Published in Australia's Longwalls
Harworth, located in North Nottinghamshire, works the Deep Soft seam at a depth of 1000m below earlier workings. The standard approach to dealing with the mudstone and siltstone overburden (vertical stress of 25Mpa) was to reduce roadway width and install secondary support, such as centre props and passive cables behind the heading machine. This caused other problems such as a reduction in air quantity, causing 80% of downtime when longwalls were gassed out.
In February 2002 the mine was under review and threatened with imminent closure when the maingate end of the DS10s longwall collapsed. The writing was on the proverbial wall.
Ongoing investigations into the roof failures had found that cement grouted cablebolts were failing due to tensile failure of the cables.
Previous practice was to install conventional cable passively and then introduce grout, which was strong in compression when cured but had little tensile strength. When subsequent load was applied to the cable it gave a little before resisting the strain. Because the grout could not stretch it was breaking into discrete sections, delineated by the strata delaminations. Mine water then had access to cables which permitted corrosion and ultimately tensile failure when influenced by the front abutment.
The failure mechanism was eventually interpreted by mining consultant Chris Adams who observed cable grouting breather tubes hanging down from the cavity, all repeatedly necked, on the DS10s face end.
The previous longwall had suffered exactly the same fate and had been sterilised after it caught fire.
Pre-tensioned Megabolts, developed in Australia, had been used at Rossington mine and a batch was awaiting trial on the mine surface at Harworth. The mine decided to use the trial bolts to assist with the recovery plan, involving back-ripping of the lowered roof and rib squeeze and supporting with passive cribs, steel and props.
Two 6m Megastrands were installed per 1.2m run of the sagging roof and tensioned to 25 tonnes in batches of four. Operators immediately found the Megastrands quicker and easier to install, one reason being that 35mm holes are quicker to drill than 48mm holes.
“The sight of the failed roof being physically pulled back up as the tension went on was extremely encouraging,” according to Adams.
Once the face was recovered mining was restarted very slowly. Clearance for the gate-chock was restricted by the steel sets, but as soon as the support entered the Megastranded roof it was apparent steels were unnecessary.
“The maingate roof, which had always caved immediately behind or over the top of the support, now proceeded to stay up,” Adams said.
Following the success of the DS10s recovery, the system was extended to other areas of the mine.
Until then standard support on drivage was straps at 0.6m spacing with alternating 7 + 6 off 2.4m bolts. Some lengths of roadway had bolt density of 13 bolts through every strap.
Passive cables in DS22s longwall maingate were replaced with Megastrands, which were installed at roughly double the speed of passive cables. This longwall has performed more consistently than any other previous longwall at the mine, according to Adams.
Prior to Megastranding this maingate, all previous longwalls had suffered 1-1.5m floor blow. The worst floor blow in DS22s was about 450mm.
Other results were unprecedented stability throughout the Megastranded gate, and clearances at the face-end were maintained for the first time, allowing engineering staff to place a toolbox on top of the stage-loader for the first time.
The third application was the three-way junction at the face end of DS19s, usually supported on bolts, cables and heavy steels. On introduction of Megastrands the roof was transformed to a flat, smooth and stable surface.
The system was then introduced into the tailgate to reduce standing support, and then to a development heading, for installation up-front by bolters.
Finally Megastrands were introduced to a face heading. All previous headings had been driven on steel support with bolts and cables, and all had experienced floor lift which had to be dinted. On DS19s face heading, with a span of 6.5m, the Megastrands controlled the roof effectively with some 65mm movement on the extensometers. The floor lift disappeared and the drivage rate picked up.
“The tensioned tendon concept proved itself at Harworth in the most adverse conditions of DS10s Maingate recovery, impressing everybody involved,” Adams said.
Only six months after its impending closure the mine achieved lowest cost UK producer and now has a five-year plan approved.
To date the system has been taken up by four UK coal mines, with a further two mines evaluating its use. The use of Megabolts and Megastrands is also well established in Australian longwall mines, with 19 sites having used the system to date.
