What follows is an abridged version of Winkel’s analysis, published in Australia’s Longwalls in September 2003.
Click here to view the full survey results, including all graphics(~450Kb).
A significant new benchmarking survey of longwall development rates at Australian mines will aid industry efforts to identify best practice and build on current performance levels.
The latest survey encompassed roadway design, technical and organisational change, and performance factors and trends. For comparative purposes the survey was developed with reference to a survey conducted in 1993 by Bruce Robertson on behalf of the Australian Coal Association.
Participation in this year’s survey was high, with some 90% of longwall mines returning responses. Further detailed analysis of the survey will appear on www.longwalls.com in October.
From the responses, a broad evaluation was conducted, showing the status of roadway development in the Australian longwall mining industry. The results are particularly helpful for low performers to ascertain best practice.
The spread of performance, measured in metres per shift, ranged from 6-23m/shift. The standard deviation is 4.5m/shift showing that there is potential for improvement. A higher longwall retreat rate is “pushing” the average development performance higher. A very clear result of this survey is that super section system configurations are under-performing industry wide.
Not only is the average advance per (productive) shift much lower than two single units, none of the mines are doing a panel extension under three shifts. This reflects very well the exposed need for infrastructure. A super section set-up should stay a niche solution if a single section cannot keep up with the longwall retreat rate.
The average pillar length is increasing steadily. For most chain pillars a length of 100m is common, however, one mine has introduced a pillar length of 155m and a cluster of mines are driving a crosscut about every 125m.
Even where shift lengths increased in the past 10 years, shifts longer than eight hours do not correlate with more metres driven in a shift. Six mines use a shift roster with 12hr/shift – probably due to organisational and union pressures rather than as a means of optimising performance. Water and the formation of slurry inbye in the panels is an issue for almost every mine. Regarding maintenance, no best practice could be detected. Most mines apply a maintenance roster that fits the production schedule, indicating that maintenance has a low priority in development.
Layout/geometry
All mines except two are developing panels with three entries. The entries have a width of 4.8-5.4m, averaging 5.1m. Roadway height varies between 2.3-4.1m, averaging 3m. A common chain pillar width is between 30-40m. However, one mine is using a pillar width of 25m, one with 45m and one with 50m. Some 73% of the mines are using a pillar length of 90–100m. Five mines have longer pillars (115m, 120m, 125m, 130m and 155m).
The relation between the ratio (roadway height/average seam height) versus roadway height shows a clear trend: if the coal seam is less than 3m mines are cutting rock. Generally, where seams are thicker than 3.5m, mines are leaving coal in the roof or in the floor.
The development to mining ratio averages 2.4 with a span from 2.3-2.5. About 80% of the mines use a crosscut angle of 90 degrees, while the rest use 70 degrees. The roadway floor type in eight mines is shale, in four mines mudstone and in eight mines coal is left as a pavement. This means that almost all mines, where the seam thickness is substantially higher than the roadway height, are leaving coal in the floor.
Organisation
For a single section a manning of 7-9 men is standard, of which five men are at the face, two are tradesmen and one is supervisor/deputy. In one mine only five men are working in the panel. (This mine achieves a development rate of 15m/shift but only four bolts are installed per metre). In a super section 13-16 men are necessary.
Roadway development operates five days per week in 50% of the mines. A shift time of eight hours is in place at 50% of the mines. A total of 70% of the mines have three shifts/day and the rest have two shifts/day. Nine mines are applying a three-day, 12-hour weekend roster. One mine mentioned a special and interesting roster. The nightshift and the dayshift as well as the dayshift with the afternoon shift are overlapping, so the shift changeovers are done as a hot-seat change. Between the afternoon shift and the nightshift is a gap of three hours.
Another very important factor is to organise the panel extension so as to minimise the number of shifts needed to get miners back to the coal as quickly as possible. Three mines are very well organised: they are doing this in one day. The best super section is carrying out the panel advance in three shifts. The distribution is almost Gaussian, as shown in the following figure.
Every mine in Australia has its own maintenance strategy. Four groups can be distinguished: about a third of the mines do maintenance only when required. Almost half of the industry has defined maintenance windows of a couple of hours each day. Another group of mines mainly use the weekends for maintenance while the last group has maintenance windows of several shifts/week.
Cutting
There are only two mines with a place change (cut and flit) operation and they are also the only mines using separate bolting rigs. The majority of the mines (65%) are using a single unit rather than a super section. The average development rate is 13.6m/shift, with a minimum of 5m/shift and a maximum of 23m/shift. The super sections' development rate averages below the single sections.
An examination of the ratio metres/shift versus metres/week shows that between 7-20 shifts are productive per week. On average, 12 shifts are productive which equals four of seven days at three shifts per day.
