23

ABSTRACT 

 Longwall mines typically use some form of standing support 

for secondary roof support in longwall tailgate entries. Although 

there have been several new support products developed for this 

application, there remains no universal design criteria to optimize 

the application of these support technologies. The requirement for 

optimization and proper support selection is to understand the 

degree of control that the support has on the ground behavior. The 

ground reaction curve and numerical modeling was used to 

evaluate the impact of standing support on ground behavior. 

LaModel was used to evaluate the impact of standing support on 

main roof and floor behavior and pillar yielding. The conclusion 

drawn from this study was that standing supports do not have 

sufficient capacity to control main roof or floor loading or prevent 

the resulting convergence of the tailgate entry. However, it is 

imperative that this “uncontrollable convergence” be considered in 

the support design to prevent premature failure of the support. A 

FLAC model was used to evaluate the near-seam roof and floor 

behavior in conjunction with the global vertical and horizontal 

stresses. The model suggests that standing roof supports can have 

some impact on the ground behavior as the elastic response of the 

rock is exceeded and rock structure deteriorates from the stress 

concentrations that develop around the tailgate opening. During 

this phase, the capacity and stiffness of the standing support can be 

critical to the stability of the opening, as eventually the rock mass 

will be transformed into a partially detached structure whose 

weight must be supported by the standing support. This work is 

part of a ground control program of research at the National 

Institute for Occupational Safety and Health (NIOSH) aimed at 

improving mine safety by reducing roof fall injuries and fatalities. 

INTRODUCTION 

 During the past decade, there has been a substantial increase in 

the development of new and innovative concepts for standing roof 

support. Currently, there are over 50 different standing support 

systems available, each with a distinct loading characteristic. They 

vary considerably in capacity, stiffness, and yield characteristics. 

So, on what basis is one support chosen over another? Which 

support will provide the most effective roof support in a particular 

mining condition? These questions define critical issues in roof 

support design. 

Statement of the Problem 

 The performance characteristics of these standing roof support 

systems have been well defined through rigorous full-scale testing 

and documented in the NIOSH Support Technology Optimization 

Program (STOP) (Barczak, 2000). A great deal of knowledge has 

been gained in recent years about the ground behavior associated 

with underground coal mining through numerous field studies and, 

more recently, improvements in numerical modeling capabilities. 

However, there is very little known about the interaction between 

the support and the ground. How much control do the standing 

supports have on roof behavior? Do they influence the stress 

changes and ground movement around the opening? Is it simply a 

matter of having sufficient capacity to ensure stable roof conditions, 

or is there a limit to the control that a support has on the roof 

behavior, and if so, what degree of control does it have? Until this 

understanding is developed, proper support design cannot be 

accomplished, and premature failures of supports or excessively 

conservative applications of support will continue. 

Technical Approach 

 Physical studies of the interaction of support and ground are 

difficult to accomplish. Since the ultimate goal is to determine the 

impact of the support on ground control, the study must examine 

the complete mine roof and floor response in relation to different 

support characteristics. Because in-mine measurements of stress 

and deformation of the ground are difficult to obtain, it is 

impractical to make these measurements, except at a few discrete 

points, which provides uncertainty in evaluating the complete 

ground behavior. Furthermore, the load conditions in the mine 

vary constantly, making it virtually impossible to evaluate the 

impact of subtle changes in loading which are likely to be present 

in every study, even if the support parameters are well controlled. 

These barriers provide justification to approach the evaluation of 

support and strata interaction through numerical modeling means, 

which is the approach used in this study. 

 Two numerical modeling software systems were utilized: 

(1) LaModel and (2) FLAC. LaModel is a displacementdiscontinuity numerical program that can determine the 

convergence over a coal seam with the overburden responding as a 

laminated elastic rock mass and with yielding elements in the coal 

seam (Heasley and Salamon, 1996). These capabilities make it 

suitable to evaluate the interaction of standing support with the 

Evaluation of the Impact of Standing Support on Ground Behavior in 

Longwall Tailgates 

Thomas M. Barczak, Mining Engineer 

Gabriel S. Esterhuizen, Mining Engineer 

Dennis R. Dolinar, Mining Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA 
24

main roof and floor subsidence and pillar yielding. However, the 

model cannot evaluate any inelastic behavior or failure of the roof 

or floor that is common in the near-seam ground behavior. FLAC 

was used for this purpose, instead, and is a finite-difference 

software program that was used to investigate the effect of rock 

failure and post peak behavior on rock-support interaction 

(HCItasca, 2003). The FLAC model is therefore able to provide a 

more detailed analysis of the near-seam rock response associated 

with particular support applications. 

Scope of Study 

 The focus of this effort is a study of standing support 

interaction with the surrounding rock in a longwall tailgate. The 

LaModel studies evaluated loading conditions both outby and inby 

the longwall face. FLAC modeling was limited to an evaluation of 

support and strata interaction for abutment loading conditions 

outby the longwall face. This simplifies the evaluation by allowing 

a two-dimensional assessment of the loading conditions, a 

requirement to keep the modeling size manageable for the detailed 

assessment of the near-seam rock mass. A geological condition 

representative of the Pittsburgh seam in a western Pennsylvania 

mine was also used as the basis for this study. 

CONCEPTUAL MODEL OF ROCK MASS BEHAVIOR AND 

SUPPORT INTERACTION 

 The interaction between standing supports and the surrounding 

rock mass can best be explained in terms of a conceptual ground 

reaction curve shown in Figure 1 (Brady and Brown, 1985). The 

curve represents roof-to-floor convergence as a function of the 

internal support pressure plotted to a logarithmic scale in an entry 

that is subject to abutment loading. The curve shows that if zero 

convergence is desired, the support pressure should equal the initial 

stresses in the rock, point A on the curve. However, if the internal 

support pressure is reduced, convergence will occur. Initially, the 

convergence occurs in response to elastic relaxation of the rock 

mass, section A–B on the curve. As the support pressure is further 

reduced, rock failure and pillar yield can occur which will increase 

the rate of convergence, section B-C on the curve. When the 

support pressure is reduced even further, the failed rock mass will 

loosen and the dead weight of the loosened material will rest on the 

support. This will result in an increase in loading of the supports, 

section C-D on the curve. The curve is known as the “required 

support line” and has been proposed as a methodology for 

understanding roof support requirements, because it represents the 

support pressure required to achieve equilibrium in the entry 

(Mucho, et al., 1999). 

 Figure 1 also shows the typical support resistance provided by 

standing supports. It can be seen that the support resistance is very 

small relative to the initial part of the ground reaction curve. The 

convergence during this part of the curve is called “uncontrollable 

convergence” because the required support line is overwhelmingly 

greater than the available support capacity. However, after 

sufficient convergence has occurred, the support capacity exceeds 

the required support line and equilibrium will be achieved in the 

entry. This is referred to as “controllable convergence”. The 

convergence can become uncontrollable if the supports yield 

excessively and loosening of the rock mass is allowed to occur. 

 The ground reaction curve shows that standing supports must 

survive the initial uncontrollable convergence and should retain 

sufficient capacity to exceed the required support line. Increasing 

the support capacity beyond that of the typical support would only 

have a minor effect on the uncontrollable convergence, owing to 

the steepness of the required support line. 

STANDING SUPPORT SYSTEMS 

INCLUDED IN THIS STUDY 

 As indicated in the introduction, there are a wide variety of 

standing support systems currently available for longwall tailgate 

application. Although their performance characteristics vary, they 

can be classified into four basic types as illustrated in figure 2: 

(1) non-yielding, (2) constant yielding, (3) load-increasing or 

strain-hardening yielding behavior, (4) load-shedding or strainsoftening yielding behavior. Supports chosen for this particular 

study that are representative of each of these types of supports were 

the concrete donut crib (non-yielding), conventional 4-point wood 

crib (load increasing), pumpable roof support (load shedding), and 

the Can1

 support (constant yielding). A comparison of their loaddisplacement characteristics is shown in figure 3. The concrete 

donut crib has the highest capacity and stiffness, but quickly looses 

its capacity after reaching its peak load, making it a non-yielding 

support. The pumpable roof support is a grout-filled support that is 

formed in place in the mine entry by pumping a specialized grout 

into a fabric bag that is hung from the mine roof. The pumpable 

roof support has the second highest capacity and stiffness of the 

four supports considered in this analysis and is classified as a load-

 

1

Mention of company name or product does not constitute endorsement by 

the National Institute for Occupational Safety and Health. 

DISPLACEMENT

LOAD

+

+

DISPLACEMENT

LOAD

+

+

DISPLACEMENT

LOAD

+

+

DISPLACEMENT

LOAD

+

+

Non yielding Constant yielding

Load shedding Load increasing

Figure 2. Four basic types of loading characteristics for 

standing roof support systems. 

Figure 1. Ground Reaction Curve approximation for outby 

loading conditions in a longwall tailgate. 

1

10

100

1000

0 1 2 3 4 5 6 7 8 9 10 11 12

Roof to floor convergence (inches)

Support Pressure (tons/ft2) .

A

B

C

Controllable

Convergence

Uncontrollable

Convergence

Standing

support

capacity

D
25

shedding support. The Can support comes in several sizes. A 24-

in diameter Can was chosen for this study. It is stiffer than a wood 

crib with a higher yield capacity and is able to sustain its peak 

loading through a large displacement. The Can support is 

considered a constant yielding support. The wood crib is the 

softest support system, but increases its load carrying capacity as it 

yields. The properties of each of these supports were determined 

from full-scale testing in the NIOSH Mine Roof Simulator. 

NUMERICAL MODELING STUDIES OF SUPPORT AND 

STRATA INTERACTION 

LaModel and FLAC models were utilized to evaluate the 

impact of roof support on ground behavior. As stated in the scope 

of work, LaModel was utilized to evaluate the response of the main 

roof and floor and pillar yielding for the full tailgate condition both 

outby and inby the longwall face, while FLAC focused on the 

response of the near-seam rock mass for outby loading conditions 

only. The experimental design and results of these studies are 

provided. 

Evaluation of the Outby and Inby Tailgate Conditions 

with LaModel 

 Input into the model includes the model geometry, the 

overburden, coal and support properties, and the mining depth. The 

model elements were 4 ft by 4 ft. A description of these input 

parameters and a parametric support study are described. 

Model Layout and Loading Conditions. Figure 4 shows the 

three-entry gateroad system with two adjacent panels that was 

evaluated in this analysis. Each longwall panel was approximately 

800 ft wide. The abutment pillars were 88 ft wide and 180 ft long 

and the entries were 20 ft wide, providing a gateroad system with a 

width of 236 ft. A 1,000-ft section of the gateroad, 500 ft inby and 

500 ft outby the longwall face, was evaluated. This area represents 

the full range of tailgate conditions that the standing support may 

be subjected to, including the front abutment pressures outby the 

face and the main roof-to-floor subsidence caused by the adjacent 

gob inby the face. The seam thickness was 6 ft. As shown, the 

tailgate entry under consideration is adjacent to panel 2. In figure 4, 

the first panel has been mined and the second panel has been 

partially mined. This represents the situation that was used to 

evaluate the standing support performance. Overburden depths of 

600 and 900 ft were modeled with the initial stress condition based 

on a stress factor of 1.1 psi per ft of overburden. 

Coal, Overburden, and Support Properties. Both elastic and 

elastic-plastic elements were used for the coal material with an 

elastic modulus of 300,000 psi and a Poisson’s ratio of 0.33. The 

elastic elements were used for coal that is a sufficient distance from 

the openings and do not yield and were considered to have infinite 

strength. The elastic-plastic behavior was applied for those 

elements closer to the opening that could yield. The coal strength 

for the elastic-plastic elements was developed from the Bienawski 

strength formula, where the base strength for the coal was 900 psi 

(Mark and Chase, 1999). The coal strength then increases with 

distance from the opening because of confinement. After yield, the 

deformation modulus of the elastic-plastic elements was zero. For 

the overburden, an elastic modulus of 3 million psi and a 

lamination thickness of 50 ft were used in the analysis (Heasley and 

Chekan, 1999). Strain-hardening elements were used for the gob 

material. The initial elastic modulus was 100 psi with a Poisson’s 

ratio of 0.25. At the 900-ft depth, the peak gob stress was 495 psi 

with a final elastic modulus of 22,500 psi, and at the 600 ft depth, 

the peak gob stress is 330 psi with a final elastic modulus of 

33,700 psi. 

 The support properties are defined in terms of strain-softening 

and elastic-plastic element behavior in the model. Elastic-plastic 

elements are used to approximate the behavior of the Can support 

and wood cribs, while the strain-softening elements are used to 

approximate the behavior of the pumpable roof support and 

concrete donut cribs. 

Parametric Study. In this study, four parameters were varied that 

include the support type, the amount of support, the pillar design, 

and the overburden depth. The four support types used in the 

models were discussed previously. The performance of each type 

of support was evaluated through changes in the other three 

parameters. The supports were placed along two rows down the 

tailgate with each row centered 6 ft from the middle of the entry. 

The support density was varied by changing the support spacing 

within a row. Two support densities were examined, a 4-ft and an 

8-ft center-to-center spacing. The supports were installed after the 

first panel had been mined but prior to mining of the second panel. 

 Two pillar designs were evaluated in the study. For both 

designs, the gateroad system width remained the same. One system 

had two stiff, abutment pillars that were each 88 ft in width. This 

0

200

400

600

800

1,000

1,200

0 2 4 6 8 10 12

VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, kips

Concrete Donut Crib 

Pumpable Roof Support Can Support 

Wood Crib 

Figure 3. Actual loading characteristics of standing support 

systems chosen for study based on full-scale testing 

in NIOSH Mine Roof Simulator. 

Figure 4. Three-entry gateroad system with two adjacent panels 

utilized in LaModel study. 

0 200

S c a le , ft

Panel 2 

LONGWALL FACE

First Panel 

Completed

Gob

Gob

Tailgate Entry 
26

was the base model where the affects of support density and the 

overburden depth were evaluated. The other system had an 

abutment-yield pillar design with an abutment pillar width of 148 ft 

and a yield pillar located adjacent to the tailgate entry with a width 

of 28 ft. Two overburden depths, 600 ft and 900 ft were modeled. 

The base depth was 900 ft. 

Roof-to-Floor Convergence Analysis. To evaluate the impact of 

the standing support on roof and floor behavior with LaModel, the 

roof-to-floor convergence in the tailgate was analyzed. Figure 5 

shows the roof-to-floor convergence of the tailgate entry with no 

standing support for a distance of 500 ft inby and 200 ft outby the 

longwall face for the two pillar geometries and the two different 

mining depths. The largest convergence occurred with the 

abutment-yield pillar system at a 900-ft depth and the least 

convergence at the 600-ft depth with the 88-ft-wide abutment pillar 

system. The convergence starts to increase about 100 ft outby the 

longwall face in the abutment zone and reaches a maximum well 

behind (inby) the face in the gob area. 

 With the addition of standing support in each case shown in 

figure 5, the convergence was reduced by less than 1 pct, and the 

convergence curves if plotted would overlie the curve shown for no 

standing support without any noticeable difference at the scale of 

the figure. Thus, the roof-to-floor convergence curves for the 

standing support are not plotted on the figure. Table 1 shows the 

roof-to-floor convergence for the case of 900-ft depth with 88-ftwide pillars and both 4- and 8-ft support spacing for selected 

positions from the face. The selected positions are in the abutment 

zone just outby the face, at the face, just behind the shields, and at 

the first crosscut inby the face. It is seen that there is very little 

difference in the convergence with or without standing support. 

Thus, it is concluded that the convergence produced by the main 

roof and floor activity and pillar yielding is uncontrollable or not 

influenced by the roof support system, regardless of the support. 

Evaluation of Outby Abutment Loading with FLAC 

 The FLAC 2D finite difference software was used to 

investigate the interaction between rock failure and standing 

support in a tailgate entry subject to abutment loading. The model 

did not consider the effects of the longwall intersecting the entry 

nor the inby loading condition since these are three-dimensional 

conditions. The FLAC code is able to model large displacements 

and deformations that are associated with rock failure. It also has 

capabilities to realistically model roof bolts and standing supports. 

Model Layout and Loading Conditions. The overall FLAC 

model geometry and boundary conditions are shown in figure 6. A 

typical three-entry gateroad design was evaluated in the study. The 

model was constructed to analyze the tailgate entry specifically, 

which is depicted in figure 6. Symmetry would satisfy loading 

conditions for the middle gateroad entry. The third entry has been 

eliminated by the first panel mining. The seam is approximately 

6-ft thick and is overlain by a weak stack rock followed by 

interlayered black shale and thin coal rider beds. The mine roof is 

typically cut just below one of the rider beds to provide a stable 

horizon. The coal riders are overlain by weak black shales, grading 

to stronger gray shales. The shales are overlain by a 22-ft thick 

limestone bed. Figure 7 shows the model detail and geological 

profile in the vicinity of the entry. 

 The initial vertical stresses in the model were based on gravity 

loading only, while the horizontal stress were determined by the 

tectonic strains, which depends mainly on the elastic modulus of 

the rock, plus a Poisson effect component (Mark and Mucho, 1994; 

Dolinar, 2003). The input parameters were selected so that the 

horizontal stress in the moderately strong gray shale was 1,160 psi, 

similar to measured values in Eastern U.S. coal mines (Dolinar, 

2003). The effect of an approaching longwall face was modeled by 

increasing the vertical stress by 1,044 psi, and the horizontal stress 

Table 1. Roof-to-floor convergence at selected locations for the case with a depth of 900 ft and 88-ft pillars 

for both 4-ft and 8-ft support spacing 

Roof-to-Floor Convergence, in 

Location, ft1 No support Concrete donut 

cribs Wood cribs Pumpable roof 

supports Can supports 

8-ft support spacing 

18 0.777 0.764 0.775 0.770 0.774 

0 1.186 1.181 1.183 1.178 1.181 

-34 2.113 2.107 2.109 2.106 2.106 

-200 5.656 5.657 5.649 5.650 5.645 

4-ft support spacing 

18 0.777 0.754 0.773 0.764 0.770 

0 1.186 1.158 1.180 1.171 1.176 

-34 2.113 2.102 2.105 2.100 2.100 

-200 5.656 5.661 5.641 5.644 5.587 1

Negative numbers refer to locations inby the longwall face. 

Inby 

0

2

4

6

8

10

12

-500 -400 -300 -200 -100 0 100 200

DISTANCE FROM FACE, feet

CONVERGENCE, inches .

Depth 900 ft -- 88-ft-wide 

abutment pillars

Depth 900 ft -- abutment-yield pillar system

Depth 600 ft -- 88-ft-wide 

abutment pillars

Outby

Longwall face 

Inby 

Figure 5. Roof-to-floor convergence calculated with LaModel. 

Negative numbers refer to locations inby the longwall 

face. 
27

by 522 psi in three equal increments. The vertical stress increments 

were based on LaModel results. The model was found to be 

sensitive to the horizontal stress increments. The values finally 

used were selected to result in roof instability without standing 

support under abutment loading. This allowed the effect of standing 

support on rock failure development in the roof and associated 

closure of the tailgate entry to be evaluated. 

 The rock mass was modeled as a strain softening/ubiquitous 

joint material, using the built-in constitutive model in FLAC. This 

model is well suited to modeling the layered coal measure rocks, 

since the bedding layers can be described as strain softening 

ubiquitous joints, while failure of the rock matrix can be simulated 

as a strain softening Coulomb material. Great care was taken in 

setting up the models to replicate the geological sequence with as 

much detail as practical. Strength data for the different rock types 

included in the models were obtained from published data (Rusnak 

and Mark, 1999). 

Applied Stress 

Applied displacement 

Entry 

180 ft 

300 ft 

50 ft 

Not to scale 

Entries 

Gob 

Figure 6. FLAC model geometry and boundary conditions. 

Figure 7. Detail of FLAC model showing element size and geological profile. 

Gray shale 

Limestone 

Shale 

Carbonaceous 

Shale/Rider

Black shale 

Coal Seam 

Limey shale 

Claystone 

Siltstone 

0 Scale 10 ft 

8 ft bolts 

Standing support 

Tailgate entry 
28

 The models included fully grouted roof bolts as well as the four 

standing support systems previously described. The grouted bolts 

were modeled using the pre-defined cable elements in FLAC. Bolts 

were 8 ft long with a yield capacity of 22 tons. An algorithm was 

developed using the internal programming language in FLAC to 

remove yielding sections of bolts to simulate bolt failure. In 

addition, roof collapse was modeled by removing elements in the 

roof that had experienced excessive plastic strain. Standing 

supports were modeled using the built-in support logic in FLAC, 

which allows the load-deformation characteristics shown in 

figure 3 to be accurately simulated. The standing support was 

installed after approximately 1 inch of tailgate closure resulting 

from the first panel mining. 

FLAC Model Results for Abutment Loading. The FLAC model 

results for the different types of standing support are presented in 

the form of stress-closure curves in figure 8. Associated support 

loading for each of the three stress increment steps is shown in 

figure 9. Closure is reported at the center of the tailgate entry, 

while the stress is the abutment stress associated with the 

approaching longwall face, shown in three steps. The FLACcalculated closures are generally larger than those determined by 

the LaModel program for abutment loading conditions. One reason 

for the difference is that FLAC simulates the rock failure and 

associated bulking in the immediate vicinity of the entry while 

LaModel does not account for this behavior. 

 Comparing the results of the model to measured field data in 

the Pittsburgh seam, the convergence determined from the FLAC 

models are higher than field measurements. A study of pumpable 

roof supports in a longwall tailgate at the Emerald Mine near 

Waynesburg, Pennsylvania measured convergence just outby the 

longwall face excluding the first foot of roof and floor rock 

movement ranging from 0.25 to 0.45 inches under a depth of cover 

of 750 ft (Barczak, et al., 2003). A previous study at the nearby 

Cumberland mine with wood cribbing and cable bolts measured 

convergence on the order of 1.75 inches in a longwall tailgate 

(Molinda, et al., 1997). The FLAC model shows 2-3 times this 

convergence at the maximum abutment stress considered in the 

analysis. This is a consequence of the selected horizontal stress 

increments used in the FLAC model. 

0

1

2

3

4

5

6

7

8

0 200 400 600 800 1,000

VERTICAL STRESS INCREASE, psi

CONVERGENCE AT CENTER OF ENTRY, in

Bolts

Wood Cribs

Concrete Donut Cribs

Pumpable Supports

Cans

Load condition after first 

panel mining

Initial horizontal stress 1,160 psi

with increments of 50% of the vertical stress increase

Step 1 Step 2 Step 3

Cans or pumpable 

roof supports

Roof Bolts - no 

standing support

Wood cribs or 

concrete cribs

Figure 8. FLAC model results for three abutment loading steps showing tailgate closure versus vertical stress 

increase for roof bolts and various standing support systems. 

Figure 9. Support loads for each of the three vertical 

stress increments analyzed in the FLAC model.

-50

0

50

100

150

200

250

300

350

348 696 1,044

VERTICAL STRESS INCREASE, psi

SUPPORT LOAD, tons

Wood cribs

Cans

Pumpable cribs

Concrete donut cribs

Step 1

Step 2 Step 3
29

 The FLAC results show that the standing support has only a 

minor impact on the closure during the first two steps of abutment 

loading. It is only when the vertical stress is increased by 1,044 psi 

that one begins to see significant differences in convergence with 

and without the standing support. At this stress level, where the 

total vertical stress is 2.44 times the virgin stress, support 

consisting of bolts only is no longer capable of controlling the 

immediate roof as the closure approaches 8 inches. In comparison, 

the maximum convergence is less than 4.5 inches when any of the 

standing support systems are installed in the tailgate. 

 Among the four standing support systems analyzed in the study, 

the difference in closure was about 1 inch. The closure was 

approximately 4.4 inches when the tailgate was supported with 

wood cribs or concrete donut cribs and was 3.5 inches when the 

tailgate was supported with the Can supports or pumpable roof 

supports. The least convergence, 3.5 inches, occurred with the Can 

support, just slightly less than the 3.6 inches that occurred with the 

pumpable roof support system. The FLAC results indicate that in a 

situation where outby closure is less than 2 inches, the standing 

supports have little impact on roof behavior for the conditions 

analyzed in the model. 

 Examination of the deformation of the immediate roof (see 

figure 10) shows that for a tailgate entry with bolt support only, 

separation of strata occurs about 6 ft above the roof horizon. This 

separation indicates the onset of instability and inability of the 

support to control roof deflections. Smaller roof displacements are 

indicated for the concrete donut crib and the wood crib, while the 

Cans and pumpable roof supports maintain a tight roof without any 

separation. The results show that extensive yield capability, 

coupled with adequate support capacity, is required to maintain 

control over the roof deformation. The wood cribs, while having a 

large yield capacity, do not have sufficient resistance to prevent 

separation in the roof. The donut cribs, while having a significant 

initial capacity, do not have sufficient residual yield capacity to 

maintain control during extended closure. 

STANDING SUPPORT DESIGN AND APPLICATION 

IMPLICATIONS 

 The results of these studies provide key insight into support 

design requirements. Some relevant issues for support design and 

application are discussed in more detail. 

Surviving Uncontrollable Convergence

 The first requirement for any standing support design is to 

recognize that in every application, there will be a component of 

the roof-to-floor closure that cannot be prevented by any standing 

support system. However, a standing support must survive this 

uncontrollable convergence without losing its support capability in 

order to be able to provide reinforcement and support of the 

immediate roof once the rock stress is relieved through further 

deformation. Hence, non-yielding support systems are not 

expected to perform well in longwall tailgate applications. Stiff 

support systems that shed load after reaching peak loading capacity 

may also have trouble in maintaining roof control if they are loaded 

beyond their peak loading outby the face and their residual load 

capacity is low. 

How Much Uncontrollable Convergence Can Be 

Expected 

 A major portion of this uncontrollable convergence is caused 

by elastic deformations of the rock mass and yielding of the coal 

pillars. As such, the amount of uncontrollable convergence will 

increase with increasing depth of cover and decreasing pillar size. 

The LaModel study showed that the "uncontrollable" convergence 

at the longwall face was about 0.6 inches for a depth of cover of 

600 ft and increased by a factor of two to 1.2 inches when the depth 

of cover was increased to 900 ft. The FLAC studies showed that 

additional uncontrolled convergence can be caused by the failure 

development of the near seam roof and floor rock mass. The 

uncontrollable convergence for the three levels of vertical stress 

considered in the analysis of outby loading conditions ranged from 

3⁄4 of inch to 4-1/2 inches. The amount of uncontrollable 

convergence will be site specific. 

Benefit of Support Stiffness 

Is there a benefit to using stiffer support systems? Stiffness is a 

measure of how quickly a support system develops its loadcarrying capacity in relation to the roof-to-floor convergence. The 

stiffer the support, the more load capacity it will have for a given 

convergence. Since equilibrium of the roof and floor is a matter of 

force balance, then in general higher support stiffness will achieve 

equilibrium of the ground at less convergence, assuming the 

support has a positive stiffness (no load shedding) throughout the 

loading cycle. But how much is the benefit of the stiffer support? 

Is it a direct relationship? Does doubling of the support stiffness 

reduce the convergence by 50%? 

 The benefit of support stiffness depends on how the support 

interacts with the ground and the degree to which it can influence 

the development of failure within the immediate roof or floor. 

Figure 11 shows the convergence determined from the FLAC 

model for an increase in vertical stress of 348 psi (Step 1 of the 

analysis) as a function of the support load. The regression line 

suggests a nonlinear relationship between the support load and 

convergence. It is seen from this figure that higher support load 

results in less convergence. Figure 9 showed the loading of the 

four support systems for each of the stress increments analyzed in 

the FLAC model. The stiffest support (concrete donut cribs), had 

the least convergence; while the softest support (wood cribs) had 

the highest convergence at Step 1 (348 psi stress increment). 

However, the difference in convergence, less than 1⁄4 of an inch, is 

small. A 662 pct increase in support load resulted in only a 24 pct 

reduction in convergence. The benefits of the stiffer supports are 

similar in the second stress increment of 696 psi shown as step 2, 

0

2

4

6

8

10

12

14

16

18

20

0 0.5 1 1.5 2 2.5 3 3.5

ROOF DEFORMATION, inches 

DEPTH INTO THE ROOF, feet .

Roof bolts -- no 

standing support

Wood cribs

Concrete 

cribs

Cans or 

pumpable supports

Figure 10. FLAC model results showing the displacement profile 

 in the roof for various support systems after an increase 

in vertical stress of 1,044 psi (Loading Step 3 shown in 

figure 8). 
30

but become more pronounced when the stress is increased by 

1,044 psi (Step 3). At the 1,044 psi stress increment, the soft wood 

cribs are allowing 0.86 more inches of convergence than the Can 

support (see Figure 9). In this case, a 53 pct increase in support 

capacity (Can compared to the wood crib), resulted in a 20 pct 

reduction in convergence. 

 In conclusion, the stiffness of the support will have a positive 

impact on controlling the ground and minimizing the convergence 

of the tailgate. The benefit is small until the near seam rock has 

strain softened and is depending on its residual load capacity for 

stability. 

Impact of Support Load Shedding 

 One of the most revealing outcomes of this analysis was the 

impact that the concrete donut cribs had on the tailgate closure. 

The concrete donut crib acts as a non-yielding support reaching its 

ultimate capacity at less than 2 inches of convergence and then 

sheds its entire load at less than 1⁄4 inch of additional convergence. 

It also has over 3.5 times the capacity of any other support and is 

the stiffest support considered in this analysis. However, it is 

somewhat surprising that at the highest loading condition (Step 3), 

equilibrium is attained at far less convergence than the tailgate 

supported only by bolts. One might expect the convergence to be 

closer to that of the tailgate supported with bolts only since the 

concrete donut crib is no longer providing any roof support. The 

roof behavior can be explained by reviewing the rock failure and 

roof bolt performance in the FLAC model. Figure 12 shows the 

development of shear bands in the surrounding rock at Step 3 for 

the tailgate supported only with roof bolts (figure 12b) and 

supported with concrete donut cribs in addition to the roof bolts 

(figure 12a). It is seen that the shear band development in the roof 

is substantially worse in the tailgate supported only by bolts, 

resulting in failure of the roof bolts. In the case of the tailgate 

supported by concrete donut cribs and bolts, the shear band 

development appears to have been inhibited by the high initial 

concrete donut crib resistance, preserving the roof bolts, which are 

still able to prevent large roof displacements at the Step 3 loading 

condition, after the concrete donut crib had failed. 

 A similar conclusion can be drawn from the pumpable roof 

support application. Examining figure 9, it is seen that the 

pumpable roof support had achieved more load than the Can 

support after Step 1. However, because the pumpable roof support 

sheds load after reaching its peak loading while the Can yields 

without shedding load, the pumpable roof support and the Can 

support have the same loading after Step 2 in the FLAC analysis. 

Comparing the convergence at Step 2 (see figure 8) shows that the 

tailgate supported with pumpable roof support had less 

convergence (1.90 inches) than the one supported with Can 

supports (2.00 inches). The early stiffness of the pumpable roof 

support compared to the Can support made a difference in the 

subsequent ground response. However, as the vertical stress 

continues to increase (Step 3) and more damage occurs to the 

immediate roof, the load shedding of the pumpable roof support 

will eventually cause more convergence to occur with this system 

than with the Can support that does not shed load (see figure 8). 

CONCLUSIONS AND RECOMMENDATIONS 

 The development of innovative roof support systems continues 

with several new products being commercialized each year, each 

with a distinct load-displacement performance characteristic. The 

purpose of this study was to gain an understanding of the impact of 

the standing roof support system on ground control in longwall 

mining so that the most effective support can be utilized to 

minimize the risk of roof falls. Several conclusions have been 

made as a result of the numerical modeling studies of this problem. 

These conclusions are summarized as follows: 

1. Standing supports cannot control or prevent convergence 

produced by main roof or floor activity or gateroad pillar 

yielding. Although this convergence most likely will not result 

in immediate instability of the roof, it is important from the 

perspective of the support design since it causes loading of the 

standing support. 

2. Standing supports must be able to survive “uncontrollable

convergence” associated with the elastic responses of the rock 

mass, particularly the main roof and floor activity, and the 

pillar response including its yielding behavior, in order to 

provide reinforcement and support of the immediate roof as the 

loading cycle continues. 

3. Standing supports do not exhibit much control over the 

progressive failure development of the immediate roof, but can 

provide some degree of control over the post-failure (strainsoftened) rock response. 

4. The contribution of standing support in ground control is 

increased when the rock stresses increase, thus causing damage 

that cannot be fully controlled by the roof bolts. 

5. The contribution of the standing support can directly impact the 

load development of the bolts, and at times, prevent or delay 

failure of the bolts. As such, the load path by which rock 

failure is developed can be altered by the standing support, 

resulting in more stable roof conditions than would be achieved 

without the application of standing supports. 

6. Increasing the support stiffness can have a positive impact on 

ground control. The benefit may not be fully realized until the 

rock mass has failed and is behaving in accordance with its 

residual loading properties. 

7. The Ground Reaction Curve is a concept that helps to explain 

the impact of roof and standing support on ground control. It 

should also be noted that the application of a particular 

standing support can alter the ground reaction curve. An 

assessment of the roof deformation and strata separations can 

provide additional insight into the capability of the support 

system to control the roof. 

Figure 11. Impact of support load on convergence after vertical 

stress increase of 348 psi in the FLAC model (Step 1 

in figure 8). 

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 50 100 150 200 250 300 350

SUPPORT LOAD, tons

CONVERGENCE, inches

Concrete Donuts

Wood Cribs

Cans Supports Pumpable Roof Supports
31

0 Scale 10 ft 

Max. shear strain 

 0.00E+00

 2.00E-01

 4.00E-01

 6.00E-01

 8.00E-01

 1.00E+00

Gray shale 

Limestone 

Shale 

Carbonaceous 

Shale/Rider 

Black shale 

Coal Seam 

Limey shale 

Claystone 

Siltstone 

Roof bolts failed

0 Scale 10 ft 

Max. shear strain 

 0.00E+00

 2.00E-01

 4.00E-01

 6.00E-01

 8.00E-01

 1.00E+00

Gray shale 

Limestone 

Shale 

Carbonaceous 

Shale/Rider 

Black shale 

Coal Seam 

Limey shale 

Claystone 

Siltstone 

Figure 12. Comparison of roof conditions after loading step 3 with concrete donut cribs (a) 

and with entry supported only with bolts (b). 
32

 Finally, it is recommended that additional studies be conducted 

to further the understanding of support and strata interaction. First, 

additional roof geologies should be investigated with the 2-D 

FLAC modeling. This is critical since this study has shown that the 

support does have some impact on the failure development of the 

immediate roof. The roof structure analyzed in this study is a 

relatively weak, laminated structure. A more competent roof 

structure may respond quite differently. Next, a three-dimensional 

FLAC model needs to be developed to evaluate the face and inby 

loading conditions. Since there will be more failure within the 

immediate roof and floor for these conditions, standing support is 

expected to have more of an impact on ground control in these 

conditions. The LaModel results have already shown that the 

uncontrollable convergence inby the face can be quite large and 

will place additional demands on a standing support system 

operating in this environment. 

REFERENCES 

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5-7, pp. 283-294. 

Barczak T.M. (2000). Optimizing Secondary Roof Support with 

the NIOSH Support Technology Optimization Program (STOP). 

Proceedings, 19th International Conference on Ground Control in 

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