Civil Engineering April 2021 | Vol 29 No 3

Civil Engineering April 2021 31 between the top of explosives columns and the collar of holes, loaded with decoupled explosives and detonated si- multaneously in advance of trim and bulk blasts. Presplitting provides a preferential fracture plane to intercept cracking pro- duced by adjacent trim and bulk blasting, while trim blasting reduces the rate of energy release against presplit planes. In an effort to limit backbreak and the dilation of sub-vertical jointing, some presplits were divided into separately fired groups. Trim blasts were taken together with bulk blasts, but only initiated after the detonation of bulk holes. In order to limit damage to presplit planes, due to high vibration levels and back thrust, trim and bulk blasts were initiated with electronic delay detonators, which provide for precise detonation timing and flexible sequencing. A robust quality control system, comprising as-drilled surveys, hole depth measurements, and charging and initia- tion inspections, were adopted to ensure adherence to blast designs. All blasts were captured via unmanned aerial vehicle (drone) for record keeping and analysis purposes. Blasting results were subjected to rigorous review to allow optimisation of subsequent blast designs. There was significant adaptation of blast design in the light of developing an understanding of the blasting results, with close commu- nication between the client, drill and blast contractor, explosives supplier, load and haul contractor, and consultants. Eire Contractors supplied drilling and blasting services, Enaex/Sasol supplied explosives and Andru Mining undertook load and haul of blasted materials. Total excavation quantity amounted to some 1.5 million m 3 with the following approximate divisions between various types of blasting: 1. Presplitting: 60 000 m 2 2. Trim blasting: 337 000 m 3 3. Bulk blasting: 1 177 000 m 3 LATERAL SUPPORT As mentioned previously, global slope stability of the rock mass over the larger stack, and on a bench scale, was adjudi- cated as satisfactory and did not require stabilising measures. However, localised structurally controlled instabilities were expected on the final highwall face. It was necessary to introduce adequate stability to the face such that safe access could be Figure 6 Various phases of the excavation process: (a) drilling of blast holes, (b) blast drilled and charged-up, (c) initiation of blast, (d) load and haul of blasted material (a) (b) (c) (d)