COMPUTER SIMULATION OF GROUND BEHAVIOUR AND ROCK BOLT INTERACTION 

AT EMERALD MINE 

Winton J. Gale, Director 

Strata Control Technology
 

Wollongong, NSW, Australia 

Christopher Mark, Section Chief, Rock Mechanics
 

David C. Oyler, Mechanical Engineer
 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory
 

Pittsburgh, Pennsylvania USA 

Jinsheng Chen, Senior Engineer 

RAG American Coal Holding, Inc.

Waynesburg, Pennsylvania USA 

ABSTRACT 

A collaborative project between RAG Emerald Mine, NIOSH, 

and SCT Operations was conducted to investigate ground 

behaviour, reinforcement performance, and stress redistribution in a 

coal mine entry subjected to a severe horizontal stress 

concentration. Field measurements indicated that the stresses 

applied to the study site nearly doubled during longwall mining, 

resulting in roof deformations extending to a height of 4.8 m (16 ft) 

above the entry. 

This paper focuses on the computer simulation that was 

undertaken to provide more insight into the roof behaviour and 

rock bolt interaction during mining. The model’s input rock 

properties were derived from extensive laboratory testing, and the 

model itself simulated a broad range of failure mechanisms. The 

effects of different bolt patterns on roadway behaviour were 

evaluated. Comparison between the model results and the field 

measurements indicated that that the model effectively simulated 

the critical elements of the actual roadway’s behaviour. With the 

confidence gained, the model was used as a baseline for additional 

simulations that evaluated the expected performance of alternative 

roof support systems. The study will also provide a benchmark 

data set for future applications of numerical modelling to U.S. coal 

underground mining. 

INTRODUCTION 

The National Institute for Occupational Safety and Health 

(NIOSH) at PRL together with RAG Pennsylvania and SCT 

Operations of Australia collaborated to conduct an extensive study 

of roof bolt strata interaction at Emerald Mine. The goals of the 

study were to: 

•	 Determine baseline U.S. rock mass and stress properties for 

input into numerical models; 

•	 Evaluate the performance of U.S. primary supports (roof 

bolts) in typical U.S. geological conditions; 

•	 Investigate the interaction between supplemental support 

systems (cable bolts) and primary supports, and; 

•	 Explore the ability of numerical modelling to aid in 

analyzing complex ground control problems. 

The study is part of a broader effort to develop methods to 

optimize roof bolt designs and prevent roof falls in a range of 

geologic and stress conditions encountered in U.S. underground 

coal mines. 

The site of the study was the tailgate of the 11 North longwall 

at the Emerald Mine, located in Greene County, Pennsylvania 

(figure 1a). The site was chosen because it was anticipated that the 

extension of 11 North beyond the start line of 10 North would 

result in a significant horizontal stress concentration (Mark et al., 

1998). From past experience at Emerald Mine, a horizontal stress 

window like the one created by 11 North could be expected to 

cause severe loading to be applied to the crosscut and tailgate entry. 

The location thus provided a unique opportunity to study the roof 

failure process as the applied horizontal stress increased during the 

progression from development through longwall mining. 

Figure 1a. Emerald Mine and study site. 
At the site, two monitoring arrays were installed in a crosscut 

and a third in the adjacent tailgate entry (figure 1b). The two 

crosscut sites, labelled Site B and Site C, are the focus of this 

paper. Conditions in these sites were more severe than in the 

tailgate site because the crosscut was oriented less favourably 

relative to the regional maximum horizontal stress. 

 

 

 

 

 

 

 

 

DOOR 

B array 

N H2 H3 H4 

H1 HI Cells
 H6
 H7 

Vertical H5 

Corehole C array Surface 

Corehole 

Stopping (overburden 198 m) 

0 5 

Scale, m T array Pumpable Cribs -35 -30 -25 -20 -15 -10 -5

05

101520

11 North Panel 

Figure 1b. Detail of study area. Distances shown are meters 

from the T array. 

Instrumentation at the sites included mechanical and sonic 

extensometers for measuring roof movement, instrumented roof 

bolts, and three-dimensional roof stress cells (HI cells). Details of 

the results of the monitoring program have been reported elsewhere 

(Oyler et al., 2004). 

The aim of this paper is to report the results of computer 

modelling and its relationship to the field site. Computer modelling 

was undertaken by SCT Operations to simulate the deformation 

mechanics of the strata and the interaction of bolting patterns with 

roof control. 

DESCRIPTION OF THE FIELD SITE 

Emerald Mine operates in the Pittsburgh coalbed in Southwest 

Pennsylvania, cutting a roadway approximately 2.1-2.4 m (7-8 ft) 

high and 4.9 m (16 ft) wide. Approximately 0.3m (1 ft) of roof 

shale is cut in the roadway. 

Primary roof support used in the area of this study consisted of 

three 22 mm (7/8 in) diameter, 2.4 m (8 ft) long combination bolts. 

They were installed with 1.2 m (4 ft) resin cartridges in a 35 mm 

(1-3/8 in) borehole. The yield load of the bolts is 19 tonne 

(21 tons) with an ultimate capacity of 28 tonne (31 tons). At the Csite, supplemental support consisting of rows of three cable bolts 

were installed between the rows of the primary bolts. The cable 

bolts were 3.6 m (12 ft) long, 15 mm (0.6-in) diameter, and 

partially grouted with 1.2 m (4 ft) of resin. 

The stress field has been measured in several locations in the 

Emerald and Cumberland Mines, but not at the specific study site. 

These measurements were reviewed to assess the range of stresses 

anticipated at the test site. The major stress was oriented N70°E. 

The regional stress field appears to be relatively consistent, with a 

lateral tectonic strain (Dolinar, 2003) of approximately 

550 microstrain. This means that the maximum stress is inferred to 

be approximately 11 MPa (1,600 psi) for a rock having a Young’s 

Modulus of 20 GPa (3 million psi). The horizontal stress within 

other rock units will be different and dependent on their elastic 

properties. The minor horizontal stress is estimated to be 

approximately half of the major stress. 

Vertical stress is approximately 5 MPa (700 psi) and related to 

overburden of approximately 200 m (650 ft). 

These have been assumed to be the background field stresses at 

the site for the purposes of the modelling study, however, past work 

indicates that significant local stress variation can occur. 

The magnitude of horizontal stress affecting roadways will vary 

depending on the direction of the roadway, and the mining induced 

stresses redirected about extraction panels. As part of this study, a 

range of horizontal and vertical stresses were applied to the model 

to explore the effects of such variation. 

 

 

 

 

 

 

 

 

Redstone 

5.5 Limestone 

5.0 Claystone 

4.5 Claystone/ 

Fireclay RQD 40% 

4.0

Fireclay RQD 36% 

w/thin 3.5 Ls, slips, 

Depth, m

& clay RQD 50% 

3.0
 veins 

Gray RQD 0% 2.5 Shale 
 

Black RQD 89% 

2.0 shale 

w/coal streaks 

1.5 Bone RQD 27% 

Rider RQD 29% 1.0 Coal 

Claystone 

0.5 w/coal RQD 0% 

Coal 

0.0 

Figure 2. Composite core log from the study site, from the 

vertical corehole drilled in the study crosscut and from a 

nearby surface corehole. 

A geologic column of the mine roof obtained from a vertical 

core hole drilled at the site is shown in figure 2. The roof may be 

roughly divided into three units: 
 

 

 

 

 

•	 A sequence of coals and weak, slickensided black shales in 

the lowest 2.7 m (9 ft); 

• A slightly stronger grey claystone sequence from 2.7 to 

5.4 m (9 to 18 ft), and; 

• A significantly stronger limestone above 5.4 m (18 ft). 

The low uniaxial compressive strength and RQD for the bolted 

horizon results in an estimated Coal Mine Roof Rating (CMRR) of 

37 (Mark et al., 2002). 

MODELING APPROACH AND MODEL USED 

1

The multi stage triaxial test aims to define the rock strength envelope for a 

number of confining pressures (2, 5, and 10 MPa) using a single sample. A 

stiff, servo controlled testing machine must be used in the deformation 

control mode. The sample is loaded progressively until the onset of initial 

fracture, and then the confining pressure is increased to the next stage and 

the test continues. At 10 MPa confining pressure the sample is taken to full 

failure and then the confining pressure is released slowly and the residual 

strength monitored to determine the post failure strength envelope. When 

angled core samples are tested, the failure takes place along a bedding 

plane, and the applied stresses are resolved into the shear and normal 

stresses as shown in figure 3. An early application of multi-stage triaxial 

testing is described in Dolinar et al. (1982). 

Detailed monitoring studies conducted in coal mines in a 

number of countries have shown that the mechanisms of failure 

about excavations can be highly complex, involving fracture of 

rock, failure of bedding or joints, buckling of parted rock, and slip 

along weak surfaces (Gale et al., 1992; Gale and Tarrant, 1997; 

Mark et al., 2000). The use of computer simulation therefore 

requires a detailed geotechnical characterization of the strata and 

stressfield, and must incorporate the many potential failure 

mechanisms. 

Beginning more than a decade ago, SCT has undertaken 

computer simulations at a wide variety of mine sites. Examples of 

the method have been published (Gale, 1998; Sandford, 1998; 

Kelly et al., 1998). SCT has found that model results which 

compare well with field validation measurements can be achieved 

if sufficient care and detail is employed. 

The input rock properties for the computer models are 

developed from detailed geotechnical testing of strata properties. 

The rock intact and post failure strengths, stiffness, in situ stresses, 

permeability and bedding plane characteristics are key factors to be 

quantified. The aim is to allow the model to simulate a wide range 

of interacting and complex failure modes in a manner which 

maximizes its independence from input generalisations. 

Generalisation of the rock properties or the rock mass section on 

the basis of averaged properties has been found to limit the 

capability to reproduce actual rock mass behavior. 

The code used in the model is FLAC which has been modified 

to employ rock failure routines developed by SCT Operations. The 

constitutive models used by SCT are very similar to the strainsoftening, ubiquitous joint model (SU) included with the latest 

release of FLAC2D (HCItasca, 2000). Rock failure is based on 

Mohr-Coulomb criteria relevant to the confining conditions within 

the ground. A broad range of potential failure modes are simulated 

including: 

• Shear fracture of intact rock; 

• Tension fracture of the rock; 

• Bedding plane shear, and; 

• Tension fracture of bedding (bedding separation). 

The stability of pre-existing jointing, faults or cleat is also 

addressed in the simulations where appropriate. The model 

simulates new or re-activated rock fracture and stores the 

orientation of the fractures. 

In the SCT constitutive model, as well as Itasca’s SU, the intact 

rock matrix exhibits strain-softening post-failure behaviour. A 

weakness plane of any orientation is also included in the model, 

and this weakness plane can also exhibit strain-softening behaviour. 

This constitutive model is most appropriate for coal measure rocks 

where the intact rock is strain-softening and one dominant 

weakness plane exists, namely bedding. In both SCT’s and Itasca’s 

constitutive models, cohesion, friction angle, dilation angle and 

tensile strength are specified as functions of the “plastic strain.” 

Depending on the nature of these functions, a variety of complex 

hardening and softening behaviors can be produced. 

Rock properties for the models were obtained from a 

combination of previous testing at the mine and from tests of 

underground core samples obtained from the study site. Of 

particular importance were angled core that were subjected to 

multi-stage triaxial testing1

 in order to determine bedding plane 

strength. The bedding plane strength test results are presented in 

figure 3 for the two shale units. 

Figure 3. Bedding plane test results from angled core. 

The in situ strength of the rock materials is reduced to 0.58 of 

the laboratory unconfined compressive strength (UCS). This labto-field scaling factor was originally suggested by Hoek and Brown 

(1980), and is routinely used in all SCT model studies. 

The model geometry is presented in Figure 4 with the various 

rock layers characterized by their laboratory UCS. The typical 

element size in the region of interest is approximately 20 cm by 

10 cm (8 in by 4 in). This element size is fine enough to capture 

geologic variations that may be important at the coal mine entry 

scale. The UCS profile and bedding cohesion profile used in the 

model is presented together with the laboratory core test results in 

figures 5 and 6 respectively. The layer-to-layer variations in the 

model profiles were derived from both the test results and 

inspection of the roof core. 

The in situ strength of coal within the model was 6.5 MPa 

(900 psi), which is typical of the bulk strength of coal. Coal cleat 

was included in the model. 

Rock bolts were included in the model. The bolts were bonded 

in the upper 1.2 m with a free length to the roofline. Yield of bolts 

was 19 tonne (21 tons) with a pullout force of approximately 

15 t/ft. Cable bolts were also modelled as partially bonded with a 
 

 

yield capacity of 25 tonne (27 tons). Care was taken to simulate 

the shear strength and stiffness properties of the resin/rock 

interfaces of the bolts. All of the bolt properties in the 2-D model 

were adjusted to account for the row spacing. 

Figure 4. Model geometry and UCS of rock units. 

Figure 5. UCS profile used in the model together with rock 

property data derived from testing. 

Figure 6. Bedding plane cohesion profile used in the model 

together with rock property data. 

The modelling sequence followed the actual mining process in 

that the entry was first excavated, then the outside roof bolts were 

installed, and finally the center bolt was installed. The model was 

allowed to respond to each of these steps. If cable bolts were used 

in a model, they were placed last. 

The stress path modelled is presented in figure 7 together with 

the anticipated in situ stresses. The stress path represents the farfield (boundary condition) stress changes (horizontal and vertical) 

applied to the study site due to the extraction of the two longwall 

panels. The modelled stress path was estimated from the stress 

measurements made at the site and measurements made at similar 

sites in past studies. One indication of the “far field” stress 

increase associated with longwall mining at this site was the 

approximately 10 MPa (1,400 psi) of stress relief that was 

measured following the passage of the longwall face (Oyler et al., 

2004). 

Figure 7. Stress path modeled representing increasing stress 

during development and longwall extraction operations. 

For this model, a plane of symmetry was used at the roadway 

centre line. The use of symmetry speeds the modelling process 

particularly in this case where a range of stress conditions or 

support systems is to be evaluated. 

Figure 8. Comparison of modeled and measured roof 

displacement profiles. 
Figure 9. Roof deformation in the model at three stress levels. 

 (A) 11 to 12 MPa, (B) 16 MPa and (C) 19 MPa. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

RESULTS AND COMPARISON WITH SITE DATA 

The model is an estimate of the strata response to the 

incrementally increasing far-field applied stresses that are believed 

to have occurred at the site. Its purpose is to obtain a greater 

understanding of the behaviour of the strata. Comparison of the 

modelled response to that of the actual field site is required to 

provide confidence in the strata characterization and the 

computational method applied. 

The comparison relies heavily on the roof extensometer data. 

The approach has been to compare the extensometer results at 

approximately equivalent total displacements at the roof line. In 

this way, the nature and style of deformation within the roof which 

causes the total displacement can be compared. If the comparison 

is good, then the model is inferred to simulate the rock deformation 

mode and location in a realistic manner. This allows the 

performance of different support systems to be evaluated using the 

same criteria that are employed in the mine—namely, how much 

roof movement is taking place. Therefore, it is not necessary to 

employ surrogates such as the maximum stress or a safety factor. 

It should be noted that the study is aimed at “normal” unfaulted 

ground conditions. The effect of faulted or structured ground 

conditions was beyond the scope of this study. Also, the strata 

geology may locally vary from that estimated, or the stress 

conditions may become more complex during mining. 

Roof Movements 

Figure 8 compares the roof deformations calculated in the 

model to those measured underground. Model results from three 

stress levels on the model loading path are shown. Also shown are 

a number of deformation profiles measured at the two crosscut 

sites. As Oyler et al., (2004) noted, the sites “differed in the timing 

of the roof deformation, but it was significant that the deformation 

process followed a broadly similar pattern in all of them.” The 

figure indicates that the model was able to capture the deformation 

pattern, showing a very close correlation in both deformation style 

and height of movement. It should also be noted that the model 

indicated that some deformation occurred even above the top of the 

extensometers. 

Figure 9 illustrates the location and nature of the rock failure 

processes that are associated with the roof deformation profiles. 

Upon initial development (figure 9a) bedding plane shear occurs 

readily within the section and early in the deformation process. As 

stress levels increase shear fracture of the weaker shale units 

occurs. Higher stress levels cause shear of the stronger materials 

together with additional bedding plane shear. However, total roof 

deformations are relatively minor until the applied far-field stress 

reaches approximately 15-16 MPa (2,200 psi; figure 9b). Beyond 

that stress level, significant roof deformation develops and 

progresses higher into the roof section (figure 9c). 

Roof Bolt Forces 

The roof bolt forces developed in the model and roof bolt load 

data from sites B and C are presented in figure 10. To make the 

comparison, the average bolt loads were determined for three levels 

of roof displacement. In general there is a fairly wide range in 

average bolt load at the monitoring sites, however, the overall 

forces developed in the model are consistent with the range as 

monitored. This provides an indication that the bolt-strata 

interaction is being simulated in a realistic manner consistent with 

the site response. 

Figure 10. Roof bolt forces developed relative to horizontal 

stress. 

The results indicate that under the in situ development stress 

state (less than approximately 11-12 MPa (1,600 psi)) roof 

conditions would be anticipated to be good and well controlled by 

the bolt pattern placed. The roof bolts continue to be well under 

yield load up to a stress level of approximately 15-16 MPa 

(2,200 psi). Once significant roof deformations begin to occur, 

however, the bolt loads rapidly increase. 

The model (figure 10) shows that the addition of cable bolts 

initially has little effect on bolt loads. However, once major roof 

movements begin to occur, the cables assume enough load to delay 

the onset of yield in the roof bolts. The cable bolts appear to 

develop their full capacity after the roof bolts yield. 

Stress Redistribution about the Cross Cut Caused by

Roadway Deformation 

The measurements made during the study showed that the 

additional stresses were redirected above the immediate roof of the 

crosscut even before significant roof deformations had occurred 

(figure 11). The model indicated that the bedding plane shear that 

developed early in the deformation process could be sufficient to 

cause significant stress redirection even though the displacement 

and visual deformation of the roadway was low. An example is 

presented in figure 12 for an equivalent displacement of 

approximately 20 mm (8 in). 

The amount of horizontal stress transferred within the initial 

2 m (7 ft) of roof at the centre of the roadway is an indication of the 

stability of the roof section and the requirement for reinforcement 

of the rock. When the bolted roof section has lost its integrity and 

become “softened,” the horizontal stress it can carry is reduced. 

Once extensive rock fracturing causes roof softening, the 

reinforcing action of the reinforcement is the primary design task. 

APPLICATION OF THE MODELING RESULTS TO MINING 

ISSUES 

It appears that the model results are consistent with the 

monitored and observed behaviour of the roadway available at the 

site. This provides confidence that the model is simulating the rock 

deformation processes influencing roadway stability and 

reinforcement interaction. This confidence allows for realistic 

assessment of various bolt patterns within the stress conditions 

anticipated at the site. 
 

 

 

 

 

 

 

 

 

 

Estimated 

softened roof 

zone 

0 2 

Scale, m 

10 

Stress scale, MPa 

0 

Study Crosscut 

H7 

H1 H2 H3 H4 

H5 

H6 

Figure 11. Principal stress changes after the completion of 

the 10 North longwall and prior to mining the 11 North 

longwall in the plane perpendicular to the study crosscut, 

with respect to initial HI cell readings (See figure 1b). 

Stress changes with arrows are tensile or stress relief, 

stresses without arrows are compressive. 

Figure 12. Stress redirection about the roadway caused by 

localized rock failure and subsequent changes in bulk material 

properties during roadway development. 

In this section, the effect of hypothetical bolt patterns were 

evaluated to demonstrate the influence of reinforcement patterns on 

roadway behaviour. 

The bolt patterns assessed in this example were: 

1.	 3, 1.8 m (6 ft), long fully grouted bolts. 

2.	 3, 2.4 m (8 ft), long combination bolts. 

3.	 3, 2.4 m (8 ft), long combination bolts with 2, 4 m (13 ft) 

cables. 

Figure 13 shows the roof displacement within the roof section. 

Each bolt system has a characteristic “limit” at which point the roof 

deformations begin to rapidly increase. It is significant that the 

deformation does not follow a smooth curve towards failure, but 

rather abruptly goes from “controlled” to “uncontrolled” 

movement. This seems to conform with underground experience, 

where “good” conditions often seem to “suddenly” go bad. 

Figure 13. Roof displacement of the roof section for various 

bolt patterns. 

For the standard 2.4 m (8 ft) bolt pattern, the roof maintains 

integrity up to approximately 15-16 MPa (2,200 psi), which is 

within the range anticipated during development at high angles to 

the regional stress field. The addition of supplemental cable bolts 

would allow the roof to cope with an additional 3-4 MPa (500 psi). 

On the other hand, 1.8 m (6 ft) bolts reach their limit at just 

12 MPa (1,700 psi), which could change a situation of probable 

roof control success during development in the cross cut direction 

to one of potentially difficult roof conditions. These results are 

consistent with expectations, and indicate that various 

combinations of reinforcement can significantly modify the 

deformation limit of the strata section. 

CONCLUSIONS 

The computer modelling developed for this site has been found 

to simulate the roof behaviour, rock bolt forces and stress 

redistribution characteristics about the roadway in a realistic 

manner. The study has shown that: 

•	 Bedding plane shear occurs early in the deformation process 

and significantly modifies the stress redirection about the 

roadway. 

•	 Modification to the bolting pattern has significant effect on 

roadway stability at the site. 

•	 Cable bolts initially have little effect on either the roof 

deformation or the roof bolt loads, but they become 

significant at higher levels of roof stress. 

The modelling indicated that the strata section would maintain 

integrity up to approximately 15-16 MPa (2,200 psi) with 2.4 m 

(8 ft) bolts. The deformation limit was extended to approximately 

19 MPa (2,700 psi) with the addition of cable bolts and elevated 

vertical stress. 

The results of this study demonstrate that sophisticated 

numerical models can obtain very realistic results, so long as: 
•	 The details of the geology and the rock properties (both preand post-failure) are replicated on a very fine scale within the 

model, and; 

•	 The model can simulate the broad and complex range of 

failure modes that occur underground. 

Modelling to assist with mine planning and support design can 

be done on a comparative basis to assess the performance of 

different support options. If ground characterisation is 

satisfactorily achieved, then the results can be extended to assess 

ground behaviour in absolute terms. Dedicated geotechnical 

monitoring and strata characterization is necessary to fully utilize, 

and extend, the benefits which can be obtained. 

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in Mines in Bedded Deposits in the Eastern and Midwestern United 

States. Proceedings, 22nd International Conference on Ground 

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