ABSTRACT

The nature of competition in the coal market tends to deplete 

the most favorable coal reserves first, and forces subsequent

development of mines in more extreme ground conditions such as 

those associated with multiple -seam mining. In fact, 70% of the 

United States coal reserves are in multiple-seam situations. The 

National Institute for Occupational Safety and Health (NIOSH) is 

conducting research to develop mine design algorithms that will 

result in safer multiple seam mines. 

This paper presents an overview of multiple seam issues in the 

central Appalachian coalfields. To date, more than 50 case

histories have been analyzed from 20 operating mines in Eastern 

KY, Western VA, and Southern WV. Each case history is

classified according to the degree of multiple-seam interaction, 

ranging from no apparent interaction to severe interaction with 

direct impacts on mine safety and resource recovery. The case 

histories are also examined with regard to amount and geologic 

characteristics of interburden and overburden, and design stability 

factors. The sequence of mining has also been found to be critical 

when the lower seam is fully -extracted.

This paper compares current case histories to traditional rules 

of thumb. In addition two situations were modeled using LaModel 

software. LaModel has been upgraded to import pillar grids and 

topographic data from AutoCAD files. The cases presented

demonstrate vertical stress capabilities of LaModel.

INTRODUCTION

Over the past decades, coal mine roof/rib falls have been a 

leading cause of underground fatalities and injuries. Previous 

NIOSH research has identified various mining practices that tend to 

pose greater risk to underground personnel as compared to other 

procedures or techniques. NIOSH is currently investigating

hazardous ground conditions that may occur when reserves are 

mined above or below other mined areas. One may perceive the 

normal sequence of mining to be a progressive sequence from the 

top to the bottom in a geologic section such that the normal 

sequence would be all undermining (figure 1). However, geologic 

conditions often place thicker and more desirable coals lower in the 

section, resulting in mining of seams out of vertical sequence. This 

results in subsequent operations that are developed to overmine 

these deeper works (figure 2). 

USING SITE CASE HISTORIES OF MULTIPLE SEAM COAL MINING TO ADVANCE MINE 

DESIGN

John L. Ellenberger, Research Geophysicist

Frank E. Chase , Research Geologist

Christopher Mark, Supervisory Physical Scientist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Figure 1. Schematic of an undermining situation. Upper 

seam mined first, lower seam mined second.

Figure 2. Schematic of an overmining situation. Lower seam 

mined first, followed by the upper seam.

John K. Marshall, Electronics Technician

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Keith A. Heasley, Associate Professor

West Virginia University

Morgantown, West Virginia USA
Researchers have investigated problems associated with

multiple seam mining for over a century (1). Similar problems 

have been discussed as mining has evolved, and multiple seam 

situations have become more and more common (2, 3, 4). 

Researchers (5, 6) have documented and analyzed a number of 

specific experiences of mines operating in multiple seam

configurations. For example, Haycocks, et al (7) analyzed various 

interburden characteristics for 25 case histories and developed 

guidelines on when interactions should be anticipated.

Mining variables and interaction mechanisms have been

recognized in earlier literature (11). Most of the problems that 

result from overmining are also common to undermining. Most 

readily recognized is the tendency to redistribute overburden loads 

where barriers have been left during earlier mining. Load transfer 

(figure 3) results from the weight of overburden being shifted from 

relatively uniform overburden loading to concentrated areas where 

the coal has not been mined. Load transfer may be seen in both 

undermining and overmining situations, but during overmining, 

subsidence may also contribute additional problems such as

fracturing and stress concentrations to further complicate mining. 

The amount and composition of interburden also affects the

interaction with stronger rocks such as sandstone distributing loads 

and creating beam behavior with lesser amounts of interburden. 

The cases for ultra-close mining (25 feet (7.5 m) of interburden or 

less) (16) require quality interburden and the rocks tend to fail in 

shear. When ultra -close mining takes place, a failure in one seam 

may result in pillar instability in the other seam.

A number of studies have identified the various geologic 

parameters and mine design variables that most probably play a 

role in determining whether interactions are likely to occur. As 

computers have become available and progressively more powerful 

engineering tools, researchers have als o developed numerical

modeling methods and designed software programs to predict 

multiple seam interactions (8, 9, 10). The LaModel software 

developed by NIOSH (13, 14, 15) is a boundary -element program 

for modeling stress in coal mines. LaModel calculates stresses and 

displacements in multiple seam situations. Recent upgrades to 

NIOSH’s LaModel program have simplified the process of

developing mine grids and importing topographic files. 

The goal of the current NIOSH research in this area is to 

determine appropriate design methodologies for mining in multiple 

seam extraction situations. In order to accomplish this objective, a 

large data base of underground case histories are being collected at 

mine sites for multivariate statistical analyses. In addition, case 

histories are being analyzed using the LaModel program for

representative overmining, undermining, and subsidence case

history situations. 

CASE HISTORY DATA

Case histories are developed in two steps, the first being a mine 

visit with underground observation, where pertinent data is

collected, with the second portion being detailed analysis of

parameters that comprise the case history, including a variety of 

general and specific characteristics of each site. Involvements of 

operations personnel after the mine visit are generally limited to 

contacts for AutoCAD files and core logs, thereby minimizing 

impacts on mining operators. 

Mine visits consist of a discussion with experienced mine 

officials to review maps and conditions encountered and to collect 

relevant information about the mines involved. This is followed by 

an examination of accessible areas of the mine to assess conditions 

and to collect Coal Mine Roof Rating (CMRR) data. One of the 

key pieces of necessary data is an accurate overlay map including 

workings of both the mine that is under evaluation and the mine 

that is causing interactions. Areas where interactions between the 

mines are identified and apparent similar areas with less or no 

interactions are also addressed. AutoCAD files are also collected 

during the visit, and core hole data are requested either during the 

visit, or as case histories are developed.

Development of Case Histories

While the CMRR values are included as one portion of the 

geologic information in each case history, additional information 

relating to the interburden and overburden is derived from core 

holes drilled in the vicinity of each case history site. Core hole data, 

supplemented by electric logs, when available, provides base data 

relating to interburden thickness, number of geologic units with 

beam forming potential, and the percentage of competent rock 

within the interburden.

The case histories are classified according to the severity of the 

observed interaction. There are three levels:

• No interaction included sections where overmining or

undermining had occurred, but conditions in the subject seam 

did not appear any different as compared to areas where mining 

had been isolated in only one seam. 

• Moderate interactions included those with notably difficult 

ground conditions, but mining was still possible. 

• Extreme interactions included those situations where parts of a 

seam were rendered unmineable due to severe multiple seam 

interactions.

A number of other factors are derived from maps for each case 

history, including mining height, loading conditions, pillar stability 

factors, angle of mining intercept, and dates of mining in each 

seam. Estimates of average seam height for old works were 

included when they did not appear on mine maps, and the dates 

when mining occurred were often estimated on the basis of the 

closure of a mine. 

Figure 3. Load Transfer from an upper seam barrier to lower 

seam entries and pillars.
Several factors posed unusual problems in the multiple seam 

evaluations. For example, mines often have their own coordinate 

system, and the accuracy of the case history analysis depends on 

correct translation of the coordinates. Abandoned workings,

relating to mines closed for more than a generation, are often 

encountered with inconsistent pillar designs, and the evaluation of 

their stability may result in a range of values rather than a single 

value for a site.

Several factors were calculated primarily from core hole data 

relating to both mines in the case history: interburden to lower 

seam thickness ratio, interburden thickness, overburden to

interburden ratio, the number of competent interburden units, and 

finally the percentage of competent units contained in the

interburden.

Summary of the Current Case History Database

The multiple seam studies currently being conducted have 

contributed more than 50 case histories to a database that will 

continue to expand. As of this writing, the cases include 12 from 

Kentucky, 29 from Virginia, and 13 from West Virginia mines. 

Twenty-nine cases are classified as undermining, and 25 cases are 

classified as overmining.

The database includes 12 different coalbeds with a range of 

mining heights of 4.0 to 6.8 feet (1.2 to 2.1 m). Overburden ranges 

from 250 to 2025 feet (75 to 620 m) with 75 percent of the cases 

having overburden of 1000 feet (300 m) or less. CMRR values 

range from a low of 41 to a high of 67. Interburden ranges from 23 

to 680 feet (7 to 210 m) with 34 cases being less than 100 feet (30 

m) and 7 cases with interburden greater than 250 feet (75 m).

Figure 4 shows the undermining case histories, where damage 

to the lower seam is caused entirely by load transfer from the upper 

seam. The data indicate that no significant damage has been 

observed when the overburden-to-interburden (OB/Int) ratio is less 

than approximately 7.0, and the depth of cover is less than 600 ft. 

Extreme conditions seem most likely when the OB/Int ratio is 

greater than 16.

The data also show that is possible to mine successfully, even 

at high cover and with large OB/Int ratios. In these cases, mining 

was carefully planned to take place in the stress shadow beneath 

fully-extracted gob areas. In these same mines, however, attempts 

to cross beneath remnant barrier pillars resulted in extreme

interactions leading to abandonment.

With overmining (figure 5), damage can be caused by both load 

transfer and subsidence. The data show that when the OB/Int ratio 

is greater than 7.0, damage can generally be expected. However, 

extreme damage may also occur at low OB/Int ratios due to the 

subsidence effect. Timing is a lso a critical factor since subsidence 

needs a period of time to stabilize. If the upper seam is developed 

first, and then the lower seam is extracted, severe damage is likely 

during the active undermining, as is illustrated by one of the case 

histories described below. It seems that when the roof is strong, 

the probability of subsidence damage is reduced. 

LaModel Updates

NIOSH has recently upgraded the LaModel program and added 

features that include auto-meshing capabilities for inputting the 

mine and topographic grids (13, 14, 15). The enhanced program 

greatly speeds analyses of the multiple-seam case histories. 

LaModel can generate the mesh directly from an AutoCAD map of 

the pillars. Since AutoCad is used by 90% of the mining industry, 

maps suitable for automatic mesh generation are typically available 

directly from mines. Therefore, any mine having their mine map in 

AutoCad can quickly and easily generate meshes of parts of their 

mine plan for analysis using LaModel. 

SPECIFIC CASE HISTORIES USING LAMODEL

The case histories presented here include an undermining and 

an overmining scenario. In the undermining scenario, the

interburden between the two seams appeared to be sufficient that 

there would be only a minimal interaction; however, the magnitude 

of interaction caused a significant delay in mining in the upper 

seam. The overmining case shows a situation where an abandoned 

channel in the lower seam limited mining, and behaved as a barrier 

with regard to stress transfer to upper seam mining. The input files 

for LaModel were generated using the default physical properties 

directly from the materials wizards in LaModel, and the

topographic and pillar grids were imported directly from AutoCAD 

files.

Figure 4. Undermining cases showing degree of interaction 

plotted by Interburden vs. Overburden (in feet).

Figure 5. Overmining cases showing degree of interaction 

plotted by Interburden vs. Overburden (in feet).
Case History of a Longwall Undermining Active Mining

When overmining reserves that have been fully extracted, it has 

been recommended that the strata be allowed to settle for two or 

more years (11). If two seams are concurrently being mined, 

dynamic interactions can occur in the upper mine when lower seam 

reserves are fully extracted (12). These interactions can cause 

difficult conditions in the upper seam due to subsidence induced 

ground disturbances and stresses. 

In this case history, the upper seam mine was in the process of 

developing mains in the 9-foot (2.7-m) thick Coalburg seam. 

Concurrently, the underlying 5.6 foot (1.7 m) thick No. 2 Gas seam 

was extracted with a longwall. As shown in Figure 6, the longwall 

panel nearly paralleled the upper seam mains. Maximum

overburden above the upper seam varied from 350-400 feet (105-

120 m) and the interburden between the two coalbeds averaged 

560 feet (170 m). This results in an overburden to interburden ratio 

of less than 1.0, and a ratio of interburden to lower seam thickness 

of 100; therefore, the fracture zone might not have been expected to 

extend to the upper seam. In addition, over 50 percent of the 

interburden consisted of massive sandstone, which might also have 

been expected to dissipate the effects of the undermining. Despite 

the optimistic prognosis, precautionary 16 ft (4.9 m) cable bolts 

were installed between the rows of primary supports in the upper 

seam entries before undermining. Cable bolts and cable slings 

were also added as supplementary support in the intersections. 

As the longwall face approached to within 70 ft (20 m) of the 

upper seam workings, small tension cracks began to develop.

Roof/rib control problems intensified as the longwall progressed 

underneath and continued beyond. The majority of the damage had 

occurred when the face had passed by the workings a distance of 

300 feet (90 m), with total damage being sustained at 1,000 feet 

(300 m). In terms of an undermining time frame, most of the 

subsidence was realized by 3.5 weeks; however, it took

approximated 7.5 months for the ground to essentially stabilize. 

Subsidence displacement in the upper seam mine ranged from 36-

42 inches (0.9-1.1 m).

In the upper seam, the competent sandstone immediate roof 

was extensively fractured. Fractures were measured with apertures 

up to 4 inches (0.1 m) and several large falls resulted. Severe rib 

spalling also occurred. It was apparent that the traveling wave of 

subsidence had caused far more damage than might have been 

expected. As the ground settled and reached equilibrium, the roof 

fractures closed and conditions significantly improved.

The LaModel program was used to determine the mining and 

multiple seam-induced stresses, and also to estimate the subsidence 

displacements. Based on the interburden characteristics and pillar 

dimensions, a 50-foot (15-m) lamination thickness and 5 foot (1.5 

m) element width were selected. The modeled area illustrated in 

Figures 7 and 8 is 2,000 by 2,000 feet (600 m). Figure 7 depicts 

the multiple seam-induced stresses generated after one half of the 

lower seam longwall panel had been extracted in the modeled area. 

As would be anticipated, the highest stresses are concentrated in the 

inby tailgate chain pillars. Another highly stressed area is outby 

the face in the front abutment zone. Conversely, figure 7 depicts an 

area of stress relief in the gob. Figure 8 displays the total stress in 

the upper seam due to topography, development, and undermining 

induced stresses. As shown in figure 8, the highest stresses occur 

in the pillar row almost immediately above and outby of the lower 

seam longwall face.

As previously mentioned, LaModel was also used to calculate 

upper seam subsidence displacements. Based on the previously 

mentioned input parameters, the maximum upper seam subsidence 

was calculated to be approximately 9.3 inches (23 cm). Upper 

seam movement was measured and ranged from 36-42 inches (0.9-

1.1 m). When using LaModel, more realistic subsidence

displacements can be attained by calibrating lamination thickness 

or by using the free surface effect. Another calibration option is to 

use a displacement adjustment factor based on site-specific 

conditions. Since the primary objective in this work was to 

examine multiple seam stresses, the subsidence was not calibrated.

Figure 6. Mine layout. Upper seam working consist of 7 

entry mains, lower seam workings consist of longwall and 

accompanying gob areas.

Figure 7. Plan view of multiple seam stress from 

LaModel when the longwall face progressed under the 

overlying mains.
Case History of Overmining a Sandstone Channel 

The second case modeled using LaModel involved a situation 

where the Upper Banner coal seam had been mined, and delimited 

the boundaries of an abandoned channel. Nearly 50 years later, 

overmining was initiated in the Splashdam seam approximately 50 

feet (15 m) above the first mine. The overburden averaged 600 feet 

(180 m) on the active mine. Both mines were recovering

approximately 4.5 feet (1.4 m) of coal. In view of the extended 

period of time between the development of the two seams, one may 

have expected to see little or no interaction; however, operations in 

the upper seam encountered frequent interaction in areas where the 

underlying channel had prohibited mining. In fact, three rows of 

pillars were abandoned in the middle of the upper panel in Figure 9. 

As a result, a portion of this mine was selected to be one of the first 

case histories modeled using LaModel.

The area selected for the study was 2500 by 2500 feet (750 by 

750 m), and an element size of 10 feet (3 m) was selected for the 

seams, and 50 feet (15 m) for the lamination thickness. The grids 

were also rotated to give the appearance of East/West and

North/South entries and cross cuts. Coal strengths and yield zone 

properties were accepted at the defaults from LaModel, and the 

final modulus for a strain hardening gob was set at 300,000 psi 

(2000 MPa). The channel was treated the same as coal while 

running the program, assuming that it may have coal underneath it 

in places, and was viewed as a barrier pillar. Lamination thickness 

was set at 50 feet (15 m).

The study area included over 600 pillars in the active workings. 

The channel outlined by the abandoned workings was represented 

as a series of pillars in direct contact with each other across the 

study area in order to facilitate gridding. Several areas of bad 

mining conditions were outlined on the map of the upper seam 

(figure 9). Pillar remnants along the edges of the channel were 

assumed to have failed, and were assigned gob values in the grid. 

For purposes of modeling, all pillars in the panels were left in place 

in the first step of mining, and in step two all the pillars that were 

recovered up to the point where three rows of pillars were lost were 

removed.

The results of the LaModel analysis of vertical stress are shown 

in figures 10 and 11. It should be noted that the areas having 

higher than normal vertical stress are located directly above the 

remnant sandstone channel with a peak stress of 4077 psi (28 MPa), 

and when the pillars are removed, a peak vertical stress of over 

6300 psi (44 MPa) is shown with its location being at the pillar line 

where pillars had to be abandoned.

Figure 8. Plan view of total vertical stress from LaModel 

when the longwall face progressed under the overlying 

mains.

Figure 9. Plan view showing room and pillar panels and bad 

mining condition areas overlying sandstone channel.

Figure 10. Plan view of total vertical stress from LaModel 

indicating stress concentrations above sandstone channel from 

development.

Figure 11. Plan view of total vertical stress from LaModel 

indicating stress concentrations above sandstone channel with 

pillars removed.
RECOMMENDATIONS AND CONCLUSIONS

Several conclusions can be drawn from the case histories and 

the specific LaModel evaluations discussed in this work.

1. No significant damage has been observed when the

overburden-to-interburden (OB/Int) ratio is less than

approximately 7.0 and the depth of cover is less than 600 ft 

(180 m). 

2. Extreme conditions seem most likely when the OB/Int ratio is 

greater than 16.

3. It is possible to mine successfully, even at high cover and with 

large OB/Int ratios when mining is carefully planned to take 

place in the stress shadow beneath fully-extracted gob areas; 

however, attempts to cross beneath remnant barrier pillars may 

be cause extremely adverse conditions relating to load transfer 

from the upper mine.

4. When the roof is strong, the probability of subsidence damage 

is reduced. 

5. Timing is also a critical factor. If active workings are

undermined, severe damage to the upper seam is likely.

6. Vertical or near vertical load transfer appears to occur

frequently in both undermining and overmining cases where a 

seam is not completely extracted. The sand channel case 

presented in this paper is an excellent example of this transfer.

With the current estimate of 70% of mines operating in

multiple seam situations, the problems encountered by the industry 

will continue for the foreseeable future, and modeling of the 

interactions will continue to have an important role. NIOSH

expects that its current research will provide the mining community 

with additional design tools to minimize exposure to the hazardous 

ground conditions associated with multiple seam mining.

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