Mining under sandstone

AFTER the occurrence of roof falls in late 2002 and early 2003 at Xstrata Coal’s West Wallsend Colliery in New South Wales, a comprehensive investigation was instigated. Investigations found it was necessary to lower the working height of the longwall working area, due to the presence of a sandstone channel overlying the coal seam.
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Courtesy of NSW Minerals department, David Barnes

Staff Reporter

West Wallsend mine is situated 25 kilometres south west of Newcastle. The thick seam longwall system was introduced in July 1997 and operates at a mining height of 4.8 metres, one of the highest operating longwalls in Australia.

A sandstone channel with a number of bedding planes overlies the seam. Vertical fractures are induced ahead of the face by the mining process. After a number of shears a periodic weighting event occurs followed by a large section of the lower channel becoming detached.

As mining proceeds the detached blocks rotate into the goaf, however in some instances a weighting event can occur and the block can fall onto the back of the supports. The resultant force of the slab hitting the goaf shield has the ability to push the support forward into the AFC and face.

This occurred in December 2002 when LW27 was working under a sandstone channel and suffered a number of large goaf falls that disrupted ventilation and caused damage to face equipment and injured an operator. Longwall production was halted until January 7.

At the time of the fall the shearer was on the main cut towards the maingate. The operators noticed the supports were yielding (>470 bar) and stopped the operation but left the AFC running. After a short period a large piece of sandstone, estimated at 6 metres thick by 15 metres wide by more than 30 metres long fell onto the rear of the supports. This forced a number of supports forward towards the face.

The pontoons of supports 44 and 45 hit the back of the AFC pans and sheared the AFC joint at 43 – 44. The alignment of the pan joint at 43 – 44 was disjointed by approximately 300mm. The AFC chain kept running and flights and chain were derailed from the top race.

The AFC chain stopped when the derailed flights reached the MG drive. The shearer was pushed tight into the face and was not able to traverse the break in the AFC pans. Significant face spall and roof coal failure overloaded the AFC.

After this incident, management worked hard to implement more stringent controls in place. Then operations manager, Jerry Wallace, said the main concern was not weighting generally but being exposed to the lower sections of the channel and unplanned movements of supports.

As part of the control planning process, meetings were conducted with SCT and CSIRO engineers with the outcome being series of trials on LW28 to assist with controlling caving using Hydrafrac technology. Surface ‘pre-fracturing’ proved unsuccessful in the stress regime above LW28.

The lowering of the face operating height to 3.5m provided the solution. This reduced the angle of the support goaf shield and ‘bulked’ up the goaf material to cushion any falling material.

From an operator safety point of view, all PM4 brackets had polyurethane security straps attached to restrict forward movement in the event of a goaf fall and all fire extinguisher brackets were securely mounted to the roof supports. All ancillary equipment (tools, jackpicks etc) was to be kept at the maingate during production.

The operating cycle was divided into two distinct processes - cutting then support advance – to allow the shearer to be parked in a safe location prior to support advance. Supports were advanced remotely (up to 20 supports away) to remove the operators from the support advance area. Hydrafracturing technology from the face was set up underground and tested away from the active LW but was not called upon during LW28 retreat to initiate falls.

Finally, improved monitoring of leg pressures via an in-house designed software package was used as a successful predictive tool. This package was submitted by the West Wallsend workforce as a Safety Initiative at the NSWMC OHS Conference held in the Blue Mountains in July and took out both the Judges and Peoples Choice Awards and will now be an entry in the National Awards.

The package was used successfully to provide a seven minute warning to operators on LW30 and is proving to be reliable as a safety tool for the mine’s longwall operations.