Rib Mechanics and Support Systems

The control of coal mine rib stability is an increasingly important issue, particularly with a trend towards narrower roadways, greater depths, more rapid development and the need to accommodate fast moving abutment stresses from retreating longwalls.  Rib control management is a priority both for reasons of personnel safety and productivity, not to mention the implications for pillar stability.

Traditional approaches to rib control and rib reinforcement have essentially taken roof support concepts rotated them through 90o and applied them to the coal ribside.  There have been very few attempts to truly understand the engineering behaviour and mechanics of rib deformation, load transfer and response to reinforcement elements.  The School of Mining Engineering at the University of New South Wales, together with ANI Arnall have been investigating the mechanics of coal rib behaviour and this has led to the identification of certain engineering parameters for potential rib reinforcement hardware.

A joint project between the School of Mining Engineering, The University of New South Wales and ANI Arnall was funded by ACARP in 1995 with the following objectives:

• To adapt recent advances in coal pillar yield modelling to quantify the mechanics of rib behaviour under the range of Australian longwall conditions.
• To apply these findings to optimising theoretical rib support requirements, and developing and demonstrating innovative yielding rib support systems and their installations.

Modelling

Modelling of yield behaviour of ribs was undertaken by computer software which identified the behaviour of ribside coal under different loading conditions.  Three zones within the pillar were identified, namely elastic, yield and crush.  Figure 1 below shows a cross section of a pillar with these zones highlighted.

The modelling phase examined how these three zones progress through the ribsides as abutment loading increases.  The edges of the abutment pillar reach yield very quickly since they are unconfined.  Yield progresses into the ribside as the excavation span increases, eroding the internal elastic core.

Quantification of these behaviour regimes gives direction to the type and length of rib support required under various loading regimes.

Two field investigation sites were selected for geotechnical monitoring, based on their very different rib conditions.  Angus Place Colliery suffered considerable rib crush and deterioration.  Dartbrook Colliery, on development, exhibited negligible apparent rib deterioration.

Angus Place Field Investigations

The objectives of the investigation at Angus Place were to define;

• The time dependent stability of the ribs;
• The nature and extent of the ribside yield zone;
• The region of influence of a single steel reinforcement bolt;
• The influence of development face proximity and subsequent longwall   abutment influence.

Dartbrook Colliery Field Investigation

The monitoring program at Dartbrook was intended to provide a contrast to Angus Place with the coal being far more competent (high strength and less structure).  The objective at Dartbrook was purely to monitor the rib (and pillar) behaviour under the standard reinforcement regime, subject to the longwall face abutment effect.

Laboratory Investigations - Prototype Yielding Supports

From the Angus Place trials, it was determined that longer rib reinforcement would not necessarily be more effective unless it incorporated a yielding component.  Laboratory studies were conducted using birdcaged cable bolts as a means of providing a yield element in an otherwise stiff system.  In these studies, it was found that each birdcage could provide up to 5mm - 10mm of yield.  Furthermore, a soft material (such as weak plaster mix) was placed into the birdcages as a means of delaying the yield process until a certain threshold load was reached, at which time, the straightening of the birdcage would crush out the plaster.  After the birdcages closed, the reinforcement cable would resume its stiff, high strength characteristics.

Prototype testing at Angus Place

Following initial monitoring at Angus Place, numerous rib bolt prototypes were designed to match the monitored conditions.  A second stage monitoring was then completed where prototype rib bolts were installed to qualitatively evaluate their performance under similar loading condition.The aim in deciding the rib bolt prototype design was to make it work effectively in conditions similar to those initially monitored.  The prototype rib bolts tested included

• Friction bolt (soft system)
• Friction bolt anchored with a Conbextra Cartridge (soft system)
• A point anchored bolt with a yielding washer (soft system)
• Minicaged Cable Bolt (combination soft and stiff)
• Strain Gauged AX Bolt (stiff system)

From visual observations, neither the stiff nor soft systems performed consistently better or worse than the other.  Both systems showed rib fracture to some degree.

Project Outcomes

The research investigations described in this report have identified the following conclusions regarding the mechanics of rib behaviour and reinforcement requirements:

• Different coal seams can exhibit markedly different behaviours ranging from highly cleated and friable coal at Angus Place to the much stronger blocky nature of the Dartbrook coal ribs.
• In the Angus Place environment, in spite of surprisingly high bolt loads, the ability to effect and maintain high levels of load transfer is extremely limited.
• Failed [crushed] and yield zones exist within the first 1.2m of the ribsides, with some deformation, but greatly reduced magnitude, beyond 1.2m at Angus Place.
• The yield zone extends to a width equivalent to approximately 45% of the mining height at Angus Place.
• The behaviour identified suggests the need for a yieldable reinforcement  element, capable of providing up to 30mm and possibly as high as 50mm yield to accommodate development loading but then retaining a reasonable level of stiffness.
• A flexible rib surface constraint, providing total coverage between bolts would be an effective supplementary support to a yielding bolt system to conditions such as those at Angus Place.  The concept of a flexible liner has great merit.
• The flexibility is a critical feature, which is not available through materials such as shotcrete which is far too rigid and brittle on failure.  Developments in Canada and South Africa of a very thin (2-3mm) sprayable, acrylic-based liner product show great promise and warrant further evaluation.
• In the case of Dartbrook, the depth of yield is slightly greater than at Angus Place (corresponding to increased mining height) but only under longwall abutment conditions and only on the opposite side of the pillar to the goaf.  (This also suggests that a wider pillar may be required to minimise the abutment conditions on the adjacent roadway).
• Rib behaviour at Dartbrook is characterised by detached blocks buckling and/or rotating from the ribside, generating very high bolt loads.  In this type of behaviour, only high capacity, stiff systems will offer any degree of rib control and they must be anchored well beyond the yield zone.
• Further development and comprehensive evaluation of a number of the yieldable/stiff prototype support elements trialled in this project should be undertaken.