Ground and Standing Support Interaction in Tailgates of Western U.S. 

Longwall Mines Used in the Development of a Design Methodology 

Based on the Ground Reaction Curve

Dennis Dolinar, Mining Engineer

NIOSH Ground Control Branch 

Pittsburgh, PA

ABSTRACT

Tailgate entries in U.S. longwalls can be subjected to high 

loads and deformations. Therefore, tailgate entries are generally 

supported with varying types and amounts of standing support. 

However, even with standing support, roof falls and blockages 

can occur in the tailgate when the support system is not properly 

designed. Safe and efficient operation of a longwall face requires 

the selection of the appropriate type and amount of standing 

support. Because of the importance of maintaining a safe and 

functional tailgate entry, the National Institute for Occupational 

Safety and Health (NIOSH) is currently developing a standing 

support design methodology based on the ground reaction curve. 

Using the ground reaction curve for design requires that the ground 

and support interaction be quantified. This can be accomplished by 

the installation and analysis of data from instrumented field sites. 

Although necessary, the field results cannot develop the complete 

ground reaction curve. Numerical models, calibrated to the field 

results, are used to establish the complete ground reaction curve. 

In the present study, data was obtained and evaluated to establish 

points for calibration of the ground reaction curve for two western 

longwall operations.

One test site, Mine A, was located at a depth of 1,550 ft. At 

this site, the floor consisted of siltstone, while the roof was an 

interbedded siltstone and sandstone. Because of the depth, a double 

row of 22-in diameter CANs1

1 Mention of any company name or product does not constitute endorsement by the 

National Institute for Occupational Safety and Health.

were installed to support the tailgate. 

Both the first and second longwall panels have mined past the 

instrumented site.

The second site in Mine B is at a depth of only 760 ft. The 

floor at this mine consists of a weak shale/mudstone and the roof 

consists of thick, top coal overlain by weak shales. At this site, a 

double row of pumpable cementitious cribs was installed. The 

second test site has only been subjected to side abutment loading 

from first panel longwall mining.

The measurements indicate that the support at both mines was 

heavily loaded when subjected to the side abutment loading from 

first panel mining. At the deeper site, the support began to yield 

when the face was more than 300 ft outby during second panel 

mining. In comparison to results from previous test sites in the 

Pittsburgh Seam and Illinois Basin, the amount of support loading 

and roof-to-floor convergence was an order of magnitude higher at 

the western sites. This has resulted in a significant difference in the 

position and shape of the ground reaction curves.

INTRODUCTION

Maintaining a functional tailgate entry is critical to the safe 

and efficient operation of a modern U.S. longwall. However, 

tailgate entries are often subjected to high loads and deformations 

because of abutment loads and caving. Unless adequately and 

appropriately supported, excessive convergence, ground falls, and 

blockages can and have occurred. To ensure that the tailgate entries 

remain functional, various types and levels of standing support are 

installed. Typically, the type and amount of support is established 

by trial-and-error or determined by what other longwall mines have 

done. To improve these approaches, NIOSH is developing a design 

methodology for standing support based on the ground reaction 

curve concept. This concept incorporates not only the ground 

reaction, but also the support load-displacement characteristics. 

The support characteristics have been or can be measured (Barczak, 

2003). Then the support characteristics and support should be 

selected to match the ground response (Mucho et al., 1999). 

However, the ground response around the tailgate has typically not 

been determined and is often ignored in the selection of the support 

system (Barczak, 2003). The key to this design methodology is to 

be able to quantify the ground reaction curve. This requires the 

establishment of instrumented tests.

Several recent studies have been conducted to evaluate the 

support and ground interaction. In these studies, instrumented 

test sites have been established to evaluate support and rock mass 

interaction (Barczak et al., 2008; Dolinar et al., 2009). To develop 

the ground reaction curves for these sites, the results from the 

underground tests sites were used in combination with numerical 

modeling. Based on these studies, ground reaction curves have 

been developed for tailgate sites in the Pittsburgh Seam and the 

Herrin #6 Seam in Illinois.

In this study, instrumented test sites were installed in the 

tailgates of two western longwall operations. The results of the 

support performance and ground reaction are presented in terms 
of the developed support loads and displacements and the tailgate 

entry convergence. The points on the ground reaction curve 

determined from the test site results are presented and discussed. At 

the two western longwall operations, there was significantly more 

roof-to-floor convergence than was measured at any of the previous 

sites. The degree of convergence observed provided a good test of 

the capabilities of these standing support systems.

DEVELOPING A STANDING SUPPORT DESIGN 

METHODOLOGY BASED ON THE GROUND 

REACTION CURVE

The design methodology for standing support is based on the 

ground reaction curve. The ground reaction curve is the response of 

the ground to varying levels of support. It was developed for civil 

tunneling applications but has been utilized in both hard rock and 

coal mining applications. (Brown, et al., 1983; 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 properties, the loading 

conditions and load path, and the support resistance. The support 

resistance is determined by the load-displacement characteristics 

of the support. A conceptual ground reaction curve is shown 

in Figure 1. 

Figure 1. Conceptual ground reaction curve.

The ground reaction curve is the support pressure 

versus the opening convergence. Prior to creating the opening, the 

support resistance is equal to the forces in the surrounding rock 

mass and there is no convergence (Point A). When the opening 

is created and the support level diminished, there is an increase 

in convergence. The first part of the curve is very steep and near 

linear. This represents the elastic response of the rock mass. To 

limit the convergence in this part of the curve requires a substantial 

amount of support, well beyond what is normally used or can be 

provided. With a further reduction in the level of the support, the 

curve becomes non-linear and begins to flatten. This is the result of 

the rock being fractured and going into yield (Point B). The level 

of support required to limit the convergence in this region is much 

less. This is the region where the support could have an impact 

on the amount of convergence. Once the low point is reached 

(Point C), the amount of support required begins to increase with 

additional convergence. This is caused by the downward deflection 

of the roof allowing more rock to loosen, and the weight of the 

rocks must then be controlled by the support system.

The support and rock interaction are also shown on Figure 1. 

Points PQB represent the load-displacement curve for a yielding 

support. The support is installed after some initial convergence, 

then develops load and yields before finally intersecting the ground 

reaction curve (Point B). Equilibrium between the support and 

the rock mass is then achieved. The minimum level of support 

necessary to achieve equilibrium is the amount required to reach 

the nadir (Point C). However, depending on the ground reaction 

curve, significant rock damage and convergence could occur as 

this point is approached. Further, there is no margin of safety in 

this design. The maximum level of support has been termed the 

support design threshold in a previous study (Barczak et al., 2008). 

This is the transition point on the curve between linear and nonlinear behavior. Levels of support greater than the design threshold 

would provide little benefit because there is no rock damage and 

limited convergence.

The ground reaction curve is dependent on the load path taken 

by the rock mass. Theoretically, in the tailgate entry, because of 

the continually changing loads for a given location as the longwall 

approaches, there are an infinite number of ground reaction curves. 

However, for tailgate standing support design and evaluation, there 

are only four loading conditions that need to be considered. These 

loading conditions are (A) development loading, (B) side abutment 

loading from first panel mining, (C) front abutment loading at 

the tailgate corner adjacent to the face, and (D) full extraction 

loading inby the face. Figure 2 shows the generalized location 

of these four points with respect to the panel layout. The ground 

reaction curves for these four load cases are then determined by 

numerical modeling.

First Panel mined out

Second Panel 

A

D C B

Longwall face Future tailgate entry

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 (Dolinar et al., 2009).

The support characteristics can be determined through 

laboratory tests conducted in such facilities as the Mine Roof 

Simulator (MRS). However, the ground reaction is generally 

not known or evaluated (Barczak, 2006). The ground reaction 

can be determined by establishing underground test sites where 

the support loads and displacements and entry convergence are 

measured. However, from such sites, only one or two points on a 

ground reaction curve may be determined.
The full ground reaction curve is developed for all four load 

cases by using numerical modeling. The finite difference code 

FLAC is used to generate the curves (Itasca Consulting Group, 

2005). The procedure for developing the curves has been detailed 

in previous studies (Barczak et al., 2008; Dolinar et al., 2009). 

Site specific loading conditions, mine layout, immediate roof and 

floor geologies, and physical properties are input into the models to 

develop the ground reaction curves for each individual site. Support 

loads are varied in the simulated tailgate entry of the models to 

produce the ground reaction curve. The field results are used 

to calibrate the model results and to establish the position of the 

ground reaction curve.

TEST SITES AND CONDITIONS

Instrumented test sites were installed in the tailgate entries of 

two western longwall mines to monitor the performance of the 

tailgate standing support. Each mine had different conditions and 

each used a different support type. In both mines, support was 

instrumented near a mid-pillar location and in an intersection.

Support Types

In Mine A, the standing tailgate support consisted of 22-in 

diameter CANs, manufactured by Burrell Mining Products. The 

CANs have a yield load of about 100 tons. Mine B used a 30-in 

diameter pumpable crib. The pumpable cribs have a peak capacity 

of approximately 160 tons.

Figure 3 shows typical load-displacement curves for each type 

of support based on tests conducted at the Pittsburgh Research 

Laboratory in the MRS. 

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20 22

Vertical load, kips

Vertical displacement, in

CAN 22 in

Pumpable 30 in 

Figure 3. Load-displacement curves for a 22-in diameter CAN 

and a 30-in diameter pumpable crib tested in the Mine Roof 

Simulator. Both supports were 6-ft high.

For both types of support, the yield or 

peak load is achieved between 1 to 2 in of displacement and 

each has a similar stiffness up to yield or peak load. The CAN 

exhibits an approximate elastic-plastic behavior with the support 

load still increasing after yield but at a reduced rate. After yield, 

the CAN will remain stable through 20 in of displacement. On 

the other hand, the pumpable crib demonstrates a strain softening 

behavior. The peak load is reached with less than 1 in of support 

convergence. As a result, in higher convergence environments, 

the peak load will be exceeded. Therefore, it is the residual 

strength and not the peak strength that must be considered in such 

conditions. In this test, the residual strength was above 200 kips. 

The pumpable crib will generally maintain a fairly high residual 

strength through about 10 in of displacement. Significant drops in 

residual strength are associated with the grout fracturing and loss of 

confinement by the containment bag and wire wrap.

Instrumentation

Instrumentation was installed to measure the support loads, the 

support, and the roof-to-floor convergence. All the instruments 

were read remotely using permissible data loggers. The support 

loads were measured by installing calibrated flat jacks with 

pressure transducers on the top of the support (Figures 4 and 

Figure 5). 

Figure 4. Load cell and displacement transducer installed on 

a CAN.

Figure 5. Load cell and displacement transducer installed on a 

pumpable crib.

Displacement measurements were made with wire 

potentiometers. These have a displacement range of about 11 in 

(Figures 4 and 5). For the CAN displacement, the potentiometers 

were installed on a piece of plywood that had been placed on top of 

the support (Figure 4). For the pumpable cribs, the potentiometers 

were mounted about 6 in from the top of the support (Figure 5). 

For both supports, the potentiometers were then connected with a 

wire to an anchor that was about 12 to 15 in above the bottom of 

the support.
Wire potentiometers mounted on the roof bolt bearing plates 

were also used to measure the roof-to-floor convergence. An anchor 

was installed in a hole drilled into the floor directly below the 

instrument. For Mine B, roof bolts 9 in long were also grouted into 

the floor below a roof bolt. The distance between the bolts was then 

measured manually with a tape measure.

Mine A Test Site

This test site was at a depth of 1,550 ft with a mining height of 8 

to 10 ft. The three-entry gate road system had 90-ft and 50-ft wide 

chain pillars with the wide pillar adjacent to the tailgate entry. The 

panel width (face length) was 1,050 ft.

The immediate roof consisted of 3 ft of siltstone overlain by 6 

ft of sandstone. In the floor, there is 2 to 3 ft or more of siltstone. 

The average compressive strength for the roof siltstone is 5,900 

psi, 12,500 psi for the roof sandstone, and 6,300 psi for the 

floor siltstone.

Figure 6 shows the layout of the test sites. A total of 8 CANs 

were instrumented. 

Panel

Pillar

Instrumented CAN

CAN (not instrumented)

x Roof-to-floor convergence

x

Mid Pillar

7

Panel

x

Intersection

8

Figure 6. Layout for instrumented test sites in Mine A.

Initially, a single row of CANs was installed 

along the panel side of the entry (instrumented CANs 1 to 6). 

The support spacing in a row was between 8 and 9 ft center-tocenter. Installed CAN heights ranged from 8 to 10 ft and were 

topped with 9 to 20 in of wood. To achieve the required heights, 

the CANs were made from 2 sections that were welded together 

with the top section being 5 ft long and the bottom section ranging 

from 3 to 5 ft long depending on the overall height of the CAN. 

The instrumentation and support were installed about 300 ft ahead 

of the first panel face. Because the depth was greater than 1,500 

ft, a second row of CANs was installed about 700 ft in front of the 

face during second panel mining (instrumented CANs 7 and 8). As 

a result, the second row of CANs was subjected to front abutment 

loading only and not to the side abutment.

Mine B Test Site

This test site is at a depth of 760 ft with a mining height of 

approximately 9 ft. A three-entry gateroad system is used with 

chain pillars 90 ft wide. The panel width is 1,100 ft.

The immediate roof consists of 2 to 3 ft of coal, overlain by 8 ft 

of a carbonaceous mudstone. In the floor there is 0 to 1 ft of coal, 

underlain by 3 ft of shale then 13 ft of sandstone. In general the 

coal measure rocks are weak, and the uniaxial compressive strength 

average is approximately 2,100 psi for the floor shale, 2,400 psi for 

the roof shale, 5,200 psi for the roof sandstone, 4,000 psi for the 

floor sandstone, and 2,000 psi for the coal.

Figure 7 shows the layout of the test sites. Two offset rows 

of pumpable supports were installed with center-to-center 

support spacing between 8 and 9 ft. A total of 10 supports were 

instrumented: 4 in the intersection and 6 at a mid-pillar site. The 

supports and instrumentation were installed about 250 to 350 ft in 

front of the first panel mining.

STANDING SUPPORT PERFORMANCE

Mine A

The support load versus time for first panel mining is shown in 

Figure 8. The face passed the test sites between February 13 and 

15. At the end of the first panel monitoring period, the average 

support loads developed in the intersection were 66 tons and at 

the mid pillar site 25 tons. In general the loads increased rapidly 

after the face passed then continued to increase but at a reduced 

rate (CAN 4 is the exception). The average support convergence 

in the intersection was 0.9 in and at the mid pillar site 0.5 in. 

All instruments used to measure the roof-to-floor convergence 

were lost, but based on this data in comparison to the support 

convergence, the estimated entry convergence in the intersection 

was 8 in and at the mid pillar site 4.5 in. (The instruments 

measuring the roof to floor convergence were lost within a week 

after being installed. During this period about 0.6 in of roof to floor 

convergence was measured.) The loads developed on the support 

during second panel mining with respect to the distance from the 

longwall face are shown on Figure 9 and Figure 10. In all cases, 

the support went into yield. Table 1 gives the yield, maximum, 

residual, and face loads, as well the distance from the face when the 

support yielded.

As the CANs went behind the face, all had one or more wrinkles 

at the top, a couple had over 5, indicating yielding was occurring. 

CAN 4 at a mid-pillar location buckled (bent over) and split along 

a weld seam. The instruments used to measure the roof-to-floor 

convergence were either lost or had reached their maximum range 

(10 to 11 in). However, it is estimated that there was between 

24 and 30 in of floor heave that occurred at the face. Bagging of 
welded wire screen due to loading from the failed immediate roof 

material between the two rows of support occurred when the face 

was in the intersection site.

x

x x

Mid-pillar site

Panel 

Pillar

Panel

x

x

Intersection

x

Flat Jack and potentiometer 

Roof-to-floor convergence (Potentiometer)

Roof-to-floor convergence (roof-to-floor bolt) manually read

CAN (not instrumented)

Figure 7. Layout of instrumented test sites in Mine B.

Mine B

Figure 11 shows the support loads with respect to time from the 

first panel mining. The face passed the sites between November 

11 and 14. The average support loads at the completion of first 

panel mining were 60 tons in the intersection and 37.5 tons at 

the mid pillar site. There was an initial rapid increase in the 

load development followed by a leveling off, and in 6 of the 10 

supports, load shedding occurred. However, there was no evidence 

of damage to the pumpable cribs that would have suggested that 

they were loaded beyond their peak capacity. The average support 

convergence in the intersection was 0.22 in and at the midpillar site was 0.6 in. Measured roof-to-floor convergence in the 

intersection was 15 in and 17.5 in at the mid-pillar site. The second 

panel has yet to be mined past these sites.

At both sites, floor heave pushed up around the supports 

(Figure 12). At the mid-pillar site, the roof started to fail in the 

middle between the two rows of pumpable supports. This resulted 

in bagging of the roof screen from loading due to failure of the 

immediate roof. Additional support in the form of Roc Props was 

then installed on 6-ft spacing.

ANALYSIS OF THE SUPPORT PERFORMANCE

Significant loads did develop on the support during first panel 

mining at both mines. For Mine A the loads ranged from 20 to 80% 

of the expected yield load for the CANs. At Mine B maximum 

loads were between 30 and 50% of the expected peak capacity 

for the pumpable support. Most of the support loads developed 

after the face had passed the test sites. This indicates that the side 

abutment causes the support loads.

Load shedding occurred on one support at the mid-pillar site 

in Mine A and on several supports in Mine B during first panel 

mining. At Mine A, one CAN reached a peak load of about 65 

tons, then the load slowly decreased to 33 tons when the face was 

well past the support. This is only about 70% of the yield capacity 

measured on the CANs. During this loading phase, the CAN had 

actually tilted about 8°. During second panel mining, the CAN 

bent over and split along a fabrication weld joining the lower and 

upper CAN sections. This load shedding may have been caused 

by eccentric loading with the floor yielding under the section of 

the CAN with the highest stress, thus causing the CAN to tilt, then 

bend and fail at the seam. Increasing the potential for this condition 

to occur was the 9-ft height of the support and the 18-in height of 

the wood material on top of the can to establish roof contacts. This 

resulted in a height-to-width or aspect ratio for the support of 5.7. 

The stability of the CANs can become an issue with an aspect ratio 

greater than 5 (STOP, 2004).

In Mine B, load shedding occurred with 4 of the 5 supports 

that had functioning load cells at the mid-pillar site and 2 out of 

the 4 at the intersection site. There was no indication that any of 

the supports had gone beyond their peak capacity, which would 

be indicated by fracturing of the cementitious grout. Pumpable 

support No. 6 had about 2 in of convergence and this amount of 

displacement could indicate that the support was loaded beyond 

its peak capacity. However, there were no signs that the bag was 

bulging due to the formation of fractures in the grout material. 

Further, some of the supports also saw a small rebound or reduction 

in the convergence. A significant amount of floor heave occurred 

(over 15 in). The yielding and failure of the floor around the 

supports may have caused the load shedding.

During first panel mining in Mine A, the intersection support 

loads were over twice those developed on the support at the 

mid-pillar site. The distance from the face where the support 

yielded during second panel mining was also much greater in 

the intersection than at the mid-pillar site. In general, it would be 

assumed that larger displacements, and therefore greater loads, 

would occur in the intersection. However at Mine B, the average 

peak loads was nearly the same for the intersections (68 tons) and 

mid-pillar (70 tons), well below the expected support capacity 

of 160 tons. This may again be related to the floor strength 

controlling the maximum loads that could develop on the support. 

The loads from second panel mining may add further clarification 

on the role of the floor heave in loading the support and the 

support performance.
Table 1. Summary of support performance at Mine A, supported by CAN supports, during second 

panel mining.

Yield load, Yield distance, Maximum Load at face, Load cell Residual load, tons tons ft load, tons tons

Mid Pillar

CAN--4 27 104 30 3 3

CAN--5 108 33 115 5 15

CAN--6 80 90 93 62 90

Average1 94 61.5 104 33.5 52.5

CAN--8 148 22 184 101 61

Intersection

CAN--1 103 209 109 6 2

CAN--2 92 171 101 0 0

CAN--3 89 334 90 14 2

Average 95 238 100 7 1

CAN--7 - - - - -

1

 Average does not include CAN-4.

0

10

20

30

40

50

60

70

80

90

2/6/09 2/16/09 2/26/09 3/8/09 3/18/09 3/28/09 4/7/09 4/17/09

Support load, tons

Date

Intersection CAN-1

Intersection CAN-2

Intersection CAN-3

Mid Pillar CAN-4

Mid Pillar CAN-5

Mid Pillar CAN-6

CAN-1

CAN-2

CAN-3

CAN-4

CAN-5

CAN-6

Figure 8. Support load versus time, side abutment loading from 

first panel mining, Figure 9. The support load versus the face position for second 

panel mining for the intersection site at Mine A. During second panel mining at Mine A, all the CANs went into 

yield well outby the face. The average yield load for the support 

along the panel was 95 tons for those in the intersection and 94 tons 

for the mid-pillar site (excluding CAN 4). The yield loads are close 

to the 100-ton yield capacity for the support as determined in the 

laboratory. The average yield distance for the intersection support 

was 230 ft and 75 ft for the mid-pillar site. Because of the stable 

yielding characteristics of the CAN, even after yielding outby the 

face, the CAN has sufficient capacity through the range of 

displacements that were measured. The yield load was sustained in 

several CANs over 100 to 300 ft of longwall panel mining (Figure 

6). However, significant load shedding had occurred before the face 

reached the support with 5 of the supports having loads only 

between 0 and 15 tons when positioned at the face. Only one 

support along the panel side continued to have a significant postyield load. For the five load shedding supports, the load shedding 

occurred very rapidly, but this was not the result of the failure of 

the load cells. By this time significant floor heave had occurred (24 

to 30 in), and this load shedding could be related to the floor 

yielding and failing around the supports.

The amount of the support load that resulted from the floor 

heave is not clearly known, but it must be significant. Much 

of the floor heave occurred between the supports. However, in 

general, the standing support system should not be designed to 

reduce floor heave. If the floor had not failed around the support, 

floor heave of the magnitude observed at both mines would have 

seriously damaged the support and little support capacity would 

have remained to provide support to the roof. This is especially 

true for the pumpable supports that strain soften after the peak 

load. To a large extent, floor heave may be considered uncontrolled 

convergence and a significant amount of support would be required 

to reduce the floor heave (Barczak, 2005; Barczak et al., 2008).
Figure 10. The support load versus the face position for second 

panel mining for the mid pillar site at Mine A.

0

10

20

30

40

50

60

70

80

90

11/3/09 11/13/09 11/23/09 12/3/09 12/13/09 12/23/09 1/2/10 1/12/10 1/22/10

Support Load, tons

Date

Pumpable-8 Pumpable-9 Pumpable-7 

Pumpable-6 Pumpable-10 Pumpable-5 

Pumpable-6

Pumpable-8

Pumpable-9

Pumpable-5

Pumpable-7

Pumpable-10

`

A

0

10

20

30

40

50

60

70

80

90

11/03/09 11/13/09 11/23/09 12/03/09 12/13/09 12/23/09 01/02/10 01/12/10 01/22/10

Support load, tons

Date

Pumpable-1 Pumpable-2 Pumpable-3 Pumpable-4 

Pumpable-1

Pumpable-3

Pumpable-2 Pumpable-4

B

Figure 11. Support load versus time for first panel mining for 

Mine B—mid pillar (A) and intersection (B).

Figure 12. Floor heave developed during first panel mining 

from the side abutment at Mine B.

The support at both mines did prevent any large roof fall from 

developing that could have caused a tailgate blockage. Therefore, 

the support performance should be considered successful. 

However, at Mine A, the amount of convergence, though limited 

by the support, was still sufficient to result in some immediate roof 

failure. From a ground reaction and design viewpoint, the amount 

of support applied could be considered to be near the minimum 

required to maintain stability.

At Mine B, the amount of support convergence was small, but 

the immediate roof still failed because of the weak roof. From a 

design viewpoint, the span between the supports may be too large 

for this roof structure. Under these conditions, standing support 

was added not so much to limit the ground reaction, but to provide 

greater distribution of the support for surface coverage over 

the roof.

Portions of the roof failed between the two rows of support at 

both mines, indicating the standing support is not fully controlling 

the unsupported area. Essentially, the standing support is only 

controlling a limited area. The immediate and intermediate roof 

that is not controlled by the standing support may still fail. This 

failed material was in part controlled by the welded wire screen 

and intrinsic roof support. However, at Mine B, additional standing 

support was added to help control the areas between the pumpable 

supports. The area of coverage may have to be increased in 

such cases.

GROUND RESPONSE IN THE WESTERN MINES WITH 

A COMPARISON TO GROUND REACTION CURVES 

FOR MINES IN THE PITTSBURGH SEAM AND 

ILLINOIS BASIN

The measured ground and support interaction for specific mining 

locations for the two western mines is given in Table 2. These are 

actual points on ground reaction curves with each set of numbers 

representing a point on a different ground reaction curve. 

Essentially, the field data results will position the ground reaction 

curve. There are also other points that can be established for the 

ground reaction curves from the field data. If floor heave between 

the supports is considered as the response of the ground without 

support, then further points can be added to ground reaction curves. 

For Mine A, after second panel mining at the face, the estimated 

amount of floor heave/convergence was 27 in. For Mine B, after 

first panel mining, the convergence from the side abutment loading 

was 17.5 in for the mid-pillar site and 15 in for the intersection site. 

However, this does not take into account the roof reaction if there 
was actually no standing support; hence it must be considered a 

minimum level of convergence.

Table 2. Average support load and convergence for the intersection and mid pillar sites at key locations 

with respect to long wall face and panel.

Entry Average support Support Support Site location loads, tons rows support 

tons/ft convergence, in

First panel—side abutment

Mine A Intersection 66 1 8 0.9

Mine A Mid pillar 25 2 3 0.5

Mine B Intersection 60 2 15 0.22

Mine B Mid pillar 33 2 8 0.25

Second panel—front abutment/face

Mine A Intersection 7 2 2 5.5

Mine A Mid pillar 23 2 6 3.4

Based on data obtained from similar test sites as established in 

this study, ground reaction curves were developed for mines in 

the Pittsburgh Seam and the Herrin #6 Seam in the Illinois Basin 

(Barczak et al., 2008; Dolinar et al., 2009). Figure 13 shows these 

ground reaction curves for the side abutment loading and front 

abutment loading at the face for mid-pillar conditions. In the 

Pittsburgh Seam case, the depth is 750 ft, and in the Illinois case it 

is 500 ft.

0

200

400

600

800

1,000

1,200

1,400

1,600

0.0 1.0 2.0 3.0 4.0

Support load, tons

Convergence, in

Herrin #6 Side Abutment

Herrin #6 Front Abutment

Pittsburgh Side Abutment

Pittsburgh Front Abutment

Mine A Side Abutment 

Mine A Front Abutment 

Mine B Side Abutment 

Mine A Front Abutment

Mine A & B Side Abutment

Figure 13. Comparing the ground reaction for the two western 

mines to that from the Pittsburgh and Herrin #6 seams for the 

side and front abutment loading conditions.

The ground reaction points for the two western mines are also 

shown on Figure 13. For the mid-pillar site at Mine A, the support 

load for the side abutment loading was 3 tons/ft (24 tons) and for 

the front abutment loading at the face it was 6 tons/ft (48 tons). 

The support loads from side abutment loading for Mine B at the 

mid-pillar site were 8 tons/ft (64 tons). For the side abutment 

case from first panel mining, when only the support convergence 

is considered, the ground reaction is similar for Mine A and the 

Pittsburgh and Herrin #6 Seam sites despite the difference in depth. 

Also, there is only slightly greater convergence in Mine B than 

in the Pittsburgh and Herrin Seams. Essentially, the position of 

the curves for the side abutment loading is similar to the Eastern 

mines. A significant difference does develop with front abutment 

loading. In this case, the position of the ground reaction curve has 

shifted significantly to the right for Mine A. This implies more 

convergence for the same level of support.

When the support loads drop to near zero in the eastern mines 

there is only a small increase in the convergence. However, for 

the two western mines, the convergence between the supports is 

at least 15 in for the side abutment loading in Mine B and at least 

24 in from front abutment loading at the face with second panel 

mining at Mine A. Therefore, as the support level is diminished 

toward zero for the two western mines, there would be a large 

amount of convergence, which this would create a long tail on the 

ground reaction curves. In comparison, there is only a small tail 

for the ground reaction curves for the eastern and mid-western 

mines indicating some non-linear behavior at low support levels. 

Essentially, there can be a significant difference in the position 

and shape of the ground reaction curves. For Mine A, the depth 

may be contributing to this development. However, Mine B has a 

similar depth as the Pittsburgh Seam and similar gate road layouts. 

Therefore, for Mine B, the difference in ground reaction must 

be caused by the weak roof and floor. Further, the roof-to-floor 

convergence is, at most, 25% greater than the support convergence 

for the eastern mines (Barczak et al., 2008; Dolinar et al., 2009). 

This can largely be ignored when evaluating support and ground 

interaction. This is not the case with the two western mines, where 

the roof-to-floor convergence is several times greater than the 

support convergence. The large amount of convergence, primarily 

floor heave between the supports, may affect the operational 

function of the tailgate entry and also the structural integrity of the 

rock where the roof and ribs could destabilize.

Minimizing or preventing floor heave across the opening, not 

just under the supports, would require a substantial amount of 

support. For Mine A, considering that the peak loads developed 

were about 95 tons per support, and based on the support area, the 

support pressure required to minimize the floor heave is 36 tons/

ft2

. If this pressure were applied across the opening, the required 

support resistance would be 720 tons/ft of entry. The capacity of 

the CAN support system at yield in this application was only 24 

tons/ft of entry. For Mine B, based on the average peak support 

load of 70 tons, the support pressure required to minimize the 
floor heave is 14 tons/ft. The necessary support pressure across the 

opening would be 280 tons/ft compared to the achieved support 

pressure at peak of 17.5 tons/ft.

CONCLUSIONS

Instrumented tests sites were established in the tailgates of 

two western longwall mines to measure the ground and standing 

support interaction. At Mine A, the supports have been subjected 

to both first and second panel mining, while at Mine B only the 

first panel mining has been completed. Several conclusions can 

be reached from the results and analysis developed from these 

test sites.

•  Significant loads can develop on the support from first panel 

mining. This is the result of side abutment loading.

•  The standing support in intersections can see much higher 

loads during first panel mining and yield at greater distances 

from the face during second panel mining than those located 

in the middle of the pillar. This justifies the need for increased 

levels of support in the intersection.

•  Significant amounts of roof-to-floor convergence did occur at 

both mines. Much of this convergence is due to floor heave. 

The standing support did not control the floor heave except 

underneath the support footprint. Essentially, the floor broke 

and heaved up around the supports. However, the standing 

support should not be used to control the floor heave even 

though the floor heave can impact the functional requirements 

of the tailgate. The primary function of the standing support 

should be to control the roof. Substantial amounts of standing 

support would be required to prevent this floor heave.

•  Load shedding occurred with the CAN supports prior to 

reaching their yield capacity. This characteristic is not seen in 

the laboratory tests. This may be caused by the floor heave, 

whereby the floor is yielding under the supports. With the 

pumpable support, load shedding also occurred that may also 

be related to floor heave and the floor yielding and not due to 

the failure of the support.

•  Support stability can become an issue as the height-to-width 

(aspect ratio) ratio goes above 5 with the CANs. This can 

result in a large reduction in support capacity.

•  The position of the ground reaction curves is similar for the 

two western mines and those from the Illinois Basin and the 

Pittsburgh Seam for the side abutment loading from first panel 

mining. However, the shape of the curves is different. The 

two western mines have long tails on the curves as a result 

of the large amount of roof-to-floor convergence between 

the supports. For the front abutment loading at the face, the 

ground reaction curve position has shifted significantly to 

the right for the one western site subjected to second panel 

mining. This is the result of greater support convergence at 

the western mine. Again, the curve would have a long tail 

indicating a large degree of non-linear behavior.

DISCLAIMER

The findings and conclusions in this paper 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.

REFERENCES

Barczak, T. M. (2003). Longwall Tailgates: The Technology 

for Roof Support Has Improved But Optimization is Still 

Not There. Proceeding of the Longwall USA, International 

Exhibition and Conference, pp. 105–130.

Barczak, T. M., Esterhuizen, G. S. and Ellenberger, J. (2008). 

A First Step in Developing Standing Roof Support Design 

Criteria Based on Ground Reaction Data for Pittsburgh 

Seam Longwall Tailgate Support. Proceedings of the 27th 

International Conference on Ground Control in Mining, 

pp. 347–357.

Barczak, T. M., Esterhuizen, G. S. and Dolinar, D. R. (2005). 

Evaluation of the Impact of Standing Support on Ground 

Behavior in Longwall Tailgates. Proceedings of the 24th 

International Conference on Ground Control in Mining, 

pp. 23–32.

Barczak, T. M. (2006). A Retrospective Assessment of Longwall 

Roof Support with a Focus on Challenging Accepted Roof 

Support Concepts and Design Premises. Proceedings of the 

25th International Conference on Ground Control in Mining, 

pp. 232–243.

Brady, B. H. G. and Brown, E. T. (1985). Rock Mechanics for 

Underground Mining. London: George Allen and Unwin.

Brown, E. T., Bray, J. W., Ladanyi, B. and Hoek, E. (1983). Ground 

Response Curves for Rock Tunnels. J. Geotech. Eng. 109:15–

39.

Dolinar, D. R, Barczak, T. M. and Gurley, H. (2009). Evaluation 

of Tailgate Standing Support Interaction in the Illinois Basin 

for the Development of a Ground Reaction Curve Based 

Standing Support Design. Proceedings of the 28th International 

Conference on Ground Control in Mining, pp. 30–37.

Hoek, E. and Brown, E. T. (1980). Underground Excavations in 

Rock. London: Inst. Min. Metallurgy.

Itasca Consulting Group Inc. (2005). FLAC Version 5.0, User’s 

Guide. Minnesota.

Medhurst, T. P. and Reed, K. (2005). Ground Response Curves for 

Longwall Support Assessment. Trans. Inst. Min. Metall., A, 

Mining Technology 114:A81–A88.

Mucho, T. P., Barczak, T. M. and Dolinar, D. R. (1999). Design 

Methodology for Standing Secondary Roof Support in 

Longwall Tailgates. Proceedings of the 18th International 

Conference on Ground Control in Mining, pp. 136–148.

STOP (2004). Support Technology Optimization Program, NIOSH 

Version 2.7.