Development of Ground Response Curves 

for Longwall Tailgate Support Design 

Esterhuizen, G.S. 

Barczak, T.M. 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, U.S.A. 

ABSTRACT: Longwall tailgates in coal mines are often subject to severe mining induced loading and deformation. Innovative 

tailgate supports have been developed over the years to provide safe and economical access and a ventilation pathway at the 

tailgate. The support capacity and yield capabilities of the supports need to be matched to the loading imposed by the surrounding 

rock mass. The ground response curve can be used to represent the rock mass response to mining and its effect on support 

systems. The FLAC finite difference code was used to supplement field results by simulating a longwall tailgate and the 

associated ground response. The ground response curve is developed by modeling tailgate excavations with different internal 

support pressures and recording the resulting convergence. Ground response curves are developed for two typical longwalls 

operating in the Pittsburgh seam with weak and strong immediate roof. An additional model is presented in which the effect of 

weak overburden strata is simulated. The ground response is shown to be significantly affected by the strength of the immediate 

roof as well as the main roof. The importance of yield capacity of standing support is demonstrated. Gob height and compaction 

are found to affect the amount of convergence in the tailgate. The potential exists to develop site specific tailgate design curves by 

combining model results with field observations. 

1. INTRODUCTION 

Longwall tailgate entries can be subject to severe 

loading and deformation associated with the 

approach and passing of the longwall face. The 

tailgate entry is required to remain open so that a 

safe escape way and a reliable return airway is 

maintained at the tailgate corner of the advancing 

longwall face. In gassy mines it is also 

advantageous if the tailgate entry remains open 

behind the face, allowing ventilation air flow to the 

first crosscut in the gob area. Depending on the 

chain pillar size, this can be up to 60 m behind the 

advancing face. Innovative support methods have 

been developed to maintain the stability of the 

tailgate under these severe conditions. At present, 

standing supports are widely used as tailgate 

support in U.S. longwall mines [1, 2]. 

The design of standing supports requires knowledge 

of the loads that the ground will impose on the 

supports and the roof-to-floor convergence that will

occur. This allows the support capacity and yield 

capability of the supports to be matched to the 

expected ground response. The load-deformation 

characteristics of standing supports are well known, 

and can be tested in the laboratory [3]. The ground

response, however, is poorly understood and is not 

easily measured in the field, especially in the gob 

area behind the longwall face. 

This paper presents the results of a study into the 

ground response around tailgate entries using 

numerical models. The objective of the study was 

to improve the understanding of both the ground 

response and the required yielding capability of 

standing supports. The work forms part of the

strategic goals of the National Institute for

Occupational Safety and Health (NIOSH) research 

program that addresses safe ground control 

practices for coal mines. 
 

2. STANDING SUPPORT CHARACTERISTICS 

There is a wide variety of standing support systems 

currently available for longwall tailgate application. 

Although their performance characteristics vary, 

they can be classified into four basic types, as 

illustrated in Figure 1: (1) brittle, (2) constant load, 

(3) strain hardening, (4) strain softening behavior. 

Examples of each of these types of supports are the 

concrete donut crib (brittle), conventional 4-point 

wood crib (hardening), pumpable roof support 

(softening), and the Can support (constant load). 

+ +

Brittle 

LOAD 

Constant load 

+ 

LOAD LOAD

Softening 

Hardening 

LOAD 

DISPLACEMENT + DISPLACEMENT +

+ +

DISPLACEMENT + DISPLACEMENT +

Fig. 1. Four basic types of loading characteristics for 

standing roof support systems. 

Two of the support types were selected for 

discussion relative to the ground response curves. 

The first is a strong support exhibiting brittle 

behavior, similar to the concrete donut crib. The 

support has a peak capacity of approximately 

500 tonnes after 25 mm of compression, but loses 

its strength after a further 25 mm of compression. 

The second is a softening support, such as the 

pumpable roof support. It is a grout-filled support 

that is formed in the mine entry by pumping a 

specialized grout into a fabric bag that is hung from 

the mine roof. This support type typically has a 

peak strength of about 150 tonnes after 25 mm 

compression, and loses about 50% of its strength 

after 300mm of compression. 

3. OBSERVED GROUND RESPONSE IN 

LONGWALL TAILGATE ENTRIES 

Longwall tailgate entries are subject to four distinct 

loading stages, shown in Figure 2. The loading 

stages are defined as follows: A) Development: 

loading condition before the effects of longwall 

retreat mining; B) Side Abutment: the entry is 

subject to an increase in vertical loading from the 

side abutment of the first panel mining, the 

horizontal stresses can decrease owing to the 

relaxation of the strata towards the gob; C) Face 

Abutment: the entry is subject to a further increase 

in loading as the second panel face approaches; D) 

Full Extraction: the loading condition after the 

longwall has passed and the entry is located in the 

gob area. 

First panel mined out

Second panel

A

D 300m

3000m

B C

Future tailgate 

entry

Second panel 

tailgate entry

Direction of

mining

Chain pillars

Advancing

longwall face

Gob

First panel mined out 

Second panel 300m 

3000m 

A 

B C D 

Future tailgate 

entry 

Second panel 

tailgate entry 

Direction of 

mining 

Chain pillars 

Advancing 

longwall face 

Gob 

Not to scale 

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

Development, Side Abutment, Face Abutment and Full Extraction tailgate loading conditions, respectively. 
Observations of tailgate entry performance in 

Eastern U.S. longwalls have shown that limited 

convergence between the roof and floor occurs 

between initial development and the Side Abutment 

loading stage. Convergence values of less than 

10mm have been measured [4]. The Face 

Abutment loading stage can result in convergence 

of up to 50 mm [1, 5]. Convergence measurements 

are difficult to obtain behind the face, but a further 

50mm is not uncommon within the first 10m behind 

the face [6], provided the support system remains 

functional. Mucho et al. [1] measured convergence 

of 100 mm at distances of up to 30 m behind the 

face in a tailgate entry supported with wood cribs. 

The success of tailgate support can be compromised 

by issues such as side loading by gob fragments, 

lateral movement between the roof and floor and 

disintegration of the roof around the supports. 

These issues, though important, were not addressed 

by the current study. 

4. GROUND RESPONSE CURVES 

The concept of a ground response curve was 

originally developed for the civil tunneling industry 

where the timing and method of ground support is 

determined by monitoring the support pressure and 

excavation convergence during construction [7]. 

The ground response approach has found 

application in both hard rock and coal mining as a 

method to better understand the interaction between 

the rock mass and the support system [1, 2, 8, 9, 10, 

11]. 

The ground response curve plots the support 

pressure against the excavation convergence, as 

shown conceptually in Figure 3. If the excavation 

boundaries are subject to support pressure equal to 

the stress in the surrounding rock, no convergence 

will occur (point A). As the support pressure is 

reduced, the excavation boundaries converge and 

the pressure required to prevent further convergence 

reduces as arching and the self supporting capacity 

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

reached (point C) where the required support 

resistance begins to increase as self-supporting 

capacity is lost and the dead-weight of the failed 

ground must be resisted (point D). 

The effect of the support system can also be plotted 

on Figure 3. Line PQB represents a yielding 

support that is installed after initial convergence (δ). 

As the convergence increases, the support resistance 

increases, in proportion to the support stiffness. 

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

support then yields and the support pressure is 

sufficient to arrest further convergence at point B. 

Ideally, support should be designed and installed to 

operate as close as possible to point C, which allows 

the available strength of the rock mass to be utilized 

while minimizing the load carried by the support 

system. 

 

A

B

C

D

δ Convergence

Support pressure

P

Q

Ground response

curve

Support line

Initial stress level

A 

B 

C 

D 

δ Convergence 

Support pressure 

P 

Q 

Ground response 

curve 

Support line 

Initial stress level 

Fig. 3. Idealized ground response curve and support line. 

In a longwall tailgate entry, the stress in the 

surrounding rock does not remain static. The 

advancing longwall causes changes in the loading 

condition as it approaches and ultimately removes 

one side of the entry excavation. These changes 

will result in a unique ground response curve for 

each loading stage. Support installed during the 

initial loading stages can therefore experience 

further convergence as the ground responds to the 

new loading condition. In this study, families of 

ground response curves were developed, each 

corresponding to one of the four loading stages of 

the tailgate entry. 

5. MODELING METHOD TO DEVELOP

GROUND RESPONSE CURVES 

The finite difference software FLAC [12] was used 

to develop ground response curves for a tailgate 

entry in various loading conditions and geological 

settings. The software is able to realistically model 

rock behavior from the initial elastic response to the 

large displacements and deformations that are
Tailgate Entry

 

associated with rock failure. It has the capability to 

model strength anisotropy found in the bedded coal 

measures and can simulate strain related weakening 

of failed rock. The software also has a built-in 

programming language that allows the user to 

control loads and displacements in the model. This 

facility was used to apply internal pressure within 

the modeled tailgate entry excavation so that the 

ground response curve could be determined. 

Being a two-dimensional method, the software 

cannot model the conditions just behind the 

longwall face where a three-dimensional geometry 

exists. The results for the fully extracted loading 

stage are only applicable at a location remote from 

the longwall face, where two-dimensional

conditions exist. Inspection of three-dimensional 

model results [11] using the Lamodel software [13] 

shows that two dimensional conditions are restored 

approximately 120 m behind the face for a longwall 

layout similar to that evaluated in this study. 

Simple linear interpolation was used to estimate the 

convergence between the Face Abutment stage and 

the Full Extraction loading stage. 

A further limitation of the modeling method is that 

the final dead-weight loading by loosened or 

detached roof rocks is not well represented. The 

software was designed to simulate continuous 

materials but does not efficiently simulate discrete 

particles such as detached blocks in the roof of the 

entry. Issues such as disintegration of the roof 

rocks around standing supports and support loading 

by detached blocks were therefore specifically 

excluded from this study. 

5.1. Model Layout 

The overall finite difference model geometry and 

boundary conditions are shown in Figure 4. A 

typical three-entry gate road design used in the 

Eastern U.S. longwall operations was evaluated in 

the study. The model was constructed to analyze 

the tailgate entry specifically, which was 5 m wide 

by 1.8 m high. The extent of the model was 180 m 

wide by 100 m high, which allowed a 300 m wide 

longwall panel as well as the adjacent tailgate entry 

and the center entry to be modeled, by employing 

symmetry. The element size was 20 cm in the 

vicinity of the tailgate entry. 

Three different geological settings were simulated, 

shown in the left margin of Figures 5 to 9. The first 

was a coal bed overlain by weak shale strata and 

alternating weak and strong beds, typical of the 

Pittsburgh seam in Western Pennsylvania. A 

significant feature of this lithology is the presence 

of thick, strong limestone beds in the roof strata. 

The second model is similar to the first, except that 

a strong sandstone bed was modeled in the 

immediate roof, typical of mining under a sandstone 

channel in the Pittsburgh seam. The third model 

simulated weak overburden, in which the limestone 

beds are absent and the overburden consists mainly 

of shale and siltstone beds. 

Overburden loading 

5m wide tailgate entry 

100m 

Chain pillar 

30m 150m 

Center entry 

180m 

Not to scale 

Fig. 4. Finite difference model showing general model layout 

and boundary conditions. 

0 10m

Scale

Black shale

Shale

Limestone

Coal

Sandstone

Shear strain > 5%

Rock matrix shear

or tensile failure

Bedding plane shear

or tensile failure

Tailgate Entry 

0 10m

Scale

Black shale

Shale

Limestone

Coal

Sandstone

Shear strain > 5%

Rock matrix shear

or tensile failure

Bedding plane shear

or tensile failure

0 10m 

Scale 

Black shale 

Shale 

Limestone 

Coal 

Sandstone 

Shear strain > 5% 

Rock matrix shear 

or tensile failure 

Bedding plane shear 

or tensile failure 

Fig. 5: Model with weak immediate roof and strong 

overburden beds. Rock failure around tailgate entry at the 

Development loading stage (Point A, Figure 2). 
Tailgate Entry

Black shale Shear strain > 5% 0 10m 

Shale Rock matrix shear Scale 

Limestone or tensile failure
 

Sandstone Bedding plane shear

or tensile failure Coal 

Fig. 6. Model with weak immediate roof and strong beds in 

the overburden. Rock failure and shear bands around tailgate 

at the Face Abutment loading stage, (Point C, Figure 2). 

Gob 

Tailgate Entry

Black shale
 Shear strain > 5% 0 10m
 

Shale
 Rock matrix shear Scale 

or tensile failure 

Bedding plane shear 

Limestone 

Sandstone
 

or tensile failure Coal	 

Fig. 7: Model with weak immediate roof and strong 

overburden, Rock failure and shear bands around tailgate 

entry at the Full Extraction loading stage (Point D, Figure 

2). 

Gob 

Tailgate Entry

Black shale Shear strain > 5% 0 10m

Shale Rock matrix shear Scale

Limestone or tensile failure

Sandstone
 Bedding plane shear

or tensile failure Coal

Fig. 8. Model with strong immediate roof and strong 

overburden, rock failure and shear bands around tailgate entry 

at the Full Extraction loading stage (Point D, Figure 2). 

Gob 

Tailgate Entry

Black shale Shear strain > 5% 0 10m 

Shale Rock matrix shear Scale 

Limestone or tensile failure 

Sandstone Bedding plane shear 

or tensile failure Coal
 

Fig. 9. Model with weak immediate roof and weak

overburden, rock failure and shear bands around tailgate entry 

at the Full Extraction loading stage, (point D Figure 2). 

5.2. Material Properties used in the Models

The rock mass was modeled as a strain softening, 

ubiquitous joint material, using the built-in 

constitutive model available in the finite difference 

software. This model is well suited to modeling the 
layered coal measure rocks, since the bedding layers 

can be described as strain softening ubiquitous 

joints, while failure of the rock matrix can be 

simulated by the strain softening Coulomb 

constitutive model. Strength data for the different 

rock types included in the models were based on 

published data for to coal measure rocks [14, 15]. 

A summary of the average material properties used 

in the models is presented in Table 1. Rock 

strength variability was simulated in the models by 

allowing variations of up to 30% above and below 

the average values. The average values of the 

bedding plane strength used in the models are 

summarized in Table 2. The bedding strength was 

loosely related to the intact rock strength, the 

weaker rocks having weaker bedding planes. 

Table 1 Average Mechanical Properties of Rock Types 

Rock class Cohesion Friction Poisson Elastic 

(MPa) angle (o

) ratio modulus 

(gpa) 

Weak rock: 4.5 25 0.25 6.0

Black shale, 

mudstone 

Moderate rock: 8.0 28 0.25 8.0

Shale 

Strong rock: 12.0 32 0.25 12.0

Sandstone 

Very strong 20.0 36 0.25 20.0

rock: 

Limestone 

Coal 1.9 31 0.25 2.5

Table 2. Average Strength Properties of Bedding Planes 

Bedding plane description Cohesion (MPa) Friction a ngle 

(

o

) 

Very weak – clay filled 0.055 21

discontinuity 

Weak – open 0.5 21

discontinuity 

Moderate – weakly healed 3.3 24

discontinuity 

Strong – healed 5.5 26

discontinuity 

Very strong – strongly 10.0 28

healed discontinuity 

Strain softening of the rock matrix and bedding 

planes was modeled by implementing cohesion 

weakening. The cohesion of all the rock types and 

bedding types was specified to reduce to 10% of the 

initial value after 0.5% plastic strain. The friction 

angle remained constant during yield. Dilation 

angle was constant at 15 degrees for all rock types. 

Owing to element size dependence of strain 

softening models, the element sizes were unchanged 

in all the analyses. 

The model simulating the fully extracted conditions 

included the gob on one side of the tailgate entry. 

The gob was modeled as a soft elastic material. The 

bulk modulus of the gob was set at 18 MPa, 

determined by trial and error to allow 

approximately 90 cm of subsidence over the center 

of the longwall panel, similar to observed 

subsidence in Pittsburgh seam longwalls [16]. 

5.3.	 Model Loading 

It was assumed that the longwall gate roads are 

aligned parallel to the major horizontal stress, thus 

reducing the out-of-plane stresses in the twodimensional models. The initial vertical stress in 

the model was set at 5 MPa to simulate a tailgate 

entry at a depth of approximately 200 m below the 

ground surface. Since the model did not extend to 

the ground surface, vertical loads were applied to 

the top of the model to simulate the overburden up 

to the ground surface. The initial horizontal stresses 

were calculated from the Poisson ratio of each rock 

layer plus a tectonic component which depended on 

the elastic modulus of the rock [17, 18]. The input 

parameters were selected so that the horizontal 

stress in the moderately strong shale beds was 

8 MPa, similar to measured values in Eastern U.S. 

coal mines [18]. 

The loading induced by the Side Abutment and 

Face Abutment stages were simulated by increasing 

the vertical loading of the model by 1 MPa and 

6 MPa, respectively. The stress increments were 

based on the results of three-dimensional models of 

longwalls using FLAC3D [19]. Out-of-plane stress 

rotation near the longwall tailgate corner could not 

be included in the two-dimensional models. 

The Full Extraction loading condition was modeled 

by simulating the extraction of the coal on one side 

of the tailgate entry and simulating gob formation. 

The gob was assumed to extend three times the 

mining height above the roof of the mined coal 

seam. The extraction of the coal and associated gob 

formation was simulated as a single step in the 

model. 

5.4.	 Procedure for Developing Ground Response 

Curves 

The ground response curves were developed by 

simulating a uniform support pressure on the roof 

and floor of the tailgate entry while sequentially 

modeling the four external loading stages. The 
model was run to equilibrium at each loading stage 

and the average convergence in the tailgate entry 

was recorded. In this manner, failure and 

convergence that occurred during the earlier stages 

are preserved and included in the later loading 

stages. Repeat analyses were carried out in which 

the internal support pressure was varied, so that the 

ground response curve could be developed. Internal 

pressures of 1.0 kN/m2

 up to 2,500 kN/m2 were 

applied to provide a range of results that would 

bracket the typical range of standing support 

capacities. Ground response curves were developed 

by plotting the support pressure against the tailgate 

entry convergence for each loading stage. 

6. MODEL RESULTS 

6.1. Rock failure and convergence 

The model results for the Development, Face 

Abutment and Full Extraction loading conditions 

are presented in Figures 5 to 7 for the model which 

simulated typical Pittsburgh seam geology. The 

support resistance for this set of results was 

75 kN/m2

, that approximates a single row of 

standing supports with 90 tonnes capacity, spaced 

2.4 m apart along the length of a 5m wide entry. 

The Figures indicate the extent of failure of the 

intact rock and bedding planes around the tailgate 

entry. Shear band development is shown by 

highlighting the zones where the strain exceeds 5%. 

It can be seen in Figure 5 that the Development 

loading stage results in bedding slip in the 

immediate roof and floor of the entry, and limited 

failure in the coal seam. Failure and convergence 

increases considerably at the Face Abutment 

loading stage, shown in Figure 6. About 32 mm of 

additional convergence occurs, which is similar to 

that observed in a Pittsburgh seam longwall using 

standing supports of a similar capacity [1]. The 

convergence in the model is largely the result of 

elastic relaxation and bulking of the failed rock into 

the excavation. 

The Full Extraction stage, shown in Figure 7, results 

in 256 mm of convergence and considerable failure 

of the surrounding rock mass. The limiting effect of 

the strong limestone beds on vertical failure 

development is clearly demonstrated. A semidetached block is formed in the roof that is held in 

place by the support pressure. In practice such a 

detached block is likely to also transfer some of its 

weight to the gob, resulting in reduced loading of 

the standing supports. Further inspection of the 

model results showed that during the Full 

Extraction loading condition, the convergence is 

largely caused by the upper strata settling onto the 

gob and the chain pillars. As a result, the gob 

stiffness will have a direct effect on the amount of 

convergence in the tailgate entry behind the face. 

Figure 8 shows the rock failure associated with the 

Full Extraction loading condition for the strong roof 

case. It can be seen that the strong sandstone roof 

has failed to a lesser degree and convergence is 

169 mm. A horizontal shear band forms within the 

sandstone, near the top of the gob. The vertical 

extent of rock failure is again inhibited by the 

overlying limestone bed. 

The results for the weak overburden case, under 

Full Extraction loading conditions, are presented in 

Figure 9. The results show extensive shear band 

development in the absence of the strong limestone 

beds. Convergence in the tailgate entry is much 

larger now, at 381 mm. Closer inspections of the 

results showed that the shear bands in the 

overburden result in a stepped displacement profile. 

Consequently the immediate entry experiences a 

greater amount of convergence. The weak 

immediate roof is seen to be failed and forms a 

detached block, delineated by the shear banding. 

6.2. Ground Response Curve Results 

The resulting ground response curves for the three 

cases analyzed are presented in Figures 10 to 12. 

Each case shows the support pressure versus 

convergence curve for the four longwall loading 

stages. Three additional curves are shown in which 

the convergence at 10m, 30m and 60m behind the 

face (inby) was estimated by linear interpolation. 

The support pressure is plotted on a logarithmic 

scale, starting at 10 kN/m2

. It is assumed that the 

primary support, which is installed during 

development, will provide this level support prior to 

the installation of standing supports. 

The two idealized standing support curves, 

previously discussed, are also shown in each chart. 

Support line A represents the idealized brittle 

support, and B represents the softening support. 

The supports are assumed to be installed after 

convergence caused by the Development loading 

stage has occurred. 

Figure 10 shows the set of ground response curves 

developed for the weak roof, strong overburden 

case. The results show that, up to the Face 
Abutment loading stage, the ground response curves 

are steep and convergence is limited to less than 

about 75 mm. At the Full Extraction loading stage 

the convergence increases to more than 100 mm for 

2,500 kN/m2 support pressure, and apparent 

collapse is indicated if the support pressure is 

reduced to less than 25 kN/m2. 

Fig. 10. Ground response curves derived from the model of a 

tailgate entry under a weak immediate roof with strong beds in 

the overburden. Dotted lines indicate interpolated curves. 

Lines A and B represent brittle and softening supports, 

respectively. 

10 

100 

1000 

10000 

0 100 200 300 400 500 

Convergence (mm) 

S upport P ressure (kN /m2 ) 

Development 

Side abutment 

Face abutment 

10m Inby

30m Inby 

60m Inby

Full extraction 

A 

B 

Considering the two idealized standing support 

curves, we see that the brittle support (A) will be 

able to control convergence up to the Face 

Abutment stage, when it reaches its maximum load. 

However, it will shed its entire load before it 

reaches the 10 m inby location. The lower capacity 

softening support (B) will provide adequate support 

up to the Full Extraction loading stage. At this 

stage the convergence will be about 260 mm. To 

control the roof up to the 60 m inby mark, if 

required for ventilation purposes, would require 

about 150 mm of yield capability. It is interesting 

to note that even if the capacity of the softening 

support is increased tenfold, it would need to yield 

by about 150 mm in response to the convergence at 

the Full Extraction loading stage. 

In the case of the strong roof model, Figure 11, the 

ultimate convergence is less than 175 mm for the 

Full Extraction loading condition. The brittle 

support (A) reaches its peak load at the 10 m inby 

condition. The yielding support (B) can arrest the 

convergence up to the Full Extraction stage by 

yielding about 145 mm and would require about 

90 mm of yield capability to support the roof up to 

the 60 m inby mark, if required for ventilation 

purposes. 

10 

100 

1000 

10000 

0 100 200 300 400 500 

Convergence (mm) 

Support Pressure (kN /m2 ) 

Development 

Side abutment 

Face abutment 

10m Inby 

30m Inby

60m Inby

Full Extraction 

A 

B 

Fig. 11. Ground response curves derived from the model of a 

tailgate entry under a strong immediate roof with strong beds 

in the overburden. Dotted lines indicate interpolated curves. 

Lines A and B represent brittle and softening supports, 

respectively. 

10 

100 

1000 

10000 

0 100 200 300 400 500 

Convergence (mm) 

Support Pressure (kN /m2 ) 

Development 

Side abutment 

Face abutment 

10m Inby

30m Inby

60m Inby

Full Extraction 

A 

B 

Fig. 12. Ground response curves derived from the model of a 

tailgate entry under a weak immediate roof with weak beds in 

the overburden. Dotted lines indicate interpolated curves. 

Lines A and B represent brittle and softening supports, 

respectively. 

The results for the weak overburden model, 

Figure 12, show that convergence will be 

substantially higher than for the previous two cases. 

In addition, the roof apparently starts to collapse 

when the support resistance drops below 75 kN/m2, 

which is higher than the strong overburden case. 

The results show that the brittle support (A) will be 
adequate up to the Face Abutment loading stage, 

but would fail soon afterwards. The softening 

support (B) is not able to arrest convergence up to 

the Full Extraction stage because the ground 

support curve exceeds its capacity. This support is, 

however, able to control the roof up to the 60 m 

inby loading stage. 

7.	 DISCUSSION AND CONCLUSIONS 

The finite difference model results presented in this 

paper have provided valuable insight into the 

loading mechanisms and associated convergence in 

longwall tailgate entries. The method can be used 

to investigate the effect of various loading 

conditions, longwall geometries and geological 

scenarios on the ground response. 

Although the two-dimensional modeling technique 

used in the study has limitations, especially for 

capturing rock response near the longwall tailgate 

corner, satisfactory agreement was obtained 

between field observations and model behavior. 

The results showed that up to the Face Abutment 

loading stage, the convergence in the tailgate entry 

is largely the result of bulking associated with rock 

failure and bedding plane shear in the surrounding 

strata. 

In the Full Extraction loading stage, convergence is 

additionally caused by settlement of the overburden 

onto the gob and chain pillars. The gob stiffness 

and height of gob formation will therefore have an 

impact on the convergence of the tailgate behind the 

face. 

The strength of the overburden strata is shown to 

have a significant effect on the amount of 

convergence in the tailgate entry and on the support 

required to restrict the convergence. Weak 

overburden strata tend to shear at the edge of the 

chain pillar, resulting in greater convergence in the 

tailgate entry and a higher load demand on the 

standing supports. 

The ground response curves, developed from the 

model results, show that convergence in the tailgate 

entry cannot be prevented by standing supports. A 

significant proportion of the tailgate convergence is 

driven by loads which far exceed the capacity of 

typical standing supports. 

The results presented in this paper should not be 

used as standing support design curves since they 

were developed for a specific set of geological and 

geometric conditions using simplified twodimensional models. In addition, further work is 

required to investigate issues such as ground 

response in the vicinity of the face corner and the 

effect of loosened or detached roof rocks. 

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