ABSTRACT 

 Multiple seam interactions are a major ground control hazard in 

many U.S. underground coal mines. The two most common types 

are: 

• Undermining, where stress concentrations caused by 

previous full extraction in an overlying seam is the 

primary concern, and; 

• Overmining, where previous full extraction in an 

underlying seam can result in stress concentrations and 

rock damage from subsidence. 

 The National Institute for Occupational Safety and Health 

(NIOSH) has completed a major study aimed at helping to identify 

the location and likely severity of these interactions. In the course 

of field visits to mines throughout the U.S., more than 300 multiple 

seam case histories were assembled into the largest data base of 

multiple seam case histories ever collected. These data were 

analyzed with the multivariate statistical technique of logistic 

regression. The study also employed LaM2D to estimate the 

multiple seam stresses, ALPS and ARMPS to determine pillar 

stability factors, and the CMRR to measure roof quality. 

 The study resulted in the development of a computer program, 

called Analysis of Multiple Seam Stability (AMSS), which can 

help mine planners to evaluate each potential interaction and take 

steps to reduce the risk of ground control failure. AMSS first 

evaluates pillar design by calculating the single seam Stability 

Factor (SFSS) using ALPS or ARMPS. It also automatically 

generates a LaM2D analysis that provides the additional multiple 

seam stress so that the final, multiple seam SFMS can be determined. 

The second part of the AMSS procedure builds upon the statistical 

findings that overmining is much more difficult than undermining, 

isolated remnant pillars cause more problems than gob-solid 

boundaries, and weaker roof significantly increases the risk of 

multiple seam interactions. AMSS quantifies these effects and 

predicts the outcome in terms of three levels of risk: GREEN 

(where a major multiple seam interaction is considered unlikely), 

YELLOW (where adding a pattern of cable bolts or other 

equivalent supplemental support could greatly reduce the 

probability of a major interaction.), or RED (a major interaction 

should be considered likely, and it may be desirable to avoid the 

area entirely). 

ACKNOWLEDGEMENTS 

 The authors would like to thank our colleagues Prof. Zacharias 

Agioutantis, of the University of Crete, and Prof. Keith Heasley, of 

West Virginia University, whose invaluable contributions made the 

development of the Analysis of Multiple Seam Stability (AMSS) 

software package possible. 

INTRODUCTION 

 Studies have estimated that 156 billion tons of coal, 

representing two-thirds of the mineable reserves in the U.S., are 

subject to multiple seam mining influences (Singh and Dunn, 1981). 

In some U.S. coalfields, particularly in Central Appalachia and the 

West, the majority of today’s mines are operating above and/or 

beneath previously mined seams. 

 The effects of multiple seam interactions can include roof falls, 

rib spalling, floor heave, and bumps which can seriously disrupt 

mining operations and threaten the safety of miners. In early 2006, 

a West Virginia coal miner was killed by rib roll that occurred in a 

high-stress zone beneath a remnant structure in an overlying mine 

(MSHA, 2006). 

 Fortunately, not every multiple seam situation results in 

hazardous conditions. Indeed, most do not. Accurate prediction of 

which interactions are likely to be higher risk allows mine planners 

to prepare for them or avoid them. 

 Over the years, multiple seam mining has been the subject of 

much research, both in the U.S. and internationally. Much advice 

on how to mitigate the risk has been presented, but unfortunately it 

is often contradictory. For example, one group of researchers 

wrote that “stresses from superincumbent workings are not 

transferred through shale strata for distances of over 110 ft” 

(Haycocks et al., 1982), while another group indicated that “a stress 

transfer distance of 760 ft has been recorded between longwalls” 

(Haycocks et al., 1992). 

 For the past several years NIOSH has been conducting research 

with the goal of developing better techniques to predict the location 

and severity of multiple seam interactions. In the course of this 

investigation, more than 40 mines were visited across the U.S. 

Analysis of Multiple Seam Stability 

Christopher Mark, Principal Research Engineer

Frank E. Chase, Research Engineer

Deno M. Pappas, Research Engineer

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA
coalfields. The study also made extensive use of numerical models, 

particularly the LaModel family of software (Heasley and 

Akinkugbe, 2004). 

MULTIPLE SEAM MINING IN THE U.S. 

 Figure 1 shows the 5 major regions for underground coal 

mining in the U.S. From the standpoint of multiple seam mining, 

by far the most significant is the Central Appalachian region of 

southern WV, eastern KY, and southwestern VA. Currently, 

underground mines in this region produce approximately 123 

million tons of coal per year, or about 33% of the total U.S. 

underground production (DOE-EIA, 2006). Mining has been 

ongoing in Central Appalachia for nearly 150 years, and recent 

studies have indicated that perhaps 70% of the ultimate reserve 

base in the region has already been mined out (Bate and Kvitovich, 

2004). 

 One consequence of the maturity of the central Appalachian 

coal fields is that nearly every remaining underground reserve has 

been impacted by past mining activity. The mountains of the 

central Appalachian coalfields are honeycombed with worked-out 

mines, located above, below, and adjacent to today’s and 

tomorrow’s operations. Full-extraction is also widely practiced in 

the Central Appalachian coalfields. While only 8 mines currently 

employ the longwall method (Fiscor, 2007), a recent survey 

indicated that approximately 315 mines, accounting for 58% of the 

room and pillar production in the region, engage in pillar recovery 

(Mark et al., 2003). The prevalence of full extraction adds greatly 

to the potential for multiple seam interactions. 

 The Western U.S. is the next most significant area for multiple 

seam mining. Here, in the states of UT, CO, WY, and NM, nearly 

95% of underground production comes from 13 longwall 

operations (DOE-EIA, 2006; Fiscor, 2007). Approximately half of 

these are operating in multiple seam configurations. In contrast to 

Central Appalachia, in the West the same mining company is 

usually responsible for all the mining on a property. As a result, a 

greater degree of multiple seam planning is normally possible. On 

the other hand, when combined with deep cover and strong roof 

and floor rock, multiple seam interactions can contribute to deadly 

bump hazards (Peperakis, 1968; Iannacchione and Zelanko, 1995). 

 In none of the other three underground mining regions are 

multiple seam interactions currently a major factor, though all three 

have historically had problems (Kohli, 1992; Paul and Geyer, 1932; 

Zachar, 1952), and they may very well have them again in the 

future. Factors that contribute to the relative lack of multiple seam 

interactions in these regions include: 

• Most longwall production in the Northern Appalachian 

and Alabama coalfields is from a single seam (the 

Pittsburgh and the Blue Creek seams respectively), 

without significant mining in other seams above or below; 

• The depth of cover, particularly for room and pillar mines, 

is relatively low in Northern Appalachia and the Illinois 

Basin, and; 

• Very few room and pillar mines engage in full-extraction 

pillar recovery in the Illinois Basin, and there is almost 

no room and pillar mining at all in Alabama. 

HAZARDS ASSOCIATED WITH MULTIPLE 

SEAM MINING 

 Ground instability is usually the greatest hazard due to multiple 

seam interaction. Interactions may be classed into four major 

categories, depending on the mining method, the mining sequence, 

and the thickness of the interburden. Other potential hazards are 

associated with inflows of water, gas, or oxygen-deficient air. 

Undermining, the first category of interaction, occurs when the 

upper seam has been mined first and the lower seam is the active 

seam (figure 2). In an undermining situation, damage is caused by 

load transfer from highly stressed remnant structures associated 

with full-extraction mining in the overlying seam. For significant 

load transfer to occur, the interburden must be relatively thin, and 

the seams must be relatively deep. 

 Two types of remnant structures can cause undermining 

interactions (figure 3). A gob-solid boundary carries a single, 

distributed abutment load, while an isolated remnant pillar is 

subjected to two, overlapping abutments. As a result, the stress 

concentration on an isolated remnant pillar is usually significantly 

larger than that on a gob-solid boundary, and its impact on 

underlying seams proportionally greater. 

 Stemple (1956) conducted a landmark study of multiple seam 

interactions which involved the collection of 61 case histories from 

the eastern U.S. He found that in all the cases where undermining 

interactions occurred, the depth of cover exceeded 500 ft, and the 

interburden was less than 110 ft. Prof. Chris Haycocks and his 

Figure 1. The five major underground coal mining regions in 

the United States. 

Central

Appalachian

Western 

Alabama

Northern

Appalachian

Illinois

Basin

Central

Appalachian

Western 

Alabama

Northern

Appalachian

Illinois

Basin

Overburden

Upper Seam

(Earlier Mining)

Lower Seam

(Current mining)

Interburden

UNDERMINING

Overburden

Upper Seam

(Earlier Mining)

Lower Seam

(Current mining)

Interburden

UNDERMINING

Figure 2. Undermining 
students and colleagues at Virginia Polytech Institute built upon 

Stemple’s work over a period of nearly two decades, beginning in 

the early 1980’s. Haycocks et al. (1982) emphasized the role of the 

interburden geology in determining the extent of load transfer. A 

softer overburden, either due to a large number of rock layers or a 

low modulus of the individual layers, results in an elongated 

pressure bulb that reaches deeper seams below. Using Stemple’s 

data, Haycock’s et al. (1982) proposed two relationships for 

predicting the critical interburden thickness (Icu) in “room and 

pillar mining”: 

 Icu = 110 - 0.42 Z (1) 

 Icu = 6.8 N + 55 (2) 

Where N is the number of interbeds, and Z is the percent of hard 

rock (sandstone) in the interburden. 

 In European mines, multiple seam interactions have been a 

major concern for many years due to the deep cover and long 

history of mining. In the 1970’s, the National Coal Board in the 

UK collected detailed data from 18 undermining case histories 

(Dunham and Stace, 1978). The study’s authors cautioned that it is 

“extremely dangerous to dismiss interaction effects purely on the 

basis of the thickness of the interval between the seams.” In one 

case in their data base, an isolated remnant pillar caused a 

disturbance 450 ft below, while in another case, a gob-solid 

boundary had no noticeable effect just 90 ft below. 

Overmining, the second type of interaction, occurs when the upper 

seam is extracted after mining is complete in the lower seam 

(figure 4). Load transfer occurs in this situation just as it does in 

undermining (in other words, gob-solid boundaries and isolated 

remnant pillars cause stress concentrations both above and below). 

In addition, however, full extraction of the lower seam normally 

results in subsidence of the overlying beds. Figure 5 is a 

conceptual model that illustrates the type of damage that can be 

expected within the overburden due to subsidence above a full 

extraction panel. Five broad zones of overburden behavior can be 

identified (Singh and Kendorski, 1981; Peng and Chiang, 1984; 

Kendorski, 1993; Kendorski, 2006). The implication of this model 

for multiple seam mining is that when the interburden thickness 

exceeds approximately 6-10 times the lower seam thickness, the 

upper seam should be largely intact, though the strata may be 

fractured or otherwise damaged. 

 Early studies of multiple seam interactions generally concluded 

that overmining could be successful with interburden distances of 

20 ft or even less (Eavenson, 1923; AIMME, 1926; Holland, 1951). 

It seems that these studies described conditions over gob areas, 

however, and did not address the effects of abandoned remnant 

structures. Stemple (1956) observed that some evidence of 

subsidence-induced damage could be observed in nearly every 

overmining case he examined. Haycocks and Zhou (1990) 

proposed several equations for predicting the critical interburden 

thickness for successful overmining, but these have not proven to 

be very useful in practice (Mark et al., 2007; Luo et al., 1997). 

Dynamic interactions occur whenever active mining occurs above 

or beneath open entries that are in use. The most severe dynamic 

interactions occur when a lower seam is longwalled or pillared, 

resulting in active subsidence of the open overlying workings. 

However, damage can also be caused by the abutment stresses 

associated with full extraction in an overlying seam, or even, in 

extreme cases, by development mining above or below. 

Figure 3. Stress concentrations in multiple seam mining. (A) 

gob solid boundary and (B) remnant pillar isolated in the gob. 

Gob Solid Boundary

Abutment 

Stress

Isolated Remnant 

Pillar

9.3 H 5.0 H

A B

Abutment Load

H Overburden Depth

Gob Solid Boundary

Abutment 

Stress

Isolated Remnant 

Pillar

9.3 H 5.0 H

A B

Abutment Load

H Overburden Depth

Overburden

Upper Seam

(Current mining)

Lower Seam

(Earlier mining)

Interburden

OVERMINING

Overburden

Upper Seam

(Current mining)

Lower Seam

(Earlier mining)

Interburden

OVERMINING

Figure 4. Overmining. 

Figure 5. Overburden response to full-extraction mining: (A) 

caving zones, (B) fracture zone, (C) dilated zone, and (D) 

confined zone (after Kendorski, 1993; and Peng and Chiang, 

1984). 
 The conditions associated with dynamic interactions are 

generally far more difficult than would have been the case if the 

open workings were developed after the full extraction was 

completed. For this reason, experts have been warning for nearly a 

century that dynamic interactions should be avoided at all costs 

(Eavenson, 1923; Paul and Geyer, 1932; Stemple, 1956; Lazer, 

1965). Unfortunately, at least three relatively recent cases have 

been reported in the literature, and the overlying entries were lost 

or severely damaged in every case (Su et al., 1986; Ellenberger et 

al., 2003; Mark, 2006). Less predictable are instances in which 

delayed subsidence of underlying works has the same destructive 

effect on overlying entries. In one instance, a set of mains was 

developed 180 ft above pillared works, and conditions were 

excellent for two years. Then the roof began to deteriorate 

dramatically, and heavy supplemental support was required to 

prevent major roof collapses (Mark, 2006). 

Ultra-close mining is the fourth type of interaction, and the only 

one in which development mining alone is significant. The primary 

concern is failure of the interburden between the two seams. The 

beam of interburden can fail either through shear caused by pillar 

punching, or by tension caused by the self-weight of the rock plus 

that of any machinery working on it (figure 6). Ultra-close 

scenarios are most likely to occur near where a thick seam splits, or 

where a rider coalbed is of mineable thickness. 

 Ultra-close interactions are unlikely when the two seams are 

more than 20-30 ft apart (Haycocks and Zhou, 1990; Singh et al., 

2002). Zhou and Haycocks (1989) determined that the minimum 

safe working thickness for a massive, unstratified sandstone was 

just 6 ft, while for shale it was 20 ft. Columnization of the pillars is 

considered the standard design practice when ultra-close 

interactions are a concern (Singh et al., 2002; Munsamy et al., 

2004). 

Other multiple seam hazards include the potential for inundation 

from an overlying, flooded mine, particularly where full extraction 

in the lower seam can create a direct pathway between the upper 

and lower seam gobs. MSHA regulations require that a permit be 

obtained prior to mining under a body of water (Michalek and Wu, 

2000). 

 A review of MSHA data indicates that of the 201 inundation 

incidents that were reported between 1996 and 2005, only four 

resulted when caving associated with full extraction in a lower 

seam intersected water-filled overlying workings. Several other 

water inundations occurred when development in a lower seam 

inadvertently cut into uncased boreholes that were connected with 

an upper seam. No injuries were associated with any of these 

incidents. Interestingly, development above gob areas has been 

associated with large, but temporary, groundwater inflows in 

several instances (Stansbury, 1981; Bauer et al., 1992; Lazer, 1965). 

In these cases, the Fractured and Dilated Zones (see figure 5) 

apparently filled with excess groundwater which was drained when 

the entries were developed. Fractures in these zones can also fill 

with methane or oxygen-deficient air, resulting in inflows of 

methane or blackdamp when they are intersected by overmining. 

NUMERICAL METHODS FOR MULTIPLE SEAM 

MINE DESIGN 

 Of the numerical techniques, the Displacement Discontinuity 

models are best suited for stress analysis of the complex threedimensional geometries of many multiple seam situations. 

Displacement Discontinuity models have undergone continuous 

development and improvement over the past two decades (Heasley 

and Su, 2006). The original MULSIM and MULSIM-PC codes 

were limited to purely elastic analyses (Donato, 1992). MULSIMNL allowed yielding of elements within the coal seams and nonlinear gob elements (Zipf, 1992), but the overburden was still 

simulated as one solid material. LaModel introduced a formulation 

that simulates the overburden as a stack of layers with frictionless 

interfaces, thereby providing a more realistic suppleness to the 

strata response (Heasley and Chekan, 1999). LaModel can also 

consider topographic relief and subsidence, and LaModel grids can 

be generated directly from AUTOCAD mine maps (Heasley and 

Agioutantis, 2001). The most recent development is a simplified 

two-dimensional version of LaModel, called LaM2D, which is 

much easier to grid and which runs in a fraction of the time 

required for the full 3D model (Heasley and Akinkugbe, 2004). 

 Chekan and Listak (1993; 1994) employed MULSIM-NL in an 

extensive series of parametric studies evaluating the effects of 

mining sequence and orientation on multiple seam interactions. 

Their most significant findings were: 

• Peak multiple seam stresses are greater when retreating 

from the solid toward the gob than they are when 

retreating from the gob to the solid (figure 7); 

• Stresses on the longwall face are greatest when the face is 

being retreated in a direction directly perpendicular to a 

remnant structure in the other seam, and; 

Figure 6. Ultraclose mining (after Chekan and Listak, 1994). 

Figure 7. Influence of retreat direction on multiple-seam

interaction: (A) retreating from solid to gob creates an

unfavorable “stress window,” while (B) retreating from gob to 

solid results in lower stress concentrations (after Chekan and

Listak, 1993). 
• Orientation relative to other seam remnant structures is 

not a major factor for development workings. 

 Hsuing and Peng (1987a) used finite element modeling to 

develop some rules-of-thumb for undermining. They concluded 

that if the interburden thickness is 2-3 times the width of the upper 

seam isolated remnant pillar, no interaction is likely to occur. On 

the other hand, when the interburden is less than 10 times the 

mining height of the upper seam, the models indicated that the 

lower seam is likely to be fractured as well as highly stressed. 

Hsuing and Peng (1987b) also indicated that it is best to retreat 

from the gob towards the solid, and that the best situation occurs 

when a longwall face maintains an approach angle of about 30 

degrees to remnant structure. 

 Some recent examples of finite element and finite difference 

model applications to multiple seam mining include 2- and 3-D 

analyses of pillar and roof stability in overmining cases from 

northern WV (Zhang et al., 2004; Morsy et al., 2006). Zipf (2005) 

focused on the effects of vertical stress, horizontal stress, stress 

reorientation, and bedding slip on failure mechanics during 

multiple seam mining. Gale (2004) evaluated different stacked 

longwall chain pillar layouts in the Australian context, and 

concluded (as have many others) that the offset arrangement is far 

superior to vertical stacking. His models also predicted that stress 

transfer might be observed up to 4 pillar widths above and below a 

chain pillar, which would be approximately 400 ft for a typical 

Australian longwall design. 

FACTORS AFFECTING MULTIPLE SEAM 

INTERACTIONS 

 Nearly a century of research has identified a number of 

qualitative factors that can affect the intensity of a multiple seam 

interaction. These include: 

• Depth of cover: The deeper the overburden, the greater 

the potential stress concentration caused by multiple 

seam mining. 

• Mining sequence: Overmining is more difficult than 

undermining, because of the potential for rock damage 

caused by subsidence. Dynamic interactions (particularly 

retreating beneath open works) should be avoided at all 

costs. 

• Interburden thickness: The smaller the distance between 

the seams, the greater the intensity of the potential 

interaction. 

• Type of remnant structure: Isolated remnant pillars that 

are surrounded by gob cause more intense interactions 

than do gob-solid boundaries. First workings are 

generally not a concern unless the seams are ultra-close. 

• Interburden geology: Stronger, less bedded interburden 

tend to distribute multiple seam stress concentrations 

more rapidly, resulting in less intense interactions. 

• Immediate roof geology: Weak roof (and floor) are more 

likely to be damaged by multiple seam interactions. 

• Angle of approach to remnant structure: Retreat mining 

should proceed from the gob towards the solid side of a 

gob-solid boundary, and a longwall should not be brought 

broadside into long remnant structure. 

 The goal of the NIOSH multiple seam study was to quantify the 

effects of these factors, so that they can be evaluated on a sitespecific basis and used in design. 

NIOSH MULTIPLE SEAM DATA BASE 

 In conducting the study, NIOSH relied primarily on an 

empirical approach. Empirical methods in ground control start 

with the concept that real-world mining experience, in the form of 

case histories, can provide valuable insight into the performance of 

very complex rock mechanics systems. In recent years, statistical 

analysis of large ground control case history data bases has led to 

the development of methods for longwall pillar design (Mark et al., 

1994; Colwell et al., 1999), roof bolt selection (Mark et al., 2001), 

retreat mine pillar design (Mark and Chase, 1997), and the design 

of rib support (Colwell and Mark, 2005). While fairly uncommon 

in mining, modern empirical research methods based on 

quantitative data analysis using statistics are the foundation of 

econometrics, epidemiology and many other scientific disciplines. 

 Past empirical studies of multiple seam mining have floundered 

because the data bases were too small for the large number of 

geologic and mining variables involved in multiple seam 

interactions, and because bi-variate analyses are inappropriate 

when there are so many variables involved. The key to the success 

of the NIOSH study was the assembly of the largest data base of 

multiple seam case histories ever collected. 

 The mines included in the NIOSH data base were identified 

through discussions with mining company personnel and MSHA 

Roof Control Specialists in each District. The study focused on 

those mines that had experienced the most difficulties with multiple 

seam interactions. A total of 44 mines were visited in the course of 

the study, nearly all from the Central Appalachian and Western 

coalfields (figure 8). 

 The key goal of each mine visit was to develop a history of 

multiple seam interactions for the operation. Care was also taken 

to collect successful case histories as well as unsuccessful ones. 

Overlay mine maps, showing both the active mine and past 

workings above and/or below, were reviewed with experienced 

mine officials who had first-hand experience of the conditions 

encountered. Every instance where the active mine had crossed a 

gob-solid boundary or a remnant pillar was discussed. The officials 

also provided their best recollection of the support used and other 

relevant information. These discussions resulted in a preliminary 

Central

Appalachian

Coalfields

Western

Coalfields 

Western

Coalfields 

Figure 8. Location of mines included in the NIOSH multipleseam data base. 
list of case histories for the operation. In general, only those case 

histories where problems might reasonably have been expected 

were documented. The many cases where the seams were clearly 

too far apart for interaction to occur were ignored. 

 Underground investigations were also conducted at nearly 

every mine. It was seldom possible to access more than a few of 

the historic interaction sites, because many were in sealed or 

otherwise inaccessible areas. However, underground observations 

provided a sample of the ground conditions associated with 

interactions at that mine. Mapping of the conditions was only 

conducted in rare instances. The underground visits also provided 

raw data on roof geology and strength for determination of the Coal 

Mine Roof Rating (CMRR). 

 The mine officials were also asked to provide Autocad files 

with mine maps for all the seams on the property, together with 

exploratory bore logs. These data were subsequently analyzed by 

NIOSH to complete the data base. 

PARAMETERS IN THE DATA BASE 

 In any empirical study, the most important parameter is the 

“Outcome” for each case history. The multiple seam study 

employed a combination of reported conditions and evidence from 

the mine map to rate the Outcome. For example, where a roof fall 

occurred above or beneath a remnant structure, but the map showed 

that nearby non-interaction areas encountered a similar density of 

roof falls, the case would be eliminated from the data base. 

 The two Outcomes were defined as follows: 

• Successful cases were those where undermining or 

overmining had occurred, but no effect was reported in 

the seam currently being mined (the “target seam”). 

Successful cases also included those where the presence 

of past mining was noticed underground (for example, 

there was slightly more rib spall or an occasional roof 

crack), but the interaction was so minor that it did not 

have any effect on mining operations. 

• Unsuccessful cases (Failures) were those where mining 

operations were abandoned, or in which mining was 

completed with significant difficulties, including such 

evidence on the mine map as roof falls, entry segments or 

crosscuts that were not developed, or pillars that were left 

unmined during retreat operations. 

The “explanatory” or “independent” variables are those that are 

thought to possibly contribute to the Outcome. Values for some of 

the variables were readily-available, including whether the case 

was: 

• Overmining or undermining; 

• Development or retreat; 

• Longwall or room-and-pillar. 

Other variable values could be obtained from the mine maps or drill 

logs: 

• Depth to the target seam; 

• Thickness of the interburden; 

• Seam heights for both seams. 

• Time lag between the mining of the two seams (obtained 

by comparing the dates of mining for each seam). 

• Angle of mining at which the active section intercepted 

the remnant structure. 

 The level of roof support was determined during the discussion 

with the mine staff. Originally, the plan was to use the NIOSH 

Analysis of Roof Bolt Stability (ARBS) method to measure the 

amount of support installed. However, it was found that in almost 

every instance the primary support consisted of 4- or 5-ft fullygrouted resin bolts. With so little variation, it did not make sense to 

include primary support as an independent variable. Similarly, 

supplemental support was almost always a pattern of 8- to 12-ft 

long cable bolts or resin-assisted mechanical bolts. Therefore, 

supplemental support was included as a “yes/no” variable. 

 A remnant structure was judged to be an isolated remnant pillar 

if its SF indicated that it concentrated the abutment stress as shown 

in Figure 3. Equation (3) was used to determine whether a pillar 

was considered an isolated remnant or a gob-solid boundary: 

 Wp = 5 H

 (3) 

where Wp is the maximum width (ft) for a remnant structure to be 

considered an isolated remnant pillar, and H is the depth of cover 

(ft). Equation (3) indicates that the maximum width of an isolated 

remnant pillar is 100 ft at 400 ft of cover, and 200 ft at 1,600 ft of 

cover (figure 9). Wider remnant structures were considered gobsolid boundaries, and very narrow pillars (those falling to the left of 

the shaded area in figure 9) were assumed to have yielded. 

 The CMRR was normally determined underground, but the 

core logs often showed that the geology varied from hole to hole. 

Using the underground unit ratings for individual rock layers, 

CMRR values were calculated for each borehole. Each case history 

was then assigned the CMRR determined for the nearest borehole. 

However, since geostatistical studies have shown that immediate 

roof geology is seldom consistent between holes (Mark et al., 2004), 

an average CMRR value was also determined for each mine. 

 None of the case histories had a CMRR of less than 44, which 

means that truly weak roof conditions are not represented in the 

data base. For this reason, and because it was believed that 

Figure 9. Upper and approximate lower limits for determining 

whether a remnant structure is an isolated remnant pillar or a 

gob-solid boundary. 

400

600

800

1000

1200

1400

1600

0 20 40 60 80 100 120 140 160 180 200

Pillar W idth (ft)

Depth of Cover (ft)

Maximum width of 

isolated remnant

Minim um 

width range of 

isolated 

remnant
increasing the CMRR from 40 to 50 has a greater effect on stability 

than an increase from 80 to 90, the CMRR used in the analysis was: 

 lnCMRR20 = ln (CMRR-20) (4)

Since this logarithmic transformation implies that impact of a 

multiple seam interaction is amplified when the roof is weak, using 

the log version means that the ultimate design equation is more 

conservative for the lower CMRR values. 

 The Percentage of Competent Rock in the interburden was 

calculated at each borehole by summing the total thickness of 

sandstone, sandy shale, and limestone, and then dividing by the 

total interburden thickness. The number of beds in the interburden 

was also estimated from the core logs. 

LAM2D AND MULTIPLE SEAM PILLAR 

STABILITY FACTORS 

 An important difference between this study and past multiple 

seam studies is the use of LaModel to obtain estimates of the 

multiple seam stresses for all the case histories. Because of the 

large number of case histories, this study employed the twodimensional LaM2D version of LaModel. Some simplifications 

were necessary in order to create 2-D grids of the case histories: 

• Gob widths were defined by the least dimension of the 

gob (in plan view); 

• Entry widths in the previously-mined seam were adjusted 

so that the model 2D extraction ratio approximated the 

true 3D extraction ratio, in order to more accurately 

simulate pillar yielding; 

• Standard LaM2D defaults for the rock and coal moduli, 

lamination thickness, gob stiffness, and yielding 

properties of the previously mined seam were employed; 

• Coal elements in the target seam were modeled as elastic 

(non-yielding) to better estimate the loads that the ground 

was attempting to apply to the critical pillars, and; 

• An out-of-plane extraction ratio multiplier was applied to 

the target seam to account for the crosscuts that could not 

be modeled in 2D. 

 Figure 10 shows a portion of a typical LaM2D model grid. The 

key stress output from LaM2D was the average multiple seam 

stress on the critical (most highly stressed) pillar. The results from 

the LaM2D analyses were then used to determine the multiple seam 

pillar stability factor (ARMPS SFMS or ALPS SFMS) determined for 

the target seam, using the following formula (using either ARMPS 

or ALPS as appropriate): 

⎥

⎦

⎤ ⎢

⎣

⎡

+ = (single seam load multiple seam load)

single seam load ARPMS SFMS ARMPS SF

 (5) 

 Finally, a SF Rating was calculated by comparing the ARMPS 

SFMS to the recommended ARMPS SF, which depends upon the 

depth of cover (Chase et al., 2002). Those cases where the ARMPS 

SFMS exceeded the recommended SF were given a rating of 1, and 

the others were rated 0. 

DESCRIPTION OF THE DATA BASE 

 The final data base included 344 case histories from 36 

different coal mines. The cases include 252 development cases and 

92 retreat mining cases. Since retreat mining cannot be conducted 

unless development mining was successful, every retreat mining 

case is also included in the development mining data base. 

 Figure 11 shows that more than half the cases in the data base 

involved undermining during development (n=190). Only about 

13% of these undermining development cases were judged to be 

failures. Retreat mining was later attempted in about 40% of the 

undermining cases, and was successful about 65% of the time. 

There are about one-third as many overmining development cases 

in the data base as undermining (n=61), and their failure rate is 

almost three times as great. Retreat mining was only attempted in 

19 of the overminining cases in the data base, with a 68% success 

rate. 

 Figure 12 shows the type of remnant structure encountered for 

the development cases only. It indicates that when undermining 

encountered a gob solid boundary, the crossing was successful 90% 

of the time. The failure rate almost doubled for undermining 

isolated remnant pillars however, from 10% to 19%. Overmining 

developments were successful 73% of the time when crossing gob 

solid boundaries, but that rate dropped to only 59% for remnants. 

0

20

40

60

80

100

120

140

160

180

200

Overmining -

Retreat

Overmining -

Development

Undermining -

Retreat

Undermining -

Development

Number of Cases

Failure

Success

Figure 11. Histogram showing the case history data base by 

type of mining (development or retreat) and type of 

interaction (overmining or undermining). 

Figure 10. A portion of a LaM2D grid. 

UPPER SEAM 

LOWER SEAM

(Seam of Interest)

Remnant Pillar

Critical Pillar

Gob Gob

0

500

1000

1500

2000

2500

0 200 400 600 800 1000 1200 1400 1600

Distance (ft)

Total Vertical Stress (psi)

(Target Seam)
 Figure 13 shows the depth of cover and interburden thickness 

for the undermining cases. In about 90% of the cases, the depth of 

cover ranged between 400 and 1,200 ft. The interburden is less 

than 220 ft in all but 20 cases (and those 20 are all successes). 

 Figure 14 shows the ARMPS SF adjusted for the multiple seam 

stress plotted against the depth of cover. Of the failed cases, about 

one-third had ARMPS SF that were below the recommended values. 

These included nearly one-half of the retreat cases. In contrast, 

more than 87% of the successful cases (including two-thirds of the 

retreat successes) had ARMPS SF that exceeded the recommended 

values. The clear implication is that many multiple seam 

interactions could be avoided simply by adjusting the pillar design 

to account for the additional multiple seam stresses. On the other 

hand, there are still many unsuccessful cases in the data base that 

cannot be explained by improper pillar design. 

STATISTICAL ANALYSIS 

 Logistic regression, the statistical technique employed in this 

study, is the most common multivariate statistical technique used 

when the outcome variable is binary (i.e., there are two possible 

outcomes). The goal of logistic regression analysis is to develop an 

equation that can predict the Outcome using a combination of the 

explanatory variables (Hosmer and Lemeshow, 2000). Further 

details on the logistic regression modeling of the multiple seam 

case histories can be found in Mark et al. (2007). 

 Of the 344 cases in the data base, the outcome in 9 of them was 

considered “borderline,” and these were excluded. An additional 

26 failed cases were excluded because their ARMPS (or ALPS) 

SFMS were less than the recommended values. It was believed that 

the poor mining conditions in these cases were likely attributable to 

inadequate pillar design, rather than to multiple seam interaction 

per se. As a result, the final data base included 309 cases histories. 

 The first step in the analysis was to weight the cases. 

Weighting was necessary because some mines provided a large 

number of case histories, while others provided only a few. To 

fairly represent all these cases, without allowing the data base to be 

overwhelmed by a few mines that contributed many cases, the 

following weighting equation was used: 

 N m

1 case weight =

 (6)

where: Nm = the total number of cases from this mine. 

In other words, the more cases there were from an individual mine, 

the smaller the weight of each individual case, but the greater the 

weight of the mine’s total experience. 

 In designing the analysis, a key issue was whether the 

undermining cases, overmining cases, development cases, and 

retreat cases should be analyzed separately or together. There are 

good scientific arguments for separating them, since the mechanics 

of the interactions may be different in the different groups. The 

disadvantage is that the four data bases would each be much 

smaller. The analyses showed that all four groups could be 

combined and analyzed together (Mark et al., 2007). 

 The final, best model is given below: 

g(x) = –0.81*TVS + 1.79*UO + 0.0233*INT + 2.02*EX 

 – 1.80*REMPIL + 1.95*lnCMRR20 – 6.47 (7)

where: g(x) = A measure of the likelihood of a case being a 

success 

 TVS = Total vertical stress on the critical pillar 

(thousands of ksi) 

Figure 13. Plot showing depth of cover and interburden 

thickness for the undermining cases in the data base. 

0

500

1000

1500

2000

0 100 200 300 400

Interburden (ft)

Depth of Cover (ft)

Failure

Success

0.00

1.00

2.00

3.00

4.00

250 500 750 1000 1250 1500 1750 2000

Depth of Cover (ft)

Actual ARMPS SF

Successes

Failures

Suggested

ARMPS SF

Figure 14. ARMPS SF for the case histories compared 

with the suggested values of the ARMPS SF. 

Figure 12. Histogram showing the case history data base by 

type of remnant structure and type of interaction. 

0

20

40

60

80

100

120

140

Overmining -

gob solid

Overmining -

remnant

Undermining

- gob solid

Undermining

- remnant

Number of Cases

 Failure

Success
 UO = 1 for undermining, 0 for overmining 

 INT = Interburden (ft) 

 EX = 1 for Extra Support, 0 for none 

 REMPIL = 1 for isolated remnant pillar, 0 for gob-solid 

boundary 

lnCMRR20 = ln (CMRR-20) 

 All of the parameters in this model are statistically significant 

at well above the 99% confidence level (Mark et al., 2007). The 

ROC (a logistic regression analog to the r-squared used to measure 

goodness-of-fit in multiple regression (Hosmer and Lemeshow, 

2000)) is 0.88 for this model. 

 For use in mine design, the “cut point” value separating 

predicted successes from failures was adjusted to g(x)=0.86 (Mark 

et al., 2007). Table 1 indicates that with this cut point, the model 

correctly classifies approximately 81% of the cases overall, 

including 79% of the failures. 

 Since equation (7) would be difficult to use directly in design, 

it was rearranged to predict the “critical interburden thickness” 

(INTcrit, ft) and the maximum allowable stress on the critical 

pillar (TVSallow, psi): 

 INTcrit = 35*TVS - 77*UO - 87*EX + 77 *REMPIL 

 83*(lnCMRR20) + 359 (8a) 

TVSallow = 1000 (2.21*UO+0.0288*INT+2.49 

 *EX-2.22*REMPIL+2.41*(lnCMRR20)-10.33) (8b) 

 One disadvantage of equation (8a) is that in some extreme 

cases it can predict a negative value of the critical interburden 

thickness. Therefore, a logistic regression model was determined 

using the log of the interburden in place of the interburden per se. 

The resulting lnINT model was very similar to equation 5, but its 

ROC was slightly less at 0.87. The lnINT model was used to 

derive a second pair of equations for the critical interburden 

thickness INTcritLN and the TVSallowLN. 

INTcritLN = EXP(0.35*TVS - 0.74*UO - 0.99 * 

 EX + 0.74* REMPIL - 0.91*(lnCMRR20) + 7.23) (9a) 

TVSallowLN = 1000 (2.11*UO+2.85*Ln(INT)+ 

 2.83*EX-2.10*REMPIL+2.59*(lnCMRR20)-20.75) (9b) 

 Figure 15 shows that in general INTcritLN>INTcrit when the 

interburden is less than about 50 ft or greater than about 170 ft. 

Since the linear interburden model fits the data slightly better, 

equation (8a) was preferred for most situations. The log 

interburden model is preferred only for the thinnest interburdens, 

where equation (9a) provides the most conservative value for the 

critical interburden. 

 When the actual interburden thickness exceeds the INTcrit 

determined by equation (8a), there is a very good likelihood that 

conditions will be satisfactory. The data in Table 1 and Figure 16 

indicate that within the NIOSH data base, only 4.3% of the cases 

(9.6 of 221.8 weighted cases) where the interburden exceeded the 

critical value defined by equation (8a) were failures. Of the cases 

where the actual interburden was less than the critical value, the 

multiple seam interaction resulted in unsatisfactory conditions 

41.5% of the time. 

Table 1. Performance of the Design Equation against the 

NIOSH data base. 

Model predictions No. of 

cases1

% of total 

cases 

Successes correctly predicted ....................... 212.2 68.7 

Failures correctly predicted ......................... 36.2 11.7 

Total cases correctly predicted .................... 248.4 80.4 

Failures predicted as success........................ 9.6 3.1 

Successes predicted as failures..................... 51.0 16.5 

Total cases incorrectly predicted .................. 60.6 19.6 

 Total cases............................................ 309 100% 1

Weighted using equation (6). 

 Equation (8a) also indicates that to maintain the same outcome 

when other factors are held constant: 

Figure 15. Comparison of Equations 8 and 9, showing that 

the logarithmic version (Equation 9) can yield the most 

conservative value of the critical interburden when the 

calculated interburden is less than about 50 ft. 

0

50

100

150

200

250

1234567

Vertical Stress (ksi)

Critical Interburden (ft)

Linear Equation (8)

Log Equation (9)

Figure 16. Performance of the design equation, comparing the 

suggested critical interburdens with the actual interburdens in 

the case histories. 

0

50

100

150

200

250

300

0 50 100 150 200 250 300

Suggested Interburden (ft)

Actual interburden (ft)

Successes

Failures

Fails: SF

Predicted

Successes

Predicted

Failures
• In order to compensate for an additional 1,000 psi of 

vertical stress on the critical pillar, an additional 35 ft of 

interburden would be required; 

• Overmining requires 77 more ft of interburden (or a 

2,200 psi reduction in the total vertical stress) than 

undermining; 

• An isolated remnant pillar requires 77 more ft of 

interburden (or a 2,200 psi reduction in the total vertical 

stress) than a gob solid boundary; 

• A CMRR = 45 roof requires approximately 50 more ft of 

interburden (or 1,580 psi less total vertical stress) than a 

CMRR = 65 roof. 

 The analysis indicates that by installing a pattern of cable bolts 

or other heavy supplemental support it may be possible to mine 

with 87 ft less of interburden than would be the case without the 

extra support. However, while supplemental support may make 

mining possible, the likelihood of encountering rib spalling, floor 

heave, or hazardous roof also increases when the analysis suggests 

that supplemental support is necessary. 

 In Figure 17, the case histories are plotted again, but this time 

each point is plotted with its suggested overburden for the no extra 

support (EX=0) condition. Three regions are defined on the graph. 

The uppermost region, where the actual interburden exceeds the 

critical interburden when EX=0 is labeled “Predicted Successes.” 

Within this “Green” region, 97% of the case histories that 

maintained an adequate pillar SF were successful. In the middle, 

“Yellow” region, success is predicted only if a pattern of 

supplemental support is installed. Within the Yellow zone, 93% of 

the cases that did install supplemental support were successful, 

while just 63% of those who did not succeeded. In the bottom, or 

“Red” region of the graph, where failure is predicted, only 52% of 

the cases were successful. 

VARIABLES NOT INCLUDED IN THE DESIGN 

EQUATION 

 The design equation does not include a number of variables 

that past studies had identified as being important to multiple seam 

analysis. This does not mean that these variables are not important, 

only that their influence was not identified in this study. For 

example, interburden competence was not significant in the NIOSH 

study. Two related factors may have contributed to this: 

• The percent of competent rock was based entirely on the 

geologic descriptions included with the core logs. In 

many cases, the description was little more than the rock 

type (shale, sandstone, etc.). In the Central Appalachian 

coalfields, however, some siltstones and even shales can 

be very strong (Rusnak and Mark, 2000). Without an 

actual geotechnical description, some weak rocks may 

have been labeled strong, and vice versa. 

• Since the case histories are all from two coalfields where 

the rocks tend to be strong, there may not be sufficient 

variability in the data base to capture the effect of 

interburden competence. 

 The time lag since mining the bottom seam was another 

variable that was not statistically significant. The data base 

contained a total of 12 overmining cases in which the time lag was 

less than 10 years. Of these, all but two were successes, indicating 

that time lag by itself is unlikely to be a major factor. However, 

one of the two failures proved to be a major outlier when compared 

with the rest of the data base. It seems quite likely, in this instance 

at least, that the settling time was important. 

 The lack of influence of the lower coal bed to interburden 

thickness ratio may also be due to the small number of relevant 

cases in the data base. There were 30 cases (21 development and 9 

retreat) in which the interburden thickness was 7.5-10 times the 

lower coal bed thickness. Of these, 13, or 44%, are failures, which 

is a relatively high failure rate. However, the effect may be 

captured by other variables, particularly the interburden thickness, 

which was less than 50 ft in all but one of these cases. It seems 

likely that the upper seam mining in these 30 cases probably took 

place in the fracture zone, above the top of the caving zone which 

is normally 6-10 seam heights above the lower bed (see figure 11). 

It may be that once the upper seam is above the caving zone, the 

lower coal bed to interburden thickness ratio may not be significant. 

However, since all of these cases (but one) came from just two 

mines in Virginia, it is possible that more trouble might be 

encountered in other geologic environments. 

 Retreat mining was another factor that was not significant in 

the final analysis. The effect of retreat mining is indirectly 

included in the total vertical stress variable, however. On average, 

the total vertical stress was 20% greater in the retreat cases than in 

the development cases. 

ANALYSIS OF MULTIPLE SEAM 

STABILITY (AMSS) 

 The results of this study have been implemented in a software 

package called “Analysis of Multiple Seam Stability” (AMSS). 

AMSS requires only that the user input a variety of easily-obtained 

geometric and mining parameters. The program automatically runs 

the necessary LaM2D and ALPS or ARMPS analyses. The 

primary output from AMSS is a three-level (green/yellow/red) 

prediction of the intensity of the multiple seam interaction that is 

likely to be encountered. The step-by-step procedure for using 

AMSS follows: 

1. Identify critical remnant structures on the maps of mining 

in seams above and below the target seam. Every 

Failures: No Extra Support Successes: No Extra Support

Success: With Extra Support Failures: With Extra Support

Figure 17. Suggested critical interburden values setting EX=0.
remnant structure that may be crossed by active mine 

workings should be evaluated. 

2. For each potential remnant structure crossing, determine 

these AMSS input parameters using the maps and core 

logs: 

• Depth of cover to the target seam; 

• Interburden thickness; 

• Seam heights (both seams); 

• Age of the older workings; 

• CMRR for the roof of the target seam. 

3. Check that the parameters of the case being considered 

fall within the limits of the AMSS data base. If the roof 

of the active seam is very weak (CMRR<45) or the stress 

is very high (>5,000 psi) then AMSS should be used with 

caution. The same is true if the case involves overmining 

and the lower coalbed thickness to interburden ratio is less 

than 10. If the interburden thickness is less than 30 ft in 

either undermining or overmining, then potential for an 

ultra-close interaction should be the primary consideration. 

AMSS will help with this by printing a "warning" if the 

data entered falls at the margins of the data base. 

4. Determine whether the remnant structure is a gob-solid 

boundary or an isolated remnant pillar. Figure 9 may be 

used if the structure is a pillar. If the remnant pillar is so 

small that it may have failed completely, it may be helpful 

to determine its ARMPS SF. 

5. Enter the AMSS parameters on the first input page of the 

program. These parameters include: 

• Whether the active mining is longwall or room and 

pillar; 

• Whether the case is undermining or overmining; 

• The interburden thickness; 

• The type of remnant structure; 

• The active seam CMRR; 

• The previously mined seam thickness; 

• The width of gob areas, and; 

• The width of the isolated remnant pillar (if present). 

6. Enter the mining parameters for the active seam into the 

ARMPS or ALPS module for the proposed section in the 

target seam. AMSS then automatically conducts a single 

seam ALPS or ARMPS analysis, as appropriate. 

7. AMSS automatically creates a LaM2D grid, and conducts 

a LaM2D analysis of the remnant structure crossing. It 

then determines the multiple seam stress applied to the 

critical pillar in the target seam. 

8. AMSS determines the ARMPS or ALPS multiple seam 

pillar stability factor (SFMS) for the target seam using 

equation (5), and compares it to the recommended 

ARMPS or ALPS SF. If the calculated SFMS is lower 

than the recommended value, then AMSS will print a 

“warning” suggesting that the pillar size should be 

increased. 

9. AMSS will use the design equations (equations 8 and 9) 

to determine the critical interburden thickness (INTcrit) 

and the maximum allowable total vertical stress on the 

critical pillar (TVSallow), both with and without 

supplemental support. 

10. AMSS compares the actual interburden and stress with 

the INTcrit and TVSallow values determined in step 9. 

Three predicted outcomes are possible: 

a. GREEN: If INTcrit is significantly less than the 

actual interburden without supplemental support, 

then a major multiple seam interaction can be 

considered unlikely. 

b. YELLOW: If the actual interburden is less than 

INTcrit without supplemental support, but greater 

than INTcrit with supplemental support, then adding 

a pattern of cable bolts or other equivalent 

supplemental support could greatly reduce the 

probability of a major interaction. 

c. RED: If INTcrit even with supplemental support is 

greater than the actual interburden thickness, then a 

major interaction should be considered likely, and it 

may be desirable to avoid the area entirely. 

 

 If desired, the pillar design in the target seam can be adjusted 

before running the program again. Changing the pillar size changes 

the value of the TVS, which can reduce it below the TVSallow 

(reducing the TVS also reduces the INTcrit.). Finally, if the case 

still falls within the “Yellow” range, it might be desirable to 

conduct a more detailed analysis using LaModel 3D. 

CONCLUSIONS 

 To conduct this study, NIOSH collected the largest data base of 

multiple seam case histories ever assembled. These data were 

analyzed with the multivariate statistical technique of logistic 

regression. The study also employed LaM2D to estimate the 

multiple seam stress, ALPS and ARMPS to determine pillar 

stability factors, and the CMRR to measure roof quality. 

 Several of the study’s findings confirm the conventional 

wisdom about multiple seam interactions. Overmining was found 

to be much more difficult than undermining, and isolated remnant 

pillars caused more problems than gob-solid boundaries. For the 

first time, however, it was possible to quantify these effects for 

protective mine design. 

 The study found that pillar design is a critical component of 

multiple seam mine planning. Many of the failed cases involved 

pillars whose SF appeared inadequate once the multiple seam 

stresses were accounted for. Weaker roof was also found to 

significantly increase the risk of multiple seam interactions. Some 

factors that were not found to be statistically significant included 

the interburden competence, the time lag between mining the two 

seams, the lower coal bed to interburden thickness ratio, and the 

angle between the active mining and the remnant structure. 

 The study resulted in the development of a computer program, 

called Analysis of Multiple Seam Stability (AMSS), which can 

help mine planners to evaluate each potential interaction and take 

steps to reduce the risk of ground control failure. The first step in 

the AMSS procedure is to evaluate the pillar design. The AMSS 

program calculates the single seam SFSS using ALPS or ARMPS, 

and then it automatically generates a Lam2D analysis that provides 
the additional multiple seam stress. If the final, multiple seam 

SFMS appears inadequate, the SFMS can be improved by increasing 

the pillar width, dropping crosscuts, or reducing the entry width. 

 The second part of the AMSS procedure is embodied in an 

equation that predicts the critical thickness of the interburden, 

and/or the maximum allowable pillar stress, required to minimize 

the likelihood of a multiple seam interaction. The program predicts 

the outcome in terms of three levels of risk: GREEN (where a 

major multiple seam interaction is considered unlikely), YELLOW 

(where adding a pattern of cable bolts or other equivalent 

supplemental support could greatly reduce the probability of a 

major interaction.), or RED (a major interaction should be 

considered likely, and it may be desirable to avoid the area entirely). 

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