Published in American Longwall Magazine
Tahmoor North, located in New South Wales, 43 miles south-west of Sydney, extracts steaming and medium-hard coking coal from the Bulli seam. Up until March this year mining was taking place in the Tahmoor mine which proved extremely troublesome towards the end of its life. Austral terminated mining of Tahmoor's last panel 163ft short of the planned length after successive roof falls damaged equipment. Mining then moved to Tahmoor North which promised a thicker coal seam, better product, and improved ground conditions with a higher capacity longwall system.
Like its Pittsburgh seam counterparts, Tahmoor North extracts from a 6.6ft thin seam and, while the face length isn’t close to that of Cumberland or Enlow Fork, the mine was looking to expand it from 738ft to 902ft. Austral contracted John T Boyd’s (JTB) Australian branch in April 2002 to undertake an analysis of the geotechnical design requirements, and advise on the design and selection of longwall equipment.
The closest Australian comparison JTB could find was the Oaky Creek No. 1 longwall in central Queensland. Oaky Creek extracts from an operating height of 5.9-7.2ft in the German Creek seam using a medium thickness set of equipment. However, JTB managing director Chris Wilkinson felt Oaky had some fundamental differences when compared with Tahmoor North, making it an unsuitable example.
“We felt they weren’t pushing the mining height envelope quite as much as Tahmoor was and with the size of the equipment they had, we felt some of the operating clearances weren’t big enough,” he said.
Instead JT Boyd turned to Pittsburgh seam mines to provide guidance for Tahmoor North. Operational issues at Consol Energy’s Cumberland, Enlow Fork and Bailey longwalls were analyzed, as well as Massey Energy’s Big Branch operation.
First up JTB embarked on a geotechnical analysis of the low roof support requirements to ensure there were no fatal flaws in the proposed face length increase to 902ft. Wilkinson determined no additional complications would arise with the additional weight on the face as its length increased.
Wilkinson looked at the principles being applied in US operations moving towards longer faces. “The principal reasons behind this trend is that most geotechnical problems manifest themselves in the gateroads, so the fewer gateroads you have, the less geotechnical problems you have,” he said.
“Secondly, we found an increase in stress associated with widening the face made little impact on the gateroads. From a geotechnical perspective the benefit of widening the face is greater than the offset of additional weight on the gateroads.” This was conditional on the face length being in excess of the critical span for forming a gob, he added.
“Say if you had a 295ft face and the critical span was 492ft – there would be a difference in going from 295ft to 574ft – but in the case of Tahmoor North its face was already in excess of the critical span so the difference in the geotechnical behavior of the goafing process would be negligible.”
JTB was also involved in the design and selection of the total longwall system – including roof supports, shearer, armored face conveyor, stageloader, pump station and monorail station. Particular emphasis was placed on determining the capacity and primary characteristics of the roof supports. Tahmoor North has significant overlying sandstone and then weaker friable shales making up the immediate roof – a recipe for localized roof control problems.
To combat this a stiff roof support was required to withstand cyclic loading. Cyclic loading is caving and/or support loading which occurs on a regular cyclic basis every 33-65ft, accompanied by roof and rib fracture. Cyclic caving typically occurs within massive sandstone units with a massive nature (strong bedding plane characteristics) or which act as a massive unit within a much weaker rock mass.
At the point of collapse overburden rebound occurs where the overburden will displace on to the face and gob area. The cyclic caving event is a combination of the formation of a fracture network and overburden convergence. The convergence of the overburden drives the convergence which may be experienced at the face.
JTB ensured the setting load was fixed as high as possible through boosting the pressure in the hydraulic system to the chock legs. Yield and setting load was maintained at a 10% difference.
DBT project manager Tim Clarke was responsible for ensuring the integrity of the design for the high-powered roof supports. DBT supplied 159 roof supports rated at 1241 tons with a width of 5.74ft and height range 4.6-8.5ft. Roof density at 7.2ft high was 112 metric tons per meter squared. At 15.75in internal diameter the support legs were the largest supplied in Australia to date.
Besides power, JTB also had to ensure maximum ergonomic efficiency of the walkway. The design criteria required increasing the width of the walkway and the height clearance as much as possible. Ideas devised in the Pittsburgh seam mines on face furnishing design and incorporation of services were utilized.
“When you are in a thin seam, if you cut a step in the roof or the floor what happens is the clearance between the canopy of the roof support and the shearer can be affected. So we tried to create a canopy profile which was very streamlined, thereby maximizing the operating distance between the underside of the roof support canopy and the shearer,” Wilkinson said.
Another issue Tahmoor had to contend with as a consequence of increasing face length was the AFC design. The longer the face the larger the load on the AFC chain and the more critical the control characteristics on the drive system. Although, this was less of an issue with the face length at Tahmoor - longwalls with faces in excess of...click here to read on.
