(Ph) 6. Ground Control
Ground control at Phalen Colliery was complicated by the following factors:
I. At depths in excess of 700 meters, sandstone strata exhibited a tendency to outburst,
II. The strata overlying the Phalen Seam did not cave regularly during longwall mining,
III. Weak immediate roof strata and elevated horizontal stress.
I. Gas outburst at the Phalen Colliery
When the main slopes of Phalen Colliery passed the 700 m depth line, some sandstone beds immediately overlying the Phalen Seam exhibited a tendency to outburst. The high gas pressure within the sandstone, its low permeability and its high modulus of rigidity contributed to the burst-proneness. The high rate of gas emission and rockmass ejection presented a major hazard to mine workers, and severely reduced the rate of roadway advance.
II. Periodic weighting at the Phalen Colliery
Longwall Panels #'s 5 East, 6 East, 7 East, 8 East, 2 Center, and 3 Center were all subject to periodic weighting phenomena. Periodic weightings occur when strong rock units in the roof strata refuse to cave readily into the gob after the longwall face has advanced. As more and more coal is removed, high levels of vertical stress are created in the longwall abutments, and within the hydraulic longwall supports. After a period of time, the strong rock unit breaks and stress levels drop quickly. This cyclic rising and falling of stresses during longwall mining is called periodic weighting. At Phalen, stress related ground problems increased as roadways and longwalls approached and undermined remnant pillars in the Harbour Seam above. The Lower Sandstone Unit in the roof strata appeared to be the main factor in causing the weightings. Its thickness, and height above the coal had a major influence on how the weighting would impact mining operations.
III. Roof falls:
Falls of roof rock often occurred on the longwall faces, and rib spalling was commonly encountered in the entries. Figure Ph8 shows a typical roof failure along a longwall gate. The following factors were believed to contribute to this problem:
(i) In the Sydney Coalfield, elevated horizontal stresses are present. The direction of the principle horizontal stress is northeast. The horizontal stress may induce bed separation in the brittle, laminated bed of sandstone overlying and underlying the coal seam.
(ii) Longwall Panels were driven East-West, i.e. at an acute angle with the major stress direction. Such a direction has resulted in spalling of the coal rib, which in turn increases roadway width and subseqently the load to be carried by the roof support system.
(iii) Remnant coal pillars left in Harbour Seam above Phalen mine workings are sites where stresses, induced by overburden loading, are concentrated. These stresses are transmitted downward to the roadways and longwall at Phalen Colliery.
When longwall Panels at Phalen Colliery passed under these pillars, the surcharge of stress may have been be sufficient to cause a roof fall. In addition, longwalls working under these pillars had experienced a higher frequency of water inrushes from the overlying flooded Harbour Seam workings. This was believed to be the result of the increased hydraulic conductivity caused by stress-induced fracturing of the interburden beneath the pillars.
(Ph) 6.1 Ground Control Method
Longwall faces were equipped with 550 tonne 2 leg Dowty Shields. Roof bolts installed through steel straps and mesh was the primary support used in the panel entries. Rib-bolting with mesh and headers was also required to control coal spalling. Bolting design was based on the confined beam method (Daws). Parameters such as beam thickness, roof rockmass rating, bolt length, strength, density and direction of horizontal stress were all used to design the support. Intersection design encorporated cable bolts to lend additional support to the bolted beam. Most slope intersections were supported with square steel sets and large collar beams for breakoffs while the slopes were supported with steel arches installed at a one meter spacing.
The typical bolting pattern at Phalen consisted of 5 bolts installed across the roadway with rows 1 m apart. Spot bolts were installed in the roof between the rows to combat horizontal stress effects. The bolts were generally 2.4 m long Grade 75, J-bar profile, with full resin encapsulation. Fast setting resin was used at the back of the hole (0.6 m) to allow immediate tensioning of the bolt to 1 to 2 tonnes Slow setting resin was used to provide for full encapsulation after tensioning. Bolts were installed through 0.3 m wide x 4.2m thin gauge metal straps which spanned the roadway. Plastic mesh was also installed to prevent spalling of roof rock in the areas between the straps and bolts. To provide rib stability, bolts were installed in the coal. 1.8m long, point anchor resin fiberglass bolts were used on the Panel side of the roadway, while 1.8 m long steel bolts (resin anchor) were used on the pillar side. Plastic mesh and wood headers were used to increase rib stability.
Cable bolts consisted of 4.9 m, 7.3 m or 9.8 m point anchor passive resin cables. The length of the cable bolt was determined by the geology of the roof and monitoring data. Telltale extensometers were used in the development entries to monitor roof movement and secondary support was installed when ground movement exceeded accepted standards. Secondary support could take the form of spot bolting, center propping (wood or steel) or cable bolts. In addition to these standard forms of secondary support, other methods such as cable trusses, solid bar trusses, coupled bolts, and wood cribs had been tried, and were used under special circumstances. In extreme cases, polyurethane resin was injected into the roof of the gateroads to help stabilize the ground prior to longwall mining through the area.