Functional specifications for in-stope support based on seismic and rockburst observations in South African mines
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Authors: Durrheim, RJ |
DOI https://doi.org/10.36487/ACG_rep/1201_03_durrheim
Cite As:
Durrheim, RJ 2012, 'Functional specifications for in-stope support based on seismic and rockburst observations in South African mines', in Y Potvin (ed.), Deep Mining 2012: Proceedings of the Sixth International Seminar on Deep and High Stress Mining, Australian Centre for Geomechanics, Perth, pp. 41-55, https://doi.org/10.36487/ACG_rep/1201_03_durrheim
Abstract:
Rockbursts pose a significant risk to workers in deep gold and platinum mines in South Africa. In-stope support systems are one of the measures used to mitigate the risk. Over the last two decades several new technologies have been introduced, such as prestressed elongates, roofbolts and nets. However, seismic theory and observations present several characteristics of seismically-induced ground motion that are not taken explicitly into account in the functional specifications for support design, i.e. multi-cyclic shaking, shear motion between hanging- and footwall, transient tensile forces, and structural resonances. Investigations of rockburst damage were reviewed to evaluate the significance of these phenomena. Similar phenomena affect surface structures exposed to earthquake-induced shaking, and earthquake engineers have developed a range of solutions to mitigate damage. These solutions are reviewed to evaluate whether the principles of earthquake-resistant design can be adapted to the underground environment. Finally, functional specifications for rockburst-resistant in-stope support are proposed.
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Abbreviations: f/w – footwall; h/w – hangingwall; M – local magnitude; RHYP – Rapid-yielding hydraulic prop; VCR – Ventersdorp Contact Reef
Date: 10 January 1994
District and reef: Far West Rand, Ventersdorp Contact Reef
Mining scenario: Extraction of island remnant at a depth of 1,840 m
Local magnitude: M 2.6 followed within 30 s by M 1.9 event. Events were external to the seismic network; hence the location accuracy was poor.
Source mechanism: Failure of parts of remnant where width was less than 11 m.
Damage mechanism: Strike gully was supported by prestressed composite packs. Co-seismic closure up to 80 cm, packs punch into f/w. Stope supported by composite packs and 400 kN RYHPs with 300 mm load spreaders. Evidence for co-seismic closure of 15 cm. Face ejections and shake-down of fragmented h/w beam to 80 cm was observed.
Support performance: Support system rendered ineffective due to co-seismic fragmentation of hard lava h/w. The use of 800 mm load spreaders was recommended.
Date: 4 May 1994
District and reef: Far West Rand, Ventersdorp Contact Reef
Mining scenario: Extraction of peninsular remnant at depth of 2,300 m
Local magnitude: M 2.1, located within the remnant.
Source mechanism: Failure of L-shaped peninsular remnant with width of 8–12 m.
Damage mechanism: Stope supported by 1.1 x 1.1 m composite packs and RYHPs with headboards. A trench had been dug to negotiate a roll in the reef, and mining continued under the brow. However, the brow had not been confined and contained many weak joints in the hard lava h/w. Co-seismic convergence estimated at 50–150 mm. H/w fragmented co-seismically. North-eastern face of remnant ejected into the stope without damage to h/w.
Support performance: Support deemed to be ineffective due to fragmentation of h/w. Brows should be confined. The use of larger headboards was recommended.
Date: 3 November 1994
District and reef: Far West Rand, Carbon Leader
Mining scenario: Longwall negotiating dyke at 3,000 m
Local magnitude: M 2.5, event located 50–100 m from the damaged panel.
Source mechanism: Slip on dyke.
Damage mechanism: Faceburst and shakedown of 1 m h/w beam. Convergence about 10 cm. Numerous joints and fractures in h/w. Stope supported by backfill within 3.5 m of the face (bags supported by pipe sticks) and single row of 400 kN 1 m/s RYHPs without headboards. Packs were installed in the area of the dyke intersection.
Support performance: Twenty-eight 40 t RYHPs props were recovered and tested. Seven were non-functional, although the condition at the time of the burst could not be determined. The other 21 props survived a 1 m/s test, though with considerable variation in energy absorption. Props that punched into the h/w did not exhibit unusual stiffness. The amount of available travel at the time of the burst could not be determined. H/w collapse was attributed to fragmentation of h/w owing to poor ground conditions and single row of props supporting 3.5 m face-backfill span. Two rows of RYHP with headboards were recommended.
Date: 4 July 1995
District and reef: Klerksdorp, Vaal Reef
Mining scenario: Extraction of final remnants near shaft at 2,220 m
Local magnitude: M 3.4.
Source mechanism: Slip on fault.
Damage mechanism: Bulking of haulage sidewall over 60 m. Failure of rebar, mesh and lacing support.
Support performance: Tunnel supported by 3 m x 16 mm shepherds crook smooth bar on a 1 x 1 m pattern with 100 mm weld mesh and 10 mm lacing. Rebars were considered to be sufficiently long to be anchored in stable rock mass. Shearing and debonding of some rebars (but not shear failure) attributed to pre-seismic deformation, as well as an instance of tensile failure of a rebar was observed. Failure of system under severe loading attributed to deterioration of rebar bounds in the fractured envelope increasing the load on the mesh and lacing leading to failure at the points where the soft fabric was connected to the stiff rebars.
Date: 18 September 1995
District and reef: Far West Rand, Ventersdorp Contact Reef
Mining scenario: Mining of remnant between longwalls at depth of 2,200 m
Local magnitude: M 2.2.
Source mechanism: Slip on face parallel rupture.
Damage mechanism: Shakedown of the hangingwall in the face area and in the wide heading about 30 m from the focus of the tremor.
Support performance: The face area was supported by two rows of RYHPs; the back area by 1.1 x 1.1 m prestressed solid timber mat packs spaced 1.5 m skin-to-skin on dip and strike; and the wide heading by fully grouted 2.2 m ripple bars, in rows 1.5 m apart. Lava h/w had blocky structure owing to interaction of vertical face-parallel fractures and subhorizontal flow surfaces. Damage confined to face areas where the distance between the face and support unit exceeded 2 m. The fallout height was 1–3 m. Fresh splitting of packs indicated co-seismic convergence in back area of 100 mm, though there was little or no fall out between packs. In the wide heading it appeared that the distance from the face to the first line of support was about 4 m. The critical factor appeared to be unsupported spans exceeding 2 m.
Date: 30 January 1996
District and reef: Far West Rand, Carbon Leader
Mining scenario: Extraction of stabilising pillar at depth of 1,900 m using face-parallel pre- conditioning
Local magnitude: M 2.2 and M 2.3 within 1 s. The closest event was within 20 m of the mining panel, and 15 m in the h/w. The second event was some 100 m away.
Source mechanism: Pillar failure.
Damage mechanism: Shakedown of strike gully h/w.
Support performance: The immediate h/w consisted of a laminated quartzite (about 1 m) with a 2 m shale band above. The gully h/w was fragmented by steeply dipping shear zones associated with ancient tectonism, fractures parallel to the pillar axis related to past mining, and fractures normal to the pillar axis related to current mining. Panel support consisted of timber packs installed within 2 m of the face, spaced 1.5 m skin-to-skin. Co-seismic convergence in the panels ranged from 50–140 mm. A Ground Motion Monitor in the stope recorded amax = 4.2 g and vmax = 470 mm/s, considerably less than the values recorded during preconditioning blasts that cause no damage. However, the strong shaking had duration of 80 ms. Only minor falls of ground had occurred between the face and first row of packs. The gully shoulders were supported by solid timber packs, spaced 1.5 m skin-to-skin. The stoping height was 1 m, although the gully packs were sometimes considerably taller owing to past fall outs. Occasional split set tendons were installed in the h/w. Three substantial falls of ground had occurred in the strike gully, with fall-out heights of about 3 m. The gully support did not cater for the highly fragmented nature of the h/w. At the very least long (8 m) tendons capable of surviving shearing combined with straps or mesh and lace would have been required. The effort to extract the pillar was abandoned.
Date: 18 November 1996
District and reef: Far West Rand, Carbon Leader
Mining scenario: Mining a longwall at a depth of 3,300 m through a 10 m wide dyke.
Local magnitude: M 3.0, event located within 50 m of working places.
Source mechanism: Slip on dyke.
Damage mechanism: Severe damage to strike gullies and minor falls of ground in the face area. There was evidence for co-seismic closure of 200–300 mm in the panels. Fall-out extended for some 60 m in the 2 m wide gully, with most fall-outs about 1.8 m high, but some as high as 4 m.
Support performance: Stope support consisted of pre-stressed timber elongates and backfill, with timber packs lining the gullies. No tendons were installed in the gullies. Mechanical props were used for temporary support. Panels protected by backfill and elongates only suffered minor damage, though about 30 per cent of the props had failed, mostly by tilting of the headboards, and had lost their capacity to absorb any further energy. The most serious shortcoming was the lack of areal coverage in the gullies.
Date: 7 May 1997
District and reef: Far West Rand, Ventersdorp Contact Reef
Mining scenario: Mining of longwall at depth of 2,700 m
Local magnitude: M 3.4, event located within 50 m of working places.
Source mechanism: Slip on fault. Several faults are present in the area, which had a history of unusually high seismicity.
Damage mechanism: Damage included face ejection, and shakedown of the stope hangingwall where face to support distances exceeded 3 m. The fall-out heights were up to 1 m. There was evidence of co-seismic closure of 150 mm. Damage was most severe in the vicinity of the reef-crosscut intersection where the sidewall had been violently ejected into the haulage.
Support performance: Mining-induced fractures were prominent. Permanent stope support consisted either of timber packs or steel elongates and backfill. Mechanical props were used for temporary support. While the steel elongates had generally performed well, some headboards were severely distorted and some steel barrels had been split.
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