A First Step In Developing Standing Roof Support Design Criteria Based 

on Ground Reaction Data For Pittsburgh Seam Longwall Tailgate Support 

Thomas M. Barczak, Senior Research Engineer 

Gabriel S. Esterhuizen, Sr. Research Fellow
 

John L. Ellenberger, Lead Research Scientist 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA
 

Peter (Yunqing) Zhang, Sr. Geotechnical Engineer
 

Pennsylvania Services Corporation (affiliate of Foundation Coal) 

Waynesburg, PA
 

ABSTRACT 

Roof support systems are designed for roof control to prevent 

unplanned falls. It sounds logical and has been the conventional 

thinking in support design since supports were first installed. In 

order to determine which support system should be used or which 

would be the most effective, the degree of control provided by the 

support system must be known. This question embodies the 

concept of a ground response curve, which is a measure of 

support control by assessment of the convergence in the mine 

entry as a function of the support capacity. In this National 

Institute for Occupational Safety and Health (NIOSH) study to 

optimize standing roof support design, ground response curves 

were developed for longwall tailgate conditions from numerical 

models of Pittsburgh Coal Seam geology. The models were 

calibrated against tailgate convergence measurements that were 

made in two Pittsburgh Coal Seam mines as the depth of cover 

varied during the panel extraction. Ground response curves were 

developed for four loading conditions: (1) development, (2) side 

abutment, (3) front abutment near the longwall face, and (4) full 

extraction inby the face. In general, the tailgate convergence and 

required support capacity increase through each of these loading 

stages. 

It was concluded that prior to failure of the rock mass, the 

support system regardless of its capacity has relatively little 

impact on tailgate convergence. Recognizing that the support 

cannot prevent much of the (pre-failure) convergence that is 

occurring is an important part of support design. Supports that 

exhibit high loading stiffness but cannot sustain that loading 

through a displacement compatible with the ground response 

curve will not provide adequate roof control because they can fail 

prematurely. The required capacity depends on the loading plus 

yielding characteristics of the support and the amount of rock 

failure that has occurred. The loading plus yielding character 

establishes where the support loading intersects the ground 

response curve. Design criterion for support capacity based on 

the identifying the onset of strain-soften rock response leading to 

“damaged roof” is proposed as a foundation for assessing support 

design requirements. The capacity without accounting for the 

loading and yielding character of one support should not be used 

to assess the capacity requirement of an alternative support 

design, particularly when the loading and yielding characteristics 

are significantly different. A sensitivity study was made to 

evaluate the impact of mining height and overburden depth. As 

expected, the results show that convergence increases with 

increase in mining height and increase in overburden depth. 

Some specific support examples are also analyzed. Although this 

is a first step and the two-dimensional numerical models have 

limitations, these initial studies provide valuable insight into the 

control provided by the roof support system and will ultimately 

lead to optimizing support design based on specific mine site 

conditions. 

INTRODUCTION 

Longwall tailgate entries can be subjected to severe loading 

and deformation associated with the approach and passing of the 

longwall face. The tailgate entry is required to remain open so a 

safe travel way and a reliable airway is maintained at the tailgate 

corner of the advancing longwall face. In gassy mines it is also 

advantageous that the tailgate entry remains open behind the face, 

allowing ventilation air flow to the first crosscut in the gob area 

for passage into the bleeder entry. Innovative support methods 

have been developed to maintain the stability of the tailgate under 

these typically severe conditions. At present, standing supports 

are widely used as tailgate support in U.S. longwall mines 

(Barczak, 2003). 

The design of standing supports requires knowledge of the 

loads that the ground will impose on the supports and the roof-tofloor convergence that will occur. This allows the support 

capacity and yield capability of the supports to be matched to the 

expected ground response (Mucho et al., 1999). The loaddisplacement characteristics of standing supports are well known 

and can be tested in the laboratory (Barczak, 2003). The ground 

response, however, is poorly understood and is not easily 

measured in the field, especially in the gob area behind the 

longwall face. 

This paper presents the results of a study into the ground 

response around tailgate entries for Pittsburgh Seam longwall 

mines using numerical models that have been calibrated with inmine measurements of tailgate convergence. The objective of the 

study was to improve the understanding of both the ground 

response and the required load and yielding capability of standing 

supports. The work forms part of the strategic goals of the 

NIOSH research program that addresses safe ground control 

practices for coal mines. 
 

GROUND RESPONSE CURVES 

The concept of a ground response curve was originally 

developed for the civil tunneling industry where the timing and 

method of ground support is determined by monitoring the 

support pressure and excavation convergence during construction 

(Brown et al., 1983). The ground response approach has found 

application in both hard rock and coal mining as a method to 

better understand the interaction between the rock mass and the 

support system (Hoek and Brown, 1980; Brady and Brown, 1984; 

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

Barczak, et al., 2005). 

The ground response curve plots the support pressure against 

the excavation convergence, as shown conceptually in figure 1. If 

the excavation boundaries are subject to support pressure equal to 

the stress in the surrounding rock, no convergence will occur 

(Point A). As the support pressure is reduced, the excavation 

boundaries converge and the pressure required to prevent further 

convergence reduces as arching and the self supporting capacity 

of the ground develops (point B). A point is reached (point C) 

where the required support resistance begins to increase as selfsupporting capacity is lost and the dead-weight of the failed 

ground must be resisted (point D). 

Figure 1. Idealized ground reaction curve and support 

response. 

The effect of the support system can also be plotted on figure 

1. Line PQB represents a yielding support which is installed after 

initial convergence (δ). As the convergence increases, the support 

loading increases in proportion to the support stiffness. The 

support reaches its peak resistance at (Q). The support then yields 

and the support load resistance is sufficient to arrest further 

convergence at point B. Theoretically, support could be designed 

and installed to operate as close as possible to point C, which 

allows the available strength of the rock mass to be utilized while 

minimizing the load carried by the support system. However, 

unless accurate knowledge of the ground response is known, a 

more prudent approach would be to leave some margin of safety 

by employing a higher support capacity to ensure that point C is 

never reached. 

In a longwall tailgate entry, the stress in the surrounding rock 

does not remain static. The advancing longwall face causes 

changes in the loading condition as it approaches and ultimately 

removes one side of the entry excavation. These changes will 

result in a unique ground response curve for each mining stage. 

Support installed during the initial loading stages can therefore 

experience further convergence as the ground responds to the new 

loading condition. In this study, families of ground response 

curves were developed, each corresponding to one of the four 

loading stages of the tailgate entry illustrated in figure 2. These 

four distinct loading stages are described as follows: 

 Development (A): The loading condition before the 

effects of longwall retreat mining is the beginning of the 

loading cycle. 

 Side Abutment (B): The entry is subjected to an increase 

in vertical loading from the side abutment of the first 

panel mining and the horizontal stresses can decrease 

owing to the relaxation of the strata towards the gob. 

 Face Abutment (C): The entry is subjected to a further 

increase in loading as the face from the second panel 

approaches creating further loading from front abutment 

pressure. 

 Full Extraction (D): The loading condition after the 

longwall face has passed and the gob area from the active 

panel extraction is formed. 

 

 

 

 

 

IN MINE MEASUREMENTS OF TAILGATE 

CONVERGENCE 

Tailgate convergence measurements on a three-entry gate road 

system were taken at two Pittsburgh Seam longwall mines 

operated by Foundation Coal Corporation. The mine properties 

border one another and have similar geological conditions. 

Convergence data was collected from two different longwall 

panels from one mine and four different panels from the second 

mine over a four-year period. The goal was to collect data at 

various locations where the depth, which ranged from 650 ft to 

950 ft, changed during the course of the study. The abutment 

(chain) pillars were 100 ft wide and 184 ft long, providing an 

Analysis of Longwall Pillar Stability (ALPS) tailgate stability 

factor ranging from 1.4 to 2.4 for the depth of cover examined in 

this study, above the minimum acceptable stability factor of 1.25 

(Mark, 1992). Both mines used pumpable roof supports for 

tailgate support, so the support was similar in all cases, except one 

mine used two rows of supports instead of one. 

The convergence measurements were made from displacement 

transducers that were connected to a permissible data acquisition 

system for sampling and collection of tailgate convergence on a 

near continuous basis. The displacement transducer was mounted 

on the outside surface near the top of the pumpable support and a 

string attached to an anchor near the base of the support in order 

to measure the displacement induced in the support by the roof-tofloor tailgate convergence (see figure 3). Load measurements on 

some supports were also made by a hydraulic cell placed beneath 

the pumpable support. Displacement measurements were also 

taken between the pumpable supports by anchoring a transducer 

on a roof bolt plate and with a steel wire attached to a plate anchor 

resting on the floor to measure roof-to-floor convergence. One of 

the support measurements was typically taken at an intersection. 

Data collection started when the supports were about 300 ft outby 

the face and continued until the face passed by the instrument 
locations, at which time data acquisition was lost or corrupted as 

the coal or gob debris interfered with the displacement sensor. 

The convergence ranged from less than 0.1 in to as high as 

3.0 in at the tailgate T-junction, position C in figure 2. Figure 4 

shows the distribution for 750-ft depth of cover. The convergence 

data is normally distributed with a mean of 0.48 in and a standard 

deviation of 0.26 in. The average data shows a correlation with 

depth of cover as shown in figure 5. One set of tailgate 

convergence data was taken as the panel approached within 20 to 

165 ft of a pre-driven recovery room at the end of the panel. The 

convergence measured at this site was somewhat higher than that 

observed at the other locations. Excluding this data, a linear 

correlation suggests that the tailgate convergence increases about 

0.6 in for every 100-ft depth of cover change. A nonlinear 

correlation indicates that the increase in convergence accelerates 

with respect to increasing depth (see figure 5). Figure 6 shows 

two other trends. The convergence increased by about 10 pct 

when only one row of supports was used instead of two. It should 

be noted that two rows were used at one mine and one row at the 

other mine; however, the geological conditions were similar. 

More convergence (25 pct increase) was measured on average 

between the supports compared to measurements taken on the 

support itself. 

10,000 ft 

1,000 ft 

 

 

 

 

 

 

 

 

 

Transducer

Bottom anchor

Figure 2. Example of a longwall panel layout showing dimensions and nomenclature. Points A, B, C, and D represent the 

Development, Side Abutment, Face Abutment, and Full Extraction (Inby) loading conditions, respectively. 

Transducer 

Bottom anchor 

Figure 3. Convergence instrumentation shown on 

pumpable roof support in longwall tailgate. 

MODELING METHOD TO DEVELOP GROUND 

RESPONSE CURVES 

The finite difference software FLAC (Itasca Consulting Group 

Inc., 2005) was used to develop the ground response curves. The 

software can be used to realistically model rock behavior from the 

initial elastic response to the large displacements and 

deformations that are associated with rock failure. It has the 

capability to model strength anisotropy found in the bedded coal 

measures and can simulate strain related weakening of failed rock 

using the Coulomb constitutive properties built within the model. 

The software also has a built-in programming language which 

allows the user to control loads and displacements in the model. 
 

 

 

 

This feature was used to apply internal pressure within the 

modeled tailgate entry excavation so that the ground response 

curve could be determined. FREQUENCY 

8
 

7
 

6
 

5
 

4
 

3
 

2
 

1
 

0
 

0.1 - 0.2 0.2 - 0.3 0.3 - 0.4 0.4 - 0.5 0.5 - 0.6 0.6 - 0.7 0.7 - 0.8 0.8 - 0.9 1.3 - 1.4 

CONVERGENCE RANGE, in 

Figure 4. Distribution of tailgate convergence for front 

abutment load condition (Point C in figure 2) at 750 ft depth 

of cover 

0 0 

-0 2 

-0.4 Near 

-0 6 recovery 

room 

-0 8 

-1 0 

-1 2 

-1.4 

-1 6 

-1 8 

-2 0 

y = -2 35E-05x2 + 0 0325x - 11.50 

R2 = 0.99 

y = -0 0065x + 4.4072 

R2 = 0 93 

600 650 700 750 800 850 900 950 1000 

DEPTH OF COVER, ft 

AVERAGE CONVERGENCE, in

Figure 5. Correlation of tailgate convergence to depth of 

cover. 

TAILGATE CONVERGENCE, in 

MEASUREMENT LOCATION NUMBER OF SUPPORT Between On the 

1 row 2 rows supports support 

-0.51 -0.53 -0.56 

-0.66 

0.00 

-0.10 

-0.20 

-0.30 

-0.40 

-0.50 

-0.60 

-0.70 

Figure 6. Convergence increased by 10 pct when 1 row of 

supports used instead of 2 rows (left portion of graph) and 

25 more convergence occurred between supports than on 

the supports. 

Model Layout and Material Properties 

A typical three-entry gate road design used in the Eastern U.S. 

longwall operations was evaluated in the study. The model was 

constructed to analyze the tailgate entry specifically. The extent 

of the model was 600 ft wide by 325 ft high, which allowed a 

1,000-ft-wide longwall panel as well as the adjacent tailgate entry 

and the center entry to be modeled by employing symmetry. The 

element size was 8 in, near the vicinity of the tailgate entry. 

The geological profile simulated in the model is shown in 

figure 7. The coal bed is overlain by weak shale strata and 

alternating weak and strong beds, typical of the Pittsburgh Seam 

in Western Pennsylvania. A significant feature of this lithology is 

the presence of thick, strong limestone beds in the roof strata. 

The rock mass was modeled as a strain softening, ubiquitous joint 

material, using the built-in constitutive model available in the 

finite difference software. Strength data for the different rock 

types included in the models were based on published data for 

coal measure rocks (Rusnak and Mark, 1999; Zipf, 2005). A 

summary of the average material properties used in the models is 

presented in table 1. The average values of the bedding plane 

strength used in the models are summarized in table 2. The 

bedding strength was loosely related to the intact rock strength, 

the weaker rocks having weaker bedding planes. Strain softening 

of the rock matrix and bedding planes was modeled by 

implementing cohesion weakening. The cohesion of all the rock 

types and bedding types was specified to reduce by 90% of the 

initial value after 0.5% plastic strain to simulate the strainsoftening response of the rock. This measure is based on 

laboratory triaxial testing of rock core specimens from one of the 

Pittsburgh Seam mines (Strata Testing Services, 2003). 

The model simulated the fully extracted conditions including 

the gob on one side of the tailgate entry. The gob was modeled as 

a soft elastic material. The stiffness of the gob was determined by 

trial and error to allow the appropriate degree of subsidence (60-

65% of seam height) over the center of the longwall panel, similar 

to observed subsidence in Pittsburgh Seam longwalls (Luo and 

Peng, 1992). 

Model Loading 

The initial vertical stress in the model was set at 825 psi to 

simulate a tailgate entry at a depth of approximately 750 ft below 

the ground surface. Since the model did not extend to the ground 

surface, vertical loads were applied to the top of the model to 

simulate the overburden up to the ground surface. The initial 

horizontal stresses were calculated from the Poisson’s ratio of 

each rock layer plus a tectonic component which depended on the 

elastic modulus of the rock (Mark and Mucho, 1994; Dolinar, 

2003). The input parameters were selected so that the horizontal 

stress in the moderately strong shale beds was 1,200 psi, similar 

to measured values in Eastern U.S. coal mines, (Dolinar, 2003). 

The loading induced by the Side Abutment and Face 

Abutment stages were simulated by increasing the vertical loading 

of the model by 20% and 120%, respectively. The Full Extraction 

loading condition was modeled by simulating the extraction of the 

coal on one side of the tailgate entry and simulating gob 

formation. The gob was modeled in two stages. The lower 

section is a low modulus material to simulate full caving, while 

the upper section is modeled with an unchanged rock matrix but 

the joint cohesion is minimized to simulate partially caved rock. 

This was done to provide sufficient confinement of the immediate 

roof resulting in more realistic inby pillar response and roof 

activity. . 
 

0 30 ft 

Tailgate entry 

Limestone 

Shale 

Sandstone 

Shale 

Gray Shale 

Black Shale 

Coal seam 

Various shales 

0 10 ft 

Scale 

Tailgate 

Figure 7. Geological profile simulated in numerical model. 

 

 

 

 

 

Table 1. Material properties used in the model. 

Rock class Cohesion 

(psi) 

Friction 

angle 

(deg) 

Poisson

ratio

Elastic 

modulus

(ksi) 

Weak rock: Black 

shale, mudstone 

Moderate rock: 

Shale 

Strong rock: 

Sandstone 

Very strong rock: 

Limestone 

Coal 

650 

1,200 

1,700 

2,900 

275 

25 

28 

32 

36 

31 

0.25 

0.25 

0.25 

0.25 

0.25 

1,700

2,300

3,500

5,800

360

The friction angle remained constant during yield. 

Table 2. Bedding plane properties used in model. 

Bedding plane description Cohesion 

(psi) 

Friction angle 

(deg) 

Very weak – clay filled 

discontinuity 8 21

Weak – open discontinuity 75 21

Moderate – weakly healed 

discontinuity 500 24

Strong – healed discontinuity 800 26

Very strong – strongly healed 

discontinuity 1,450 28

Procedure for Developing Ground Response Curves 

The ground response curves were developed by simulating a 

uniform support pressure on the roof and floor of the tailgate 

entry while sequentially modeling the four external loading stages. 

In this approach, intrinsic support (i.e., roof bolts) is not explicitly 

modeled. The model was run to equilibrium at each loading stage 

and the resulting convergence in the tailgate entry was recorded. 

In this manner, failure and convergence that occurred during the 

earlier stages are preserved and included in the later loading 

stages. Repeat analyses were carried out in which the internal 

support pressure was varied, so that the ground response curve 

could be developed. Internal pressures of 0.08 tons/ft2

 to 23 

tons/ft2

 equating to unit support loads of 10 to 3,000 tons were 

applied to provide a range of results that would bracket the 

possible range of standing support capacities. Ground response 

curves were developed by plotting the support pressure against 

the tailgate entry convergence for each loading stage. 

GROUND RESPONSE CURVES 

Figure 8 shows the suite of ground response curves for the 

four different load stages for the base case condition at 750 ft of 

overburden and a 7.5-ft coal seam height. Average in-mine 

tailgate convergence measurements are also shown on the graph. 

The curves shift to the right in the plot indicating increases in 

convergence with each loading stage. Again, these curves 

represent snapshots in time at specific points in the loading cycle 

of a longwall tailgate (see figure 2). The migration of the curves 

occurs because the loading on the tailgate is not constant but 

changes with time as the panel is mined. Another notable 

observation is that (at this scale) the development, side abutment, 

and face loading curves are nearly vertical (very high slope) and 

fairly linear. This indicates that the standing support resistance 

has very little impact on the tailgate convergence for these stages. 
 

 

 

 

 

 

 

 

SUPPORT RESISTANCE, tons

3,000 

2,500 

2,000 

1,500 

1,000 

500 

0 

Full extraction (inby) 

Front abutment 

Side abutment 

Development 

In mine 

measurment In mine 

measurment 

0 00 1.00 2 00 3.00 4.00 5 00 6.00 7 00 8.00 9 00 

CONVERGENCE, in 

Figure 8. Suite of ground reaction curves for the four 

loading stages for the base configuration at 750 ft of 

overburden and a 7.5 ft coal seam height. 

Conversely, the full extraction (inby) loading curve is nonlinear 

with decreasing slope as the support resistance decreases, 

meaning increases in support capacity from low capacity to higher 

capacity can produce significant reductions in convergence. It 

can also be seen that model responses are less consistent for the 

inby loading than they are for other three loading stages. 

These differences in the inby behavior can be explained by the 

rock mechanics associated with the different loading stages. 

Outby the advancing longwall face, the tailgate is always flanked 

by a pillar on one side and an unmined longwall panel on the 

other side. A softened or damaged rock zone develops above and 

to a degree below the tailgate as the stress is transferred to the 

solid abutments on each side (see figure 9a). This roof beam 

bridging across the tailgate entry and supported by the solid 

abutments on each end will deform downward into the tailgate 

entry producing convergence. The deformation will generally 

increase as the rock strength is exceeded resulting in a strain 

softened rock response. Likewise, the floor can move upward, 

also creating convergence. Inby the longwall face, the panel 

abutment is replaced by the gob material, which exhibits a much 

softer response. In this condition, the roof is acting more like a 

cantilevered beam (see figure 9b), supported on one end by the 

pillar abutment. Here, larger deflections occur creating more 

damage to the immediate roof and more opportunity for the 

support system, which tries to act as a breaker line, to influence 

the roof failure process. As such, the impact of the support 

resistance is greater inby the face. In general, the impact of the 

support on the ground response will increase once the rock is 

damaged and strain softened. Prior to this, the elastic response of 

the rock or coal is not significantly influenced by the support 

system. 

A sensitivity study was conducted with the numerical models 

to evaluate the impact of depth of cover, seam height, and applied 

stress. These results are summarized as follows. 

 

Face abutment condition (cross 

section of tailgate entry) 

0 2 

Longwall 

tailgate 

Rock matrix 

failure 

development 

Full extraction (inby) condition (cross section) 

Longwall 

tailgate 

Rock matrix 

failure 

development 

Fully caved 

gob 

Low cohesion 

gob debris 

Gateroad 

Middle pillar 

gateroad 

Figure 9. Rock response around tailgate 

opening for face abutment (a) and full 

extraction (b) loading conditions (750-ft 

overburden and 7.5-ft mining height). 

Depth of cover: The overburden stress directly increases with 

increasing the depth of cover at a rate of 1.1 psi/ft. Examining the 

response for the front abutment loading near the longwall face as 

shown in figure 10, it is seen that the ground response curves 

become further apart as the depth of cover increases, indicating 

that the tailgate convergence grows progressively larger as the 

depth of cover increases. With 500 tons of support resistance, the 

modeling indicates that the convergence increases by 0.5 in as the 

depth of cover increases from 500 to 750 ft, and increases by 0.6 

in as the depth of cover increases from 750 to 1000 ft and by 

nearly 0.7 in from 1,000 to 1,250 ft. It is also seen that the 

nonlinearity in the ground response curves progressively increases 

at support resistances below 500 tons with increasing depth of 

cover. This indicates that the impact of the support capacity 

relative to reducing tailgate convergence increases as the depth of 

cover increases. This supports the hypothesis that the capability 

of the support to control the ground response increases as the rock 

damage intensifies. Inby the face, the tailgate convergence 

increases in proportion to the depth of cover, but the ground 

response curves have a similar profile. This suggests that the 

support control is about the same regardless of the depth of cover 

at least for the ranges evaluated in this model. 
 

 

 

 

 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1,000 

500 ft 

overburden 

750 ft 

overburden 

1,000 ft 

overburden 

1,250 ft 

overburden 

SUPPORT RESISTANCE, tons

0.00 0.50 1 00 1 50 2 00 2.50 3.00 3.50 

CONVERGENCE in 

Figure 10. Ground response curves become further apart as 

the depth of cover increases, indicating that the tailgate 

convergence grows progressively larger as the overburden 

depth increases with 7.5-ft mining height. 

 

Mining (coal seam) height: The base case mining height of 7.5 

ft was increased and decreased by 2.5 ft for the sensitivity study 

to evaluate the impact of pillar response. The ground response 

curves near the longwall face location for these three mining 

heights are shown in figure 11. The tailgate convergence also 

increases with increasing mining height, approximately 0.25 in for 

each 2.5-ft increase in mining height. As such, the impact of 

mining height over this range was less than that observed for 250-

ft increments in depth of cover. The ground response curves also 

have the same profile, indicating the support impact is about the 

same regardless of seam height. Inby the longwall face, the 

ground response curves tend to coalesce (see figure 12), 

indicating that seam height does not influence convergence much 

in this area and has significantly less impact than other factors. 

Abutment Stress: The face abutment stress was increased from 

the base case of 2.2 times the overburden load to 2.5 times the 

overburden load and then incrementally increased by an 

additional 0.5 factor to a maximum of 5.0 times the overburden 

load. This is done to evaluate the impact of pillar deformation 

and yielding. The tailgate convergence at the face location 

increased linearly with these stress multipliers until the factor 

reached 4.0 (see figure 13). Beyond this, the convergence began 

to increase to a greater degree. Beyond a stress multiplier of 4.5, 

the pillar failure was extensive as was the immediate roof and 

floor damage in the tailgate entry. 

800 

1,000 

5 ft 

coal seam 

7.5 ft 

coal seam 

10 ft 

coal seam 

900 

SUPPORT RESISTANCE, tons

600 

500 

400 

300
 

200
 

100 

0 

0.50 0.75 1.00 1.25 1.50 1.75 2.00 

CONVERGENCE, in 

700 

Figure 11. The tailgate convergence increases with increasing 

mining height, approximately 0.25 inches for each 2.5-ft 

increase in height with 750 ft of overburden. 

Inby (750 ft overburden - 5 ft coal seam) Inby (750 ft overburden - 7.5 ft coal seam) 

Inby (750 ft overburden - 10 ft coal seam) 

1,000 

900 

800 

700 

7.5 ft 

coal seam 

5 ft 

coal seam 

10 ft 

coal seam 

SUPPORT RESISTANCE, tons 

600 

500 

400 

300 

200 

100 

0 

4.00 5.00 6.00 7.00 8 00 9.00 10.00 

CONVERGENCE, in 

Figure 12. Inby the longwall face, the ground response 

curves tend to coalesce, indicating that seam height does not 

influence convergence much in this area. 

CONVERGENCE, in 18 

16 

14 

2.0 2.5 3.0 3.5 4 0 4 5 5 0 5 5

ABUTMENT LOAD MULTIPLIER 

12 

10 

8 

6 

4 

2 

0 

Figure 13. The tailgate convergence increases linearly with 

face abutment stress up to a multiplier of 4.0. Beyond this, the 

convergence began to increase to a greater degree. 

MODELING DISCUSSION 

Modeling of the elastic response of rock masses has been 

effectively achieved, but modeling of rock failure remains a 

complex and daunting task even with the advancements in model 

capabilities in recent years. The development of ground response 

curves requires modeling of the rock response beyond the elastic 

range of loading. FLAC has several properties that facilitate this 

task, including representing the inelastic response and strain 

related weakening of failed rock, but limitations remain in being 

able to capture the behavior of the rock structure through failure. 

The final dead-weight loading by loosened or detached roof rocks 

is not well represented. The software was designed to simulate 

continuous materials but does not efficiently simulate discrete 

particles such as detached blocks in the roof of the entry. Issues 

such as disintegration of the roof rock structure around standing 

supports and support loading by detached blocks were therefore 

specifically excluded from this study. Likewise, failure that 

involves buckling of delaminated strata is also not considered in 

this analysis. Buckling occurs at lower stress and through a 

different failure mode than the intact rock matrix allows in this 

analysis. 

Some of the other issues that were examined in this study are 

discussed as follows. 
 

 

 

 

 

 

 

 

 

Gob modeling: The formation of the gob and reaction of the 

caved material is not an easy process to model. After several 

approaches were examined, it was concluded that a two-stage gob 

model provided the most realistic results. The lower section of 

roof is fully caved and is modeled with a low modulus material, 

while the upper gob has an unchanged rock matrix but the joint 

cohesion was removed (see figure 9). 

Material modulus: The rock matrix consisted of four primary 

rock materials as depicted in table 1. A range of material 

properties exists for all rock materials. The elastic modulus was 

varied by a factor of 2 as part of the effort to calibrate the model 

to the observed tailgate convergence during the field studies. The 

higher modulus values were found to provide convergence most 

closely related to the field measurements. 

Horizontal stress: It was assumed that the longwall gate roads 

are aligned parallel to the major horizontal stress, thus reducing 

the out-of-plane stresses in the two-dimensional models. 

Horizontal stress can significantly impact the roof response. For 

this study, the horizontal stress was decreased by 20% during the 

side abutment loading and then increased 20% during the face 

abutment loading. 

Two-dimensional limitations: Only two-dimensional models 

were utilized in this study. Aside from the fact that the twodimensional modeling cannot account for the spacing of the 

support along the tailgate entry, it is believed that the twodimensional approach is reasonably accurate for the development 

and side abutment stages of the loading cycle. The most severe 

limitations occur at the tailgate corner as the longwall face passes 

by the area of interest. Here the stress field can be complex and 

the intersection of the front and side abutments can create out of 

plane stresses that are not properly modeled in a two-dimensional 

environment. As a result, the face loading stage is best described 

as behavior just outby the face line. The behavior just inby the 

face directly behind the shield line where the gob forms also has 

these same issues. Likewise, the inby behavior modeled by the 

two-dimensional approach is best described after the gob has fully 

formed. Although not an issue in this situation, two dimensional 

modeling would also have limitations regarding panel geometry 

effects, such as sub-critical conditions that could occur for deeper 

cover conditions. 

SUPPORT DESIGN DISCUSSION POINTS 

Uncontrollable convergence: Clearly, prior to failure of the rock 

mass, the support system regardless of its capacity, has relatively 

little impact on tailgate convergence. This behavior has been 

referred to as “uncontrollable convergence” in previous 

publications (Barczak et al., 2005). Referring back to figure 8, 

increasing the unit support capacity from 100 to 500 tons (400% 

increase) reduced the convergence near the longwall face by only 

5% (0.06 inches). Recognizing that the support cannot prevent 

much of the convergence that is occurring in these conditions is 

an important part of support design. Supports that exhibit high 

loading stiffness but cannot sustain that loading by yielding 

through a displacement compatible with the ground response 

curve will not provide adequate roof control. The support must be 

able to survive the “uncontrollable convergence” in order to 

provide roof support if failure of the rock mass occurs later in the 

loading cycle. Several examples of very high capacity support 

systems with limited yield capability that fail prematurely due to 

the “uncontrollable convergence” component have been 

documented in previous publications (Barczak, 2006). 

Establishing capacity threshold based on the ground response 

curve: One approach to establishing the support design capacity 

requirement is to identify the nonlinearity portion of the ground 

response curve. Then assuming this area represents strain 

softened or “damaged rock” response, the required support 

capacity to prevent the onset of this condition can be determined. 

Figure 14 illustrates this concept for the front abutment (face) 

loading condition. In this example, the support design threshold 

would be approximately 225 tons, equating to a support load 

density of 28 tons/ft of tailgate entry for an 8-ft support spacing. 

Providing capacity above the support design threshold provides 

little improvement in ground control since the convergence is 

reduced by only a slight amount. Conversely, providing capacity 

less than the support design threshold can lead to progressively 

higher risk of roof instability and the potential for roof falls.. 

Damaged rock 

Elastic rock 

response 

Support design 

threshold

Little 

benefit

Increased risk 

of instability

500 

450 Elastic rock 

Little 

benefit 

response 

Support design 

threshold 

Damaged rock 

Increased risk 

of instability 

SUPPORT RESISTANCE, tons 400 

350 

300 

250 

200 

150 

100 

50

0 

1.00 1.10 1 20 1.30 1.40 1.50 

CONVERGENCE, in 

Figure 14. Support capacity design threshold established from 

onset of strain-soften rock response from ground response 

curve. In this example for face abutment loading condition, 

the support capacity requirement is approximately 225 tons. 

Required capacity dependent upon support design: The 

required capacity based on intersecting the ground response curve 

depends on the loading and yielding characteristics of the support. 

In general, a support with a higher stiffness will require a higher 

capacity than a support with a lower stiffness. Therefore, the 

capacity of one support should not be used to assess the capacity 

requirement of an alternative support design, particularly when 

the loading characteristics are significantly different. 

Timing of support installation can affect design requirements: 

Since supports develop load in response to convergence, the 

timing of the support installation can affect the capacity and yield 

requirements. Some longwall supports are installed in the active 

tailgate while others may be installed prior to it becoming an 

active tailgate. Since convergence occurs throughout the mining 

cycle, as illustrated in the four loading stages of the ground 

response curves, the earlier in the loading cycle that the support is 

installed, the more convergence it will see and must survive to 

maintain control as the panel mines by the support location in the 

tailgate. However, since the side abutment loading from first 

panel mining produces relatively low convergence, this is not 

likely to be a major concern except in stiff support systems that 

have very limited yield capabilities. 
 

 

 

 

 

 

 

 

 

 

 

 

 

IMPLEMENTING THE GROUND RESPONSE CURVES 

INTO THE NIOSH STOP PROGRAM 

The ultimate goal is to transfer this information into the 

NIOSH Support Technology Optimization Program (STOP) to 

facilitate use as a design tool (Barczak, 2000). The program 

currently allows ground reaction data to be entered into the 

program, although additional plans are being made to facilitate the 

process presented in this paper. For example, the face and full 

extraction ground response curves for the base case condition of 

750 ft of overburden and 7.5-ft coal seam height were entered into 

the program. Using this as the design criterion, the performance 

of single and double rows of conventional 4-point wood cribs and 

30-in-diameter pumpable supports was examined. Both support 

systems were installed on an 8-ft center-to-center spacing in the 

analysis. 

The output chart showing the support performance curves and 

the ground reaction design curve are shown in figures 15a (face 

abutment condition) and 15b (full extraction condition). 

Examining the face abutment condition first, the single row of 

pumpable supports intersects the ground response curve very 

closely to the 225-ton threshold referenced in figure 14 since the 

supports are installed on 8-ft center-to-center spacing. Both of the 

wood crib support systems intersects the ground response curve 

near the bottom of the curve, indicating significant rock damage 

has occurred and the probability of unstable ground is much 

higher. It is also interesting to see that adding another row of 

wood cribs makes very little difference. Conversely, adding 

another row of pumpable supports moves the intersection 

significantly above the support design threshold. Although the 

added capacity moves the rock response to the elastic range, the 

reduction in convergence is very small and the support capacity is 

probably not necessary in this case. 

Despite their high capacity, the pumpable supports cannot 

control the ground movement to prevent yielding of the support 

system as the full extraction condition develops inby the longwall 

face. As seen in figure 15b, the single row of pumpable supports 

now intersects the ground response curve well into the damaged 

rock zone, while the double row intersects the ground response 

curve near the support design threshold. Therefore, inby the face, 

the added capacity provided by the second row of pumpable 

supports can be justified. The additional row of wood cribs is 

also more beneficial inby the face compared to previous analysis 

outby the face, and due to yielding of the pumpable supports, the 

double row of wood cribs is comparable to the single row of 

pumpable supports. The single row of wood cribs is again on the 

low end of the ground response curve, allowing about 7 in of 

convergence, and as such provides the most risk for poor ground 

conditions of the supports analyzed in this example. 

CONCLUSIONS 

The ground reaction concept provides a means to design 

standing support systems based on the control the support system 

has on the ground response. Since the goal of support application 

is to prevent roof falls, this is a logical design philosophy. The 

ground reaction concept is not new; the difficulty in applying the 

concept has always been obtaining the curve. In this NIOSH 

research project, FLAC numerical models were constructed that 

develop the ground response from simulated support loading in a 

longwall tailgate and stress conditions induced by the longwall 

extraction process. The models were calibrated against tailgate 

convergence measurements during the front abutment loading in 

two Pittsburgh-Seam longwall mines and inby measurements 

made at one of two mines, which are valid points on the ground 

reaction curve for those specific conditions. 

This study, along with others, has clearly shown that prior to 

failure of the rock mass, the support system regardless of its 

capacity has relatively little impact on tailgate convergence. 

Recognizing that the support cannot prevent much of the 

convergence that occurs is an important part of support design. 

Supports that exhibit high loading stiffness but cannot sustain that 

loading by controlled yielding through a displacement compatible 

with the ground response curve will not provide adequate roof 

control. The support must be able to survive the “uncontrollable 

convergence” in order to provide roof support if failure of the 

rock mass occurs later in the loading cycle. It is also clear that the 

required capacity depends on the loading plus yielding 

characteristics of the support and the capacity of one support 

should not be used to assess the capacity requirement of an 

alternative support design without also assessing its loading and 

yielding characteristics, particularly when they are significantly 

different. 

Design criterion for support capacity based on the identifying 

the onset of strain-soften rock response leading to “damaged roof” 

has been proposed as a foundation for assessing support design 

requirements. Under 750 ft of overburden with a 7.5-ft mining 

height, this translates into a support design requirement of 225 

tons per row of support, if installed on an 8-ft spacing for supports 

within the row down the tailgate, for the face abutment loading 

condition for the study areas. About 255 tons per row for the full 

panel extraction inby loading condition is required for support 

design. With less capacity than identified by these thresholds, the 

risk of instability increases and grows progressively larger as the 

support density approaches the bottom of the ground reaction 

curve. 

Assessing the level of risk as the support response falls within 

the soften rock response portion of the ground reaction curve 

remains part of the research challenge. The rock failure response 

is the most difficult part of the process to model. The FLAC twodimensional models used in this study have capabilities to 

simulate strain related weakening of failed rock using the 

Coulomb constitutive properties built within the model, but as a 

continuum model there are still limitations in this regard. With 

few exceptions, the ground response curves come to equilibrium 

in the models, implying that “stable ground conditions” can be 

achieved without any significant support. Perhaps that is true for 

the conditions analyzed in the model, but it is likely related to the 

inability of these models to fully simulate discontinuous rock 

mass. Nonetheless, the ground response curves produced in this 

study are believed to be a reasonable representation of the ground 

response and provide a major first step at examining support 

design from the perspective of ground control. This can help 

improve the application of roof supports. In the future, additional 

geological conditions will be examined. The immediate goal is to 

build a data base of baseline curves for various coal seams and 

then to refine them through advancements in the numerical 

models with field measurements for calibration and validation. 
 

Ground 

reaction curve

Pumpable supports 

1 row

Pumpable supports 

2 rows

Wood cribs

Wood ribs 1 row

2 rows

Damaged rock 

Support design 

threshold

Ground 

reaction curve 

Pumpable supports 

1 row 

Pumpable supports 

2 rows 

Wood cribs 

Wood cribs 1 row

2 rows 

Damaged rock 

Support design 

threshold 

Ground reaction curve

Pumpable supports 

1 row

Pumpable supports 

2 rows

Wood cribs - 1 row

Wood cribs - 2 rows

Damaged rock 

Support design 

threshold

Ground reaction curve 

Pumpable supports 

1 row 

Pumpable supports 

2 rows 

Wood cribs - 1 row 

Wood cribs - 2 rows 

Damaged rock 

Support design 

threshold 

Figure 15a. Support performance assessment for front abutment (face) loading condition (750-ft overburden and 7.5-ft mining 

height). 

Figure 15b. Support performance assessment for full extraction (inby) loading condition (750-ft overburden and 7.5-ft mining 

height). 
 

 

 

 

 

 

 

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 any 

agency determination or policy. 

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