Underground » Mining Technology and Production
This executive summary is written to summarise the key outcomes, however it is difficult to transfer the context of those outcomes with the executive summary. It is essential to read the report to understand the action and limitations of face supports within the mining environment.
The aim of this project has been to:
· Obtain an understanding of the ground fracture mechanics about longwall panels and the geotechnical requirements of a face support within weak strata;
· Investigate factors which will optimise face support performance; and
· Review operational factors to optimise the support performance within a weak strata section.
The data used in the study has been obtained from previous field monitoring studies and computer simulations of modified longwall support characteristics within weak geotechnical environments for which SCT Operations had developed pre-existing computer models. The work has concentrated on geological sections of weak to moderate strength from the Bowen Basin and Hunter Valley.
The design concept and support capacity requirement for longwall supports evolved out of the British and European coal industries during mid to late 20th century. The British coal standard concept was to evaluate the support capacity on the basis of a detached block of rock above the canopy. This material therefore had to be supported. The dead weight and moments related to the block were then calculated and the leg load and configuration were assessed.
This concept has been modified by Smart (1986) and Barzack and Tadolini (2007) to better reflect the caving process whereby the strata above the immediate caved zone behaves as an interlocked but fractured mass which lowers onto the goaf.
The face supports react to vertical movement of the interlocked strata as well as horizontal movement of the ground at the face area, toward the goaf. The overall stability of the face area under this concept is related to providing confinement to the fractured ground and maintaining the interlock between the fractured blocks. This then allows the ground to subside gradually onto the goaf and not dislodge as a "dead weight" above the supports. Face support capacity in this situation is less definable, and is related to contributing to the natural interlocking capacity of the larger caving blocks. If this interlocking capacity were to be lost, then the load on the supports could change from a passive load related to overall ground convergence, to a "dead weight" load of the zone where the upper roof is not interlocked.
It is common that the strata ahead of the supports is fractured as a result of the stress redistributions ahead of the faceline.
The face supports are required to confine and control the fractured roof as the coal is being cut. The requirement is two fold.
Firstly to laterally confine the high angle fractures and reduce the likelihood of dropout ahead of the canopy.
The second requirement is to vertically confine the fractured rock and bedding. This allows the vertical stress to be transferred into the roof to enhance the integrity of the immediate roof section and to create a goaf break off line. If this break off line is not developed at the rear of the supports the potential exists for caving to progress over the canopy to the face line.
In order to allow such confining stresses to develop, the supports need to minimise dilation of the ground on bedding and fracture planes.
Dilated and loosening ground above the supports is not conducive to efficient stress transfer (horizontal and vertical) from the canopy to the roof section. This is due to the requirement for additional canopy or rock movement to compact or stiffen the strata on the canopy prior to stress transfer being effective.
Compaction of the dilated ground need not be uniform and will relate to the geometry of the dilated ground and the canopy-leg geometry. Biased compaction of the roof toward the rear of the canopy is common and may cause the rear to push up and the tip to rotate down off the roofline. This reduces horizontal stress at the tip and causes an uneven bearing surface for successive shears.
The support requirement is typically that which will maintain the ground as an interlocking network of fractures which can transfer stress across from the solid onto the goaf. This will ensure a secure goaf break off line and control goaf override.
Determination of this requirement has been an ongoing challenge and often based on a pre-conceived concept of the caving mechanics.
Over time there has been a tendency to increase the capacity as an "insurance" against possible caving anomalies. Such caving anomalies are typically related to perceived detached massive blocks. A study of longwall support behaviour under adverse caving was made by Gale (2001) and found that maintenance of the interlocking nature of the roof was the main control in difficult caving environments, and this was best achieved by ensuring good face confinement and stress transfer in to the roof section. This view is supported by other workers Barzack and Tadolini (2007) reflecting on the US coal sector.
Fundamentally, a face support is a passive structure in the ground, in that it reacts to ground movement toward the goaf. This is an important conceptual point as often supports are conceived as the active element controlling the overburden. In reality, they are reacting to overburden caving movements which can be variable depending on the geological sequence. Therefore the performance of a support can be variable within a panel depending on the nature of the roof.
Face supports will operate well within ground which allows the stress transfer from the support to the surrounding strata. This is essentially their operational bandwidth.
Outside of that bandwidth, the face supports loose their capability to confine and stabilise the ground.
A range of support variants were assessed. These included tip to leg distances from 2-4m, a split canopy and lateral thrust rams on the canopy. The support capacity was evaluated in terms of a ground reaction response for weak to moderate roof sections from the Bowen Basin and Hunter Valley.
The results of this study indicate that:
· The yield capacity to control the caving line and provide confinement to the fractured material is recommended to be in the 100-110t/m2 range with set of approximately 80t/m2;
· The canopy balance is recommended to be less than 2.4 and preferably less than 2. In most instances this relates to a tip to leg distance in the range of 3-3.5m; and
· The tip to face distance to maximise roof integrity and limit dilation should be less than approximately 0.6m. The smaller the distance the better the result.
If these key requirements are met, then the ground about the face and above the supports should be controlled under most geological scenarios.
In an operational sense, making the decision as to when the supports can operate to develop suitable stress transfer and when they cannot is a key aspect of successful mining. That is, can they operate within their operational bandwidth.
In sheared or faulted ground, forward confinement is often difficult to achieve. This is typically due to the frictional and stiffness characteristics of the ground, coupled with the propensity for the forward rib to fall and increase the tip to face distance. In such situations, the face supports cannot provide the required confinement (i.e., are outside the operational bandwidth) and ground injection or reinforcement is required for stabilisation.
Therefore a recognition of the action and limitations of what a support can achieve is an important part of determining the design criteria and the method of operation.