ABSTRACT

Pumpable roof supports provide an alternative longwall tailgate 

roof support and have grown in usage during the past few years. 

Heintzmann Corporation has been installing pumpable roof

supports at the RAG Resources Emerald Mine in western PA since 

1998, where they have provided effective roof control in their 

longwall tailgates. Despite the success of these supports in this 

application, questions remain regarding critical design issues for 

optimizing the use of this support technology. The support loading 

profile is characterized by a high initial stiffness with peak loading 

occurring at less than one inch of convergence, followed by 

significant load shedding with a post failure capacity comparable to 

that of wood cribbing. Therefore, the key to optimizing the support 

utilization is to provide sufficient load density to prevent

convergence from occurring beyond the peak loading capacity. 

This requires an understanding of how the supports interact with 

the ground conditions, hence measurement of the ground reaction 

curve. In order to obtain this information, pumpable roof supports 

were instrumented to measure support loading. Roof deformation 

and roof-to-floor convergence measurements in the vicinity of the 

instrumented supports were also made. The experimental

parameters for the installation were the support spacing and waterto-solids ratio of the grout, which controls the grout strength and 

ultimately the maximum capacity of the support. The study clearly 

shows that at this mine a 24-inch diameter support is fully capable 

of providing adequate ground control under depths of cover of 750 

ft as only 50 pct of the available support capacity was utilized 

outby the longwall face. It was also shown that the 2.00 to 1 watersolids ratio, despite providing a slightly softer support response, is 

sufficient for maintaining the same degree of roof control provided 

by the traditional 1.75 to 1 grout mix. The 10-ft spacing of the 

supports did not cause any ground control problems outby the face. 

However, inby the face the performance of the support is degraded 

once the peak capacity of the supports is exceeded. It appears that 

the large load shedding behavior, which is characteristic of this 

support following peak loading, allows the immediate roof to 

separate. When this happens, the support is unable to regain 

control of the roof in time to prevent failure of the immediate roof 

beam.

INTRODUCTION

Conventional wood cribbing has been the traditional form of 

secondary roof support in longwall tailgates. In recent years,

several alternative support technologies have been developed,

including pumpable roof support systems. As the name implies, a 

pumpable roof support is one in which the support is formed in 

place in the mine entry by pumping material, such as a

cementitious grout, into some form of a containment structure, 

typically a bag that is hung from the mine roof (figure 1). One 

major advantage of the pumpable roof support system is that it 

reduces the material handling difficulties that are common with 

most other forms of standing roof support, thereby expediting the 

installation process as well as reducing material handling injuries 

which are prevalent with many other support systems (1-2). 

Heitech1, an affiliate of Heintzmann Corporation, has been 

installing pumpable roof supports in the Emerald and Cumberland 

(RAG American Resources) mines near Waynesburg, PA for the 

past four years. The Heitech pumpable roof support uses a two -

component grout that has successfully been pumped for distances 

of over 15,000 ft through surface boreholes at these mines. The 

key ingredient in the grout is a calcium sulfo -aluminate (CSA) 

component that provides a fast set up time with a high yield 

solution of cementitious material with a compressive strength of 

approximately 1,000 psi after cementation. The CSA material is 

not available domestically and is currently imported by Heitech 

from England. This has prompted an effort to optimize the design 

 

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

by the National Institute for Occupational Safety and Health.

PUMPABLE ROOF SUPPORTS: DEVELOPING DESIGN CRITERIA BY MEASUREMENT 

OF THE GROUND REACTION CURVE

Thomas M. Barczak, Geophysicist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Figure 1. Pumpable roof support being installed in a longwall 

tailgate.

Dr. Jinsheng Chen, Senior Engineer

RAG America

Waynesburg, Pennsylvania USA

Joe Bower, Northern Sales Manager

Heintzmann

Morgantown, West Virginia USA
and utilization of this support system, both to improve its

performance and reduce the installation cost. Efforts to optimize 

the bag design, reduce the diameter of the support from 30 to 24 

inches, and improve the logistics of the support installation have 

helped to reduce the costs to its current level of about $120 per 

meter of entry when the supports are installed in a single row on 8-

10 ft center-to-center spacings. At this cost, the support system is 

competitive with other alternative support technologies that are 

used to replace conventional wood cribbing in longwall tailgates in 

several other mines. The goal of this study was to focus on the 

performance aspects and to provide data that will help to further 

optimize the support design.

Figure 2 illustrates the loading characteristics of the Heitech 

pumpable roof support from full-scale testing conducted in the 

National Institute for Occupational Safety and Health’s (NIOSH) 

Mine Roof Simulator. The support loading profile is characterized 

by a high initial stiffness with peak loading occurring at less than 

one inch of displacement, followed by significant load shedding 

with a post failure capacity comparable to that of wood cribbing. 

Hence the key to optimizing the support utilization is to prevent 

convergence beyond the peak loading capacity from occurring in 

areas where critical roof support is needed. Control of the roof 

depends on a certain level of support capacity. The question then 

becomes “what needs to be done to ensure that the peak loading 

does not occur?” If the loading environment is load-controlled 

within the realm of the available support capacities, meaning the 

equilibrium of the roof and floor is related to the support resistance, 

then this can be accomplished by installing the proper number of 

supports to provide a support load density that will establish 

equilibrium of the rock mass before the convergence reaches a 

level where peak support capacity will occur (3). However, all 

longwall mining operations have some degree of displacementcontrolled loading, meaning that some converg ence will occur

regardless of the support system in use. For example, pillar

yielding cannot be prevented by any man-made standing support. 

Hence, pillar yielding produces roof-to-floor convergence of the 

tailgate, which induces displacement on the pumpable support, 

regardless of its size, design, or the number and arrangement of 

supports installed in the longwall tailgate (4). Floor heave and 

compressional (buckling) failures of the bolted roof beam due to 

horizontal stress may also have elements of displacementcontrolled loading. A critical question then becomes, “how much 

displacement-controlled loading is occurring in a particular

longwall tailgate?” If it is more than 1 inch, then the pumpable 

roof supports will be pushed beyond their peak capacity, leaving 

only their residual capacity to support the mine roof, and this may 

not be adequate in all circumstances.

Another issue is “how far inby can the pumpable roof supports 

maintain stability of the immediate roof in gassy mines that require 

ventilation be maintained in the active tailgate to the first open 

cross cut alongside the caved gob?” Does the roof fail once the 

peak loading capacity of the support is exceeded or does the post 

failure support capacity help to maintain roof stability and form an 

open air course to the first open cross cut alongside the caved gob?

In an effort to answer these questions and establish design 

criteria for pumpable roof supports, a study was conducted at the 

Emerald mine where support loading, roof deformation, and roofto-floor convergence were measured. This paper documents the 

results of this study, assesses the degree to which the supports are 

loaded relative to their peak loading capacity, and draws

conclusions regarding the overall design requirements necessary to 

provide effective roof support with this support technology.

PROJECT DESCRIPTION

Objectives

The focus of this investigation was to evaluate the interaction of 

the pumpable roof support with the strata in the longwall tailgate, 

both outby and inby the longwall face. The specific objectives of 

the underground monitoring were: (1) to determine the loading of 

the support as a function of face position relative to the capacity of 

the support, (2) evaluate the roof deformation and roof-to-floor 

convergence and resulting stability of the immediate mine roof in 

the longwall tailgate, (3) determine the difference in behavior in the 

intersections compared to the entry areas adjacent to the pillars, and 

(4) determine trends in loading profiles and roof control relative to 

the three instrumented test sites where the grout strength and 

support spacing were varied. 

Support Specifications and Control Parameters

Three test sites consisting of a single row of 24-in-diameter, 

Heitech pumpable roof supports spanning over a distance of three 

pillars were evaluated in the study. The supports were located in 

the center of the longwall tailgate. The locations of the

instrumented supports for each of the three test sites in the study 

area are illustrated in figure 3. As shown in figure 3, both pillar 

areas as well as intersections were evaluated in the study, with a 

sufficient transition zone between the instrumented supports to 

ensure that the impact of the experimental parameters was properly 

observed. The experimental parameters for the roof support system 

were the water-to-solids ratio, which controlled the grout strength, 

and the spacing of the supports. The specifications for the

individual study areas are defined as follows: 

NIOSH SAFETY STRUCTURES TESTING LABORATORY

HEINTZMANN CORPORATION -- PUMPABLE CRIBS -- DEC 13, 2001

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0 1 2 3 4 5 6 7 8 9

VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, tons

Pumpable Crib -- 24 in diam -- Improved Bag (Blue)

Figure 2. Loading profile of 24-inch diameter (Heitech) 

pumpable roof support.

Figure 3. Location of instrumented tailgate supports in the 

study areas.
• Site A (conventional design) – 24 in diameter supports – 1.75 to 

1 water-solids ratio – 8 ft center-to-center row spacing.

• Site B (lower strength and softer design) – 24 in diameter 

supports – 2.00 to 1 water-solids ratio – 8 ft center-to-center 

row spacing.

• Site C (lower support density) – 24 in diameter supports – 1.75 

to 1 water-solids ratio – 10 ft center-to-center row spacing.

Mine and Tailgate Description

The Emerald mine, where the study was conducted, is located 

in Greene County in western Pennsylvania where it is mining the 

Pittsburgh coal seam. The immediate roof in the study area was 

typical for the Pittsburgh seam consisting of thinly interbedded 

rider coal and weak shale or claystone layers with a CMRR of 40. 

The seam height varied from 6.5 – 7.0 ft in the study area under a 

depth of cover of approximately 750 ft. The goal was to select the 

area with the highest depth of cover to evaluate the worst case 

overburden loading. The pillars in the three-entry gate roads were 

on 184-ft cross-cut centers and 100-ft entry centers (figure 3)

providing an ALPS stability factor of 1.82, compared to the 

minimum recommended stability factor of 1.2 for these conditions. 

The location of the study site was at blocks 15, 16, and 17,

positioning the instrumented supports approximately 2,700 ft from 

the end of the 10 North longwall panel. Site C was mined through 

first, followed by sites B and A respectively as the longwall

advanced.

Instrumentation Description

The instrumentation arrangement for each of the three study 

areas is shown in figure 4. Instrumentation was at the mid-pillar 

and intersections. A permissible Campbell Scientific multiplexor 

and data logger was used at each of the three test sites to provide 

continuous recording of the instrumentation. The cables for the 

sensors were run outby to allow monitoring of the instrumentation 

inby the face on Sites B and C while site A was monitored until the 

face area reached the intersection. The cables were laid against the 

coal pillar and covered with the grout used to fill the pumpable 

supports. This grout layer would protect them from damage due to 

rock falls and pillar sloughage. In the intersections, the cables were 

covered with a foaming agent, typically used to seal around the 

perimeter of ventilation seals and stoppings, to provide protection 

to the cables in these areas. 

Load was measured on five supports at each of the three study 

sites: (1) two adjacent supports at the mid-pillar area, (2) two 

adjacent supports in the entry at the intersection, and (3) one 

support in the crosscut corner at the intersection. Loading was 

measured by a 26-in-diameter hydraulic flatjack cell. The flatjacks 

were filled with water and calibrated to a pressure of 600 psi in the 

NIOSH Mine Roof Simulator. The flatjack was placed on top of 

the support prior to being fully filled with grout, and then it was 

pushed tightly against the mine roof as the support was completely 

filled. The flatjack was instrumented with a 1,000 psi pressure 

transducer and gage to record the pressure as the support load 

developed.

Roof-to-floor convergence was measured at each of the supports 

that were instrumented to measure support loading. The

convergence was measured approximately 1 ft from the pumpable 

roof support. Twelve-inch-long rebar bolts were grouted into the 

mine roof and floor to provide anchors which would eliminate 

movements of the skin of the mine roof and floor in the

convergence measurements. A displacement transducer was

mounted to the plate on the roof anchor and connected to a floor 

anchor by a wire to measure the roof-to-floor convergence. Roofto-floor convergence was also measured between the two, flat-jackinstrumented supports located at the pillar area and intersection to 

help evaluate the zone of influence of the pumpable support.

Roof deformations were also made near three of the

instrumented supports: (1) one in the pillar area, (2) one in the 

intersection area, and (3) one in the crosscut. Three-point 

extensometers were installed at levels of 2, 7, and 10 ft from the 

roof horizon in each location. These measurements were also made 

approximately 12 inches from the support. 

MATERIAL PROPERTY TESTS AND FULL-SCALE 

LABORATORY SUPPORT TESTS

Laboratory tests were conducted on 3 x 6 in grout samples 

collected during the pumping of the supports. The samples were 

tested by NIOSH approximately 28 days from the pouring of the 

supports and by an independent laboratory a week prior to the time 

the instrumented areas were mined through (approximately five 

months from the time when the samples were taken during the 

pumping of the supports). The averages of the three-sample test 

results are shown in table 1. As expected, the material in Site B 

exhibited the least strength because the water content in these 

supports was higher. Since Site A and C had the same water 

content, it was expected that they would produce similar

compressive strengths. This was true in the 28-day test conducted 

at NIOSH, but in the tests conducted when the longwall mined by 

Intersection Direction of 

face advance

Cross cut support

Intersection-inby 

support

Intersection-outby 

support

Pillar-outby support Pillar-inby support

Mid – Pillar Area

Pumpable Roof Support with Load 

Cell

3 Point Extensometer

Roof to Floor Convergence

Tailgate 

Direction of 

face advance

Figure 4. Instrumentation arrangement for each pumpable 

roof support area of study.
the instrumented areas, Site C exhibited a 21 pct higher strength 

than the average of Site A. Another unexpected result was that the 

material apparently lost strength from the tests in November, 2002 

to March, 2003. Since the tests were conducted at different labs, 

the difference in results may be due to sample preparation (i.e., 

sample size and end conditions). 

It is expected that the grout with the higher water content would 

not only lower the compressive strength of the grout, but also 

reduce its modulus of elasticity, thereby reducing the initial

stiffness of the full-scale support. Figure 5 illustrates the loaddisplacement response of full-scale supports tested in the NIOSH 

Mine Roof Simulator. It can be seen from this graph that the initial 

stiffness of the support with higher water content was lower. The 

lower support stiffness indicates that the support with a 2.00 to 1 

water-solids ratio would require more roof-to-floor convergence to 

provide the same load as the support with a 1.75 to 1 water-solids 

ratio. The supports with the higher water content also produced 

lower peak loads in this full-scale test, which is expected since the 

grout strength was lower. However, the water content does not 

influence the post failure characteristics of the support, since the 

bag confinement controls the post failure response.

NIOSH SAFETY STRUCTURES TESTING LABORATORY

HEINTZMANN -- PUMPABLE CRIBS -- NOV 9, 2001

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0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, tons

1.75 to 1 water-grout ratio

2.00 to 1 water-grout ratio

Figure 5. Loading profile of 1.75 to 1 and 2.00 to 1 watersolids ratio showing reduced stiffness and peak capacity of 

support with the higher water content.
Table 1. Summary of grout compressive strength tests for tailgate performance inby and outby the face.

Cure time is 28 days Test at time of longwall passing (Cure time is approx. 5 

months)

Site 

Identification

Avg Compressive strength, psi Standard Deviation Avg Compressive strength, psi Standard Deviation

Site A 1126 108 852 79

Site B 912 88 758 50

Site C 1272 64 1032 76

Table 2. Summary of support loading for all three instrumented support sites.

SITE A SITE B SITE C

Support ID

(see fig. 4)

Face 

loading, 

tons2

Pct of 

peak 

load3

Peak 

load, 

tons

Peak load 

(distance 

Inby, ft)

Face 

loading, 

tons

Pct of 

peak load

Peak load, 

tons

Peak load 

(distance 

Inby, ft)

Face 

loading, 

tons

Pct of 

peak 

load

Peak 

load, 

tons

Peak load 

(distance 

Inby, ft)

Pillar Area Supports

Pillar Inby 36 30 N/A 49 42 116 39 18 15 121 26

Pillar Outby 62 52 N/A 43453 55 97 42 12 10 N/A

AVG 49 41 51 49 106 40 15 13 121 26

STD 18 16 3 9 13 2 4 4

Intersection Area Supports

Intersection 

Inby 18 15 N/A N/A N/A 4 5 78 49

Intersection 

Outby 38 32 N/A 30 25 N/A 17 14 N/A

Xcut 25 21 N/A 17 29 58 67 17 16 108 53

AVG 27 23 24 27 58 67 9 12 93 51

STD 10 9 9 3 7 6 21 3

Summary of All Instrumentation

Overall 

AVG 36 30 43 35 38 90 49 12 12 102 43

Overall STD 17 14 21 14 30 15 6 5 22 15

2Face loading refers to the support loading when the support is at the longwall face as it is being mined. This represents the maximum outby 

loading on the supports.

3Peak loads a re the maximum loads measured underground, which always occur inby the longwall face. Peak loads were not obtained at Site 

A. The pct of peak load values for Site A are an approximation using an assumed peak load of 120 tons which is representative of the 

maximums observed in this underground study.

4Loading was approaching peak load of other supports at this distance inby the face.
MINE TEST RESULTS AND ANALYSIS

An assessment of the underground study is broken down into 

the following areas: (1) tailgate performance outby the face, and 

(2) tailgate performance inby the face. Table 2 provides an 

overall summary of the support loading and figures 6a, 6b, and 6c 

show the support loading as a function of the face position for 

each support in Sites A, B, and C respectively. Table 3

summarizes convergence measurements taken at the longwall 

face. The study applies only to the active tailgate loading, with 

inby and outby referenced to the face position (inby referring to 

the mined out or gob area and outby referring to the un-mined 

section of the panel). In all figures referring to face position, 

inby is denoted as a positive tailgate location and outby is 

denoted as a negative tailgate location with the face being 

represented as 0. The supports were not instrumented in time to 

collect data during the first panel mining prior to the entry 

becoming an active tailgate for the study panel.

Table 3. Summary of maximum outby convergence.

SITE A SITE B SITE C Support ID

(see fig. 4) Convergence, 

inches

Convergence, 

inches

Convergence, 

inches

Pillar Area Supports

Pillar Inby .457 .364 .209

Pillar Middle .332 .480 .265

Pillar Outby .392 .500 .267

AVG .394 .448 .247

STD .063 .073 .033

Intersection Area Supports

Intersection Inby .450 .639 .406

Intersection Middle .400 .352 .155

Intersection Outby .430 .881 .215

Xcut .250 .500 .336

AVG .383 .593 .278

STD .091 .225 .114

Summary of All Instrumentation

Overall AVG .387 .531 .265

Overall STD .074 .182 .084

Tailgate Behavior Outby the Face

Overa ll, the support loading outby the longwall face was 

minimal, and the tailgate conditions were excellent. The chart in 

figure 7 shows the average support load and convergence for 

each of the three test sites as the longwall face passed the 

instrumented supports. Site A and B had similar load

developments, approximately 35 tons on average, but the

convergence in Site B was on average higher than Site A. This is 

consistent with the pumpable crib material properties for Site B 

has the higher water content, which as previously shown, reduces 

the material modulus and initial stiffness of the support. As 

shown in figure 7, Site C exhibited considerably less loading and 

convergence outby the face than the other two sites. It is also 

seen that Site C felt the effects of the front abutment later (closer 

to the face) than did site A or B (figure 8). The response 

observed in Site C suggests that there was some difference in the 

ground conditions at Site C compared to A and B. It would be 

expected that Site C with the widest spacing (10 ft compared to 

8 ft for Sites A and B) and consequently the lowest support 

density, would have the highest loading and convergence. The 

precise differences in conditions remain unknown. Factors that 

could result in less convergence would include (1) less pillar 

yielding, and (2) more competent roof and/or floor. 

Unfortunately, the uncertainty of conditions makes a direct

comparison of the outby performance of Site C with Sites A and 

B more tenuous.

Analysis of the roof deformation data showed that there were 

essentially no significant separations of strata within the first 10 

feet of the roof outby the face in the instrumented areas. Some 

movement was detected at the 2 and 7 ft horizon as the face 

approached to within 25 feet of the instrumented supports in a 

few areas indicating some local roof loading was beginning to 

occur. The magnitude of the deformation at the face was still less 

than 0.05 inches, which is not considered to be significant in 

terms of roof stability or pumpable support loading. Figure 9 

shows the data from the mid-pillar area roof extensometer in Site 

Figure 6a. Support loading for Site A.

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE

SITE A -- 1.75:1 WATER-GROUT RATIO -- 8 FT SPACING 

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DISTANCE FROM FACE, feet

SUPPORT LOAD, tons

Pillar inby

Pillar outby

Intersection inby

Intersection outby

Cross cut

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE

SITE C -- 24 IN DIA -- 1.75:1 WATER-GROUT RATIO -- 10 FT SPACING 

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DISTANCE FROM FACE, feet

SUPPORT LOAD, tons

Pillar outby

Pillar inby

Intersection 

outby

Cross cut

Intersection 

inby

Figure 6c. Support loading for Site C.

Figure 6b. Support loading for Site B.

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE 

SITE B -- 24 IN DIA -- 2.00:1 WATER-GROUT RATIO -- 8FT SPACING

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DISTANCE FROM FACE, feet

SUPPORT LOAD, tons

Pillar outby

Pillar inby

Intersection 

outby

Cross 

cut

Intersection inby
C as an example of the most severe roof deformation outby the 

face.

Another trend was that the pumpable roof support loading in 

the mid-pillar area was considerably higher (87% on average for 

all three sites) compared to the support loading in the

intersections. Other mine sites have reported less closure in 

intersections compared to the pillar line in fair to good roof 

conditions which would account for the lower support capacity. 

In these cases, it appears to be a pillar influence situation where 

the stresses imposed by the pillar react on the roof and floor. At 

mid-pillar, the close proximity of the pillar/panel generates more 

closure than within the intersection where the open cross cut can 

not generate loading and consequently less roof-to-floor closure 

in this area. However, in this mine, the convergence in the 

intersections was comparable, actually slightly higher, than that 

observed in the pillar line. But the support response was softer 

and this is what resulted in the lower support loadings for the 

intersection supports compared to the pillar area supports. It 

appears that for whatever reason, the full roof-to-floor closure 

was not causing displacement of the supports in the intersection 

areas.

Tailgate Behavior Inby the Face

As previously discussed, the inby support behavior can be 

important if there is a need to maintain ventilation inby the face 

into the active longwall’s gob. While this is not always critical at 

the Emerald Mine, the study was designed to evaluate the inby 

loading behavior since this will provide valuable information for 

other mine sites. In addition, the inby behavior was studied to 

provide fundamental information about the capability of the 

supports to function beyond their peak capacities, which is a 

critical issue for this type of support system. Although an effort 

was made to protect the instrumentation from the coal and gob 

debris and falling roof rock, not all of the instrumentation 

survived the inby conditions.

The first question to answer was, “how far inby did the 

tailgate remain open?” Visual observations indicated that the 

tailgate remained opened for a distance of 75 to 100 ft inby the 

face. The peak support loading for the pillar area occurred 37 ft 

inby the face while the peak support loading for the intersection 

area occurred 56 ft inby the face. Therefore, it appears that the 

peak support loading occurred about half the distance that visual 

observations indicated that the roof had collapsed. 

A related question is, “at what load were the supports failing 

(exceeding their peak load capacity)?” In order to determine if 

the supports were failing or simply unloading prior to reaching 

their peak capacity, the support response must be plotted as a 

function of convergence. Figure 10 shows data from Site B. The 

pillar area supports exhibited a response similar to the full-scale 

lab tests, suggesting that these supports were failing although at 

lower loads. The following discussion is intended to evaluate 

this conclusion. The data presented in figure 10 represents active 

tailgate loading only, however, there was only about 8 tons of 

load remaining on the supports from the first panel mining, so 

this is not sufficient to account for the difference between the 

tailgate measurements and full-scale laboratory tests. Since the 

support loading data were collected every 30 minutes, it is 

possible that the true peak loads were not captured. However, the 

post failure loading which occurs over several hours is also 

proportionally lower than that observed in laboratory testing, 

suggesting the measured peak loading is fairly accurate. Hence, 

it appears that the conditions in the mine, most likely the roof or 

floor contact conditions and the resulting loading conditions were 

responsible for the lower capacity observed in the mine. It is 

likely that roof is producing eccentric loading of the pumpable 

support inby the face due to differential closure of the tailgate 

between the pillar side and caved gob material on the other side. 

Therefore, rather than a uniform stress as would be applied to the 

support by a “flat and parallel” roof and floor as would occur in 

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE

SITE C -- 24 IN DIA -- 1.75:1 WATER-GROUT RATIO -- 10 FT SPACING 

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

-50 -40 -30 -20 -10 0 10 20 30

DISTANCE FROM FACE, feet

ROOF DEFORMATION, inches

Pillar - 2 ft anchor

Pillar - 10 ft anchor

Pillar - 7 ft anchor

Figure 9. Example of the largest roof deformations outby the 

face. Overall, there were very little separations within the 

immediate roof.

PUMPABLE ROOF SUPPORT STUDY

EMERALD MINE -- INITIAL DATA STARTING FROM 13-NOV-2003 

0

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DISTANCE FROM FACE, feet

SUPPORT LOAD, tons

Site A

Site B

Site C

Figure 8. Site C felt the effects of the front abutment later 

(closer to the face) than did Sites A and B.

Figure 7. Average support load and convergence recorded at 

the three instrumentation sites.

0

5

10

15

20

25

30

35

40

SITE A SITE B SITE C

Support Load, tons

0

0.075

0.15

0.225

0.3

0.375

0.45

0.525

0.6

Convergence, inches

Support Load

Convergence
the laboratory, the pressure acting on the supports in the mine is 

most likely non-uniform with higher pressures acting on one side 

of the support than the other. However, while this may explain 

the lower peak load observed, it would be expected that the 

residual loading would not be as significantly affected by the 

non-uniform loading since once the material fractures, the

residual loading is controlled by the bag confinement.

The post failure loading is primarily a function of how brittle 

the grout is, how large the pieces are when it fractures, and the 

capacity of the bag to confine this fractured material. Hence, one 

would expect the post failure behavior in the mine to be similar to 

that observed in the laboratory. But here again, lower capacity 

was observed in the mine than was demonstrated in the

laboratory. The primary difference with the laboratory test that 

may account for the lower observed capacity is the loading rate. 

The full-scale tests conducted in the Mine Roof Simulator are at a 

constant displacement of 0.5 inches per minute. In the mine, the 

loading rates are more variable and likely to be much slower a 

large portion of the time. Typically higher load rates of brittle 

materials equate to higher loads since the mechanics of the crack 

growth governing the material failure are altered. This may also 

be contributing to the lower peak loading observed in the mine 

compared to the laboratory. A reduced loading rate may also 

explain the lower post failure behavior since the bulked grout 

material and the bag can creep under a more statically applied 

load. A similar effect is seen in displacement-controlled 

laboratory testing, where the support load drops considerably 

when the displacement is held (figure 11). Without the constant 

pressure applied by the load frame, the bulked grout material is

incapable of maintaining the same load. The other variable 

which has not been tested is the age of the grout. The full-scale 

laboratory tests have all been conducted within 30 days of the 

support pumping. If the material becomes more brittle with time 

and fails into larger more jagged pieces, the post failure capacity 

may be less than that observed in the lab. Rips in the bag could 

also be responsible for lower residual capacity. Although it was 

difficult to see well inby the face, no rips were seen.

This brings up another question, “what about the supports 

that exhibited essentially no residual loading?” Of the six

supports in which peak loading was measured, three appear to 

have no significant residual loading. Data was lost on another at 

a post failure load about 10 tons below the residual loading 

observed in the two pillar area supports. This suggests that this 

support probably was not going to have any significant residual 

loading as well. The lack of residual loading must equate to 

dissipation of the roof loading and no residual loading most likely 

equates to loss of roof control by the support. This may not 

necessarily imply an immediate roof fall, although that would 

certainly explain the absence of roof loading. The roof could be 

breaking up to the point where it is no longer acting as a load 

bearing member that can transfer load to the roof support. 

An examination of the roof deformation data may help to 

resolve this issue. Figure 12 shows the roof deformation data for 

the pillar supports in Site B plotted in conjunction with the 

support loading. It is seen that the roof deformation, which was 

mostly limited to the lower 2 ft of roof, did not begin to occur 

until the peak loading of the support was exceeded. The grout in 

the bag fractures into blocky pieces when the pumpable support 

reaches its peak load carrying capacity. As the bag tries to 

confine the grout, the bag bulges and allows the support to 

compress. This can account for the lowering of the immediate 

roof. Once the bag develops sufficient confinement and develops 

some post residual load capacity, the movement of the immediate 

roof tends to slow down as denoted by the change in slope of the 

roof deformation curve as the support goes further inby the face. 

A different behavior was observed for the other supports which 

exhibited no residual loading. Two things appear to be different: 

(1) the roof movement occurred prior to the peak loading of the 

support, indicating it was contributing to the loading of the 

support, and (2) the separation or movement occurred higher up 

(to 7 ft) within the roof. Examination of figures 13 and 14 

indicate that the immediate roof is delaminating prior to the 

supports reaching peak loading. This might also suggest that the 

roof had broken up and essentially lost its capability to transfer 

load which caused the reduction in support loading rather than 

the failure of the support grout. However, since there is not 

sufficient deformation after the support peak loading to validate 

this claim, and since the peak loading of the supports are similar 

to the supports in Site B which did exhibit a residual capacity, it 

is still likely that the supports were indeed failing (loaded beyond 

their peak capacity). 

The cross cut support in Site B which exhibited the low peak 

loading (60 tons compared to 120 tons for the other nearby 

supports) shows an interesting behavior (figure 15). Here again 

the roof movements at the 2-ft and 7-ft anchor showed roof 

movement early in the support loading cycle and this movement 

caused an increase in the support loading. However, the support 

increase in support loading was not as dramatic (3 tons increase 

in support loading for a 0.19 inches or roof movement) as that 

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE 

SITE B -- 24 IN DIA -- 2.00:1 WATER-GROUT RATIO -- 8 FT SPACING

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4 5 6 7 8

CONVERGENCE, inches

SUPPORT LOAD, tons

`

Full-scale lab 

Pillar area 

supports Cross cut 

support

Intersection support

Figure 10. Load-convergence plots of supports in Site B 

compared to full-scale laboratory test.

HEINTZMANN CORPORATION -- PUMPABLE CONCRETE SUPPORT

NIOSH SAFETY STRUCTURES TESTING LABORATORY

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8

DISPLACEMENT, inches

SUPPORT LOAD, tons

30 In diameter support

Drop in capacity when 

displacement was held.

Figure 11. Load drops when displacement is held in this 

full-scale laboratory test.
observed in figure 12 for the pillar area support, where a very 

small increase in roof movement (0.02 inches) resulted in an 

immediate and large increase in support loading (22 tons). This 

most likely accounts for the low support capacity observed in this 

support, but why it happened in this particular support remains 

unknown. It may be that the sudden roof movement damaged the 

support in some way that degraded its capability to develop 

loading. The roof movement decelerated following this initial 

large increase, and during this period the support loading

accelerated until its peak capacity was reached. Again, this 

suggests that the support was losing its capability to sustain 

loading during this period.

The roof-to-floor convergence inby the face was consistent 

with the load development in the support. Essentially, the 

convergence followed the gradual nonlinear increase that began 

when the abutment effects were felt outby the face, continuing 

inby the face for some 25-50 ft (depending on whether it was an 

intersection or along the pillar), and then rose sharply for about 

10-20 feet of additional face advance. Measured convergence in 

excess of 5 inches was occurring well inby the face.

DISCUSSION OF PUMPABLE ROOF SUPPORT DESIGN 

CRITERIA ISSUES

The basic design requirements for any standing roof support 

system entail three factors (1) stiffness, (2) maximum capacity, 

and (3) yield and residual loading capability. A general

assessment of design criteria relative to these three basic

requirements will be evaluated based on what was learned in this 

study. 

Stiffness

The pumpable roof support is one of the stiffest standing 

support products currently available for longwall tailgate

installations (6). The 24-inch-diameter pumpable support as used 

in this study develops nearly 150 tons of load carrying capacity in 

about 0.5 inches of displacement. This compares to a four-point 

wood crib which would develop only about 10 tons, an order of 

magnitude less load capacity, at 0.5 inches of displacement. A 

high stiffness is thought to be beneficial in load-controlled 

environments where the support capacity can limit (control) the 

deformation of the immediate roof and associated convergence of 

the tailgate. Overall (average for all supports), the pumpable roof 

support system in this study provided a support load density of 

4.38 tons per foot of roadway at the longwall face at a

convergence of less than 0.45 inches. This is relatively low, 

indicating that not much support loading was needed to provide 

good roof control. However, “was the good roof control due to 

the high stiffness of the support?” Two observations from the 

study may suggest that this is true. First, it appears from the 

larger convergence observed in Site B with the lower modulus 

grout compared to Site A with the stronger and higher modulus

grout, that the stiffness of the support may have had some effect 

in reducing overall convergence. Second, there were essentially 

no significant separations in the immediate roof strata outby the 

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE 

SITE B -- 24 IN DIA -- 2.00:1 WATER-GROUT RATIO -- 8FT SPACING

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

-100 -50 0 50 100 150 200

DISTANCE FROM FACE, feet

ROOF DEFORMATION, inches

0

12

24

36

48

60

72

84

96

108

120

SUPPORT LOAD, tons

Support load

2 ft roof anchor 

7 and 10 ft anchors

Figure 12. Roof deformation data in conjunction with 

support loading show that roof deformation did not begin 

until the peak loading of the support was exceeded.

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE

SITE C -- 24 IN DIA -- 1.75:1 WATER-GROUT RATIO -- 10 FT SPACING 

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

-50 -40 -30 -20 -10 0 10 20 30 40 50

DISTANCE FROM FACE, feet

ROOF DEFORMATION, inches

0

15

30

45

60

75

90

105

120

135

SUPPORT LOAD, tons

Support load

7 ft anchor

2 ft anchor

10 ft anchor

Figure 13. Roof deformation appears to be contributing to 

the loading of this support.

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE

SITE C -- 24 IN DIA -- 1.75:1 WATER-GROUT RATIO -- 10 FT SPACING 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

-100 -50 0 50 100

DISTANCE FROM FACE, feet

ROOF DEFORMATION, inches

0

15

30

45

60

75

90

105

120

135

150

SUPPORT LOAD, tons

Support load

7 ft roof anchor

2 ft and 10 ft 

roof anchors

Figure 14. Another example of roof deformation occurring 

during the support development with no significant post 

failure load capacity in the pumpable roof support.

PUMPABLE ROOF SUPPORT STUDY -- EMERALD MINE 

SITE B -- 24 IN DIA -- 2.00:1 WATER-GROUT RATIO -- 8FT SPACING 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

-100 -50 0 50 100 150

DISTANCE FROM FACE, feet

ROOF DEFORMATION, inches

0

10

20

30

40

50

60

70

SUPPORT LOAD, tons

Support 

Load

2 ft 

anchor

7 ft anchor

10 ft 

anchor

Figure 15. A different loading behavior appears to have 

taken place in this support which exhibited low peak loading 

and no significant residual load.
face in any of the pumpable roof support areas. Longwall 

tailgates previously supported by wood cribs were reported to 

have poorer ground conditions outby the face than were observed 

at these sites (5). Hence, it appears that the stiffness of the 

pumpable support may be a contributing factor improving ground 

control. There is some quantitative data to suggest this, but a 

control study on a significantly softer support system such as 

wood cribbing was not conducted. In the study panel, a section 

of four-point cribs in the tailgate at the completion of the panel

approaching the recovery area also provided good ground control 

conditions based on visual observations. This might suggest that 

the loading environment was displacement-controlled and that 

the support was contributing little to the ground conditions, but 

convergence measurements were not made and since the loads 

were small, visual observations are not likely to be accurate 

enough to make this claim. 

The support response inby the face is even more ambiguous 

and requires an understanding of the mechanics of the roof

activity. As the longwall passes, the immediate roof over the 

longwall panel caves behind the longwall shields. The immediate 

roof, which was previously spanning across the tailgate to the 

gate road pillars, tends to shear off in the tailgate entry near the 

row of standing supports as the gob forms behind the face. The 

remaining immediate roof is acting as a cantilevered beam from 

the coal pillar, with additional support provided by the standing 

roof support. The stiffness of the pumpable roof support may be 

beneficial in preventing deformation of this cantilevered roof 

beam and separations of the laminated strata within the

immediate roof. However, the intermediate or main roof is still 

bridging across the entry to the gob, but since the gob is 

significantly softer than the pillars, there is likely to be

considerable ground movement, and most likely the convergence 

associated with this will be uncontrollable by the pumpable roof 

support system. Therefore, there would appear to be conflicting 

support stiffness requirements relative to the deformation to be 

controlled. The ideal design would be to match the stiffness of 

the support to respond to the intermediate or main roof behavior. 

The ideal design would allow the support to yield due to the 

convergence which will occur irregardless of the presence of the 

pumpable roof support without overstressing the immediate roof 

or allowing large separations in the immediate roof to occur. The 

pumpable support functions well until it is “pushed” beyond its 

peak capacity, where load shedding appears to cause the support 

to lose control of the roof in some cases. If the loading is 

displacement-controlled, then the peak loading cannot be

prevented from occurring, but could be delayed if the support 

response was softer. However, as previously described, a softer 

support response outby the face may cause more problems and 

offset any advantage that might occur inby the face. It is also 

possible that high stiffness coupled with the capacity of the 

support may overstress the immediate roof during the load

development causing it to break up or “yield” above or in the 

vicinity of the supports, which may contribute to the inability of 

the support to maintain roof control when the peak loading is 

exceeded. 

Maximum Capacity

Historically, the maximum capacity requirement for a

standing roof support system has been equated to the weight of a 

detached block of roof rock where separations in strata occur 

above the bolted horizon. Although this factor should be

considered in the determination of the maximum support capacity, 

this study clearly indicates that such a failure may be the 

exception rather than the rule. The maximum load developed by 

the pumpable roof supports in this study outby the face was on 

average about 50 tons (for the pillar area supports), or about 40 

pct of the available capacity of the support. More appropriately 

quantified, the support load density provided by this arrangement 

of supports was 6.25 tons/ft of tailgate. Does this mean that this 

is the maximum load requirement for all pumpable roof supports 

or for any standing support? Although the generic answer is no, 

this remains a difficult question to definitively answer. If the 

stiffness of the support is indeed helping to maintain the integrity 

of the immediate roof beam (meaning load-controlled roof

behavior), then a softer support system that allows more roof 

deformation would require a higher ultimate capacity if the added 

deformation leads to additional roof weighting. In a purely 

displacement-controlled loading environment where the

convergence is uncontrollable and the support is simply reacting 

to the closure of the tailgate, a stiffer support system would also 

require higher capacity since it would be more heavily loaded 

from convergence than a softer support. Some examples might 

help to clarify this discussion. If four-point wood cribs would be 

required to provide 6.25 tons per ft of entry support capacity at 

0.45 inches convergence to control an overhead block of roof 

rock, two rows of cribs made from 6x6x36-inch, mixedhardwood timbers, wedged tightly against the mine roof with 

hardwood wedges, would need to be installed on a center-tocenter spacing of about 48 inches (half the distance of the 

pumpable supports) (6-7). In other words, four times as many 

wood cribs compared to pumpable supports would be required. 

However, if a four-point wood crib was used to replace the 

pumpable roof support (i.e. same arrangement as the current 

pumpable roof support system at Emerald), then convergence of 

approximately 3.5 inches upon the wood crib would be needed to 

generate 6.25 tons of support capacity per ft of tailgate. If only a 

full displacement-controlled loading environment is assumed,

then at the 0.45 inches of convergence measured in this study, 

four-point wood cribbing installed in the same arrangement as the 

pumpable roof supports would only provide approximately

1.5 tons of support capacity per ft of entry. This represents a 

capacity capable of supporting only the first foot of roof rock. 

Yield Capability

One of the goals in the development of the pumpable roof 

support system was to maintain a useful residual load carrying 

capacity through several inches of convergence. From this study, 

it appears that this is not critical for roof control outby the 

longwall face, at least in these conditions. Less than 50 pct of the 

available support capacity was used outby. However, this again 

brings up the issue of whether the ground activity is

displacement-controlled or load-controlled. Since longwall

mining is likely to be a combination of both, then there are likely 

to be conditions where the support will be “pushed” beyond its 

peak capacity. While this did not happen in this study outby the 

longwall face, the maximum capacity is exceeded inby the face 

and the inability of the support to sustain its peak loading appears 

to have significant consequences on the supports capability to 

maintain roof control under these conditions. 

As the previous data analysis of the support behavior inby the 

face showed, the load shedding of the support once the grout 

material fractures at the peak capacity of the support allows the 

roof movement to increase causing additional separations and 

delaminating of the immediate roof structure. Since the

immediate roof is now heavily loaded by the uncontrollable 

convergence of the main roof deforming from the gob formation, 
the acceleration of the immediate roof deformation by the loss of 

support capacity appears to result in failure of the immediate roof 

beam. By the time the support is able to re -establish its residual 

load capacity, it is too late. The roof beam has already been 

destroyed and the functional capability of the support to provide 

roof control is now lost. 

It is also important to understand that this is (assuming 

displacement-controlled roof activity) a yield capability problem 

and not just a peak capacity problem. If the convergence is 

uncontrollable, then the support will be “pushed” beyond its peak 

capacity regardless of the peak capacity. However, the support 

must to be able to sustain some load carrying capacity to support 

the immediate roof as the convergence continues. Therefore, 

yield capability is important, particularly in the case of the inby 

loading situation. In this regard, it should be noted that

premature failure of stiff fiber-concrete cribs outby the face 

occurred at this mine at higher depths of cover (900+ ft). 

Unfortunately, the lack of any residual load capability in these 

supports resulted in tailgate ground falls outby the face (5). If the 

peak loading of the pumpable support was exceeded outby the 

face, poor roof conditions may also occur. 

SUMMARY AND CONCLUSIONS

Pumpable roof supports have done a good job of providing 

roof control outby the longwall face at the Emerald mine. For the 

past 2-3 years, efforts have been ongoing to optimize the support 

design by reducing the size of the support, the water content in 

the grout mix, and most recently the spacing of the supports. 

This study was conducted to help provide answers to the impact 

of these optimization efforts and to provide insight into

fundamental design issues for pumpable roof support systems.

The study clearly shows that at this mine a 24-inch diameter 

support is fully capable of providing adequate ground control 

under depths of cover of 750 ft as only 50 pct of the available 

support capacity was utilized outby the longwall face. It was also 

shown that the 2.00 to 1 water-solids ratio, despite providing a 

slightly softer support response, is sufficient for maintaining the 

same degree of roof control provided by the traditional 1:75 to 1 

grout mix. The 10-ft spacing of the supports also did not cause 

any problems outby the face, but as noted in the paper, the load 

development in the 10-ft spacing area was lower than expected 

suggesting there may have been a difference in ground conditions 

at this site that prohibit a direct comparison of this site with the 

other two test sites.

An evaluation of the inby performance raised more questions 

than it answered; partly due to the difficulty in maintaining data 

transmission in this environment. Visual observations indicated 

that the tailgate remained open somewhere between 75 and 100 ft 

inby the face, while peak support loading occurred less than 50 ft 

inby the face. Several supports (at least half of those where peak 

loading was observed) did not develop any significant residual 

loading. This suggests that these supports had lost their

capability to provide roof control. It appears from the roof 

deformation data that the large load shedding behavior of the 

support following peak loading allows the immediate roof to 

separate. When this happens, the support is unable to regain 

control of the roof in time to prevent failure of the immediate 

roof in many cases, particularly when the roof separations begin 

to occur during the loading of the support. If true, this has 

significant consequences on the support design for application in 

these conditions. First, considerable effort has been made to 

ensure that the support is capable of providing a residual loading 

through several inches of convergence. If the load shedding 

following peak loading cannot be significantly decreased, and it 

is unlikely that it can be with the current system, than it is 

unlikely that this support will provide full control of a thinly 

laminated weak roof inby the longwall face. Given these

circumstances where the support is not achieving control of the 

roof, the need for any residual loading capacity is questioned. If 

the requirement for residual loading is dropped, then the bag can 

simply act as a form to fill the support and need not confine the 

material once it fails. Relaxation of the need for residual loading 

capacity may also allow other grout materials to be used. 

However, it should also be stated that some supports did develop 

residual loading and provide some degree of roof control. Hence, 

it is difficult to say with certainty that no residual loading 

capability on all of the supports would not result in poorer ground 

conditions than were observed here. 

Perhaps the more important question is whether a support 

system that does not shed load when yielding would provide 

significantly better roof control. This question brings back the 

primary objective expressed in the title of the paper of

determining the ground reaction curve for these conditions. 

Although there were some apparent differences in the stiffness of 

the three systems evaluated in this study, the differences were not 

large enough to define a ground re action curve. In order to 

achieve this goal, other support systems will need to be included 

in such a study. One possibility would be evaluate the behavior 

of a Can support, which unlike the pumpable roof support has 

sufficient confinement capability to sustain its load carrying 

capacity once the peak loading is exceeded. Evaluating the 

performance of the Can support would determine if the unloading 

of the pumpable support inby the face was indeed the result of the 

load shedding characteristic of the support, and more importantly, 

if roof control could be maintained for a longer period of time or 

greater distance inby the face. 

For the outby conditions, most people would agree that the 

low convergence was largely responsible for the excellent

conditions that were observed outby the face in these weak roof 

conditions. But, how significant is the stiffness of the support in 

controlling the roof outby the face? With less than 0.5 inches of 

convergence observed in this study, the two extreme possibilities 

are presented. If the loading is completely displacementcontrolled, then only 0.5 inches of convergence would occur no 

matter what support was used and it could be argued that for roof 

control outby the face, no standing support is needed. On the 

other hand, if the environment is load-controlled, then the

stiffness of the support most likely played a critical role in 

maintaining the convergence to such a low level. Again, defining 

the ground reaction curve would answer these questions. 

Although the Can support is softer than the pumpable support 

used in this study, a support system softer than the Can would be 

preferred to evaluate the ground reaction curve outby the face. A 

conventional 4-point wood crib would be useful in this regard. 

ACKNOWLEDGEMENTS

The authors wish to acknowledge Chester Huff, Sr. Mining 

Engineer for RAG Emerald Resources, for his contribution in 

coordinating the underground activities including overseeing the 

installation of the instrumentation and data collection. 

Acknowledgement is also given to John Marshall, Mining
Engineering Technician, NIOSH, for his work and dedication in 

installing the instrumentation and collecting the data.

REFERENCES

1. Barczak, T.M. Material Handling Considerations for

Secondary Roof Support Systems. U.S. Department of

Health and Human Services, Public Health Service, Center 

for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, IC 9453, 2000, pp. 165-193.

2. Barczak, T.M., Mucho, T.P., Dolinar, D.R., Bower, J. and 

Bryja, J. Longwall Tailgate Support: Consideration, Design, 

and Experience. Proceedings, Longwall USA, 1999, pp.79-

105.

3. Mucho, T.P., Barczak, T.M., Dolinar, D.R., Bower, J. and 

Bryja, J. Design Methodology For Standing Longwall

Tailgate Support. Proceedings, 18th International Conference 

on Ground Control in Mining, 1999, pp. 136-149.

4. Barczak, T.M. Longwall Tailgates: The Technology for Roof 

Support has Improved But Optimization is Still Not There. 

Proceedings now being printed.

5. Chen, J., Mishra, M., Zahl, E., Dunford, J. and Thompson, R. 

Longwall Mining-Induced Abutment Loads and Their

Impacts of Pillar and Entry Stability. Proceedings, 21st 

International Conference on Ground Control in Mining,

2002, pp. 11-17.

6. Barczak, T.M. Optimizing Secondary Roof Support With 

The NIOSH Support Technology Optimization Program

(STOP). Proceedings, 19th International Conference on 

Ground Control in Mining, 2000, pp. 74-84.

7. Barczak, T.M. Updating the NIOSH Support Technology 

Optimization Program (STOP) with New Support

Technologies and Additional Design Features. Proceedings, 

20th International Conference on Ground Control in Mining, 

2000, pp. 337-347.