The punch longwall approach is one rarely used, with the only other current example being Xstrata's Beltana mine which started in 2003. However, the punch preference by Broadmeadow, and Beltana’s adoption of the system has indicated a growing trend by coal companies to exploit highwalls.
The new punch longwall is being developed on the Goonyella lease for production in mid-2005. The choice by BHP Billiton Mitsubishi Alliance (BMA) to adopt punch longwall mining as its optimum layout was made to mitigate geotechnical risk.
The main issues associated with the geological structures was the zone of disturbed roof around the structure, the seam dislocation and the potential for wedge failures on the longwall face.
Broadmeadow project director Guy Mitchell told the AJM Longwall conference recently the objective of Broadmeadow mine design was to optimize the orientation of the gateroads and longwalls to the structures so the gate roads were not sub parallel to the structure and the retreat direction minimised the length of face retreat through these structures.
The Broadmeadow team also had to face up to the fact place changing was not the best development method in structured ground that may include jointing in the roof and rib cleats sub parallel to the heading direction.
Mitchell said during original mine planning two geotechnical layouts were evaluated; north south (conventional) and east west (punch).
It was found conventional layout had a prolonged exposure to thrust faults and the punch longwall layout had the least exposure to the faults and the shortest face relocation distance. Also, where thrust faults and other faults intersect, the conventional layout had increased potential exposure during longwall retreat.
Exploration work was an all important part of the process to mitigate geological risk. Mitchell said 3D Seismic techniques supplied by Velseis had located 100% of faults of two to three metres and the seam floor picked up +/-2.5 metre elevation during exploration.
The exploration phase involved developing 11,250 metres of roadways over a linear distance of 3,200 metres down to a depth of 280 metres. Whilst the Exploration Adit showed major challenges in geological structure, it also revealed the good news of a low stress regime on the coal seam, good floor conditions, low gas, little groundwater, low spontaneous combustion likelihood and a manageable geothermal gradient.
In addition to assessing faulting on layout, the Broadmeadow team also calculated insitu stress. They found the major stress on the 7.5 metre thick seam was vertical. Mitchell said the major and minor horizontal stress was 70% and 50% respectively of the vertical stress. The orientation of the major horizontal stress was 15-20 degrees east of north.
“As the longwall will extract the bottom 4.5 metres, the strength and thickness of the remaining coal beam for development and longwall mining purposes is critical for roof control purposes,” said Mitchell.
It was determined as the bottom two plies (3 and 4) and 0.5 metres of ply 2 were extracted, 1.5 – 2 metres thick of ply 2 would be left with strength of 7-10 MPa.
The remaining ply 2 was three to four times thicker than the minimum recommended by research, enabling it to better handle insitu stress levels with increasing depth, stress concentration in gateroads and abutment loading. However, Mitchell said the intermittent presence of parallel sub vertical structures in this coal roof beam needed to be considered when designing secondary support to avoid the loss of the coal beam structural and strength continuity.
“By extracting punch longwalls north to south, the structural integrity of the coal beam can be maintained to at least 300 metres depth of cover compared to 250 metres for the reverse south to north panel extraction sequence due to reduced stress condition at the goaf edge,” he said.
BMA concluded following an exhaustive process, the punch longwall layout was proven to be the preferred and optimum layout that would provide greatest production continuity and reliability.
Adapted from “Broadmeadow Mine – Punch Longwall Mining: Simpler, More Productive and Lower Cost” by Guy Mitchell.