Underground » Strata Control and Windblasts
This project was commissioned to develop a better understanding of the underlying strata mechanics of high deformation tailgate environments and to determine the required engineering characteristics of artificial supports to control such environments.
The project findings provide mine operators with a methodology to evaluate support strategy in difficult gateroad conditions or to apply the findings to mine layout design to optimise pillar dimension (width) against desired gateroad serviceability and support effort.
Poor tailgate serviceability has a major impact on longwall productivity and safety. The consequences of a major gateroad fall can include:
- Abandonment of the longwall.
- Inability to recover longwall equipment.
- Disruption to ventilation in the goaf with potential to initiate spontaneous combustion.
Whilst these events are potentially catastrophic, on-going poor serviceability more commonly impacts on longwall productivity, safety of mine personnel, particularly since the tailgate is typically a second means of egress, and the cost of tailgate support.
Advances in pillar design (formula based, measurement/numerical based or empirical (ALTS/ALTS) have provided a first pass means to identify those mining geometries and geological conditions that have contributed to poor tailgate serviceability in the past. However despite these advances, excessive roadway convergence, productivity losses and hazardous conditions for mine personnel still occur. This is also reflected in overseas experience where Barczak (2003) and Mucho et al. (1999) lament that tailgate support design in the US is still largely based on trial and error.
The development of more rigorous engineering based support design requires:
- Determination of the mining, geological and geotechnical aspects controlling strata behaviour within high deformation tailgates.
- Determination of the engineering aspects (stiffness, strength, positioning, timing etc) required of artificial supports (cables as well as standing supports) to control such environments.
- Requires specification of the engineering characteristics required of future support products.
- Confirmation or otherwise that the load/deformation characteristics of support products measured in the laboratory were being achieved in the field.
The support design methodology outlined is necessarily site specific however general concepts are applicable not just to tailgates but are transportable to other roadways adjacent to longwall extraction, to other mines, and to conditions outside the existing experience base.
One of the key outcomes of the project was the development of a working model of a strata deformation process coined ?skew roof deformation mechanism'. ?Skew roof' refers to the propensity of a gateroad roof to skew towards the adjacent or approaching goaf. Generally the distance that a strata unit moves towards the goaf increases with height into the
roof which may lead to shear along weak interfaces and further degradation of the roof and pillar integrity.
Skew roof has implications for support design within high deformation tailgates, mine layout (tailgate positioning) and indeed, all roadways within the vicinity of longwall extraction, including the faceline itself. Consideration of the skew roof mechanism is a critical component for successful support design within heavy gateroad environments. The mechanism and key geotechnical factors influencing the behaviour are discussed in detail in the report.
Field monitoring of standing supports has revealed significant differences in field versus laboratory support loading characteristics. Improvements to current installation practices and guidelines for new product development were identified. The major finding of the project is that existing support products are often too soft and of insufficient capacity to effectively interact with the strata deformation mechanisms operating. In other words, existing supports essentially ?go along for the ride' rather than limiting convergence and thereby improving faceline conditions.