Stress/Strata Control

Along the Longwall Face

Along the face, the roof control function is a matter of providing sufficient resistance to control the dead load of the mass of strata which is breaking, or has broken, away from the bulk of the overlying strata. The chocks are not resisting the whole vertical stress field, most of which is redistributed to the solid coal ahead of the face (the "front abutment load") and to re-compacted material in the goaf behind the face (the "rear abutment load"). Provided the chock load capacity is sufficient to control the movement of this dead load so that the roof in front of (and preferably above) the chocks remains intact, face conditions will remain good.
Horizontal stress is not really a major factor in roof control along the face. There will be zero stress in the direction perpendicular to the face once a goaf has formed and stress on the long axis of the face will not create problems. Horizontal stress in the floor however can give rise to floor heave under the pans which can become a major issue. The angle of the pans controls the attitude of the shearer and if the rear of the pans is lifted it often becomes necessary to straighten them before continuing, a difficult and time consuming operation.
Because reliance is being placed on controlling roof lowering which is expected to occur, major problems may arise if a face is stopped for any length of time. It is possible that a stable condition may be attained where movement ceases. It is equally possible that movement will continue to the point where the roof starts to fail ahead of the face and even to the point where the chock legs are fully closed and cannot yield further, a state known as being "iron bound".
Ideally a longwall face should operate as near to continuously as possible and at a steady rate in order to maintain good face conditions. It has often been the case that a relatively minor problem which has caused the face to stand for a period leads to a deterioration in conditions which then compounds into a major event.
Some degree of movement not only ahead of the chocks but slightly ahead of the face can be useful. This movement and resulting load on the uncut coal can cause crushing and fracturing leading to a much reduced power demand on the shearer, sometimes to the extent that it becomes a loading machine rather than a cutting machine. While this effect can be beneficial it should not be a design aim as it occurs near the point where roof control is lost and this would not be an ideal operating state. Even if roof control is maintained initially, too much load on the face coal will lead to slabbing, creating a wider expanse of unsupported roof with roof control problems possibly ensuing, which is exacerbated in high seams.

 

In the Gateroads and Associated Roadways

The gate roads are frequently major causes of concern with regard to strata control around a longwall panel. As well as having to have sufficient support to remain stable during development and for the period before the longwall passes, they have to withstand the redistributed stresses and abutment loads which arise ahead of, behind and to the sides of the advancing face
Around the face ends there is also a redistribution of horizontal stresses. It is often the case that one end of the face is protected by an adjacent goaf (in a "sress shadow" area) and horizontal stresses are carried in higher strata horizons away from the immediate roof. If the current longwall is extended beyond the old goaf, the stress shadow effect is lost and a stress concentration is likely instead. There will be a stress concentration at the other end of the face and such stress concentrations can be up to twice the normal stress levels and can cause compressive failure of the roof strata. It is preferable for the face to be oriented so that such concentration is applied to the minor principal stress rather than the major, but this is not always practical, especially where stress fields change orientation.
During development, roof strata support design is based on high residual horizontal stress levels. These dissipate after the longwall has retreated, often resulting in roof failures due to the lack of confining stress.
Additional support to that required during development is often needed and is usually provided by means of extra long bolts of some kind (or even trusses in weak strata), sometimes in the floor as well as the roof.
In the tailgate it is often practical to install passive support before it is affected by the face, usually in the form of "cans" (large diameter, thin walled steel tubes filled with lightweight concrete) or timber chocks. It is preferable for these to be set to one side of the roadway and still allow vehicle access to the tailgate drive area. If this is not possible, then it is best to leave the installation until just before the effects of the face become apparent.
Because of the conveyor and equipment in the maingate and associated roadway(s) passive support is not usually an option and fairly heavy secondary bolting is often carried out. It is best if this can be done well in advance of the approach of the longwall, ideally between the completion of development and before the longwall start, if there is sufficient time available.
It is ideal if an assessment of strata conditions can be made during development and the extent of secondary support required in different sections planned in detail rather than a blanket approach to the full length of the roadways. More dense support, which may include full mesh coverage of ribs is often applied adjacent to longwall start and finish positions, particularly the latter which has to remain stable under the abutment load for an extended period during longwall recovery.
As the longwall retreats, passive support is often installed in cut-throughs as the face passes when access is no longer required. This is to prevent the goaf running down the cut through and affecting the intersection in the remaining roadway, if it is required to remain open.
Another important issue affecting the stability of the gate roads is the design of chain pillars. These have to be such that they remain intact during the life of two longwalls (assuming longwall retreat and re-use of the second roadway at the maingate end as a tailgate for the second longwall).
LVA - Longwall Visual Analysis – commercial website for company dealing in face & strata data analysis