Longwall top coal caving accounts for around 40% of China’s coal production output and is a method that has been used for the last decade. Some 85 top coal caving mines operate in China, mining seams of 4.5m to 10m thick. One of the leading proponents of the method is Yanzhou Coal Mining Co with seven major mines, each producing around 4-5 million tonnes per annum, according to J. Jiang of Yanzhou, speaking at the Aachen International Mining Symposia in June.
In 1999 Yanzhou formed a joint venture company with Strata Control Technology (SCT) , Jinan Mining and Engineering Technology, to introduce roofbolting technology to its operations. Prior to that roofbolting in Yanzhou’s top coal caving mines was uncommon or ineffective; roadways were traditionally supported using conventional standing support while existing rockbolting systems were low capacity and had low stiffness. Severely deformed roadways were common and retreat rates were slow.
Yanzhou’s mining area is characterised by a moderate to high stress environment with horizontal stresses higher than vertical stress. Mines typically have main and tailgates 4.2m-4.6m wide and 3-3.5m high while longwall installation gateroads are usually as wide as 8-10m to accommodate equipment.
In top coal caving mines, gateroads are driven along the floor strata of coal seam and the roof is therefore top coal. Stresses are often high enough to cause this top coal to deform or fail as soon as a cut is mined and sometimes even in front of the heading face. The thickness of top coal is usually beyond the range of a roofbolt making it difficult to use roofbolts alone to support the roof.
In Chinese coal mines single entry development is the practice with the tailgate driven alongside the goaf of the neighbouring mined out panel with a small barrier pillar of 3-5m wide left. This results in pre-existing fractures above the roadway as a result of the goaf caving and stress abutments from the adjacent panel. Two experiments with bigger pillars of 15m wide were ‘nightmares,’ Jiang said, with roadway conditions worse than with smaller pillars.
“A possible explanation for this is that having a 15m wide pillar simply put the roadway closer to the peak value position of the abutment pressure caused by the adjacent mine out panel, and the mining depth is 600-700m!”
Subsequent guidelines developed for small pillar geometry include: waiting six months for the goaf to settle before driving the tailgate; keeping the width of the small panel at least equal to the roadway span; keeping height/width ration of pillar less than 1; no handing roof on the goaf side; and reinforcing small pillar with high capacity high elongation bolts installed at high density.
A major outcome of SCT’s joint venture with Yanzhou, is the introduction of fully encapsulated high strength rockbolts. Initially, a key issue was being able to source a secure local supply of bolts. Working in conjunction with a local steel mill, Yanzhou undertook to upgrade rockbolting consumables. A new rebar for making rockbolt was developed that gives good load transfer properties and better resin mixing.
Bolting consumables are typically 24mm (22mm root) diameter with a yield strength of 220kN and an ultimate strength of 300kN. Field testing shows these bolts to have load transfer capacities in coal roof of 12 tonnes per 300mm encapsulation. These performance characteristics are equivalent to the high capacity bolting systems used in Australian mines.
Other than rockbolts, supplemental support is also used in some maingates, all tailgates driven alongside goaf and installation roads. Resin anchored plain strand cablebolts are used along with groutable cablebolts.
As rockbolting in top coal caving operations is relatively recent, there is a limited experience base to draw from. Conventional rockbolting designs cannot be used given that the height of the top coal is beyond the range of a normal roofbolt; and bolts cannot be designed to work in suspension mode.
The design approach SCT introduced is closely aligned with Australian systems and is based on monitoring based feedback. This is based on initial geotechnical evaluation followed by systematic monitoring of reinforcement performance and roadway behaviour. Once the initial design is implemented deformation is measured in top coal roof and ribs using sonic probe extensometers. Bolt load monitoring is conducted in-situ using instrumented rockbolts which allows the initial design to be verified.
Routine field monitoring is conducted with wire extensometers telltales installed every 20-30m of roadway.
“The use of these devices has proved to be an effective way to detect top coal movements within and above the bolted horizon, especially in the most active underground workplaces and provide a good visual indication of roof displacement so that miners can be warned in time of any conditions that might lead to a roof fall,” Jiang said.
Roofbolts are typically installed with compressed air driven rotary drill. The top coal roof, due to its comparatively low strength, is difficult to support but easier to drill. Drilling a 2.4m long roofbolt hole takes 2-3 minutes.
Jiang said future developments for Yanzhou will be simultaneous cutting and bolting which the company has considered for some time.
“While bolter/miner technology has been proved successful abroad, its suitability to gateroads of top coal caving longwalls in Chinese coal mines needs to be carefully investigated.”
Floor heave remains an ongoing problem for these mines with no effective measures yet available. Broken bolts are another problem and Jiang said suitable instruments for checking roofbolt integrity were required. New types of cuttable rib bolts need to be developed he said because cuttable steel bolts cause problems for the shearer, conveying system and in the washery.
