After the water intersect in March 2012 standpipes to drill through were used that could withstand the high water pressure at the development face. Standpipes were installed and drilling of holes for high pressure grout injection was conducted through the collar of the standpipes. If large water flows were encountered the drill was withdrawn from the hole and the water flow was readily controlled by closing the valve on the standpipe. After installation and before drilling through the standpipes commenced, the standpipes were pressure tested by pressurising to 100 bar to assure that they would withstand the high water pressure at the development face. Initially, the standpipes were grouted into position during installation. This installation method resulted in a failure rate for pressure testing of 50%. Consequently resin was trialled for standpipe installation. The use of resin improved the success rate of pressure testing to 90% for initial resin injections and to 100% once the resin injection methods were refined. The use of resin for stand pipe installation allowed to significantly increase the cycle time during development. This was not only due to the higher success rate during pressure testing, but also due to the reduction in setting time. Grout has a 12 hour setting time, while resin sets in 90 seconds. To decrease the development time in areas that required grouting, hole patterns for grout injection were adjusted from patterns used in tunnelling to patterns suitable for the steeper declines and higher water seepage tolerances of mining environments. To allow successful grouting on the steep decline (1/7), which even at minor water intersects can quickly get flooded, the work area set up and grouting patterns had to be adjusted. The work area was optimised to allow water to flow away from the face by backfilling the decline before the face to level ground. The standpipe pattern was adjusted to maintain the required 5 metre grout halo around the designed decline for grouting in a backfilled state. Accurate monitoring of the pumped grout volume and of the reduction in water flow for each grouting ring allowed optimising standpipe and grouting patterns. Monitoring showed that 75% of the grout uptake was achieved with the 9 perimeter holes highlighting that 75% of voids were sealed through these outer holes. This observation allowed sequential reduction of the number of standpiped holes for grout injection from initially 23 to 17, next to 13 and finally to 9 while still staying below the set limit for water seepage of 2 l/s. The drill angles for the standpipe pattern were simplified from 9 bearings to 3 bearings, and from 23 angles to 3 angles, which aided in reducing driller errors. The length of the drill holes for grout injection and hence the length of the grout curtain was increased from 13.2 m to 20.4 m and finally to 31.2 m, which further aided in decreasing the development time. In total 301 m of development were completed at this heading before the northern boundary of the highly conductive zone of the fractured aquifer was reached. Throughout these 301 m of development 727,525 litres of grout were injected within 18 separate grouting zones. SUMMARY AND CONCLUSIONS Analysis of hydrographs from 56 Vibrating Wire piezometers allowed identification of eight separate zones in the fractured aquifer at EHM. The aquifer zone with the highest risk of significant water intersects was identified base on the occurrence of past water intersects and a high inter zone hydraulic connectivity. High inter zone connectivity was indicated by the strong similarity in hydrograph response for all piezometers in this zone. Targeted dewatering of the highest water risk zone allowed lowering the water pressure at the main development heading from 4200 kPa to 700 kPa within 2 month. The depressurization was achieved with minimum drilling effort of only six 30 m long drill holes drilled with a production drill rig. The use of standpipes to drill through allowed controlling high water flows during development. Moving from grout to resin for stand pipe installation allowed significantly increasing the cycle time during development. The reduction in cycle time was achieved through an increase in the success rate of pressure testing of the standpipes after installation (from 50 to 100%) as well as through a reduction in setting time (Grout has a 12 hour setting time, resin sets in 90 seconds). Hole patterns for pressure grouting were modified from patterns used in tunnelling to account for the higher water seepage tolerance and the steeper declines of the mining environment. Accurate monitoring of the volume of the pumped grout and the reduction in water inflow for each grouting ring allowed decreasing the number of standpiped holes in the grouting pattern from 23 to 9 and the length of the grouting curtain from 13.2 to 31.2 m while not exceeding the acceptable leakage limit.
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