12

ABSTRACT 

 This paper summarises the results of a research project whose 

goal was to provide the Australian coal industry with a rib support 

design methodology and software tool that could be utilised by 

suitably qualified colliery staff. The project was primarily funded 

by Australian Coal Association Research Program (ACARP) and 

further supported by several Australian longwall operations. 

 The outcome of the project is a design methodology and 

software tool called, Analysis and Design of Rib Support (ADRS). 

ADRS is an empirical technique, which recognises that several 

geotechnical and design factors affect ribline performance and in 

addition that operational and safety issues essentially dictate the 

level of performance required. Therefore, the design 

recommendations associated with ADRS are specific to the 

Australian coal industry; however the procedure(s) for data 

collection and analysis could be applied to other countries' 

'underground coal industries. 

 Case history data were compiled from 34 longwall and two bord 

and pillar operations resulting in 204 case histories with each case 

history data set being defined by approximately 130 individual data 

fields. In addition monitoring (incorporating stress cells and 

extensometers) was undertaken at 10 collieries to assess and 

quantify the effectiveness of different rib support patterns and 

hardware on rib performance. The monitoring sites allowed for an 

improved understanding of the mechanisms of rib failure and 

degradation in terms of its interaction with the installed support and 

at the various stages of the mining cycle. 

 The design methodology deals with both mains development 

and gateroad development specific to Australian longwall mines. 

This paper focuses on longwall gateroads subjected to abutment 

loading and in particular the travel road which becomes the tailgate 

of the subsequent panel. The statistical analyses associated with 

these cases suggested that the level of rib support should be based 

primarily on the development height and the pillar stress level. The 

ADRS software guides users through the design process, allowing 

them to develop rib support plans based on sound science and a 

broad base of in-mine experience. 

 To the best of the authors’ knowledge ADRS is the first 

systematic rib support design technique to be developed for any 

country’s underground coal industry. The development and use of 

empirical models in mining have substantially contributed to 

improving safety and productivity. ADRS further demonstrates 

that empirical techniques are particularly relevant and beneficial in 

dealing with the complexities of geotechnical design associated 

with underground coal mining. 

INTRODUCTION 

 Over the last decade many aspects of strata management have 

evolved, such that risk management has become the “core” of the 

strata management process at most Australian collieries. This is 

borne out with the advent of Strata Management Plans (SMP), 

roadway and longwall hazard plans and the categorisation of 

various roof zones as a part of the Mine Manager’s Support Rules. 

 This evolution has had a significant impact on pillar design, 

longwall face design/management, roof control and the use and 

implementation of instrumentation to monitor the strata behaviour. 

The one area that has severely lagged behind is rib control. It is 

assessed that the primary reasons for this disparity have been a lack 

of understanding of rib failure mechanisms and the significant 

factors that affect rib behaviour. 

 Coal mine rib instability is a significant safety and productivity 

issue facing the Australian underground coal industry. There are 

currently no formalised or widely accepted engineering techniques 

in relation to rock mass classification of a coal mine rib and for rib 

support design. In many cases rib support is installed in a reactive 

manner and changes to rib support patterns/hardware often occur 

following unsatisfactory conditions that are highlighted by 

personnel injury and/or extensive rib failure. 

 Information supplied by the Queensland (Qld) Department of 

Natural Resources and Mines and New South Wales (NSW) Coal 

Services Pty Ltd shows that for years 1997 to 2003 (inclusive) there 

were 175 Lost Time Injuries (LTIs) as a result of roadway rib spall. 

During this period there were unfortunately three fatalities, one in 

NSW and two in Qld. Figure 1 shows that approximately 41% of 

the rib related LTIs occurred during the installation of ground (roof 

or rib) support off the continuous miner (CM), 22% took place in 

the vicinity of the CM, while the remainder occurred away from the 

CM. 

Analysis and Design of Rib Support (ADRS) 

A Rib Support Design Methodology for Australian Collieries 

Mark Colwell, Principal 

Colwell Geotechnical Services 

Caloundra, Australia 

Christopher Mark, Chief, Rock Mechanics Section 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA 
13

 In addition to safety, rib instability can negatively impact on 

productivity or development rates in a number of other direct and/or 

indirect ways such as: 

• Unplanned (i.e., reactive) installation of primary and/or 

secondary rib support. 

• Longer transit times for men and machinery and/or roadway 

"clean-up" due to excessive rib spall. 

• Increased roof span requiring secondary roof support and/or 

resulting in roof instability. 

• Unnecessarily over-supporting the ribs. 

 The aim of this project was to provide the Australian coal 

industry with a rib support design methodology and computerbased design tool. The safety of personnel was the "key driver" for 

this research project and a safer workplace should result in a more 

productive workplace. 

BACKGROUND 

 In comparison to other strata control issues, such as pillar and 

roof support design, there has been comparatively little research 

undertaken in relation to rib support design. However, four 

significant research projects have been completed in Australia 

during the past two decades (O’Beirne, et al., 1987; Fabjanczyk, et 

al., 1992; Frith and Ditton, 1993; Hebblewhite, et al., 1998). 

 While there is some minor difference of opinion between the 

four studies in relation to the driving force behind rib degradation, 

all would appear to agree that buckling is a common failure 

mechanism. Figure 2 is an excellent example of ribline buckling. 

The studies also agree that efficient strapping, mesh and plate 

systems assist in maintaining the integrity of the immediate rib in 

most conditions, with the face plate being a vital component. 

 Cuttable (fibreglass and plastic) rib bolts are widely used in 

Australian mines for support in the longwall blockside riblines. 

The studies of O’Beirne, et al (1987), Fabjanczyk, et al (1992) and 

Frith and Ditton (1993) all highlighted significant limitations in 

relation to cuttable bolts. Roof and rib bolts, both steel and cuttable, 

are strong in “pure” tension, but when acting as rib support are also 

subjected to large bending moments. When cuttable bolts are 

called upon to resist bending, their poor elongation properties make 

them susceptible to rupture. 

 The results from this project would appear to confirm many of 

the findings of these four prior Australian studies. While these 

prior studies advanced our knowledge they ultimately did not 

provide the Australian coal industry with a rib support design 

technique. 

MONITORING STUDIES 

 As part of the project, 11 monitoring sites were established at 

ten different collieries, with seven of those collieries providing 

sufficient information for which an individual report was prepared. 

Those seven sites covered a wide variety of geological conditions 

and a total of 30 different support patterns. Of the seven sites, four 

were located in Bowen Basin coalfield of Central Queensland with 

one each in the Newcastle, Southern and Western coalfields of 

NSW. 

 The principle objectives of the monitoring program within the 

project were: 

1. To assess and quantify the effectiveness of different rib 

support patterns and hardware on rib performance and, 

2. To develop a better understanding of the mechanisms of rib 

failure and degradation in terms of its interaction with the 

installed support and at the various stages of the longwall 

extraction cycle. 

Figure 1. Rib injury experience in Australian underground 

coal mines, 1997-2003. 

All Other 

Activities

Working Near 

CM

Installing 

Support

37% 41%

22%

Figure 2. Photograph illustrating rib failure by buckling. 
14

 The monitoring sites at each colliery were located within the 

travel road of the maingate development (with one additional site 

located in a belt road). In each instance the instrumentation was 

installed some months after development but generally well before 

the site was subject to abutment loading. Each monitoring site 

incorporated the use of hydraulic stress cells, rib extensometers and 

at least one roof extensometer. Figure 3 depicts the rib support and 

instrumentation layout at Kestrel Colliery and is reasonably typical 

of the instrumentation layout. 

 The extensometers were used to compare rib behaviour, 

primarily in terms of Total Rib Displacement (TRD) and Depth of 

Softening (DOS) for the various support patterns employed. TRD 

was simply defined as the horizontal displacement of the ribline 

surface. Assessing the DOS was more subjective particularly when 

using a 4 or 5 point extensometer. In general the overall shape of 

the plots was more useful in assessing the DOS than attempting to 

define specific values. 

 The DOS in this study is deemed as the practical, rather than 

absolute, extent of horizontal displacement or fracturing within the 

coal rib. In terms of the pillar behaviour model proposed by 

Hebblewhite, et al (1998), the absolute extent of softening would 

actually extend into the elastic core of the pillar, where horizontal 

displacement is a result of Poisson’s Effect. In terms of rib 

extensometry this would equate to using zero horizontal 

displacement to define the DOS. It is assessed that using the 

absolute extent of movement to define the DOS is impractical and 

this is best illustrated with the response of the pillar side stress cell 

and adjacent rib extensometer associated with the RIMA dowel 

pattern at Kestrel Colliery (refer Figure 3). 

 In this instance the stress cell is positioned 3.5 m into the rib, 

while the rib extensometer has anchors located at 1, 2, 4 and 8 m 

into the rib. The output in relation to said extensometer is detailed 

in Figure 4 while the pillar side stress cell response is illustrated in 

Figure 5. 

 If the absolute extent of horizontal displacement were used to 

define the DOS, then in relation to the extensometry information 

presented in Figure 4 (and in terms of the final set of readings – 

Tailgate Loading) the DOS would be somewhere between 4 to 8 m. 

However, the pillar side stress cell response (refer Figure 5) clearly 

indicates that at the stress cell’s depth of 3.5 m, this section of the 

Figure 3. Typical instrumentation plan for a monitoring field site. 

INSTRUMENTATION AND SUPPORT LAYOUT

MAINGATE 205 TRAVEL ROAD - KESTREL

A - 25m section of chain pillar ribline supported with 

2 x 1.2m AVH steel bolts/m with 600mm medium set

resin and 300mm x 280mm Butterfy Plates.

5c/t B - 25m section of chain pillar ribline supported with 

2/1 x 1.5m Split Sets @ 1.5m spacing with Butterfy Plates

securing 2.2m wide Tensar Mesh (approx. 1 split set/m).

C - 25m section of blockside ribline supported with 

2 x 1.2m Dupadowels/m with 600mm medium set resin

and 230mm diameter plastic plates.

 A 32.5m

Remaining sections of ribline (pillar & blockside)

supported with 2 x 1.2m point anchored RIMA dowels/m

with 230mm diameter plastic plates.

 B 67.5m C

1 & 2 - Stress Cells positioned 3.5m into rib.

6c/t 3,4,5,6 & 7 - 8m 4-point GEL Rib Extensometers

⊗ - 8m 4-point GEL Roof Extensometer

⊗

55m

 80m

20m

45m

55m

80m

3 1 2

4 5

6 7
15

pillar is still well and truly capable of supporting a significant 

change in load, as evidenced during the approach of LW 206 or the 

Tailgate Loading phase of the extraction cycle. Therefore, in 

practical terms the DOS is certainly no more than 4m. 

 All seven sites provided sufficient information to assess the 

effect on travel road ribline performance at the Maingate (MG) 

Loading stage of the longwall extraction cycle. In addition three of 

the collieries were also able to provide data to fully assess the 

Tailgate (TG) Loading ribline performance. Figure 6 details five 

specific stages of the chain pillar loading cycle utilising a typical 

Australian longwall mining layout. Those stages are described 

below: 

1. Development Loading: The vertical loading condition of 

the chain pillars subsequent to development while prior to 

any adjacent longwall extraction. 

2. Front Abutment (MGB) Loading, occurs when a chain pillar 

is first subjected to longwall retreat and the longwall face is 

parallel with the chain pillar. This is a transient loading 

phase and its impact (for the purposes of this study) is 

specific to the belt road riblines about the maingate 

intersection with the longwall face. 

3. Maingate (MG) Loading, is when the side abutment load has 

stabilised after the passage of the first adjacent longwall 

face. This is essentially a static loading phase and is 

specific to the travel road riblines. 

4. Tailgate (TG) Loading, is when the face of the second 

adjacent panel is parallel with the chain pillar. Once again 

this is a transient loading phase and its impact is specific to 

the riblines about the tailgate intersection with the longwall 

face. In this instance the current tailgate would have acted 

as the travel road of the previous longwall panel. 

5. Double Goaf Loading, is when the pillar is isolated between 

two goafs. For rib support design purposes this loading 

phase is not required to be considered. Figure 4. Chain Pillar Rib Extensometer – RIMA 

0

1

2

3

4

5

6

7

8

9

-10 0 10 20 30 40 50 60 70 80 90 100

Rib Displacement (mm)

Distance Into Rib (m)

LW205 1070m inbye

LW205 18m outbye

LW205 80m outbye

MAINGATE LOADING

LW206 41m inbye

TAILGATE LOADING

Figure 5. Stress Cell Vertical Pressure Change versus LW’s 205/206 Face Position, Kestrel Colliery. 

0

1

2

3

4

5

6

7

8

-450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450

LW 205/206 Relative Face Position (Distance, m)

Vertical Pressure Change (MPa)

LW 205 Retreat

LW 206 Retreat
16

 While full details on the monitoring studies are available 

elsewhere (Colwell, 2005), the most significant conclusions are 

summarised below: 

• The nature of the rib movement appears to be consistent 

with the buckling of thin coal plates or slabs. While a 

precursor to buckling may be tensile failure in the form of 

vertical splitting along cleat or Mining Induced Fractures 

(MIF) or the formation of new cracks, it is clear from the 

extensometry that the nature and magnitude of the lateral 

displacement is a result of buckling. 

• Total Rib Displacement would appear to be primarily driven 

by the change in vertical load applied to the pillar (or more 

accurately the pillar rib) due to longwall abutment loading 

and appears to be largely independent of the variation to the 

ground support installed. This finding is supported by the 

research of Fabjanczyk, et al. (1992), who indicate, “that 

long tendons (50 tonne units) placed at a density of 1 

tendon/m length of rib can assist in maintaining the integrity 

of the rib, but do not significantly reduce the total rib 

deformation under extreme vertical stresses”. 

• The use of mesh (particularly steel mesh) offers confinement 

to the ribline, which reduces the depth of softening and in 

some instances appears to delay the onset of initial rib 

displacement; however, mesh does not have a significant 

impact on the TRD. In addition, mesh greatly assists in the 

effective maintenance of gateroad serviceability throughout 

the longwall extraction cycle. 

• When not utilising mesh it would appear that the bolting 

density of a support pattern is directly related to the level of 

spall. Based on observation, it is assessed that the likely or 

primary reason is the greater areal plate coverage which 

simply offers greater restraint to the skin of the ribline. This 

strongly suggests that the face plate should be as large as 

possible, while considering handling, installation and 

stiffness issues, which will limit its size. 

• Most (if not all) steel rib support systems offer greater collar 

integrity (which is critical to rib maintenance) than any 

cuttable system. 

• Stone bands within a seam appear to have several roles. 

Where present they tend to act as the ‘hinge’ or apex in 

relation to bulging in the riblines and/or the end point of the 

buckling slab. When acting as the end point, these stone 

bands typically modify the end condition allowing lateral 

movement. This increases the effective length of the coal 

plates or slabs dramatically lowering the critical load or 

stress for which buckling can occur. In addition, stone 

bands often delineate the extent of rib deterioration. 

• The orientation of the face (or dominant) cleat in relation to 

the driveage direction also appeared to be an important 

factor in terms of ribline behaviour. It seems that when the 

face cleat is sub-parallel to the driveage direction then the 

greater the depth of softening. Cleat orientation was 

considered in more detail when analysing the database and 

formulating the design methodology. 

THE INDUSTRY REVIEW 

 The aim(s) of the industry review were to: 

1. Construct both a contemporary and historical database of rib 

performance. 

2. Conduct statistical analysis of the data to determine the 

significant predictors of rib performance and, 

3. In combination with the monitoring exercises, to assist in 

developing a rating classification system for coal ribs in 

relation to their structural competence and a rib support 

rating in terms of the amount and type of rib support 

installed. 

Figure 6. Chain pillar loading cycle, showing location of Development, MG/TG, and MGB loading conditions.

Solid Coal or Barrier

GOAF GOAF

Longwall

GOAF Retreat

Direction

Unmined Panel

c

a

e d

b

Position a – Development Loading

Position b – Front Abutment (MGB) Loading – Belt Road Riblines

Position c – Maingate (MG) Loading - Travel Road Riblines

Position d – Tailgate Loading - Tailgate Riblines

Position e – Double Goaf Loading

Tailgate

Belt Road

Travel Road
17

 During the course of the project, 25 longwall mines and two 

bord and pillar operations were visited. Several of these collieries 

were visited on more than one occasion and in total 44 underground 

inspections were conducted. The underground inspections were 

carried out to evaluate rib performance at specific localities within 

an operation (i.e., mains, belt and travel roads and tailgate) and for 

the various stages of the longwall extraction cycle (refer Figure 6). 

In addition, information collected during previous underground 

investigations undertaken as a part of the ALTS (Analysis of 

Longwall Tailgate Serviceability) research (Colwell, et al., 2003 

and Colwell, 1998), was included within the database. 

 During the site inspections information was collected on factors 

affecting rib performance including coal seam properties (i.e., 

quality, strength and structure), roof/floor contacts, geometric 

details (i.e., development height, pillar dimensions, cover depth, 

etc.), in situ and mining induced stresses, as well as the type, timing 

and quantity of rib and roof support installed. Subsequent to an 

underground inspection, a site inspection report was prepared which 

was then forwarded to the respective colliery for review and 

confirmation. This process was undertaken to ensure the integrity 

of the information contained in the database. 

 In addition, discussions were held with colliery personnel to 

ascertain how current rib performance compared to past experience. 

On many occasions this resulted in a detailed description of the 

gradual development of the rib support system currently employed, 

which allowed for a greater appreciation of some of the difficulties 

faced by the collieries satisfactorily managing rib behaviour. 

The Database 

 The final database consisted of 204 case histories obtained from 

26 collieries. Of the 204 case histories, 13 are related to main 

headings, two are in relation to bord & pillar panels while the 

remaining cases are related to longwall gateroads. Of the 189 

gateroad cases, 42 are associated with the belt road, 69 with the 

travel road, and 75 were observations from the tailgate. Only three 

cases are related to longwall gateroad cut-through ribline 

performance and this was where the reorientation of the face cleat 

to the driveage direction clearly had a significant effect on ribline 

behaviour at the development stage. 

 On a number of occasions, subsequent to longwall retreat, cutthrough ribline performance was worse than that associated with the 

travel road. However, these cases were not included in the database 

as there is a distinct difference in ribline abutment loading between 

the two conditions. Away from the intersections ribline loading 

should be reasonably constant along the heading whereas it would 

vary significantly within the cut-through moving from the goaf to 

the travel road. 

 Of the 189 gateroad cases, 62 are associated solely with the 

blockside ribline, 82 directly with the chain pillar and 45 cases with 

both. In terms of those 45 cases where both riblines are designated, 

this only applied if the level of spall between the opposing 

blockside and chain pillar riblines is essentially the same and in 

addition the same level and type of rib support had been installed. 

 In terms of assessing risk there was significant difficulty in 

comparing the rib performance about the CM as opposed to those 

work-related activities associated about the maingate belt road 

intersection with the longwall face (refer Position b – Figure 6) and 

those associated with the travel road/tailgate when subject to 

maingate or tailgate loading (refer Positions c and d – Figure 6). 

 For example, in relation to operational activities within the 

gateroads about the longwall face, it was generally found that rib 

spall posed a greater safety risk in relation to those activities 

conducted about the belt road intersection with the face. The stage 

loader and conveyor belt structure firstly places men in close 

proximity to the riblines when entering or leaving the longwall face, 

or when working adjacent to the blockside ribline during a belt 

retraction, and secondly can restrict an underground worker’s 

ability to take evasive action in the event of a rib fall. 

 In these instances minor levels of spall or the unpredictable 

manifestation of that spall (observed on a number of occasions 

particularly in relation to the blockside ribline) present a clear and 

additional risk to personnel as compared to most work related 

activities conducted in the travel road or tailgate. 

 Essentially, one is not comparing apples and apples in terms of 

risk when comparing the belt road ribline performance to that 

associated with the travel road and tailgate. It was decided to 

separate the maingate belt road (MGB) loading cases from the 

MG/TG cases when analysing the effects of longwall abutment 

loading and required support levels. The database was refined as 

necessary to assess ribline performance at the various stages of and 

locations related to the mining/chain pillar loading cycle (refer 

Figure 6). 

Statistical Analysis 

 Logistic regression was the primary statistical technique used in 

the analyses. Logistic regression allows for the classification of the 

case histories into two (or more) populations based on a particular

outcome. Logistic regression was used to determine which 

predictive variables were most significant in determining the 

outcome being essentially the ribline condition’s impact on safety 

and productivity. 

 The result of a logistic regression analysis is an equation (in 

terms of the predictive variables) that acts as a boundary of 

separation between the two populations in terms of the outcome. 

We refer to the equation that separates the two groups as the 

Discriminant Equation. The Discriminant Equation can be used to 

formulate quantitative guidelines for the design of rib support 

systems in order to achieve the desired outcome. The statistical 

software package SPSS was used for the logistic regression 

analyses. 

 Statistical analyses were conducted to assess ribline behaviour 

for the following: 

1. Development Ribline Condition (1), which is defined as the 

ribline condition just prior to and/or during the installation 

of roof and/or rib support or essentially the condition of the 

exposed ribline about the CM that the miners encounter 

while preparing (i.e., drilling) and then installing roof and 

rib support. 

2. Development Ribline Condition (2) which is defined as the 

ribline condition subject to Development Loading (refer 

Position a – Figure 6). These analyses were conducted to 

assist in determining the required levels of primary support 

to maintain satisfactory ribline conditions prior to abutment 

loading effects. 

3. MGB Loading Condition, which is defined as the ribline 

condition about the maingate belt road intersection with the 

longwall face. These analyses were conducted to assist in 
18

assessing appropriate total (primary and secondary) rib 

support levels for belt road riblines. 

4. MG/TG Loading Condition (travel road/tailgate riblines 

subject to longwall abutment loading). These analyses were 

conducted to assist in assessing appropriate total rib support 

levels for travel road (and subsequently tailgate) riblines. 

 In this study the ribline performance was assessed in three ways: 

1. Satisfactory or Unsatisfactory (criteria based). 

2. Safety/Productivity Risk (five categories ranging from low 

through moderate-low, moderate, moderate-high to high). 

The initial risk categorisation was based on the author’s 

own experience in consultation with colliery personnel. 

3. The volume of spall per lineal metre of ribline (m3

/m). 

 To be classified as unsatisfactory, a case had to meet one of six 

criteria: 

• Management changed the rib support design (hardware or 

pattern density) in response to poor rib conditions. 

• Unplanned use of secondary rib support. 

• Unpredictable/Unsafe rib failure in terms of when and/or 

where it will happen (but it does happen). 

• Excessive rib spall. 

• Excessive remedial clean-up of roadway or ribline. 

• Resultant rib spall presents a clear risk to personnel (typically 

associated with a Hazardous Zone). 

 Where the resultant rib condition is unsatisfactory then 

generally more than one of the above criteria is met. A Hazardous 

Zone typically refers to a location/work practice that by its nature 

places men in close proximity to the riblines and where an 

underground worker’s ability to take evasive action in the event of a 

rib fall is restricted. 

The Rib Support Rating (RIBSUP) 

 For logistic regression to be successful, the number of case 

histories must be significantly larger than the number of predictive 

variables. Therefore, it is often necessary to combine groups of 

related variables into rating scales. In this study, the most 

important rating scale was the Rib Support Rating (RIBSUP). 

There are four components to RIBSUP, being: 

1. A measure of the bolting capacity per square metre of 

ribline – RBOLT 

2. A measure of the relative effectiveness/confinement offered 

by the face plate – FPLATE 

3. A measure of the confinement offered by a liner – 

Confinement Factor (CF) 

4. A methodology to combine these values, for both primary 

and secondary rib support, into one rating (RIBSUP). 

 RBOLT incorporates the shear strength, rather than the tensile 

strength, as the most appropriate indicator of a bolt or dowel’s 

performance in terms of modifying rib behaviour. However given 

the sometimes wide discrepancy in shear strength values between 

dowels and bolts (e.g. the shear strength of a glass reinforced nylon 

Dupadowel is 25 kN or approximately one-ninth that of an X grade 

24 mm steel bolt) it was judged that the square root of the shear 

strength resulted in a more appropriate comparison. 

 RBOLT is defined as: 

(1) 

where L = Length of the rib bolt/dowel (m) 

 N = Average number of bolts/dowels per vertical row 

 Sh = Typical shear strength of the rib bolt/dowel (kN) 

 S = Spacing between vertical rows of bolt/dowels (m) 

 h = Development Height (m) 

 The primary rib bolt length varied from 0.9 to 1.8 m within the 

database, with 1.2 m being by far the predominant value. There 

was a wide variation in the grade or type of bolt used, however the 

average steel bolt UTS and Shear Strength was 240 and 160 kN, 

respectively. On average two bolts are installed every 1.2 m, 

however, once again there is a wide variation in bolting density. 

 

 FPLATE starts with the Standard Butterfly Plate (with an 

approximate area = 300 x 280 mm = 0.084 m2

) that is the 

predominant face plate used at Australian collieries in terms of 

chain pillar riblines and mains development. FPLATE has a value 

of 1.0 for the Standard Butterfly Plate, and is adjusted for other 

plates in proportion to their area relative to the standard, such that: 

FPLATE = [(Area of Face Plate)/ 

 (Area of Standard Butterfly Plate)]1⁄2 (2) 

 In terms of the database the area of the face plates ranged from 

a minimum of 0.01 m2

 to a maximum of 0.123m2 resulting in an 

average FPLATE of 0.84 with a standard deviation of 0.19. 

 CF starts with the premise that the confinement factor owing to 

a liner operates as a multiple within the RIBSUP calculation. 

Therefore, the CF=1 where the rib support does not include some 

type of liner. The CF increases in proportion to the surface area 

covered by the liner up to a maximum value of 4.0. 

 The use of straps as a liner typically results in a 1< CF < 2. In 

terms of the analyses and design methodology, straps are defined as 

being less than or equal to 400mm wide and can be made of 

polymer or steel as well as being solid (i.e., W Strap) or mesh type 

construction. Where mesh sheets/rolls were used the resultant 

CF ≥ 2, essentially meaning that at least one-third of the ribline 

surface area was covered. Mesh sheets/rolls can be of polymer or 

steel construction. 

 In relation to face plates and liners, the methodology only 

distinguishes between the different products in terms of their areal 

size, not their strength. It is the colliery’s decision as to whether 

the face plate or liner chosen satisfies the operational purpose for 

which it is required. If the face plate or liner chosen does not 

satisfy operational requirements then it simply needs to be replaced 

by a product that does. 

 For example, steel mesh is clearly superior to cuttable polymer 

mesh types in terms of tensile strength and resisting tear, as well 

steel mesh/face plates are generally stiffer than polymer products 

offering greater potential confinement and collar integrity. If a 

cuttable face plate or liner does not satisfy the operational 

requirements (i.e., because the product is not stiff enough or of 

sufficient size to resist movement or maintain a satisfactory level of 

surface contact) then steel products may have to be considered and 

used. 

 In terms of face plates and liners the requirement within the 

methodology is that, “the face plate or liner maintains effective 

surface contact with the ribline.” It may be that the face 

plate/liner is not the only or primary cause of losing effective 

contact with the ribline and that the nut/thread tensile strength (or RBOLT = L x N x Sh1⁄2

S x h
19

head load bearing capacity in relation to Thrust Dowels) is also a 

factor. The size of the face plate, its material properties and the 

nut/thread tensile strength can essentially be grouped together in 

terms of the collar integrity of the rib support system when 

assessing the issue of the face plate or liner maintaining effective 

surface contact with the ribline. 

 RIBSUP combines the other three ratings as follows, for two 

different situations: 

 (1) Where a liner is utilised and securely fitted to the rib by the 

bolts/dowels and face plates then: 

 RIBSUP = CF x RBOLT (3) 

(2) If there is no liner but simply the face plates then: 

 RIBSUP = FPLATE x RBOLT (4) 

In relation to the above equations, essentially where there is a liner 

and as long as it is securely fitted then the size of the face plate has 

far less impact on the overall rib performance and is, therefore, 

ignored. In certain instances when combining primary and 

secondary rib support, some modifications to the above equations 

are necessary. 

MG/TG Loading Condition 

 The following discussion presents those findings associated 

MG/TG Loading Condition. Full details of the analyses associated 

with the other loading conditions have been reported elsewhere 

(Colwell, 2005). 

 A total of 116 MG/TG gateroad cases were available, in which 

the entries had been subjected to significant longwall abutment 

loads. Of these, 78 were related to the Maingate Loading (MG) 

condition within either the travel road or tailgate, and the other 38 

cases were specifically related to ribline performance in the 

immediate vicinity of the tailgate intersection with the longwall 

face (i.e., Tailgate or TG Loading condition). 

 This sub-set of the database included 25 longwall mines with 

only one of those collieries not currently operating. Developments 

heights range from 2.4 m up to 4.1 m, while depth of cover ranges 

from 105 to 510 m. 

 As previously stated risk management has become the “core”

of the strata management process at most Australian collieries. 

Therefore a risk based outcome was considered necessary so as to 

emphasise that if a colliery intends to utilise ADRS, it needs to do 

so in the context of risk management. It is recognised that the risk 

based outcome utilised in this study of low through to high (with 

five categories) would be different to a colliery based assessment 

(where risk = probability x consequence), however the criteria can 

be adapted to suit a colliery’s risk assessment process and 

operational requirements. 

 Assessing risk is not an exact science (neither is underground 

coal mining) and while guidelines and some set criteria are 

necessary, utilising engineering judgment is just as important. In 

terms of the MG/TG cases the risk based outcome, which was 

utilised, incorporated all three ribline performance assessment 

criteria, such that: 

1. If the rib performance was assessed as unsatisfactory then 

risk must be classified moderate or higher. 

2. If the change in the volume of spall (subsequent to 

development) was greater than 0.88 m3

/m (average) then 

risk must be classified moderate or higher. 

3. If the rib performance was assessed as unsatisfactory and

the change in the volume of spall was greater than 

0.88 m3

/m then risk must be classified moderate-high or 

high. 

4. If the rib performance was assessed as satisfactory and the 

change in volume of spall was less than 0.55 m3

/m (average 

minus 1 sd) then risk would typically be classified low or 

moderate-low. 

 The above set of criteria rightly allows for a certain amount of 

engineering judgment to be incorporated in the process of deciding 

which of the actual five categories best assesses the risk. Using the 

above criteria the 116 case histories were grouped as follows: 

• 9 cases were assessed as low risk. 

• 27 cases were assessed as moderate-low risk. 

• 26 cases were assessed as moderate risk. 

• 35 cases were assessed as moderate-high risk. 

• 19 cases were assessed as high-risk. 

 For the logistic regression analyses, the case histories were 

divided into two outcome groups. The first group consisted of 

those cases with low & moderate-low risk (36 cases). The other 

group included those cases with moderate-high & high risk (54 

cases). The moderate risk cases were initially excluded to have a 

clear distinction in the outcome (being beneficial to the analyses). 

Therefore the resultant discriminant equation would potentially 

represent a design equation for approximately moderate risk. 

 Three separate measures of the vertical load were initially 

employed in the analysis, the cover depth (H), average pillar stress 

(σP), and the Pillar Factor (PF = Pillar Strength/Average Pillar 

Stress). Since these three measures are strongly correlated with 

each other, only one could be retained for the final equation. It was 

found that the average pillar stress (calculated using tributary area 

and abutment angle concepts) resulted in the greatest predictive 

success rate. The average pillar stress has the additional advantage 

that it, unlike PF, is totally independent of the development height 

(h). 

 At a significance level of “alpha” = 0.05, it was found that four 

variables were significant predictors of rib performance, being: 

1. Average Pillar Stress (σP, ranging from 4.8 to 42.5 MPa) 

2. Development Height (h, ranging from 2.4 to 4.1 m) 

3. RIBSUP (ranging from zero to 80.8) 

4. Hardgrove Grindability Index (HGI, ranging from 30 to 88). 

 In terms of the individual impact or weighting on the outcome 

of the four significant predictor variables, σP has a 35.2% impact on 

the outcome, followed by RIBSUP with 29.3%, h with 25.2% and 

HGI with 10.3%. The discriminant equation was calculated as: 

 RIBSUP = 41.0 h + 2.58 σP + 0.47 HGI – 175 (5) 

Equation 5 successfully classified 47 of the 54 moderate-high/high 

risk cases (87% correct) and 27 of the 36 moderate-low/low risk 

cases (75% correct) for an overall classification success rate of 
20

82.2%. Equation 5 essentially represents a design equation for rib 

support in terms of operating with a moderate level of risk. 

 It was found that if the constant in equation 5 was adjusted by 

+6 (refer equation 6) then all 54 of the moderate-high/high risk 

cases were successfully classified. This equation resulted in a drop 

in the successfully classified moderate-low/low risk cases from 27 

to 22 of the 36 cases (i.e., 61% correct). However, the overall 

classification success rate increased from 82.2% to 84.5%. In 

addition when assessing the moderate cases with equation 6, 22 of 

the 26 cases are successfully classified. Equation 6, which is 

illustrated in Figure 7, essentially represents a design equation for 

rib support in terms of operating with a moderate-low level of risk. 

 RIBSUP = 41.0 h + 2.58 σP + 0.47 HGI – 169 (6) 

 During the MG/TG analyses several other variables were found 

to have a secondary to minor impact on rib performance, including 

the dominant/face cleat orientation to driveage direction (Ф). 

While Ф is not a significant predictor of ribline performance for the 

MG/TG Loading Condition, it is important to note that for 

Development Condition (1), Ф is a significant predictor of ribline 

performance. 

 There are sound technical reasons that help to explain why cleat 

orientation was not found to be a significant predictor of the 

outcome at the MG/TG stage. Firstly in contrast to the 

development stage where the principal vertical stress trajectories 

would “surround” the development face; during and after longwall 

extraction the principal vertical stress trajectories associated with 

the blockside and chain pillar riblines (away from the chain pillar 

corners) must run parallel with the travel road orientation. 

Therefore vertical cracking as a result of stress increase will 

preferentially develop parallel to the roadway orientation. 

 In addition to the above, existing cracking within the ribline 

(whether natural or mining induced) will act as stress raisers and it 

takes significantly less energy to extend an existing crack than to 

develop a new crack. For example, even if the butt/minor cleat is 

only moderately or poorly developed and were sub-parallel to the 

heading then it would preferentially act as a stress raiser and under 

abutment stress may readily be extended. 

 In the above situation the dominant or face cleat would be near 

perpendicular to the ribline which would appear to be favourable 

until one realises that the butt cleat (or MIF for that matter) can also 

act to “split” the coal. In addition each said cleat direction actually 

represents the statistical mean of a spread of cleat directions. 

Fundamentally coal is a relatively highly fractured rock type that 

can more readily split in several directions (as compared to other 

rock types), while preferentially splitting parallel with the stress 

trajectories. 

ANALYSIS AND DESIGN OF RIB SUPPORT (ADRS) 

 The results of the statistical analyses have been incorporated in 

the rib support design methodology Analysis and Design of Rib 

Support (ADRS). The following discussion relates to the design 

0

20

40

60

80

100

120

10 15 20 25 30 35 40

Average Pillar Stress (MPa)

Design RIBSUP

HGI = 80

HGI = 35

h = 4.0m

h = 3.3m

h = 2.5m

For moderate-low risk a minimum Design RIBSUP of 11 is recommended when σp > 10MPa

Figure 7. Design equation for the MG/TG Rib Condition, for Moderate-Low Risk Level. 
21

issues associated with the more challenging matter of controlling 

the travel road/tailgate riblines subjected to the MG and TG 

Loading Conditions. The impact of future longwall abutment 

loading can and often does place a severe demand on the rib 

support requiring a significant increase in RIBSUP over and above 

that necessary for development. 

 The design methodology does not differentiate between the 

blockside and chain pillar side riblines in relation to what type of 

support hardware is utilised. In general, Australian collieries would 

clearly prefer to use either no rib support or cuttable support 

without mesh in relation to the blockside ribline so as to reduce the 

risk of certain issues associated with the longwall face 

extraction/cutting process. For example (but not limited to): 

• Coal contamination 

• Belt damage. 

• Blockages due to mesh. 

• Sparks associated with cutting steel bolts. 

• Mesh getting caught around the shearer. 

 It is the colliery’s decision on how to “balance” the 

safety/productivity risks associated with the longwall extraction 

process and those safety/productivity risks associated with gateroad 

ribline performance. ADRS should provide sufficient information 

so that a colliery can more rationally assess the risk in relation to 

rib performance and therefore make a more informed decision. 

 In terms of the travel road riblines, once the colliery has decided 

what abutment loading condition (MG or TG) and what level of 

risk (moderate or moderate-low) is appropriate for design purposes 

then the next step is to calculate the suggested RIBSUP utilising 

equations 5 or 6. However, the design methodology also needs to 

provide guidelines in relation to the use of liners and cuttable/steel 

support to provide the end user with a workable technique. In 

addition, minimum limits need to be imposed in some 

circumstances. 

 The relationships between the type of liner (i.e., none, straps or 

mesh) and rib bolt (i.e., none, cuttable or steel) versus the four 

significant predictor variables (RIBSUP, h, σP and HGI) were 

assessed in a unique and simple manner to provide realistic limits in 

terms of hardware selection and minimum levels of rib support. It 

was found that the RIBSUP (suggested for design purposes) and σP

had the most influence on what hardware should be selected to 

satisfy the recommended levels of rib support. 

 The results of these analyses in relation to RIBSUP and σP (the 

two predictors having the greatest impact on the outcome) are 

summarised in Table 1. Further recommendations apply in relation 

to h and HGI and the interested reader is referred to Colwell (2005) 

for that additional information and a more detailed explanation of 

said analyses. To assist the reader in the interpretation of Table 1 

the following example is provided. 

Example 

 A colliery is operating at a cover depth of 325 m with a 

development height of 2.9 m, roadway width of 5 m, chain pillar 

width of 35 m (rib to rib) and longwall panel width of 250 m. The 

representative HGI of the coal seam is 42. Following a review, the 

colliery has selected the MG Loading condition for rib support 

design purposes. Utilising tributary area concepts, the abutment 

angle model (for a 2D “slice”) and incorporating a vertical pressure 

gradient of 0.025 MPa/m and an abutment angle of 21°, it is 

calculated that the average pillar stress (σP) is 21.1 MPa. 

 If a Moderate risk level is used for design purposes then ADRS 

would utilise equation 5 resulting in a Design RIBSUP of 18 and 

provide the following Design Statement (based on Table 1) to guide 

the user in the selection of the hardware: 

“Consideration should be given to including some form of 

liner as a part of the rib support system utilising steel in 

preference to cuttable support.” 

 However, if a Moderate-low level of risk is used for design 

purposes then ADRS would utilise equation 6 resulting in a Design 

RIBSUP of 24 and provide the following Design Statement (based 

on Table 1) to guide the user in the selection of the hardware: 

“The rib support system should incorporate mesh 

(preferably steel with a CF ≥ 2.5) that is firmly secured to 

the ribline with steel bolts and plates.” 

Table 1. ADRS Recommended Hardware and Support Levels 

Risk Level 

Moderate Moderate-low 

RIBSUP σP (MPa) RIBSUP σP (MPa)1

Suggested Rib Support 

Hardware/Level 

- > 11 - > 8 Rib support should be installed 2, 3

> 6 > 13 > 0 > 10 Steel bolts & plates are preferred to cuttable support 4

> 40 > 23 > 6 > 13 Steel bolts & plates should be utilised 

>10 >15 > 6 >10 Some form of liner (i.e., straps or mesh) is preferred 5

> 20 - - > 13 Some form of liner (preferably mesh with a CF ≥ 2.5) should be utilised

> 50 > 23 > 20 > 20 Mesh (preferably steel with a CF ≥ 2.5) should be utilised 1

If σP > 22MPa then even with the use of mesh and steel bolts the colliery should adapt their SMP to accommodate operating with a 

moderate (or possibly greater) level of risk in relation to rib performance based on current industry support levels and hardware. 2

For Moderate risk a minimum Design RIBSUP of 2.5 is recommended when σP > 11 MPa. 3

For Moderate-low risk a minimum Design RIBSUP of 5 is recommended when σP > 8 MPa. 4

For Moderate-low risk a minimum Design RIBSUP of 11 is recommended when σP > 10 MPa 5

For Moderate risk a minimum Design RIBSUP of 11 is recommended when σP > 15 MPa 
22

 The increase of 6 in the Design RIBSUP moving from 

Moderate to Moderate-low is straight-forward, however the two 

Design Statements are radically different in their intent when 

utilising ADRS as part of the risk assessment process. 

Risk and Abutment Loading Selection for Design 

 It is for the colliery to decide what abutment loading condition 

(MG or TG) and what level of risk (moderate or moderate-low) is 

appropriate for design purposes. A preliminary risk assessment or 

at least discussions involving several colliery personnel should be 

undertaken in evaluating both these aspects. If there is uncertainty 

at this initial design stage, then possibly all four scenarios need to 

be assessed. The following discussion and examples are provided 

to assist with that process. 

 It was found during the underground inspections that at a 

number of collieries the vast bulk of the overall rib deterioration 

associated with the travel road/tailgate occurred as a result of 

Maingate Loading and in relation to Tailgate Loading the additional 

deterioration was comparatively minor (if any) and confined to the 

immediate tailgate intersection. In this situation or with this 

general experience (at an individual mine) it would be reasonable 

for a colliery to utilise MG Loading for design purposes. 

 Conversely, there were also many instances where Tailgate 

Loading had a significant impact on ribline performance with the 

resultant ribline deterioration extending 20 to 50m outbye of the 

longwall face (sometimes further). In these instances it may be 

more appropriate to use TG Loading for design and would be 

essential if an increase in the roadway span could lead to roof 

instability. 

 In terms of risk, two examples or scenarios are provided. 

Firstly, a colliery may wish to use the future tailgate for longwall 

equipment relocation purposes, if this were the case then a 

moderate-low/low level of risk may be required along the roadway 

during the relocation process due to the man traffic and productivity 

issues (i.e., minimising the risk of delaying the longwall relocation). 

Therefore in this instance the design condition may be MG Loading 

for moderate-low risk. 

 The second example relates to a colliery where there are 

discernable Tailgate Loading effects in relation to ribline 

performance, however, the roof is quite “strong” (i.e., CMRR > 55) 

and largely unaffected by moderate levels of rib spall increasing 

roadway width. In this situation utilising the TG Loading condition 

with a moderate level of risk may be the most appropriate for 

design purposes. 

 The preceding discussion is only meant to highlight a few 

factors that may affect a colliery’s decision in relation to assessing 

the appropriate risk level and abutment loading condition for design 

purposes. There are numerous other factors to consider particularly 

in relation to the subsequent longwall extraction process, 2nd egress 

issues, safety of men when working in the tailgate (i.e., installing 

secondary support) and productivity factors that can only 

satisfactorily be assessed within the forum of a properly facilitated 

mine site risk assessment. 

CONCLUSIONS 

 The aim of this project was to provide the Australian coal 

industry with a rib support design methodology and computer based 

design tool that can be utilised by colliery engineers and geologists 

who have sufficient experience and training in relation to 

underground coal mine strata mechanics. These aims have been 

achieved and the design methodology and software package is 

referred to as Analysis and Design of Rib Support (ADRS). The 

intended benefits to underground operations, in the provision of this 

information and resource, are a safer and more productive 

workplace. 

 ADRS provides a Design Rib Support Rating (Design RIBSUP) 

and guidelines in relation to the use of liners and cuttable/steel 

support to provide the end user with a workable technique. The rib 

support hardware component of the software assists the strata 

control engineer with the selection of suitable products and rib 

support patterns to satisfy the Design RIBSUP. Also within the 

software package a database search can be conducted to assist in the 

design process. 

 Assessing risk is not an exact science (neither is underground 

coal mining) and while guidelines and some set criteria are 

necessary (which are clearly defined), utilising engineering 

judgement is just as important. Therefore, ADRS is not a 

prescriptive technique, but a tool to assist collieries in assessing 

their rib support requirements in the context of the risk assessment 

process associated with the development of a colliery’s Strata 

Management Plan. 

 Therefore, if a colliery intends to use ADRS it is critical that the 

recommended levels of rib support and how that support would be 

implemented is assessed within the framework of a properly 

facilitated risk assessment being a team exercise that draws on the 

experience of the colliery staff and employees. 

REFERENCES 

Colwell, M.G., Hill, D.J., and Frith, R.C. (2003). ALTS II – A 

Longwall Gateroad Design Methodology for Australian Collieries. 

Proceedings, First Australasian Ground Control in Mining 

Conference: Ground Control in Mining: Technology & Practice: 

Sydney, November 10-13, pp. 123-135 

Colwell, M.G. (2005). Analysis and Design of Rib Support (ADRS) 

– A Rib Support Design Methodology for Australian Collieries. 

Final Report - ACARP Project C11027, in press. 

Colwell, M.G. (1998). Chain Pillar Design - Calibration of ALPS. 

Final Report - ACARP Project C6036, 67 pp. 

Fabjanczyk, M.W., Tarrant, G.C. and Guy, R.J. (1992). Summary 

Report No.2 – Factors Affecting Design of Rib Reinforcement, as 

part of Final Report for AMIRA Project No. P207S – Optimisation 

of Coal Mine Roof/Rib Reinforcement and Design Methods. 

Frith, R.C. and Ditton, S. (1993). Stage 1 and 2 Monitoring Report 

for Geomechanics of Rib Failure and Development of Appropriate 

Support Technology. ACIRL Report EGE 3087/1. 

Hebblewhite, B.K., Walker, R. and Lin, B. (1998). Rib Mechanics 

and Support Systems. Final Report – ACARP Research Project 

C3059. 

O’Bierne, T., Shepherd, J., Rixon, L.K. and Napper, A. (1987). 

Instability and Support of Coal Mine Ribs. ACIRL Published 

Report 87-3.