SEVERE floor heave at the Tahmoor colliery has resulted in the development of a series of floor reinforcement measures.
After experiencing a combination of high horizontal stress and poor geology in the belt road of LW 21 during retreat, the Austral Coal owned Tahmoor colliery in New South Wales turned to its geotechnical consultants for a cost-effective design solution.
Gate roads at Tahmoor are typically driven only 2.4m high, resulting in little margin for vertical closure in and around the crusher and stage-loader. In order to lessen the risk of production downtime resulting from equipment becoming trapped between the floor and the roof, mine manager Peter Wynne and longwall coordinator Peter Vale instigated a series of floor reinforcement measures.
The aim of the measures was to ensure vertical closure remained within operationally acceptable limits; despite the fact that the roof in the belt road was also reinforced with 2.5-3 Megabolts every metre.
Floor geology in LW21's maingate consisted of several metres of shale and/or mudstone. The orientation of LW21's maingate panel with respect to the in-situ stress field was considered unfavourable and as a result, several hundred millimetres of roof and floor closure was anticipated during longwall retreat.
Once the face had retreated beyond square, the floor heave in the belt road became so severe that in some areas clearance between the stage-loader and the roof reduced to zero. This was of particular concern to the mine since it was considered likely that as the face progressed outbye of the protective influence of LW20's goaf, the magnitude of the horizontal stress and the floor heave would increase further.
In order to address the problem the mine's geotechnical consultants, Strata Engineering were asked to provide a design solution which satisfied the following criteria:
No delays or stoppages to the face;
Any additional support should not interfere with the ongoing installation of secondary roof support;
No protrusions into the roadway;
Limited changes in support hardware (ie. from that installed in the roof).
Strata Engineering's extensive experience with both the mine's geotechnical environment and operational constraints enabled them to determine a floor support design that satisfied these requirements.
The design required the installation of cable bolts in the floor of the roadway at varying densities depending on the anticipated stress regime. The cable lengths were determined by the need to anchor the cables into a competent sandstone unit situated between 4-5m below the floor of the roadway, and to ensure the cables were located beyond any likely zone of fracturing.
Initially a cable length of 8m was selected, though this was later reduced to 6m, in order to avoid drilling into the gassy Balgownie Seam situated immediately below the targeted sandstone unit.
Cable installation was restricted to a 30m distance inbye of each cut-through (termed the "pinch-point") with the support density varying between one and two cables per metre. Pinch-points are areas where horizontal stress increases between two excavated areas. In the case of LW 21's maingate belt road, this occurred immediately inbye of cut-throughs where the stress is "pinched" between the cut-through and the approaching longwall.
As the majority of the floor heave was biased to the blockside rib, half the cables were angled under the belt. Drilling was carried out using a small track mounted Gardner Denver drill rig which has the capacity to drill each of the 6m long holes in about 15 minutes.
Initial trials in several areas of the belt road were made using 60 tonne capacity cables normally supplied by Megabolt for use as face reinforcement dowels. The cables were untensioned and fully grouted using Degussa supplied CB02 thixotropic grout.
The integral grout tube within the Megabolts allowed grouting to be carried out from the bottom of the hole up to floor level ensuring full encapsulation. The cable heads were seated some 100mm below the floor so they did not protrude into the roadway.
Results from instrumentation installed within the cabled areas indicated a significant reduction in the magnitude of fracturing within the floor strata when compared to similar measurements made in an unreinforced area of floor. The results suggested the cables had enabled the structural integrity of the floor to be maintained.
Figure 1 illustrates the measured clearance over the crusher, stage-loader and No.1 chock as the face retreated into eight and nine cut-throughs. Clearly evident is the increasing clearance coming into eight cut-through where the floor had been reinforced with cables compared to the unreinforced nine cut-through.
The engineered application of untensioned cable bolts has successfully addressed the problems at Tahmoor. In all four areas where cables were installed floor heave was visibly reduced and no operational problems were experienced due to excessive floor heave.
The success of floor bolting in the maingate of LW 21 has convinced both Wynne and Vale of the benefits of floor cabling in the unique geotechnical environment at Tahmoor. As a result, floor cables have since been installed in the inbye areas of the new LW 22 in Tahmoor North.
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