Civil Engineering April 2021 | Vol 29 No 3

28 April 2021 Civil Engineering In order to ensure the safe operation of the new underground mine, the existing Pit 2 highwall was redesigned to a slope geom- etry appropriate to the projected 20-year life of the new underground mine. Exxaro appointed specialist drilling and blasting contractor, Eire Contractors, to design the new highwall and execute the drilling and blasting thereof. Eire Contractors in turn retained the collaborative services of Rockland Geoscience and ARQ Geotech to design the highwall and monitor blasting and excavation works. GEOLOGY The new underground mine will exploit the no. 2 and 4 coal seams hosted within the coal-bearing sandstones and siltstones of the Permian Vryheid Formation, Ecca Group, Karoo Supergroup. The rocks of the Vryheid Formation either rest con- formably on diamictites and associated glaciogenic sediments of probable Dwyka age, or unconformably on basement rocks of the Lebowa Granite Suite, which in turn is underlain by volcanic rocks of the Loskop Formation. A stratigraphic column of the geology at the mine is presented in Figure 2. GEOTECHNICAL INVESTIGATION A geotechnical investigation was under- taken to generate geotechnical parameters as input to slope stability analyses. The geotechnical investigation consisted of a desktop study of all available geotechnical information pertaining to the develop- ment; 3D terrestrial laser scanning of mining slopes to obtain the dip, dip direction, spacing and persistence of dis- continuities; manual scan-line mapping of selected faces to ground truth the laser scanned data and inspect joint surface roughness profiles and infill; double tube rotary core drilling; down-hole acoustic and optical televiewer surveys of bore- holes to determine the dip and dip direc- tion of discontinuities in boreholes; and logging, sampling and laboratory testing of the borehole core. Laboratory testing comprised uniaxial compressive strength testing including Blasting and excavation of a new highwall for Exxaro Exxaro Resources is developing a new bord and pillar underground mine at its Dorstfontein Coal Mine East (DCME), 10 km north-east of the town of Kriel in Mpumalanga. The new underground mine will be accessed via seven portals driven from the base of the existing Pit 2 open cast operation. Gerrie van Jaarsveld Principal Engineering Geologist Rockland Geoscience gerrie@rockland.co.za Werner Britz Geotechnical Engineer ARQ Geotech werner@arq.co.za Thomas O’Brien Geotechnical Engineer, HoD ARQ Geotech thomas@arq.co.za Figure 1 DCME locality Figure 2 Stratigraphic column 0 Depth (m) 10 20 30 40 50 60 70 80 No. 5 Seam No. 3 Seam No. 4 Seam No. 1 Seam No. 2 Seam Alluvium Sandstone Siltstone Coal Limestone Diamictite Basement