93

ABSTRACT 

 Multiple seam mining interactions caused by full extraction 

mining, whether due to undermining or overmining, frequently 

involve tensile failure of the affected mine roof. The adverse 

ground control conditions may prevent mining for both safety and 

economic reasons. Prior researchers have identified the geometric, 

geologic and mining factors controlling multiple seam mining 

interactions. This numerical study examines the mechanics of 

these interactions using a modeling procedure that 1) incorporates 

the essential constitutive behavior of the rock such as strainsoftening of the intact rock and shear and tensile failure along 

bedding planes and 2) captures the geologic variability of the rock 

especially the layering of weak and strong rocks and weak bedding 

planes. 

 Specifically, the numerical study considered the effect of 

vertical stress, interburden thickness, and the immediate roof 

quality of the affected seam in both undermining and overmining 

situations. The models show that for overburden-to-interburden 

thickness (OB/IB) ratios of less than 5, interactions do not occur, 

and that for OB/IB more than 50, extreme interaction is a certainty. 

In between, the possibility of an interaction was found to depend on 

gob width-to-interburden thickness ratio, site specific geology and 

horizontal stress to rock strength ratio in addition to the OB/IB ratio. 

 The models also showed that horizontal stress was profoundly 

altered well above or below a full extraction area and that these 

changes are likely to influence the success or failure of multiple 

seam mining. The role of horizontal stress in multiple seam mining 

interactions has received little attention in prior investigations. 

 Four factors control the mechanics of multiple seam mining 

interactions: 

1. vertical stress concentration 

2. horizontal stress concentration 

3. stress re-direction 

4. bedding plane slip bands. 

A combination of vertical and horizontal stress increase and high 

stress gradients in the vicinity of full extraction areas re-orients 

principal stresses into a very unfavorable direction. This seemingly 

small stress re-orientation has a profound adverse effect on bedded 

rock. 

INTRODUCTION 

 Most underground coal mines face multiple seam mining 

situations with the potential for interactions that can pose 

challenging ground control conditions. Knowing the location of 

prior mining, planning engineers may seek to access and mine new 

reserves above or below old workings. Two common questions 

arise: 

1. Will workings above or below cause excessive stresses in the 

proposed workings that lead to difficult ground control 

conditions? 

2. Will subsidence from workings below cause ruinous ground 

control conditions in the upper seam? 

 In addition to ground control issues, multiple seam mining 

interactions can create other safety issues. For example, an 

interaction can produce pathways for air, gas or water migration 

that can cause spontaneous combustion and inundation hazards. 

This study is part of the NIOSH ground control research program 

that seeks to reduce ground control failures resulting from multiple 

seam mining interactions through the development of design-based 

control technology. 

 Whether an adverse multiple seam mining interaction occurs or 

not depends on numerous factors that are well documented by 

many ground control experts (1-7). These factors include: 

1. Mining geometry – overburden depth, interburden thickness 

and seam thicknesses. 

2. Mine design – layout, sequence and % extraction. 

3. Geology – immediate roof rock quality and interburden rock 

strength. 

Combination of these factors may lead to various degrees of 

multiple seam mining interaction ranging from none to additional 

ground support required to abandonment of an area. 

 This study re-examines the failure mechanics of multiple seam 

mining interactions using a new numerical modeling approach 

under development at NIOSH. In this approach, numerical models 

are created with sufficient geologic detail and proper constitutive 

behavior. With these two conditions met, numerical models can 

predict the behavior of the rock mass and indicate whether an 

adverse multiple seam mining interaction might occur. 

Failure Mechanics of Multiple Seam Mining Interactions 

R. Karl Zipf, Mining Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA
94

 This study examines three fundamental types of multiple seam 

interaction as shown in figure 1: 

1. Undermining – This situation represents classic top-down 

multiple seam mining. The upper seam is mined first and 

abandoned, prior to mining the lower seam. Gob-solid 

boundaries, barrier pillars, pillar remnants or other structures 

left over from full extraction mining in the upper seam may 

cause adverse stress concentrations in the lower seam. 

2. Overmining – This situation represents classic bottom-up 

multiple seam mining. Full extraction mining in the lower 

seam causes the upper seam to fully subside prior to its 

development. In addition to stress concentrations due to pillar 

remnants or gob-solid boundaries in the lower seam, the upper 

seam and surrounding rock may suffer damage from 

subsidence-induced displacement and fracture. 

3. Simultaneous mining – This situation implies that both seams 

are active simultaneously. In the worst case, workings are 

developed in the upper seam followed by full extraction 

mining in the lower seam. Subsidence of the upper seam 

occurs after the lower seam workings are extracted. 

NIOSH INPUT PARAMETERS FOR NUMERICAL 

MODELING 

 Ground control researchers at NIOSH follow a philosophy 

developed by Gale (8-10) of “letting the rocks tell us their 

behavior.” Numerical models that are constructed with sufficient 

geologic detail and proper constitutive behavior can predict 

response of the rock mass including deformation, stress 

redistribution, failure modes and support requirements. For general 

modeling of rock behavior in coal mine ground control, Itasca’s 

FLAC program (11) contains many useful features, in particular, 

the SU constitutive model. SU stands for the strain-softening, 

ubiquitous joint model and is ideal for simulating laminated coal 

measure rocks. In essence, this constitutive model allows for 

strain-softening behavior of the rock matrix and/or failure along a 

pre-defined weakness plane (in this case bedding planes). Failure 

through the rock matrix or along a bedding plane can occur via 

shear or tension, and the dominant failure mode can change at any 

time. Conveniently, the “state” variable within FLAC tracks the 

failure mode in each model element as either shear or tensile failure 

through the rock matrix or along a bedding plane. 

 The SU constitutive model requires four major input 

parameters, namely, cohesion, friction angle, dilation angle and 

tensile strength for both the rock matrix and the bedding planes. 

Each of these parameters begins at some peak value and decreases 

to a residual value as post-failure strain increases. It is this 

decrease in parameter value with post-failure strain that gives rise 

to strain-softening behavior of both the rock matrix and the 

weakness planes. FLAC permits an infinite combination of these 

requisite input parameters; however, in order to facilitate rational 

numerical modeling, NIOSH researchers created an organized suite 

of material input parameters. Figure 2 summarizes the names for 

this suite of “numerical rocks” and the corresponding values for the 

unconfined compressive strength (UCS) of the rock matrix and the 

strength of the bedding planes. 

 The strength values indicated in Figure 2 are laboratory-scale 

values determined from standard UCS tests. Alternatively, the 

point load test provides excellent, economic estimates of the UCS. 

These UCS values require scaling to reduce the laboratory values to 

the field values needed by the numerical model. Following the lead 

Figure 1b. In overmining, the lower seam is mined first and 

abandoned followed by upper seam mining. The upper seam 

is fully subsided prior to its mining. 

Figure 1a. In undermining, the upper seam is mined first and 

abandoned followed by lower seam mining. 

Figure 1c. In simultaneous mining, the upper seam is 

developed first followed by lower seam mining. Full 

extraction mining of the lower seam subsides existing upper 

seam workings. 
95

of Gale (8), laboratory values of UCS are reduced by 0.56 

universally. 

 The material suite shown in figure 2 includes very weak soils 

and clay-like materials with a UCS of 0.02 MPa and weak, medium 

and finally strong rocks with a UCS of about 150 MPa. Also 

included is coal, which ranges from the most friable with a UCS of 

2 MPa to a strong coal with a UCS of 12 MPa. The soil material 

models are isotropic, that is the soil matrix properties are the same 

as those for the horizontal weakness plane. However, the rock 

models exhibit anisotropy since the strength along bedding planes 

is less than the UCS of the rock matrix. Following results of point 

load tests by Molinda and Mark (12), weak rocks are the most 

anisotropic with the strength along bedding planes about 50% of 

the rock matrix UCS, while stronger rocks have less anisotropy 

with the strength along bedding planes being about 90% of the rock 

matrix. The coal models have a similar trend in strength anisotropy 

with the stronger coal less anisotropic than the weaker coal. 

 Note that in proposing this suite of numerical rocks, the UCS of 

the rock matrix is independent from the strength of the bedding 

planes. In the absence of specific data, the user will usually specify 

the rock matrix and bedding plane strength as a pair with strength 

ratio similar to that noted by Molinda and Mark (12) for an 

extensive database of axial and diametral point load tests. However, 

the strength values for the rock matrix and bedding planes are 

independent in the material property suite, and the user can specify 

any value for the bedding plane strength up to that of the rock 

matrix UCS. 

 Also note that the material model suite has a relation to the Unit 

Ratings in the CMRR system. Mark, et al. (13) proposed that the 

CMRR Unit Rating for a coal measure rock layer is comprised of a 

UCS rating for the rock matrix strength and a discontinuity rating 

for the bedding plane strength. The UCS rating ranges from 5 to 30 

points for a rock matrix strength ranging from 0 to 138 MPa as 

determined from axial point load tests. Similarly, the discontinuity 

rating ranges from 25 to 60 for a bedding plane strength ranging 

from about 6 to 52 MPa as determined from diametral point load 

tests. The proposed material property suite correlates to CMRR 

Unit Ratings from 30 to 90 and represents the range from the 

weakest to the strongest coal measure rocks. Figure 3 shows these 

relations. Given CMRR Unit Ratings from core logging, the 

relations shown in figure 3 provide approximate material choices 

for input to numerical models. 

 The material model suite and UCS values shown in figure 2 

imply a range of cohesion and friction angle values for the rock 

matrix and bedding planes. Based on a Mohr-Coulomb strength 

model, the UCS of a rock depends on cohesion and friction angle as 

 (1) 

where c is the cohesion and φ is the friction angle. Friction angle 

for the different materials in the suite is assumed to vary as shown 

in figure 4. Soil- and clay-like materials have friction angles of 21°, 

while progressively stronger rocks have friction angles up to 36°. 

This assumption for friction angle along with equation (1) implies 

the values for peak cohesion shown in figure 5. Thus, the UCS of 

the rock matrix and the bedding plane strength provide two of the 

four major input parameters to the SU constitutive model in FLAC. 

 Other major assumptions within this material model suite are as 

follows: 

1. Moduli for the materials range from 1 to 20 GPa as shown in 

figure 6. Weaker materials have a lower modulus, while 

stronger materials have a higher modulus. The ratio of 

modulus to UCS of the rock matrix varies from about 10,000 

for the weakest to about 100 for the strongest materials. 

2. Cohesion decreases from its peak value given in figure 5 to a 

residual value of 10% of peak over 5 millistrains of post-failure 

strain. 

3. Friction angle remains constant at the values shown in figure 4, 

even in the post-failure regime. 

4. Tensile strength is equal to cohesion for the soils materials and 

decreases to 0 over 1 millistrain of post-failure strain. 

5. Tensile strength ranges from about 10% of UCS for the 

weakest rocks to about 2% of UCS for the strongest rocks. It 

also decreases to 0 over 1 millistrain of post-failure strain. 

6. Dilation angle is initially 10° and decreases to 0° over 

5 millistrains of post-failure strain. 

φ

φ

1 sin

2 cos

− = c UCS

0.01

0.10

1.00

10.00

100.00

1000.00

Soil_0

Soil_1

Soil_2

Soil_3

Soil_4

Soil_5

Rock_A

Rock_B

Rock_C

Rock_D

Rock_E

Rock_F

Rock_G

Rock_H

Rock_I

Rock_J

Coal_1

Coal_2

Coal_3

Coal_4

Strength - MPa .

Laboratory strength of soil

Laboratory strength of rock matrix

Laboratory strength of rock bedding planes

Laboratory strength of coal matrix

Laboratory strength of coal bedding planes

Figure 2. Laboratory-scale strength of matrix and bedding planes for suite of “numerical rocks.” 
96

Figure 3. Correlation of “numerical rocks” to unit ratings of CMRR system. Weak rocks have a CMRR Unit Rating from 30 to 

45; moderate rocks have a CMRR Unit Rating from 45 to 60, and strong rocks have a CMRR Unit Rating from 60 to 85. 

0

10

20

30

40

50

60

70

80

90

100

Rock_A Rock_B Rock_C Rock_D Rock_E Rock_F Rock_G Rock_H Rock_I Rock_J

Rating .

Matrix Strength Rating

Discontinuity Rating

CMRR Unit Rating

0

5

10

15

20

25

30

35

40

Soil_0

Soil_1

Soil_2

Soil_3

Soil_4

Soil_5

Rock_A

Rock_B

Rock_C

Rock_D

Rock_E

Rock_F

Rock_G

Rock_H

Rock_I

Rock_J

Coal_1

Coal_2

Coal_3Coal_4

Friction angle (degrees) .

Friction angle of soil

Friction angle of rock matrix

Friction angle of rock bedding planes

Friction angle of coal matrix

Friction angle of coal bedding planes

Figure 4. Friction angle for matrix and bedding planes in suite of “numerical rocks.” 

0.01

0.10

1.00

10.00

100.00

Soil_0

Soil_1

Soil_2

Soil_3

Soil_4

Soil_5

Rock_A

Rock_B

Rock_C

Rock_D

Rock_E

Rock_F

Rock_G

Rock_H

Rock_I

Rock_J

Coal_1

Coal_2

Coal_3

Coal_4

Cohesion (MPa) .

Cohesion of soil

Cohesion of rock matrix

Cohesion of rock bedding planes

Cohesion of coal matrix

Cohesion of coal bedding planes

Figure 5. Cohesion of matrix and bedding planes in suite of “numerical rocks.” 
97

 This suite of material models provides a convenient method for 

the modeler to go from a geologic log to a numerical model in a 

rational, systematic and efficient manner. Figure 7 illustrates this 

process and shows the level of detail needed for geologic logging. 

On the left is a typical core log with geologic description. Geologic 

features at a scale as small as 50 mm are typically recorded for this 

log. Of particular importance to note are soft clay layers or major 

bedding planes with weak infilling. In the middle are UCS 

estimates for each unit from axial and diametral point load tests. 

Finally, on the right are property codes for generating input 

parameters to numerical models using the material model suite 

presented herein. The modeling approach described has been 

verified against detailed monitoring of a coal mine entry done by 

Oyler, et al. (14). 

MULTIPLE SEAM MINING INTERACTION MODEL 

CONSTRUCTION 

 Using the material input parameters described above, models 

were created for the three interaction types as shown in figure 8. 

All models examine mining either above or below a gob-solid 

boundary which is representative of most interactions including 

mining above or below pillar remnants or barrier pillars. For the 

undermining case, a longwall is mined first in the upper seam 

followed by room-and-pillar development in the lower seam. With 

overmining, a longwall is mined first in the lower seam followed 

by room-and-pillar development in the upper seam. With 

simultaneous mining, room-and-pillar development is done in the 

upper seam followed by longwall mining in the lower seam. These 

model types enable detailed examination of the failure mechanics 

of coal mine entries subject to multiple seam mining interactions 

with the focus being on the transition zone either above or below 

the gob-solid boundary. 

0

5

10

15

20

25

Soil_0

Soil_1

Soil_2

Soil_3

Soil_4

Soil_5

Rock_A

Rock_B

Rock_C

Rock_D

Rock_E

Rock_F

Rock_G

Rock_H

Rock_I

Rock_J

Coal_1

Coal_2

Coal_3

Coal_4

Modulus (GPa) .

Modulus of soil (GPa)

Modulus of rock (GPa)

Modulus of coal (GPa)

Figure 6. Modulus of matrix in suite of “numerical rocks.” 

Stratigraphic column 

Rock type Thickness (m) 

UCS from axial point 

load tests (MPa) 

UCS from diametral 

point load tests (MPa)

Rock matrix 

strength code 

Bedding plane 

strength code 

Strong sandstone 2.4 90 80 Rock_I Rock_I 

Siltstone (stackrock) 1.2 35 5 Rock_E Rock_B 

Black shale 1.9 10 5 Rock_B Rock_B 

Soft clay 0.05 0.2 0.2 Soil_3 Soil_3 

Grey shale 1.8 25 10 Rock_D Rock_C 

Figure 7. Going from core log to numerical model input parameters. 

Figure 8. Multiple seam mining interaction models for three 

interaction types. In undermining, a longwall is mined in the upper 

seam first followed by room-and-pillar mining in the lower seam. 

In overmining, a longwall is mined in the lower seam first and the 

upper seam subsides fully followed by room-and-pillar mining in 

the upper seam. In simultaneous mining, the upper seam is 

developed first and then a longwall mines the lower seam causing 

subsidence of the existing workings in the upper seam. Dark 

shading indicates intact rock. Lighter shading indicates matrix 

failure or slip along bedding planes. 

Undermining

Overmining

Simultaneous mining

Longwall first

Room-and-pillar second

Longwall first

Longwall second

Room-and-pillar second

Room-and-pillar first
98

 The models consider a slice of the rock mass 160-m-wide and 

up to 75-m-high. Each model contains 450 elements along the 

width and up to 515 elements along the height. Coal seam 

thickness is 2 m; entry width is 6 m, and entry height is also 2 m. 

The thickness of each geologic layer is about 0.15 m on average 

and ranges from 0.05 to 0.25 m. Most of the rock mass layers are 

assigned properties corresponding to CMRR Unit Ratings of 45 to 

60 which is equivalent to a moderate strength rock mass; however, 

as discussed later, the immediate roof of the affected seam is 

considered separately. The stratigraphy used in the model is 

extracted from a detailed core log similar to that shown in figure 7. 

Weak, moderate and strong sections from this log were assembled 

as needed. While the models are artificial, they have a basis on a 

real geologic log. 

 Following recent work on horizontal stress by Dolinar (15), an 

average horizontal stress of 10 MPa is applied to the model via the 

equivalent horizontal strain. Thus, horizontal stress varies 

according to the relative stiffness of each geologic layer. Stiffer 

layers have higher horizontal stress than softer layers. Vertical 

stress is applied at the top of the model to simulate cover load. 

 The full extraction area is approximated as a strain hardening 

material using the DY or double yield constitutive model that can 

simulate irreversible compaction. Thus, a gob layer replaces the 

mined coal seam over a height of three times the coal seam 

thickness. This approximation leads to subsidence over the full 

extraction area of about 50% of the seam thickness. 

 As shown in figure 9, each model type examined the effect of 

three factors on the mechanics of multiple seam mining interaction. 

 Each variable in the matrix has just two values. Vertical stress is 

either 3 MPa or 9 MPa, which implies an overburden depth of 120 

m (shallow) or 360 m (deep). Interburden thickness is either close 

(7 m) or intermediate (24 m). Finally, the immediate roof quality is 

either weak (CMRR Unit Rating of 30 to 45) or strong (CMRR 

Unit Rating of 60 to 80). 

 Preliminary analysis of case history data presented by 

Ellenberger, et al. (16) suggests that multiple seam mining 

interactions are possible when the overburden-to-interburden 

thickness (OB/IB) ratio exceeds 7 for both undermining and 

overmining cases. By implication, this modeling matrix considers 

3 ratios of OB/IB ratio, namely, 5 where no interaction is expected, 

about 15 to 17 where an interaction is possible and last, 51 where 

an interaction is likely. 

MULTIPLE SEAM MINING INTERACTION MODELS 

COMPARED 

 Figures 10, 11 and 12 compare vertical stress, horizontal stress 

and failure state images, respectively, for the undermining and 

overmining type of multiple seam mining interactions. The top 

image is for an OB/IB ratio of 5 where no multiple seam mining 

interaction is expected. Vertical stress is low (3 MPa) and 

immediate roof rock quality is weak. These models lie near the 

lower left corner of the matrix shown in figure 9. In the middle 

image, the OB/IB ratio is 17 so a multiple seam mining interaction 

is possible. Vertical stress is medium (6 MPa) and immediate roof 

rock quality is medium. These models lie at the center of the 

matrix. Finally, the lower image is for an OB/IB ratio of 51 where 

an adverse multiple seam mining interaction is highly likely. 

Vertical stress is high (9 MPa) and immediate roof rock quality is 

weak. These models lie at the upper left corner of the matrix. 

 The following general observations are noted. 

1. Vertical stress concentrations occur in a narrow band above and 

below the gob-solid boundary. This band is inclined about 20° 

toward the gob both above and below the full extraction seam. 

A vertical stress shadow occurs above and below the gob, and it 

diminishes slowly about 50 m from the gob-solid boundary 

where the gob has fully reconsolidated. 

2. Full extraction mining produces horizontal stress changes 

several tens of meters above and below the mined area. The 

horizontal stress changes occur much farther above and below 

the mined area than do the associated vertical stress changes 

laterally away from that mined area. The horizontal stress 

concentrations may in turn induce rock failure in select 

geologic layers well above or below the mined area that can 

further amplify horizontal stress concentrations in nearby layers. 

3. Bedding plane slip and tensile failure through the rock matrix 

occurs in a narrow band directly above and below the gob-solid 

boundary. This band is more extensive above the extracted 

seam; however, it also extends a considerable distance below it. 

4. For coal mine entries in moderate strength immediate roof rock 

(CMRR Unit Rating of 45 to 60), the extent of rock failure 

through bedding plane shear or tensile failure of the rock matrix 

is about 1 times the entry width. 

 With respect to undermining, the following additional 

observations are noted. 

1. A zone of vertical stress relief occurs under the full extraction 

mining area beginning past the gob-solid boundary and 

extending several tens of meters under the gob. This zone is 

well understood and correlates well with the best practice of 

offsetting gateroads under the gob for optimal stability in 

multiple seam mining. 

2. There is increased bedding plane slip in entries close to directly 

below a gob-solid boundary. The additional failure is slight 

and should not correspond to significant additional support 

requirements. 

3. There is a small increase in the amount of pillar failure in the 

zone below the gob-solid boundary. This increase might 

correspond to additional pillar spalling and nothing more. As 

before, the additional pillar failure is slight and not indicative of 

severe ground control conditions. 

Interburden thickness

Immediate roof rock quality

Vertical stress 

Deep 

(9 MPa) 

Shallow 

(3 MPa) 

Weak 

(30 - 45) 

Strong 

(60 - 80) 

Intermediate 

(24 m) 

Ultra-close 

(7 m) 

Figure 9. Three variables for numerical modeling matrix of 

multiple seam mining interactions. 
99

Undermining OB/IB ratio = 5; vertical stress = 3 MPa Overmining 

 Undermining OB/IB ratio = 17; vertical stress = 6 MPa Overmining 

 Undermining OB/IB ratio = 51; vertical stress = 9 MPa Overmining 

 

Figure 10. Vertical stress comparison. Dark shading indicates high stress, and light shading indicates 

low stress. Applied horizontal stress is 10 MPa. Hatching indicates longwall. 
100

Undermining OB/IB ratio = 5; vertical stress = 3 MPa Overmining 

 Undermining OB/IB ratio = 17; vertical stress = 6 MPa Overmining 

 Undermining OB/IB ratio = 51; vertical stress = 9 MPa Overmining 

 

Figure 11. Horizontal stress comparison. Dark shading indicates high stress, and light shading 

indicates low stress. Applied horizontal stress is 10 MPa. Hatching indicates longwall. 
101

Undermining OB/IB ratio = 5; vertical stress = 3 MPa Overmining 

 Undermining OB/IB ratio = 17; vertical stress = 6 MPa Overmining 

 Undermining OB/IB ratio = 51; vertical stress = 9 MPa Overmining 

 

Figure 12. Failure state comparison. Dark shading indicates intact rock. Lighter shading indicates matrix failure or slip 

along bedding planes. Applied horizontal stress is 10 MPa. Hatching indicates longwall. 
102

 With respect to overmining, the following additional 

observations are noted. 

1. A significant increase in bedding plane slip and tensile failure 

occurs in the interburden and immediate roof rock along with 

pillar failure in the upper coal seam within a narrow band above 

the gob-solid boundary. This observation correlates with 

known decreases in entry and pillar stability in the transition 

zone above a gob-solid boundary (17). 

2. There is no apparent increase in bedding plane slip or 

additional tensile failure in the immediate roof above coal mine 

entries developed in a coal seam that has subsided due to prior 

mining below. This numerical observation also correlates well 

with the good stability generally observed in entries driven in 

fully subsided coal seams. The failure zone induced by entry 

development in subsided ground is not substantially different 

from that of an entry in completely undisturbed ground. 

 With respect to simultaneous mining, the following additional 

observations are noted without showing the associated models. 

1. The horizontal and vertical stress distribution is virtually 

identical to that shown for simple overmining. 

2. The failure mode situation is completely different. When the 

longwall is created in the lower seam, calculations show a wave 

of tensile failure that propagates upward through the rock mass 

and completely envelopes the developed entries within this 

failure zone. Thus, these entries are likely to experience 

deteriorating ground control conditions. This situation is 

completely different from the prior situation of entry 

development in a previously subsided coal seam. 

 Tables 1 and 2 compare notes about the undermining and 

overmining models within the modeling matrix described by 

figure 9. These tables compare conditions around an entry not 

subject to any multiple seam mining interaction to an entry subject 

to full interaction in the area directly above or below a gob-solid 

boundary. The comparisons examine relative changes on vertical 

stress, horizontal stress and failure mode as the OB/IB ratio 

increases from 5 to 51. Figures 10, 11 and 12 help illustrate this 

semi-quantitative comparison of changes in relative stress and 

failure zone size. As expected, the tables show interesting trends 

as the OB/IB ratio increases from 5, where no interaction is 

expected to over 50 where a serious multiple seam mining 

interaction is expected. 

Vertical Stress Comparison 

 In tables 1 and 2, the vertical stress comparison documents 

stress changes in pillars above or below a gob-solid boundary 

compared to a pillar far from the interaction area. As indicated in 

table 1, when undermining with low OB/IB ratio of 5 (figure 10 – 

top), there is little change in vertical stress within pillars near the 

gob-solid boundary compared to pillars far away, no matter 

whether the immediate roof rock is weak or strong. As the OB/IB 

ratio increases to 17 or 51 (figure 10 – middle and bottom), the 

relative vertical stress concentration increases significantly. Again, 

the strength of the immediate roof rock makes little difference in 

this increase. 

Table 1. Comparison of Undermining Models 

OB/IB 

ratio Weak immediate roof rock Strong immediate roof rock 

Vertical stress comparison 

5 little change little change 

15-17 increases about 25 to 50% increases about 50% 

51 increase more than 50% increase more than 50% 

Horizontal stress comparison 

5 increases less than 10% increases about 25% 

15-17 decreases due to failure increases about 50% 

51 decreases due to failure increases more than 50% 

Failure size comparison 

(remote entry and under gob-solid boundary) 

5 slight increase in size 0.1 * entry width and no 

increase in size 

15-17 0.5 to 1 * entry width to 

1 to 1.5 * entry width 

0.1 to 1 * entry width to 

0.25 to 1 * entry width 

51 1 * entry width to 

more than 2 * entry width 

1 * entry width to 

1.5 * entry width 

Table 2 – Comparison of Overmining Models 

OB/IB 

ratio Weak immediate roof rock Strong immediate roof rock

Vertical stress comparison 

5 increases about 20% increases about 20% 

15-17 increases about 25 to 50% increases about 25 to 50% 

51 increase more than 50% increase more than 50% 

Horizontal stress comparison 

5 increases about 10% increases about 50% 

15-17 small increases and 

decreases due to failure 

increases about 50% 

51 decreases due to failure increases more than 50% 

and decreases due to failure

Failure size comparison 

(remote entry and under gob-solid boundary) 

5 0.3 * entry width to 

1 * entry width 

0.1 * entry width and 

no increase in size 

15-17 0.5 to 1 * entry width to 

1 to 1.5 * entry width 

0.1 to 1 * entry width to 

0.1 to 1.5 * entry width 

51 1.5 * entry width to 

more than 2 * entry width 

1 * entry width to 

1.5 * entry width 

 When overmining, it appears there is more upward transmission 

of vertical stress concentration. As indicated in table 2 and shown 

in figure 10, at low OB/IB ratio, a significant relative increase in 

vertical stress does occur. At higher OB/IB ratios of 17 and 51, the 

relative vertical stress concentration increases, but it is not 

substantially different from that seen with undermining. 

Subsidence and the extent of the broken gob above the seam 

horizon may account for the difference at low OB/IB ratio. As with 

undermining, the strength of the immediate roof rock makes little 

difference on the magnitude of the relative vertical stress changes 

in the pillars. 

 The overburden and interburden rock in all models is medium 

strength with CMRR ranging from 45 to 60. Changing the physical 

nature of the interburden rock will change the vertical stress 

distribution; however, it will not change the relative vertical stress 

changes as noted in this comparison. 
103

Horizontal Stress Comparison 

 The horizontal stress comparison documents stress changes in 

the immediate roof rock of an entry above or below a gob-solid 

boundary compared to an entry far from the expected interaction 

area. As shown in tables 1 and 2 and figure 11, at a low OB/IB 

ratio of 5, where no interaction is expected, horizontal stress does 

increase slightly above the background level, and that increase 

depends on the strength of the immediate roof rock. A weak 

immediate roof rock sees a small increase, while strong immediate 

roof rock sees a much larger increase in horizontal stress. The 

relative increase in horizontal stress is more pronounced in 

overmining compared to undermining for reasons of subsidence 

and gob formation noted earlier. 

 As the OB/IB ratio increases, the change in horizontal stress 

from background depends on the strength of the immediate roof 

rock. With weak roof, horizontal stress decreases in the interaction 

area when the immediate roof rock fails and stress is distributed 

elsewhere. With stronger roof, horizontal stress in the interaction 

area can increase dramatically over background (50% or more). 

However at sufficiently high OB/IB ratio, even strong immediate 

roof rock can be made to fail in the interaction zone with a resulting 

decrease in horizontal stress. Table 2 for overmining shows this 

possibility. 

 The role of horizontal stress is crucial for further understanding 

of multiple seam mining interaction. As seen in figure 11, a full 

extraction area induces horizontal stress changes many ten’s of 

meters above and below the mined seam. The magnitude of these 

changes depends on several factors, namely OB/IB ratio, site 

specific geology, the ratio of extraction area width to interburden 

thickness and the ratio of horizontal stress to immediate roof rock 

strength. 

 The OB/IB ratio affects the geometry and the vertical stress 

level of the particular situation. Closer proximity of the affected 

seam to undermining and overmining has the expected effect on 

horizontal stress magnitude. With respect to geology, the major 

variable is the percentage of strong rock in the interburden and 

where that strong rock is located relative to the affected roof. A 

suitably placed strong bed can shield the immediate roof rock of a 

seam from adverse multiple seam mining interaction. The ratio of 

extraction area to interburden thickness is another geometry factor 

that controls how far above or below a full extraction area the 

horizontal stress might change. There are limits on this ratio that 

depend on the whether the full extraction area exceeds it “critical 

width” at which maximum subsidence is achieved and vertical 

stress in the middle of the full extraction area returns to in situ 

value. Finally, the ratio of applied horizontal stress (in situ plus 

induced) to the strength of the immediate roof rock controls the 

degree of multiple seam mining interaction. A higher ratio due to 

either high horizontal stress or low immediate roof rock strength 

increases the chance of an adverse interaction. Horizontal stress 

has been found by Mark and Mucho (18) to be a major factor in 

many ground control problems, especially in the Eastern U.S. 

Failure Size Comparison 

 The failure size comparison documents the extent of rock 

matrix or bedding plane failure in either shear or tension within the 

immediate roof of the affected seam. The entry itself induces a 

failure zone in the immediate roof whose extent depends on 

overburden depth and immediate roof rock quality. This 

comparison notes how much additional failure occurs due to nearby 

multiple seam mining. Failure extent is gauged relative to the entry 

width. 

 When undermining at low OB/IB ratio (table 1), the overlying 

gob-solid boundary is far away, and it induces little additional 

failure about an entry in the potential interaction area (figure 10). 

For high OB/IB ratio, the overlying gob-solid boundary is close, 

and the size of the failure zone grows by more than a factor of 2 in 

weak rock as shown in figure 12. Strong rocks show a similar 

trend as indicated in table 1. At low OB/IB ratio, the interaction is 

negligible, while at high OB/IB ratio, the added interaction is 

severe even with strong immediate roof rock. 

 At intermediate values of OB/IB ratio, induced changes in the 

failure zone extent due to multiple seam interaction can vary 

greatly. For one case in weak roof, it changes from 1⁄2 to 1 times 

the entry width, while in another case it changes from 1 to 11⁄2 times 

the entry width. With stronger rocks, the variability is even more 

pronounced. In one case, failure zone size changes from 1/10 to 1⁄4 

times the entry width and in another it remained the same size at 1 

times entry width. 

 Failure zone size in the overmining models (table 2) showed a 

similar trend. At low OB/IB ratio in weak rock, failure size is 

somewhat larger initially and grows more than in the undermining 

models, while in stronger rock there is no difference. At high 

OB/IB ratio, failure size grows significantly in both weak and 

strong rock, which is indicative of a substantial, multiple seam 

mining interaction. At intermediate OB/IB ratio, failure zone 

extent and its changes vary greatly as in the undermining models. 

 The observed changes in failure zone size reflect similar trends 

as seen with horizontal stress. For OB/IB ratio less than 5, the 

chance of a multiple seam mining interaction is very low, even 

under a weak immediate roof rock. An adverse interaction is 

expected for high OB/IB ratio greater than 50, even under strong 

immediate roof rock. For an intermediate OB/IB ratio of around 17, 

the chance of an adverse interaction depends on the vagaries of the 

interburden rock, in particular site specific geology and the ratio of 

horizontal stress to rock strength, and geometric factors such as the 

ratio of full extraction width to interburden thickness. 

MECHANICAL FACTORS IN MULTIPLE SEAM MINING 

INTERACTIONS 

 The simple modeling matrix reproduces successfully many 

practical observations of multiple seam mining interactions lending 

credibility to the numerical model and the NIOSH input parameters. 

 Close inspection of the models considered here suggests four 

underlying factors controlling the failure mechanics of multiple 

seam mining interactions: 

1. vertical stress concentration 

2. horizontal stress concentration 

3. stress re-direction 

4. bedding plane slip bands. 

 Vertical stress concentrations (figure 13) occur in the vicinity 

of gob-solid boundaries, pillar remnants and similar structures as 

vertical stress is diverted around full extraction areas. The lateral 

extent of these increases is indicated on figure 13 along with stress 

relief areas. The degree of vertical stress concentration decreases 

quickly with lateral distance from this boundary. The extent of 

vertical stress relief above and below the full extraction area 
104

depends on the width of that area. There is also an associated 

vertical stress gradient near a gob-solid boundary. 

 Horizontal stress concentrations (figure 14) also develop 

around full extraction areas; however, their behavior is much more 

complex. Horizontal stress concentration depends on both distance 

above and below the full extraction area and the relative stiffness of 

the geologic layers. Furthermore, horizontal stress concentrations 

can be expected much further above or below a full extraction area 

than vertical stress concentrations can be expected left or right of 

that area. Horizontal stress re-distribution is seen much further 

away, and it may induce failure in certain weaker layers leading to 

even more horizontal stress redistribution. It appears that the effect 

of horizontal stress on multiple seam mining interactions has not 

been explored in any prior studies. The extent of horizontal stress 

concentration and associated failure of select layers may explain 

select cases of successful and unsuccessful multiple seam mining in 

otherwise similar conditions. 

 The combination of vertical and horizontal stress increase in the 

vicinity of a full extraction area and in particular stress gradients 

will re-orient the principal stresses as illustrated in figure 15. This 

seemingly small stress re-orientation has a profound effect on 

bedded rock. In the absence of mining, principal stresses are 

usually oriented parallel and perpendicular to geologic strata 

(figure 15 top) which is a more favorable orientation for strength. 

Full extraction mining re-orients principal stresses to the weaker 

orientation shown in figure 15 bottom. Coal mine entries 

developed in nearby seams in this rotated stress field are much 

more likely to experience unfavorable ground control conditions. It 

vertical stress 

concentration 

vertical stress 

relief area 

Overmining 

vertical stress 

concentration 

vertical stress 

relief area 

Undermining 

Figure 13. Vertical stress concentration above and 

below gob-solid boundaries. Dark shading indicates 

high stress, and light shading indicates low stress. 

Hatching indicates longwall. 

Figure 14. Horizontal stress concentration above and 

below full extraction areas. Dark shading indicates high 

stress, and light shading indicates low stress. Hatching 

indicates longwall. 

horizontal 

stress 

concentrations 

horizontal 

stress 

relief 

Overmining

Undermining

horizontal 

stress 

relief 

horizontal 

stress 

concentrations 
105

is also noted without illustration that re-orientation of the principal 

stress occurs in a fairly narrow vertical band adjacent to the gobsolid interface. 

 The rotated stress field also leads to bedding plane slip in 

narrow, sub-vertical zones above and below gob-solid boundaries 

as seen in the failure state plots in figures 10, 11 and 12. These 

zones of bedding shear are also more likely areas for unfavorable 

ground control conditions. 

TOWARDS DESIGN GUIDELINES FOR MULTIPLE SEAM 

MINING 

 This research seeks to provide design guidelines that enable 

mine planning engineers to correctly assess the safety risk of an 

adverse multiple seam mining interaction based on mine geometry 

factors, mine layout factors and site specific geologic conditions. 

Preliminary analysis of case studies by Ellenberger, et al. (16) 

suggested that for both undermining and overmining, when the 

OB/IB ratio was less than 7, there was little risk of adverse 

interaction. For OB/IB ratio above 16 there was a possibility of 

extreme interaction; however, it became evident that other factors 

in addition to OB/IB also became important. 

 Numerical studies conducted during this research examined the 

effect of OB/IB ratio and the immediate roof rock quality of the 

affected seam on the degree of multiple seam mining interaction. 

The numerical models utilized contain great geologic detail and the 

proper constitutive behavior and are able to capture the essential 

failure modes of the rock mass, in particular, shear or tensile failure 

through the rock matrix or along bedding planes. 

 The numerical models confirm aspects of the initial multiple 

seam mining interaction guidelines above. When the OB/IB ratio is 

less than 5, the models clearly show little if any interaction between 

mining in nearby seams. When the OB/IB ratio exceeds 50, the 

models clearly show an extreme interaction, even with strong roof 

conditions in the affected seam. For the intermediate OB/IB ratios 

considered (15 to 17), the models show that an adverse interaction 

is possible, and they provide some insight into the controlling 

factors. 

 Numerical models show how vertical stresses divert around a 

full extraction area in a seam above or below an active mining 

seam. The lateral extent of vertical stress increase is relatively 

narrow compared to the width of the full extraction area. In 

addition, a zone of vertical stress relief occurs above and below the 

full extraction area. It will extend to a distance up to the “critical 

width” of the extraction area. Horizontal stresses also divert 

around the full extraction area; however, the distance that such 

stresses increase above or below the seam is much larger than the 

lateral extent of vertical stress increase. This distance may be 

approximately equal to the minimum width of the full extraction 

area up to the “critical width.” Thus, the size of this zone of 

vertical stress relief in conjunction with horizontal stress increase 

defines the extent to which adverse multiple seam mining 

interaction could occur. 

 Numerical modeling suggests 4 factors that control multiple 

seam mining interactions and should be considered explicitly in 

design guidelines. 

1. Overburden-to-interburden thickness ratio 

2. Gob width-to-interburden thickness ratio 

3. Site specific geology 

4. Horizontal stress to rock strength ratio 

 As mentioned earlier, the OB/IB ratio affects the geometry and 

the vertical stress level of the particular situation. Greater depth or 

closer proximity of the affected seam to undermining and 

overmining are both known to increase the chance of an interaction. 

 The minimum gob width of the extraction area relative to the 

interburden thickness is another geometric factor that controls how 

far above or below a full extraction area the horizontal stress might 

change. There are limits on this ratio that depend on the whether 

the full extraction area exceeds the “critical width” at which 

maximum subsidence is achieved, and the vertical stress in the 

middle of the full extraction area returns to its original in situ value. 

 With respect to geology, the major variable is the percentage of 

strong rock in the interburden and where that strong rock is located 

relative to the affected roof. A suitably placed strong bed can 

shield the immediate roof rock of a seam from adverse multiple 

seam mining interaction. Finally, the ratio of applied horizontal 

stress (in situ plus induced) to the strength of the immediate roof 

rock strength controls the degree of multiple seam mining 

interaction. A higher ratio due to either high horizontal stress or 

low immediate roof rock strength increases the chance of an 

adverse interaction. While horizontal stress has been found to be a 

major factor in many ground control problems, especially in the 

Eastern U.S. (18), the role of horizontal stress in multiple seam 

mining interactions has received little attention in prior 

investigations. 

BEDDING

FH

FV

Figure 15. Re-orientation of principal stresses leads to failure due 

to multiple seam mining interaction. In single seam mining, farfield principal stresses are generally aligned parallel to the bedding 

planes in which a test specimen is relatively strong. If the far-field 

principal stresses are rotated due to nearby multiple seam mining, 

the bedding planes are oriented in an unfavorable direction in 

which a test specimen is relatively weak. 
106

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