An almost disastrous fall-of-ground (FoG) incident took place on the night of April 21, 2015, at gold mining company Gold One International’s Modder East Operations (MEO), in Springs, on the East Rand, it revealed recently.

The FoG covered an area of about 63 m on dip by 43 m on strike, in the back areas of E2-E10, E11 and E12 panels of the E2 raise-line. Gold One GM and executive director Isaac Tshabalala says, on investigation, it was established that the FoG was caused by overmining of the pillars, which caused the pillar design safety factor of 1.6 to drop to as low as 1.01. “About seven-million litres a day of additional fissure water was subsequently created by the FoG, thus requiring urgent strategic intervention to save the mine from possible flooding,” he highlights.

MEO is a shallow underground gold mine 300 m to 500 m mining depth. Tshabalala comments that, in its “pristine” hydrogeological settings, the mine’s major groundwater characteristics were seasonal high-low water level fluctuations, with most of the groundwater activity taking place in the upper levels of the Malmanie dolomites.

During both decline- and shaft-sinking operations, water was encountered, with a volume of up to 72 000 ℓ/h at 3 MPa at one stage during the sinking of the decline, or 150 000 ℓ/h during shaft sinking. Tshabalala remarks that such intersections prompted a systematic double cover drilling regime right at the start of the mine’s life.

He states that, during the prefeasibility phase of the mine design, it was estimated that five-million litres a day of fissure water would be pumped from underground when the mine reached its peak operation.

“This was, however, not the case, as we were pumping on average 12-million litres a day (inclusive of three-million litres of service water from surface) prior to the FoG. All the water was pumped through a cascade system from W2 dam, through four pump stations located at various elevations.

“The water was pumped through a set of 10'' columns, driven by PR150 Battlemax dirty water centrifugal pumps at a rate of 100 ℓ/s. This was sufficient to cater for peak periods, as it provided a capacity of 200 ℓ/s,” Tshabalala elaborates.

However, he points out that the sudden major water influx not only tested the mine’s level of preparedness to deal with an imminent danger of that magnitude; it also placed the mine at great risk of flooding and consequential closure.

“This would have had dire consequences for job security for our 3 000 employees and contractors and, most importantly, the return on investment for the shareholders.”

Management responses to the FoG emergency included the establishment of an emergency control room and 24-hour, continuous monitoring with accurate and detailed reporting; daily issue-based risk assessment of the water situation and the monitoring of the FoG area for further falls; channelling the water to nonessential excavations, flooding of the decline, and continuous pumping with a 90 kW submersible pump and the immediate discontinuation of the supply of service and drilling water to the underground areas of the mine.

Additionally, management stopped and contained the FoG area to the west, north and east, using active permanent support (mine poles prestressed with jackpots and 313 prestressed five-block Apollo matpacks), which were all installed from a safe area towards the fall.

Further long-term solutions taken included drilling a 167 mm diameter hole from surface directly into the FoG to place backfill in the immediate vicinity of the fall; the commissioning of a third dirty water pipeline to surface and the drilling of a second hole from surface after blockages started to occur with the backfill at the FoG area, with the primary purpose of establishing a connection with the water source, for sealing purposes.

Progress Report

Meanwhile, Tshabalala remarks that the placing of backfill from surface into the FoG void and surrounding area extended over 10 months, during which 14 456.4 m³ of backfill was placed.

“A progressive total of 102 200 kg of Portland cement, 600 kg of Versene, 1 625 kg of calcium, and 1 500 ℓ of modifier had been used. The total costs associated with this fall amounted to R11.8-million.”

He adds that sealing from the underground workings also commenced, whereby relief holes were drilled into the water source, intersecting a total of 133 971 ℓ of water an hour with eight “strategically planned” holes at about 30 m to 50 m in the hanging wall. All the holes were sealed, thus reducing the influx of water to 276 230 ℓ/h measured on the footwall.

“It is just a matter of time before the water source is sealed completely. However, with the recent commissioning of the mine’s underground pump station and settlers, we are in a better position to handle the risk of flooding. We remain true to our conviction that ‘nothing is so important that it cannot be done safely’,” Tshabalala concludes.