The supporting of roof in coal mines in the Illawarra began in the late 1950s. This method of support was installed mainly to supplement the cross supports used in the mines at that time, namely wooden half round bars, round baulks and in heavy and adverse conditions, steel rails and RSJ sections were erected. All of these were supported to the roof by wooden props of specified diameter. Distance between advanced supports was between 1m to 1.8.
In the Wongawilli Seam mines, roof bolts were installed after a sequence was mined. The bolts were usually of the expansion shell point anchor types designed to hold the roof plies between cross supports. The bolts were installed using purpose designed mobile electric roof bolting machines manufactured by Joy and Fletcher companies from America.
These machines were fitted with trailing cables giving them versatility to instal bolts throughout the panel. Holes being drilled dry required the machines to be fitted with dust extraction systems to ensure airborne dust was not carried into working places.
The Joy machine used hollow drill rods to remove drilling dust by using suction into a bag filter system. The Fletcher bolter used scroll drills which passed through a foam rubber seal fitted with a suction hose to a bag filter system.
Expansion shells came in various configurations and lengths. The advantage of these bolts are that in low height seams they were set flush to the roof whereas split wedge bolts could protrude up to 150mm from the roof.
In some mines at this time half round timber cross bars were bolted to the roof using split wedge point anchor bolts. These were installed fairly close to the face, using compressed air percussive “Falcon” bolters. Props were then set under these bars when clear of the continuous mining machine. This system allowed props to be removed when pillar extraction was commenced.
As the coal seam depth of cover increased, the above systems of support required upgrading as roof failures started to occur. It was decided by most mines in the area to increase the use of roof bolts as a primary method of roof support. Bulli Seam mines, having rock strata above the seam were able to use roof bolts in conjunction with a W-Section steel strap, 5m to 5.2m in length. Four to six split wedge bolts, usually 1.8m in length were set to the roof. In addition, most mines then set cross supports as described above.
Case Study – Kemira Colliery
In the 1960s, longwall operations were being introduced at Coal Cliff and Kemira Collieries. Kemira Colliery at this time utilised roof bolting with W-Section steel straps as their main method of roof support. A prop was set every second cross support as a warning and also to hang brattice. Strict rules for setting these supports were set, width of headings monitored and roof bolts tested on a regular basis to ensure proper torque by the roof bolting machine was maintained. Again, point anchorage was relied on to achieve this.
The longwall tail gate roadway was driven using 20ft (6.1m) W-Straps with 8 x 1.8m bolts. Straps were set at 900mm centres. The tail gate in this case was used as track access and electrical gear, namely transformers, gate end boxes and other associated equipment. The main gate roadway was driven using 16ft (4.8m) W-Straps with 6 x 1.8m bolts. The main gate accommodated the conveyor, stage loader and cross-conveyor.
This system of roof support was highly successful, was refined and then used in the other sections of the mine.
In 1977/78 Kemira Colliery developed the Wongawilli Seam for longwall operations. Roof bolting had to be reviewed due to the coal and stone roof strata. Point anchor split wedge roof bolts were not suitable as they would not anchor in any soft coal plies. At this time, resin anchored roof bolts were being introduced into the local mines. Longer bolts and longer encapsulation anchorage of these bolts were achieved as was the drilling of holes by rotary hand-held compressed air roof bolting machines. Further advances were made by being able to fully encapsulate roof bolts using twin set resin anchors.
Case Study – Corrimal Colliery
Corrimal Colliery supported the roof using split half round bars successfully until increasing depth of cover required heavier support. This was provided by using heavy round baulks every second or third set. These baulks were held to the roof by “Specials”, props 150mm diameter at the small end and “Joggled” to fit the round section of the baulk. This was only a temporary measure and as depth of cover increased, roof bolting was introduced. This consisted of placing expansion shell bolts between timber sets after the mining sequence was completed.
In the mid 1960’s, roof conditions in the working areas of the mine progressively deteriorated. Roof bolting using “Falcon” compressed air percussive stopers were used to install four 6ft. split wedge steel bolts through a 14 ft. w-iron in addition to half round bars and full round baulks as cross supports as part of the mining process. Conditions improved slightly, but as the depth of cover further increased supporting the roof at the face was becoming difficult.
At this stage it was decided on the advice of a prominent geotechnical engineer that the mining direction was changed with an immediate improvement of roof conditions in the panel headings. Cut-throughs were still heavily affected and had extra cross supports accordingly set.
In later years when resin anchored roof bolts were introduced and proved that better roof control was achieved, the mine decided to rely solely on roof bolting, but setting of wooden props every meter or so on both sides of the headings was still insisted upon by the workforce.
Case Study – Cordeaux Colliery
Cordeaux Colliery commenced production in 1979 mining the Bulli Seam with the Wongawilli Seam to be developed at a later stage. A decision was made to support the roof solely with roof bolts for all development and longwall roadways. The seam lay at a depth of 435m at the shaft sites. Roof strata was expected to be sandstone and mudstone, similar to that at Corrimal and South Bulli collieries. Depth of cover would increase as the mine advanced into the lease.
Pit bottom development was in sandstone roof intermixed with some conglomerate. Roof support consisted of installing 6 x 1.8m resin anchored roof bolts through 5.2m W-Irons (Straps) . Headings and cut-throughs were driven 5.5m wide. A double track road 8m wide was driven over a length of 300m, using 8 x 1.8m resin anchored roof bolts set through a w-Iron. All w-Irons were set every 1.2m apart. The double track road was still secure when the mine closed.
The roof supports were supplied by the local Titan Manufacturing Company whose mining engineer carried out numerous tests and operator training actually “did the job” to ensure the sole roof support of resin anchored bolts were set correctly.
Tests consisted of
- Ensuring hole depth was adequate.
- Hole diameter allowed for adequate encapsulation.
- Resin mixing times were strictly adhered to.
- Length of encapsulation was measured.
- Roof bolting machines imparted the adequate torque to the bolt
- Torque tests were carried out.
- Pullout tests were carried out.
A high standard of setting roof bolts was set by the operators. Any roof bolt suspected of not being properly installed had another bolt set adjacent to it.
As the mine developed into a mudstone area, the roof deteriorated and an intersection fell in. An inspection of the roof bolts during clean-up of the fall revealed that;
- Adhesion between rock and the bolt in some cases lacking
- The drill hole was not round but in the shape of a helix. This affected the length of encapsulation.
- The plastic was not completely destroyed by the bolt as it was being mixed, thus affecting adhesion between bolt and resin.
These were all addressed by the Titan representative.
- Longer resin cartridges used
- Rebar steel bolts used instead of smooth bar to ensure optimum mixing.
- Plastic friction washers introduced to fit between nut and plate to improve torque setting.
- Dome nuts and washers introduced.
Roof conditions improved markedly. When areas of mudstone were encountered, 2.4m roof bolts were installed at the rib side holes of the W-Iron.
Mining at this depth also caused a problem with ribs spalling and causing possible injury to workmen. Various types of rib support systems were tried by a group of operators, trades and supervisors. After various attempts were trialled, improved upon or rejected a system of a light-weight multi hole W-Iron set to the rib at mid height continuously set with 1.2m bolts supporting was decided on. On longwall blocks the bolts were replaced by plastic bolts and wooden ply straps.
The next step in roof support was the introduction of bolting steel mesh to completely cover the exposed roof. Mesh was also installed to cover ribs.
In the 1990s continuous mining machines were fitted with roof bolting rigs and later with rib bolting rigs.
Compressed Air Rotary Roof Bolting Machines
With the introduction of quick setting resin anchored roof bolts, the use of “Stoper” percussive type drilling machines became unsuitable. Higher speed hand held drilling machines were developed, some by Illawarra manufacturers. Makes of bolters were, Vernier, Pixie, Gopher, Wombat. Earlier models were fairly heavy so to lighten these, plastic and fibre glass legs were developed and aluminium motors, suitably covered, made these more ergonomically acceptable.