisting of 15 m high 80° bench faces, and 5 m bench widths. Both designs were found to offer accept- able long-term global stability, and poten- tial instability was found to be governed by structurally controlled mechanisms on the face, such as toppling rotation, planar and wedge sliding. The design with the shallower overall stack angle (design 1 above) was selected and taken forward as the additional width on the benches afforded compliance with mine safety protocols, which impose 5 m and 3 m stand-off distances from bench faces and bench crests respectively. This design thus provides pedestrian access to the intermediate bench and vehicle access to the lowest bench for inspection and maintenance purposes. The design slope is shown in Figures 4 and 5. EXCAVATION The reactionary forces produced by blasting cause a deterioration of rock mass quality in faces adjacent to blasting, which is greatly exacerbated by poor control and the selection of inappropriate drill and blast methods and materials. Cost, pro- gramme, and slope life need to be taken into consideration when selecting drill and blast methods for final wall control. Although the temptation to apply lowest cost rapid drill and blast methods is great, these often impart substantial damage to final slopes which then require costly remediation. The optimal approach is best arrived at through multidisciplinary collabora- tion between the client, drill and blast contractor, load and haul contractor, and slope and blast designers. Given the influ- ence of geology on blasting outcomes, it is important that ground conditions are monitored as drilling and blasting progresses to allow appropriate alterations to blast designs where required. This approach was successfully adopted during the project. Excavation of the design profile, shown in Figure 5, was achieved through presplitting and trim blasting techniques. Presplitting consists of a single row of closely spaced holes which are left un- stemmed i.e. without backfilling the void Figure 5 Isometric view presenting slope design and transverse and longitudinal gradients 41 Portals 1 560 1 550 1 540 1 530 1 520 1 510 –10 0 10 20 30 40 50 60 70 80 90 100 110 120 Safety factor 0.000 6.000+ 5.500 5.000 4.500 4.000 3.500 3.000 2.500 2.000 1.500 1.000 0.500 2.255 Figure 4 2D LE and FE analyses to assess the global stability of the design slope; (a) shows results of a LE analysis and (b) shows the results of a FE analysis 1 560 1 500 1 510 1 520 1 530 1 540 1 550 –20 –10 0 10 20 30 40 50 60 70 80 90 100 110 MaximumShearStrain min(stage):0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 max (stage):0.001 Critical SRF: 2.77 (a) (b)