Dolinar, D.

Barczak, T.

Gurley, H.

Evaluation of Tailgate Ground and Support Interaction in the Illinois 

Basin for the Development of a Ground Reaction Curve Based Standing 

Support Design 

Dennis Dolinar, Lead Research Engineer
 

Thomas M. Barczak, Branch Chief
 

NIOSH Pittsburgh Research Laboratory
 

Pittsburgh, PA
 

Harrold Gurley, Researcher
 

Dept. of Mining and Minerals Resources Eng.
 

Southern Illinois University
 

Carbondale, IL
 

ABsTrAcT 

Improperly designed support systems have led to unplanned roof 

falls in longwall tailgates. To prevent such falls requires appropriate 

support and adequately designed support systems. The National 

Institute for Occupational Safety and Health (NIOSH) is currently 

developing a standing support design methodology based on the 

ground reaction curve. To design the support system with this 

methodology requires a quantitative assessment of the ground and 

support interaction, which is developed from instrumented test sites 

installed in longwall tailgates. To compliment and expand the field 

data results and to optimize the support design, numerical modeling 

is also conducted. The models are calibrated from the test site data. 

In the present study, data was obtained from a longwall tailgate in 

the Illinois basin. 

The test site was located in the Herrin No. 6 coal seam at a depth 

of 500 ft. At the site, the immediate roof was a weak shale overlain 

by a competent limestone. Above the limestone, sandstone formed 

the intermediate roof. The floor consisted of a weak underclay. In 

the study, the performance and interaction with the surrounding 

rock mass of two different standing supports, an engineered crib 

and a conventional 4-point wood crib were monitored. The two 

cribs have similar load-displacement characteristics at least through 

6 in of displacement. Beyond 6 in, the engineered crib becomes 

unstable and begins to strain soften and shed load. 

The measurements and developed ground reaction curves 

indicated that the tailgate at the mine was a low convergence 

environment until well inby the face. Because the convergence 

was well below their capacity, both supports provide more than 

adequate levels of support to the tailgate. Significant convergence 

and support loading did not occur until 90 to 100 ft inby the face. 

The results of this study were also compared with results from 

similar studies conducted in the Pittsburgh seam to further advance 

the development of the “ground reaction curve” based support 

system design. 

InTroDucTIon 

Standing support is used extensively in US longwall mines 

to provide secondary support to the tailgates. The tailgate entries 

can be subjected to high loads and large deformations and the 

standing support is required to prevent ground falls that could 

block the tailgate. There are a number of different supports that 

can be used to provide support in the tailgates. These supports have 

various load-displacement characteristics, including pre and post 

yield behavior that are important in the design and selection of an 

appropriate and adequate support system (Barczak 2003). Ideally, 

to maintain the tailgate entry requires the selection of a standing 

support with performance characteristics that will match the ground 

response or the roof-to-floor convergence (Mucho, et al., 1999). 

Therefore, a design methodology based on the ground reaction or 

response concept is being developed for longwall tailgate standing 

support. The key to this design methodology is to quantify the 

ground reaction curves for several different coal seams, mining and 

ground conditions and support types. 

The laboratory load-displacement characteristics of the standing 

supports are well known (Barczak 2003). However, the ground 

response, the developed support loads and the support and ground 

interaction in the tailgate must be determined for each individual 

site. This in part can be remedied though in situ measurements. 

However, in a given tailgate situation, field measurements will only 

develop one or two points on the ground reaction curve because 

of the limited variation in the type or level of support under the 

same ground conditions. This is not sufficient to develop the entire 

curve. However, numerical modeling can be used to extend the 

field results to develop the full ground reaction curve. Therefore, 

the approach for developing the ground reaction curves that can be 

used in standing support design in tailgates is to use a combination 

of field measurements with numerical modeling. 

Several field studies related to tailgate standing support have 

been conducted in the Pittsburgh seam with numerical modeling 

being used to complete the development of the ground reaction 

curves (Barczak, et al., 2008). As part of an effort to obtain more 

field data on the tailgate and support interaction, a study was 

conducted at a longwall mine in the Herrin No. 6 coal seam in 

southern Illinois. Convergence measurements related to the tailgate 

standing support and ground interaction from the test site at the 

mine and subsequent numerical modeling were used to develop the 

full set of ground reaction curves. 
BAckGrounD 

The ground reaction concept was developed originally 

for the design of support systems used in the civil tunneling 

industry (Brown et al., 1983). To establish the ground reaction 

curves for tunneling, the impact of the timing and degree of 

support are developed by measuring the support pressure and 

opening convergence. The concept has also been applied to 

support and ground interaction in the hard rock and coal mines 

(Hoek and Brown, 1980; Brady and Brown, 1985; Mucho, et 

al., 1999; Barczak, 2003; Medhurst and Reed, 2005; Barczak, 

et al., 2005). The ground reaction will depend on the rock mass 

characteristics, loading conditions and the support resistance. The 

support resistance will depend on the support load-displacement 

characteristics. 

Figure 1 shows a conceptual ground reaction curve. The ground 

reaction curve is the support pressure plotted against the opening 

convergence. At the initial stress state, the support resistance is 

equal to the forces in the surrounding rock mass and there is no 

convergence (point A). This is equivalent to the force on the rock 

mass before the opening was created. As the level of support is 

reduced, there is increased convergence. Initially the curve is steep 

and nearly linear. This represents the elastic response of the rock 

mass. In this portion of the curve it takes a substantial change in the 

amount of support to limit or change the amount of convergence. 

Even standing support can do little to affect the amount of 

convergence in this region of the curve. As the support level is 

reduced, the ground reaction curve becomes nonlinear and begins 

to flatten indicating the rock is yielding or fracturing (point B). In 

this region of the curve much less support pressure is required to 

reduce or change the amount of convergence. It is in this region 

that the support can limit the convergence. After the nadir (point 

C) the amount of support pressure required begins to increase with 

continued convergence. The downward deflection of the roof allows 

for more rock to loosen and the weight of this additional material 

must then be controlled by the support (point D). 

Figure 1. conceptual ground reaction curve. 

The interaction of the support and the rock can be seen in 

Figure 1. The load-displacement curve for a yielding support is 

represented by the points PQB. The support was installed after 

some initial convergence, delta. The support yields prior to 

intersecting the ground reaction curve but still has enough post 

yield capacity to intersect the curve at point B. The minimum 

amount of support required to maintain the opening is achieved 

by designing the supports to intersect the curve at point C. 

However, there is no margin of safety in such a design. Further, 

a very accurate knowledge of the ground and support behavior 

would be required. A previous study has suggested another point 

on the ground reaction curve that could be used for design, the 

support design threshold (Barczak, et al., 2008). This is the point 

on the ground reaction curve where the curve changes from being 

linear to nonlinear or where the rock begins to fracture and yield. 

Another important consideration in support design is the amount 

of uncontrolled convergence that can occur. The uncontrollable 

convergence is the convergence that the support cannot control or 

limit (Barczak, 2006 and Barczak, et al., 2008). The convergence 

that occurs along the linear portion of the ground reaction curve 

to a large extent cannot be controlled by the levels of support that 

are used. Ground reactions that cannot be resisted include the main 

roof-to-floor convergence. However, the support must be designed 

to withstand this convergence and still provide the required 

resistance to intersect the ground reaction curve at the appropriate 

location. 

The ground reaction curve depends on the load path taken by the 

rock mass. Since in a longwall tailgate situation the loads on the 

rock mass continually change especially as the face approaches, 

theoretically there are an infinite number of loading conditions and 

therefore ground reaction curves that would need to be considered. 

However, for tailgate support design and evaluation, four 

fundamental loading conditions can be identified for which ground 

reaction curves could be developed. These loading conditions are; 

development loading, side abutment loading from adjacent panel 

mining, front abutment loading at the face and full extraction 

loading inby the face (Barczak, et al., 2008). Figure 2 shows the 

generalized location of the four loading condition points with 

respect to the panel layout. Essentially, for tailgate standing support 

design only these four loading conditions need to be considered. 

MInInG AnD GeoloGIc sITuATIon 

The mine where the study was conducted is located in the Herrin 

No. 6 coal seam in southern Illinois at a depth of 500 ft with a 

seam thickness between 7 and 8 ft. The immediate roof consists of 

about 3 ft of black carbonaceous shale that is overlain by a 5-ftthick limestone (Brereton). On top of the limestone is about 40 ft 

of the Anvil sandstone. Above the sandstone the main roof consists 

mainly of shale with some coal seams up to the glacial deposits that 

are about 60-ft thick. The immediate floor consists of about 3 ft of 

weak underclay. 

The mine uses a three-entry gate road system with abutment 

pillars. The crosscuts at the test site area were on 150-ft centers 

while the pillar width was 70 ft. The longwall panels are 1,050 ft 

wide. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

First Panel mined out

Second Panel

A

B D C

Future tailgate

entry

Longwall face

Direction of

mining

2nd panel

tailgate entry

First Panel mined out

Second Panel

A

B D C

Future tailgate

entry

Longwall face

Direction of

mining

2nd panel

tailgate entry

First Panel mined out

Second Panel

A

B D C

Future tailgate

entry

Longwall face

Direction of

mining

2nd panel

tailgate entry

First Panel mined out

Second Panel

A

B D C

Future tailgate

entry

Longwall face

Direction of

mining

2nd panel

tailgate entry

First Panel mined out

Second Panel

A

B D C

First Panel mined out

Second Panel

First Panel mined out 

Second Panel 

A

B D C

A 

D 

Future tailgate

entry

Longwall face

Direction of

mining

2nd panel

tailgate entry

Future tailgate 

entry 

Longwall face 

Direction of 

mining 

2nd panel 

tailgate entry 

Figure 2. Generalized longwall panel layout indicating the 

four locations where the ground reaction curves should 

be determined. The points represent the following loading 

conditions: A-development, B- side abutment, c-front abutment 

at the face, and D-full extraction. 

Figure 3. The ATlAs 100 engineered crib installed in the 

tailgate. 

Tailgate support 

Tailgate support at the mine normally consists of a double 

row of 4-point conventional wood cribs constructed using mixed 

hardwood with a 6x6x36-in block size. The cribs were spaced on 

approximately 10-ft centers. 

As part of the study, an engineered wood crib, identified as 

the ATLAS 100, were also installed along a 150-ft section of the 

tailgate. These cribs were placed in a double row configuration and 

spaced on about 10-ft centers. These ATLAS cribs were developed 

by Southern Illinois University (SIU) (Chugh, 2008). This crib 

is similar to a 4-point wood crib but with a large portion of the 

connecting wood between the load points having been removed 

(Figure 3). The load contact points are 5.75 in by 7.75 in. This crib 

design reduces the weight of the crib blocks for material handling 

while the open crib design allows for improved ventilation. 

The load-displacement curves for the ATLAS 100 crib and for 

a conventional 4-point mixed hardwood crib are shown in Figure 

4. These curves were developed by testing the support in the Mine 

Roof Simulator (MRS) at the Pittsburgh Research Laboratory. 

In these tests, a vertical load was applied to the support under 

displacement control at a rate of 0.5 in per min. The tested cribs 

had a similar height to those installed at the test site. Up through 

about 6 in of displacement, the performance of the two cribs is very 

similar. However, beyond 6 in, the ATLAS crib becomes unstable 

and begins to strain soften and shed load. In comparison to other 

types of standing supports used in tailgates, the conventional and 

ATLAS cribs can be considered a relatively soft and lower capacity 

support (Barczak, et al., 2008, STOP, 2004). 

 

 

0 

50 

100 

150 

200 

250 

VERTICAL LOAD, kips 

4-pt mixed hardwood crib 

ATLAS Series 100 

0 2 4 6 8 10 12 14 

VERTICAL DISPLACEMENT, in 

Figure 4. The load-displacement for the ATlAs 100 and 

conventional 4-point mixed hardwood crib as determined in the 

laboratory. The crib height is 7 ft. 

Test site 

To evaluate the standing support performance and ground

reaction, both roof-to-floor and support convergence was measured

in the tailgate. The roof-to-floor measurements were made in

between the standing supports and provide a measure of the amount

of local convergence the support was controlling. Figure 5 shows

the location of the instrumentation used to monitor the standing

support and tailgate convergence. To measure the support

convergence, displacement transducers were installed near the top 

of the cribs with a wire connecting the transducer to an anchor point 

located near the bottom of the support. The loads on the support 

were estimated based on the measured convergence and the loaddisplacement curves developed from the laboratory testing of the 

support. Roof-to-floor convergence was measured by attaching the 

displacement transducer to a roof bolt with a wire connecting the 

transducer to an anchor installed in a shallow drill hole in the floor. 

Because of the relatively soft nature of the floor, only one roof-tofloor convergence anchor survived inby the longwall face. A 

permissible data acquisition system was used to record the data. 

The standing support and instrumentation were installed prior to 

the adjacent panel being mined with the test site on the headgate 

side of the first panel. This allowed for the side abutment loading 

effects to be measured. Convergence measurements were also 

obtained for the front abutment loading and inby the face where the 

effects of full extraction could be observed. 
130 ft Coventional 4-pt crib 

Atlas 100 Crib 

Sand Prop 

7+25 8+75 Convergence Station on Crib 

x Roof to Floor Convergence 

x 

x x x

x 

Figure 5. Tailgate test site with the location and type of the cribs 

and instrument location being noted. The sand props were not 

part of the study. 

resulTs oF THe converGence MeAsureMenTs 

Based on the support convergence measurements a comparison 

between the two different standing supports can be made. Figure 

6 shows the average support convergence developed from the side 

and front abutments along with the combined total both for the 

conventional wood and ATLAS 100 cribs. For the side abutment 

load, the convergence on the conventional wood cribs was about 

0.07 in higher than the SIU cribs. For the front abutment loading, 

the difference was only about 0.01 in. Further, there is only about 

0.5 to 0.6 in of support convergence from both the side and front 

abutments when the support was adjacent to the longwall face for 

both cribs. 

0.6 

0.7 

Conventional 4-pt Crib 

Atlas 100 Crib 

0.5 

SUPPORT CONVERGENCE, in 0.4 

0.2 

0.3 

0 

0.1 

Side Abutment Front Abutment (Face) Total 

Figure 6. convergence from the side abutment, the front 

abutment with the support at the face and the total from both 

abutments measured on the support for both the ATlAs 100 

and the conventional 4-point wood cribs. 

Because the convergence resulting from the two supports systems 

was so close, a combined support convergence could be developed. 

The combined support convergence is given in Figure 7 along with 

the roof-to-floor convergence for three loading conditions including 

the side and front abutment and the inby loading conditions. The 

roof-to-floor convergence was higher than the support convergence 

but only by about 0.1 in for the side and front abutment loading 

phases. Further, there was twice as much convergence from the side 

load as from the front abutment load when the test site was in the 

tailgate. Inby the face, there was over 3.5 in of support convergence 

and over 4 in of roof-to-floor convergence. However, significant 

levels of convergence did not occur until well inby the face. Figure 

8 shows the support convergence on two of the conventional wood 

cribs and roof-to-floor convergence near these cribs. At the face, 

there was less than 0.5 in of roof-to-floor and less than 0.1 in of 

support convergence from the front abutment. The large and rapid 

increase in convergence did not begin to occur until the supports 

were between 90 to 100 ft inby the face. 

4.5 

4.0 Support Convergence-Combined 

SUPPORT CONVERGENCE, in 3.5 Roof-to-Floor Convergence 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0
 

Side Abutment Front Abutment (Face) Full Extraction
 

Figure 7. Average convergence measured on both types of cribs 

for three different loading conditions, side abutment, front 

abutment for support at the face and full extraction with the 

support inby the face. Roof-to-floor convergence is also given. 

0.50 

-0.50 

CONVERGENCE, in 

-1.50 Station 13 - Conventional 4 pt crib 

Station 15 - Conventional 4 pt crib 

Station 16 - Roof-to-Floor -2.50 

-3.50 

-4.50 

-5.50 

1200 1000 800 600 400 200 

DISTANCE FROM LONGWALL FACE, ft 

0 -200 

Figure 8. Measured support and roof-to-floor convergence 

plotted with respect to the longwall face position. Positive distance 

numbers indicate the measurements are outby the longwall face 

and negative distance numbers indicate the measurements are 

inby the longwall face. 

GrounD reAcTIon curves 

The full ground reaction curves for the four different loading 

conditions were developed by numerical modeling. The finite 

difference model FLAC was used to develop these ground reaction 

curves (Itasca Consulting Group Inc., 2005). A detailed discussion 

of the numerical modeling to develop the ground reaction curves 

was given by Barczak (Barczak, et al., 2008). Figure 9 shows the 

basic layout for the model designed to specifically evaluate the 

tailgate. The model extended from the surface above the seam 

to 100 ft below the seam, a distance of 600 ft. Horizontally, the 

model simulated a section of the panel and gateroads that extended 

from the center of the middle entry to the middle of the panel for a 

distance of 650 ft. There was an axis of symmetry about a vertical 

axis through the center of the middle entry. 
 

Pillar Tailgate Entry Tailgate Entry Panel/Gob Panel/Gob

Figure 9. Basic numerical model layout used to develop ground 

reaction curves for the four different tailgate loading conditions. 

only a portion of the layout is shown. 

 

For the development loading condition, a vertical load was 

applied to the model that resulted in a vertical stress at the coal 

seam of 550 psi. The side and front abutment loading conditions 

were simulated by increasing the vertical load by 10 pct and 60 

pct respectively. These levels of vertical load were developed 

by matching the model results to the in situ convergence 

measurements. For the full extraction condition, the panel in the 

model was removed and replaced by a soft elastic gob. The stiffness 

of the gob was based on matching the amount of subsidence in the 

model to the typical subsidence that occurs over the Herrin seam 

(subsidence factor of 65 to 70 pct). A horizontal stress of 1,100 

psi was applied to the model based on the low strain model for 

horizontal stress developed for the eastern United States (Dolinar, 

2003). 

A Coulomb constitute model for the physical properties was used 

to simulate failure within the models. The model properties used 

for the coal seam and immediate and intermediate roof and floor 

are given in Table 1. A strain softening model was used for both the 

rock and bedding planes where the cohesion for all rock types and 

bedding planes was reduced by 90 pct of initial values at 0.5 pct 

plastic strain. 

To simulate support in the models, vertical support pressure was 

applied to the roof and floor in the tailgate entry that ranged from 

10 to 3,000 tons. This was equivalent to 1.25 tons/ft to 375 tons/ft 

of support along the entry. This resulted in a total of 9 points being 

used to establish each ground reaction curve. The models were run 

to equilibrium with the opening convergence being noted. For the 

four loading conditions, the convergence and the support pressure 

were then plotted to develop the ground reaction curves. The field 

results were used to calibrate the models. 

Figure 10 shows the ground reaction curves that were developed 

for the four loading phases that needed to be considered. 

Exponential regression curves were fit through the numerical 

modeling results. The convergence and support loads that were 

measured and calculated from the test sites are also shown. Support 

loads were developed as the average load for the two supports for a 

given convergence based on the laboratory tests conducted on the 

two supports. From the test site data, the support loads per foot of 

entry for the side abutment were 17.6 tons (2.2 tons/ft), for the front 

abutment at the face, 26.4 tons (3.3 tons/ft) and for full extraction 

96.0 tons (12.0 tons/ft). Both the side and front abutment conditions 

produced very steep, near linear ground reaction curves. Only the 

full extraction curve showed significant convergence and nonlinear 

behavior. 

The ground reaction response developed in the Illinois basin 

can be compared to those developed from the Pittsburgh seam 

(Barczak, et al., 2008). Figure 11 shows ground reaction curves 

from the two regions for a depth of 500 ft for the front abutment 

and full extraction conditions. The Pittsburgh seam case for a 

depth of 500 ft was based on numerical modeling only with no field 

confirmation. The front abutment ground reaction curves are nearly 

identical with only slightly greater convergence for the same level 

of support in Illinois. For the full extraction condition, there is a 

much large difference in the position of the ground reaction curves 

but in this case, the curve developed for the Pittsburgh seam shows 

more convergence for the same amount of support. 

suPPorT DesIGn consIDerATIons BAseD 

on GrounD reAcTIon curve 

The tailgate entry in this study was a low convergence 

environment until well behind the face. This resulted in very steep 

ground reaction curve with only a small amount of nonlinear 

convergence when the support was at the face and subjected to 

the full front abutment. The small amount of convergence even 

at low levels of support indicates that the tailgate conditions were 

in general very stable with very little damage or yield in the rock. 

With such near linear behavior for the side and front abutment 

curves, it was difficult to locate the support design threshold, the 

point where the curve becomes nonlinear because of damage to 

the rock. Further, with little nonlinear behavior there should be 

little rock damage and with the damage occurring at low support 

loads, the support design threshold was of limited importance. 

Even though the two supports were relatively soft and limited in 

capacity, they had more than adequate capacity and stiffness in 

such an environment to maintain tailgate stability. To increase the 

level of support, even significantly, would have resulted in only 

a small reduction in the convergence because of the steepness of 

the curve. The stability of the ATLAS 100 cribs was not a factor 

with this limited amount of convergence. It should be noted that the 

convergence measured may be normal or average but there must be 

sufficient support capacity to withstand unusually adverse ground 

and geologic conditions that may be encountered. 

The amount of uncontrollable convergence from the side and 

front abutments that the support had to handle when the face passed 

was just over 0.5 in based on the ground reaction curve. Because 

the tailgate support was not installed until after the development 

convergence occurred, it did not see convergence from development 

loading. From the front abutment only, the uncontrolled 

convergence was just under 0.25 in. In this case the standing 

support would not have to resist the development and side abutment 

convergence. However, the support system must be able to survive 

the uncontrolled convergence and still provide adequate support. 

These levels of convergence though were well below the 
yield and total load capacity of both the conventional and ATLAS 

cribs.

A significant increase in the rate and amount of support 

convergence from the full extraction condition only began when 

the support was at about 90 to 100 ft behind the face. In the case of 

the full extraction loading, the support design threshold was fairly 

easily located (Figure 10). To prevent most of the damage to the 

rock would require a high level of support. However, if there is no 

need to maintain the tailgate entry that far back into the gob there is 

no reason to design the support system to the full extraction ground 

reaction curve. 

2500 

3000 

Development 

Side Abutment 

Face Abutment 

Full Extraction 

SUPPORT LOAD, tons 2000 Measured 

1500 

Support Design Threshold

1000 

500 

0
 

0
 1 2 3 4 5 

CONVERGENCE, in 

Figure 10. Ground reaction curves from the tailgate of a longwall 

in the Herrin no. 6 seam in Illinois developed for four loading 

conditions: development, side abutment, front abutment and 

full extraction. The support loads from the actual measurements 

are also given. The dashed lines indicate exponential regression 

curves fit through the data. 

Table 1. Physical properties of the immediate and intermediate roof and immediate floor rocks used in the 

numerical models. 

Rock Cohesion, 

(psi) 

Friction angle 

(deg) 

Poisson 

ratio 

Elastic 

Modulus 

(ksi) 

Coal 275 31 0.25 360 

Immediate Floor: 

Underclay 175 22 0.25 870 

Immediate Roof: 

Black Shale 290 23 0.25 1,160 

Immediate Roof: 

Limestone 650 25 0.25 1,740 

Intermediate Roof: 

Sandstone 2,300 28 0.25 2,300 

The amount of convergence measured on the two supports was 

very similar. This was to be expected since the load displacement 

characteristics of the two supports were very similar up through 

6 in of displacement. As a result, only one point on the ground 

reaction curves for each load phase could be determined. 

The ground reaction curves for the front abutment loading 

condition at the face were essentially the same for the Herrin seam 

and Pittsburgh seam for the depth of 500 ft (Figure 11). However, 

the vertical load factors used in the numerical models were 10 pct 

for the side abutment and 60 pct for the front abutment as compared 

to 20 pct and 120 pct for the Pittsburgh seam (Barczak, et al., 

2008). This indicated that in the Pittsburgh seam there was more 

load transfer to the pillars and longwall face from the side and front 

abutments than occurred in the Illinois mine. This may be the result 

of relatively stronger and stiffer beds in the overburden above the 

caving of the Pittsburgh seam than those above Herrin seam.

3000 

2500 Front Abutment-Illinios Basin 

Front Abutment-Pittsburgh Seam 

Full Extraction-Illinios Basin 

SUPPORT LOAD, tons 2000 Full Extraction-Pittsburgh Seam

1500 

1000 

500 

0 

0 1 2 3 4 5 6 7 8 9 

CONVERGENCE, in 

Figure 11. comparison of the ground reaction curves developed 

for the Herrin no. 6 seam in Illinois basin and the Pittsburgh 

seam for the front abutment loading and full extraction. 

For the full extraction loading condition, the Herrin seam showed 

somewhat less convergence than the Pittsburgh seam. However, in 

both cases the ground reaction curves showed similar behavior with 

large amounts of convergence and nonlinear behavior with a long 

flat tail developing as the level of support was reduced. With such 
 

 

behavior, additional support could limit the amount of convergence 

if the support characteristics were properly matched to have 

sufficient support capacity after a large amount of convergence 

occurred. 

conclusIons 

Ground reaction curves for a longwall tailgate were developed 

for a mine in the Herrin No. 6 seam in Illinois. A combination of 

in situ measurements of the support and roof-to-floor convergence 

and numerical modeling was used to produce these curves. Four 

ground reaction curves were developed that can be used for support 

design. These were for development loading, side abutment loading 

from first panel mining, front abutment loading at the face and 

full extraction loading inby the face. Actual measurements were 

obtained from the side and front abutment and full extraction 

loading phases. At the test site where the measurements were made, 

two different types of cribs, a conventional 4-point wood crib and 

an engineered wood crib, the ATLAS 100, supported the tailgate. 

The laboratory load-displacement characteristics for both 

types of supports were very close at least through the first 6 in of 

displacement. This resulted in nearly equal amounts of convergence 

being measured on the two support systems. Essentially, the 

performance of the two types of cribs was the same. 

The tailgate was a low convergence environment until well 

behind the face. The estimated amount of uncontrolled convergence 

was only around 0.5 in from the side and front abutment loading 

phases and about 0.25 in from the front abutment only. These 

low levels of convergence were produced by ground and loading 

conditions that had side and front abutment ground reaction 

curves that were steep and showed little nonlinear behavior and 

convergence. This indicated that there was little rock yield or 

failure. In such an environment, both support systems were well 

below their capacity and were more than able to adequately support 

the tailgate through front abutment loading at the face. Because the 

process of full extraction loading did not begin until well behind the 

face, standing support design should be based on the front abutment 

ground reaction curve unless there would be a reason to maintain 

the tailgate far inby the face. 

The support capacity for other such low convergence 

environments that have similar side and front abutment ground 

reaction curves as measured in the Herrin No. 6 seam would need 

only a low load capacity for the standing support. In the case of 

this study, just over 3 tons/ft of support was needed to maintain 

the tailgate. This low level of support can be used because there is 

little rock damage or failure. Because of the steepness of the ground 

reaction curves, a substantial amount of support would be required 

to limit the convergence even by a small amount. 

Although the front abutment ground reaction curves were nearly 

identical for the Herrin and Pittsburgh seams at a depth of 500 ft, 

the vertical loading factors used for the side and front abutment 

phases in the numerical models for the Herrin seam were only 

half of those for the Pittsburgh seam. Because of the difference in 

physical properties of the rock mass and coal of the two seams, a 

lower amount of increase in the vertical load used to simulate the 

side and front abutment loading must be applied to the models 

for the Herrin No. 6 seam to generate the convergence that was 

measured. This suggests that the amount of load transfer to face 

and pillars from the side and front abutments was much less in 

the Herrin No. 6 seam. The reduction in the vertical loads resulted 

in less rock failure and yield and this in combination with the 

shallow depth may have contributed to the development of a low 

convergence condition in the tailgate. Under such conditions only a 

low capacity support would be required to maintain the tailgate. 

DIsclAIMer 

The findings and conclusions in this report have not been 

formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent agency 

determination or policy. 

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