1 

Extreme Multiple Seam Mining in the Central Appalachian Coalfields 

C. Mark, NIOSH, Pittsburgh, PA 

Abstract 

 Coal has been mined in the central Appalachian coalfields of 

southern West Virginia, western Virginia, and eastern Kentucky for 

more than a century. The dwindling reserve base consists in large part 

of coal that would have been considered unmineable by earlier 

generations. Nearly every current operation is working on a property 

where coal has been extracted in the past, from seams either above, 

below, or both. 

 NIOSH is conducting research aimed at helping mine planners 

prevent hazardous conditions due to multiple seam interactions. Todate, more than 300 case histories have been collected from 

underground mines, mainly from central Appalachia. This paper 

focuses on several of the more challenging situations that have been 

encountered, including: 

• Room-and-pillar development 20 ft (6 m) beneath full extraction 

workings at a depth of 1,000 ft (300m) of cover (Virginia) 

• Pillar recovery 45 ft above full extraction workings at 900 ft (270 

m) of cover (Virginia) 

• Near-simultaneous room-and-pillar mining with pillar recovery 

with 40 ft (12 m) of interburden and 1,500-2,000 ft (450-600 m) 

of cover (Kentucky) 

• Longwall mining directly beneath main entries in overlying 

seams (West Virginia) 

•

 Some of these operations have been highly successful in 

overcoming the challenges, others less so. The lessons learned from 

their experience will help ensure that these and similar difficult 

reserves can be mined safely. 

Introduction 

 The central Appalachian region of eastern Kentucky, western 

Virginia and southern West Virginia has produced more than 17 billion 

tons of coal since mining began there nearly 150 years ago. 

Production peaked in the late 1990’s at approximately 275 million 

tons/yr (250 million tonnes/yr), and has since dropped to about 240 

million tons/yr (215 million tonnes/yr). Recent studies have indicated 

that perhaps 70% of the ultimate reserve base in the region has 

already been mined (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 colafields are honeycombed with worked-out mines, 

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

operations. 

 

 In making future mine projections, mine planners must evaluate 

the potential impacts of these multiple seam interactions. When 

undermining (mining beneath old workings), the new developments 

may be subjected to excessive load transfer (figure 1). In the 

overmining situation (figure 2) load transfer can also occur, and in 

addition the ground may have been damaged by subsidence. In some 

cases, multiple seam interactions can be so severe and hazardous that 

mining is impossible. In others, it may be possible to develop pillars 

but not recover them. In many cases, however, the interaction may be 

barely noticeable1

. 

 

Figures 1 and 2. Undermining and Overmining. 

 Some rules of thumb are available to aid in planning for 

multiple seam interactions. Westman, et al. (1997) cite traditional 

reserve estimation criteria which state that when the interval to mining 

 

1

 Other hazards may arise when mining under abandoned works that 

are flooded or contain bad air (Michalek and Wu, 2000), but these are 

not discussed in this paper. 
2 

above or below is less than 40 ft (12 m) the coal is considered to be 

sterilized, but it is considered accessible otherwise. Haycocks (1990) 

found that load transfer interactions were unlikely when the interburden 

between the seams exceeded 110 ft (33 m), but that some factors 

(such as strong sandstone or a limited number of interbeds) could 

reduce this to as little as 60 ft (18 m). With thinner interburdens, 

interactions could be expected. Haycocks and Zhou (1990) also wrote 

that columnization of pillars “is considered the traditional approach to 

multiseam mining, especially when the interburden is less than 50 ft 

(15 m).” 

 

 For overmining situations, Luo et al. (1997) developed a technique 

for calculating a damage rating based on the lower seam extraction 

ratio. However, Lazer (1965) reported that if the lower seam has been 

completely extracted, the upper seam can often be easily mined. The 

overburden mechanics model developed by Kendorski (1990) indicates 

that mining might be difficult within the “caving zone” (where the 

interburden (I)-to-seam-thickness (t) ratio (I/t) is less than 6-10) or the 

“fracture zone” (I/t<24). 

 The time lapse between mining has also been cited as an 

important factor. Intervals of at least two years have been suggested 

to allow the gob to fully consolidate (Haycocks and Zhou, 1990). 

Some studies have indicated that the longer the time lapse, the better 

the conditions that are anticipated. 

 For the past several years, NIOSH has been studying multiple 

seam interactions with the goal of providing mine planners with more 

precise guidelines than are currently available. Nearly 40 mines have 

been visited, and a total of nearly 300 individual case histories have 

been documented. Approximately 80% of these case histories are 

from central Appalachian mines, with the remainder from northern 

Appalachia and from the western coalfields. 

 Each case history in the database has been classified according 

to the severity of the observed interaction. There are four levels: 

• No interaction where conditions appear to be no different from 

those in areas where no past mining has been conducted; 

• Minor interactions where minimal pillar spall or roof cracks 

indicate that there are some changes that can be attributed to 

past mining, but they had no significant effect on mining; 

• Difficult interactions where conditions were severe enough to 

require supplemental support, design changes, or (on retreat) 

abandonment of a few pillars, and; 

• Severe interactions where the area was abandoned and judged 

unmineable. 

 In the course of collecting the case histories, NIOSH has found 

that mining is being conducted under many “extreme” situations, where 

previous mine workings are close by the target seam. Currently 

available rules of thumb imply that mining should be severely restricted 

at these operations, but in many cases the majority of the target 

reserves are being mined with some success. Severe interactions 

have been encountered in some areas of nearly all these mines, 

however. The goal of this paper is to focus on a few of these extreme 

situations, identify those factors which have contributed to severe 

interactions, and discuss the control techniques which have proved to 

be successful. 

Case No. 1: Undermining With 20 Ft (6 M) of Interburden 

 In southwestern VA, NIOSH visited two mines that have exploited 

seams that lie just 20 ft (6m) beneath previously-worked seams. In 

one instance, the Marker seam is being mined beneath the Taggart 

seam, while in the other, the Tiller is being mined beneath the 

Jawbone. The interburden geology is similar in both situations, 

consisting primarily of competent sandstone and siltstone. Lower 

seam Coal Mine Roof Rating (CMRR) values are typically in the mid 

60’s. The depth of cover (H) ranges from 600-1,000 ft (180-300 m). 

 One of the mines is a two-seam operation, with both seams being 

worked by the same operator. All mining has been development, with 

no pillar recovery. The pillars have been stacked directly above one 

another. More than 800 pillars have been developed in this fashion, 

reportedly without serious incident. 

 At the other operation, mining in the upper seam was completed 

approximately 30 years ago, and included large areas where pillars 

were recovered. Due to the variety of upper seam pillar sizes, and 

uncertainty about the surveying, there has been no attempt to 

columnize the pillars. Nevertheless, lower seam mining has been 

largely successful beneath upper seam first-workings. In some areas, 

it has even been possible to partially extract pillars in the lower seam. 

Roof support beneath first-workings consists of 4-ft, no. 5, full-column 

resin bolts supplemented by 6-ft “superbolts” in the intersections. 

 Problems have been encountered when attempting to cross upper 

seam gob lines. On at least two occasions, mining had to be 

abandoned despite the use of longer bolts, cribs, and steel posts. 

Conditions were particularly difficult above an 80 ft wide (24 m) barrier 

pillar separating two gob areas. Where the gob line was successfully 

crossed, the pillar sizes were increased in addition to extensive 

supplemental support being installed (figure 3, see Appendix). 

Case No. 2: Overmining With Minimal Interburden 

 Three mining operations, also in VA, are extracting seams that lie 

less than 45 ft (14 m) above previous workings. In two instances, the 

Jawbone is being mined above Tiller seam workings, in the third, the 

target seam is the Splashdam above Upper Banner workings. The 

Jawbone-Tiller interburden again consists of strong, competent rock, 

while the Splashdam-Upper Banner interval is somewhat weaker 

(CMRR=45 for the immediate roof). The cover is mainly in the 800-

1000 ft (240-300 m) range for the Jawbone-Tiller mines, and is 500-

600 ft (150-180 m) at the other operation. 

 

 At one operation, the interburden is just 20 ft (12 m). Here both 

seams are being mined by the same operator, and pillars have been 

columnized. There has been no second mining in the lower seam, but 

pillars have been fully extracted in four upper seam panels with 

abandoned first workings directly underneath. No problems were 

reported. 

 At the other two operations, the interburden is 35-45 ft (11-14 m). 

Extensive second mining has been conducted in the lower seams, but 

was completed at least 10 years ago. Recent upper seam mining has 

been largely interaction-free above first workings or over gob areas. 

These mines typically use 5-6 ft (1.5-1.8 m) resin bolts for roof support, 

supplemented by “superbolts” or cables when crossing lower seam 

structures (isolated remnant pillars or gob-solid boundaries). 

 

 The lower seams are 4-5 ft (1.2-1.5 m) thick, so the I/t ratio is 

approximately 9. Above lower seam structures, the roof can be 

severely cracked or even “pulverized” into small pieces that fall out 

upon mining and require short cuts. Several types of lower seam 

structure have been encountered: 

• Gob-solid boundaries, where the “solid” can be either unmined 

coal or development pillars, and is at least 150 ft (45 m) wide; 

• Isolated remnant pillars and narrow barriers, approximately 50-

100 ft (15-30 m) wide, with gob on at least two sides, 

• Sandstone channels that were left between gob areas. 

Depending on the width of the channel, these cases were 

classified into one of the previous two categories. 

 Figure 4 illustrates these different kinds of structures. 

 At the two mines, a total of 23 crossings or attempted crossings 

were analyzed. In 11 of these cases, the panels were subsequently 

pillared. Such panels were analyzed twice, first as successful 

development cases, and then as retreat mining cases. 
3 

 Tables 1 and 2 show the results. During development, difficult or 

severe interactions were encountered above only two of the 16 gobsolid boundaries (13%). In contrast, of the seven isolated remnants 

that were overmined, 4 caused interactions that were so severe that 

they stopped mining completely, and a fifth resulted in very difficult 

conditions on advance (71% total). The two relatively successful 

crossings were both at relatively shallow cover. 

 Nine of the successful gob-solid crossings were subsequently 

pillared, about half with minimal problems. When pillars were retreated 

above the two successful isolated remnant crossings, one resulted in 

difficult conditions. 

Table 1. Conditions encountered crossing lower remnant 

structures: Development (Case History No. 2).

Condition 

Structure Type Minor/None Difficult Severe 

Gob-Solid Boundary 9 5 2 

Isolated Remnant Pillar 2 1 4 

Table 2. Conditions encountered crossing lower remnant 

structures: Retreat (Case History No. 2).

Condition 

Structure Type Minor/None Difficult Severe 

Gob-Solid Boundary 4 5 0 

Isolated Remnant Pillar 1 1 0 

 Clearly, isolated remnants pose much more significant hazards at 

these mines than do gob-solid boundaries. The obvious explanation is 

that isolated remnants can result in much greater stress concentrations 

in the adjacent seams. First, they normally carry two stress abutments 

(one from each gob), while gob solid boundaries usually only carry 

one. Perhaps just as important is the load distribution that develops 

within the remnant. As Chase, et al. (2005) pointed out, three different 

kinds of pillars may be defined based on their load distributions: 

• Small, yielded structures that carry relatively small loads (figure 

5a); 

• Wide pillars or gob-solid boundaries, that have localized highstress zones but distribute the load (figure 5b), and; 

• Isolated remnants that are highly stressed throughout (figure 5c). 

 Wide pillars may carry the same (or even greater) load as a 

remnant, but because their load is distributed over a much larger area, 

their “footprint” is less noticeable in seams above or below. 

Case No. 3: Nearly Simultaneous Mining 

 A mining complex in Kentucky is extracting the Kellioka seam and 

Darby seams, which are separated by 40-70 ft (12-21 m) of 

interburden. The interburden consists largely of sandstone. The depth 

of cover reaches 2,000 ft (600 m), so the H/t ratio can be as high as 

50. 

 Both mines are room-and-pillar with full pillar extraction. Mining is 

sequenced from the top down. During the 15 years since mining 

commenced on the property, a number of lessons have been learned 

and incorporated into mine planning. 

 Figure 6 (see Appendix) shows three early attempts to develop 

production panels beneath fully-extracted Darby seam works. In each 

case severe roof conditions above thin barrier pillars isolated between 

two gob areas. The problems were encountered in spite of 

modifications to the pillar size and supplemental roof support. 

 Subsequently, Kellioka workings have been laid out to parallel the 

overlying workings. The width of the Kellioka retreat panels, including 

slab cuts, exactly matches that of the Darby gobs. With this panel 

stacking design, most of the lower seam panel development and pillar 

recovery takes place under the Darby gob. The potential difficulty with 

panel stacking is that the development must cross a gob-solid 

boundary in order to access the reserve beneath the gob. At the time 

NIOSH visited the complex, seven lower-seam panels had been 

successfully extracted using the stacking design. Although some 

difficulties conditions were encountered at gob-solid boundaries, that 

they were much less severe than those associated with the thin 

isolated barrier pillars. 

 

 In the early planning, the mine tried to wait at least 6 months after 

completion of the upper seam retreat mining before developing the 

lower seam works. However, experience showed that “settling time” 

did not have a large effect on the conditions encountered. Recently, 

some lower seam developments have begun as early as one month 

after the overlying panel was extracted. 

Case No. 4: Undermining Pre-Existing Workings 

 In southern WV, mining on several properties has been conducted 

in as many as ten seams. Longwalls have mined large portions of the 

Powellton seam, and are currently working near the bottom of the 

geologic column, in the Eagle and the No. 2 Gas seams. 

 NIOSH studies found that there are numerous instances of 

successfully mining above previously longwalled areas. In most of 

Figure 5. Stress distributions within three types of remnant 

structures. 

A B C

Figure 4. Different types of remnant structures in the 

underlying workings. 
4 

these cases, the interburden between the target seam and the longwall 

gob is at least 180 ft (54 m). 

 

 There have been several instances in which longwalls undermined 

open entries, usually mains, in overlying mines. The results have 

almost always been unsatisfactory. In one instance, a mine was 

maintaining main entries in the 9 ft (3.7 m) Coalburg seam, 560 ft (170 

m) above the 6 ft (1.8 m) No. 2 Gas. The I/t ratio in this case was 

nearly 100, and the overburden-interburden ratio was less than 1.0. In 

addition, over 50% of the overburden was sandstone, and the 

immediate roof consisted of competent sandstone (CMRR=70). 

Finally, 16 ft (4.8 m) vertical cable bolts and cable straps were installed 

together with standing supports (steel props). 

 

 The longwall directly undermined the mains, as shown in figure 7. 

Within days, the Coalburg seam subsidence measured 36-42 inches 

(0.9-1.1 m). The immediate roof was severely fractured, with some 

open apertures of 4 inches (100 mm). Numerous large roof falls 

resulted (figure 8). 

Figure 7. Longwall mining that subsided open entries 560 ft (170 

m) above. 

 

Figure 8. Damage caused when open entries were subsided. 

 This example, and at least 4 others in the data base, show that 

the normal subsidence prediction rules are completely inapplicable 

when open entries are involved. The reasons are not hard to 

understand. Referring again to Kendorski’s overburden mechanics 

model, the Coalburg seam would normally have been safely within the 

“confined zone” within which the ground subsides but no new fracturing 

takes place. However, the entries removed the compressive confining 

pressure, so the rock around the mine openings was subjected to 

severe tensile stress. Had the mains been developed after the 

longwall had been extracted, there might have been no obvious 

evidence of its passage. 

 Another curious case in the data base involved a mining complex 

in Kentucky. A room-and-pillar panel was retreated in the Pond Creek 

seam, and approximately two years later a set of main entries was 

developed in the Cedar Grove seam, 180 ft (54 m) above. The I/t ratio 

was about 5, and conditions were initially excellent with just 4 ft (1.2 m) 

fully grouted bolts. After about two years, however, the roof began to 

deteriorate dramatically. Extensive supplemental support, including full 

cable bolting, wood cribs, and polyurethane injection, eventually had to 

be installed. The most likely explanation is that ground between the 

two seams had not fully subsided when the upper seam entries were 

developed. When it did subside later, it apparently caused the same 

kind of damage as in the longwall case described above. 

Conclusions 

 The case histories presented in this paper, and others contained 

in the NIOSH data base, clearly show that the existing multiple seam 

guidelines should be refined. One general point is that it is seldom 

possible to evaluate the mineability of an entire reserve with broad 

criteria based on factors like the extraction ratio or interburden 

thickness. Multiple seam interactions are highly localized, so it is 

necessary to evaluate the interaction potential from each structure left 

in the previously mined seam. 

 Experience seems to show that where the previous mining has 

been limited to development, it may have little impact on reserves 

separated by interburdens of as little as 20 ft (6 m). In one instance 

described in this paper, columnization was not even necessary. 

Similarly, mining beneath gob areas, where the ground has been 

largely destressed, seldom presents serious problems. 

 When mining above gob areas, some roof fracturing can be 

expected up to a distance of perhaps 24 times the lower seam height. 

Yet the mines described here have encountered few difficulties even 

just 40 ft (12 m) above gob areas. 

 Difficult ground conditions are often encountered when crossing 

from the solid into the gob (or vice-versa). However, by employing 

control techniques including longer pillars, narrower entries, and 

additional roof support, the mines described in this paper have been 

able to cross most of these structures. 

 The most serious interactions occur above or beneath isolated 

remnant pillars, normally 40-100 ft wide, located between two gob 

areas. These types of structures are apparently too wide to have 

yielded, but too narrow to effectively distribute the load. The high 

stresses associated with these types of structures have often stopped 

mining completely. A reserve area that contains many such isolated 

remnants may indeed be unmineable. 

 Severe interactions are also likely if an open entry is undermined 

by longwall or pillar extraction. A large interburden thickness, a low 

depth of cover, and even strong roof may be no protection from the 

damage caused when an open entry is subsided. 

 

 The case histories appear to indicate that the necessary time lag 

between mining the two seams may not be fully understood. In one 

instance, as little as one month appeared to be an adequate “settling 

time,” but in another case, four years may not have been enough. The 

important factor may not be the elapsed time, but whether the 

 
5 

subsidence is complete. Observations from longwall mining indicate 

that subsidence at the surface is often complete within weeks, while 

some abandoned mines have collapsed decades after mining 

(Iannacchione and Mark, 1990). 

 NIOSH is continuing its evaluation of the entire multiple seam 

case history data base. The results will be used to develop suggested 

guidelines for analyzing potential multiple seam interactions. It is 

hoped that these guidelines will help mine operators to more safely 

extract the increasingly difficult reserves in central Appalachia and 

elsewhere. 

References 

1. Bate, R.L. and J.F. Kvitovich (2004), "Quantifying the Coal 

Reserve Dilemma in the Central Appalachian Mining Region", 

SME preprint 04-108, 8 pp. 

2. Chase, F.E., P. Worley and C. Mark (2005), "Multiple Seam 

Mining Interactions: Case Histories from the Harris No. 1 Mine", In 

24th Intl. Conference on Ground Control in Mining, Morgantown, 

WV, pp. 79-86. 

3.

4. Haycocks, C and Y.Z. Zhou (1990), "Multiple Seam Mining: A 

State-of-the-Art Review", in 9th Intl. Conference on Ground 

Control in Mining, Morgantown, WV, pp. 1-11. 

5. Iannacchione, A.T. and C. Mark (1990), "Possible Mechanism for 

Surface Vibrations Near Maxwell Hill, West Virginia", Bulletin of 

the Association of Engineering Geologists, Vol. xxvii, No. 3, pp. 

341-353. 

6. Kendorski, F.S. (1993), "Effect of High-Extraction Coal Mining on 

Surface and Ground Waters, in 12th Intl. Conference. on Ground 

Control in Mining, Morgantown, WV, pp. 412-425. 

7. Luo, J, C. Haycocks and M. Karmis (1997), "Gateroad Design in 

Overlying Multiple Seam Mines. SME preprint 97-107, 4 pp. 

8. Michalek, S.J. and K.K. Wu (2000), "Potential Problems Related 

to Mining Under or Adjacent to Flooded Workings", in 19th Intl. 

Conference on Ground Control in Mining, Morgantown, WV, pp. 

199-203. 

9. Westman, E., J. Luo, C. Haycocks and M. Karmis (1997), 

"Ground Control Criteria for Coal Reserve Estimation in Multiple 

Seam Mines", in 16th Intl. Conference on Ground Control in 

Mining, Morgantown, WV, pp. 311-315. 

10. Lazer, B. (1965), "Mining Seams Above Mined-Out Lower 

Seams", Mining Engineering, Sept., pp. 75-77. 
6 

Appendix 

Figure 3. Interactions resulting from undermining remnant structures with just 20 ft (6 m) of interburden. 
7 

Appendix (cont’d) 

Figure 6. Interactions caused when trying to undermine thin isolated barrier pillars in an overlying seam.