ABSTRACT 

 Highwall mining is an important coal mining method. It 

appears that upwards of 60 highwall miners are presently in 

operation, and they may account for approximately 4% of total U.S. 

coal production. A review of the Mines Safety and Health 

Administration (MSHA) data over the 20 year period from 1983 to 

2002 identified 9 fatalities attributable to auger and highwall 

mining of which inadequate ground control accounted for 1/3. In 

the past 5 years, 1 fatality occurred in highwall mining. Estimates 

of the manpower requirements in highwall mining suggest that its 

fatality rate is essentially the same as for surface coal mining, thus 

highwall mining appears to be a very safe modern mining method. 

 Highwall stability is the major ground control related safety 

concern, and operators are required to develop and follow an 

appropriate highwall mining ground control plan. The plans 

usually specify the following geotechnical parameters: hole width, 

maximum hole depth, maximum overburden depth, seam thickness, 

web pillar width, barrier pillar width and number of holes between 

barriers. Calculated web pillar stability factor exceeded 1.3 for 

most designs evaluated. 

 This study examined records from 5,289 highwall miner holes 

with a total completed footage of about 2,560,000 feet to 

understand the reasons for early pull out. Average lost footage is 

typically about 20% of planned footage. Only 35% of the holes 

reached planned depth, and 20% were short due to rockfalls. Water 

and adverse geology accounted for 15% of the losses. 

Mechanical/electrical problems, guidance, and slope stability 

problems accounted for the remaining 30%. Web pillar stability 

factor for these holes also exceeded 1.3 in 95% of cases. 

 Best practices to avoid trapped miners include: avoid mining in 

stream valleys, avoid mining near outside corners, careful 

alignment of each hole and the use of an onboard guidance system. 

Several issues in highwall mining ground control require further 

investigation including highwall mining through old auger 

workings, highwall mining near old underground mines, multipleseam and multiple lift highwall mining and finally the size and 

frequency of barrier pillars. 

INTRODUCTION 

 Auger and highwall mining continues to grow in importance as 

a coal production method from surface mines in the U.S. 

Numerous recent articles in World Coal and Coal Age magazines 

(1-9) discuss recent developments in the technique. According to 

Walker (1), at least 150 auger units are still at work throughout the 

eastern U.S. coal fields. 

 The Superior Highwall Miner Company (5, 6, 10) manufactures 

the SHM1

 shown in figure 1. In its present design, the miner head 

is a modified Joy Mining Machinery miner that feeds a series of 

20-foot-long pushbeams containing counter-rotating augers for coal 

transport. Maximum penetration depth is approximately 1,000 feet. 

 Mining Technologies Inc. (1, 2, 5, 8, 9, 11) builds the Addcar 

highwall miner system shown in figure 2. MTI developed its own 

highwall miner heads that are similar to conventional continuous 

miners. The miner head discharges onto a series of 40-foot-long 

cascading conveyor sections known as the Addcars. Penetrations 

up to 1,600 feet have been achieved. Addcar systems are equipped 

with cameras that allow the operator to view face conditions, and 

the latest models feature a gyroscopic guidance system (HORTA) 

for improved navigational control. 

 

1

Mention of company name or product does not constitute endorsement by 

the National Institute of Occupational Safety and Health.

ANALYSIS OF PRACTICAL GROUND CONTROL ISSUES IN HIGHWALL MINING 

R. Karl Zipf, Jr., Mining Engineer

Suresh Bhatt, Mining Engineer

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania USA 

Figure 1. Superior Highwall Miner under construction. Note 

thin seam cutter-head, control cab and cable reel. 
 Table 1 provides an estimate of the number of mining 

machines, productivity and estimated total production for 2003. 

Total auger and highwall mining production is estimated at 

65,000,000 raw tons. Estimates are that 80% of the highwall 

miners are operating in the central Appalachian coal fields, mainly 

in southern and central West Virginia (MSHA District 4) and 

eastern Kentucky (MSHA Districts 6 and 7). This raw tonnage 

may reduce to about 45,000,000 clean tons, which is about 4% of 

total U.S. coal production. 

Table 1 - Estimated Auger and Highwall Mining 

Production for 2003 

Machine 

Approximate 

number in 

operation 

Productivity

(raw tons 

per year) 

Production 

(raw tons) 

Superior Highwall Miners 30 650,000 20,000,000

Addcar Highwall Miners 30 1,000,000 30,000,000

Augers 150 100,000 15,000,000

 TOTAL (raw tons) . . . . . . . . . . . . . . . . . . . . . . . . 65,000,000

 TOTAL (clean tons) . . . . . . . . . . . . . . . . . . . . . . . 45,000,000

MSHA INCIDENT STATISTICS FOR AUGER AND 

HIGHWALL MINING 

 Researchers at the National Institute for Occupational Safety 

and Health (NIOSH) Pittsburgh Research Laboratory (PRL) polled 

the MSHA accident/injury/illness (AII) file for all incidents under 

the Subunit number 04 which identifies auger/highwall mining 

operations within surface coal mines. In the 20 year period from 

1983 to 2002 the search identified 605 incidents reportable to 

MSHA at auger and highwall mining operations distributed as 

follows: 

- 9 fatalities 

- 460 non-fatal days lost injuries (NFDL) 

- 136 no days lost injuries (NDL) 

 Figure 3 shows that groundfalls accounted for 70 (12%) of the 

605 MSHA-reportable incidents. As shown in figure 4, groundfalls 

accounted for 1/3 of the 9 fatalities. Close examination of the 70 

groundfall related incidents shows that approximately 3/4 of the 

incidents resulted in serious days-lost injuries (figure 5). In fact, 

more than 2/3 of these incidents resulted in at least a week off work 

to recover, and more than half the victims required one month for 

recovery from their injuries. 

 Based on the brief descriptions of the groundfall incidents in 

the MSHA database, the rock fall sizes were classified as large 

(more than 1 yd3), medium (between 1 yd3 and 1 ft3) or small (less 

than 1 ft3). Figure 5 shows that 12 of the incidents (less than 20%) 

were large rock falls and that all the fatalities were associated with 

the large falls. Another 16 incidents were from medium-sized rock 

falls, and the rest, 42 incidents, were from small rock falls, 

although most of the incidents reported resulted in days lost 

injuries. About 2/3 of the injury-producing rock falls were 

classified as small (less than 1 ft3), which is a key piece of 

Figure 2. Addcar Highwall Miner in operation. Note: Addcar 

in launch vehicle, control cab and discharge conveyor. Figure 3. Distribution of 605 incidents in auger and highwall 

mining. 

Handling Material - 24%

Slips and Falls - 20%

Machinery - 16%

Groundfalls - 12%

Handtools - 11%

Powered Haulage - 8%

Ignition (CH4 or Dust) - 2%

AII Other - 7%

Figure 4. Distribution of fatality causes in auger and 

highwall mining. 

Groundfalls (3)

Powered Haulage (2)

Machinery (1)

Other Injuries (3)

Figure 5 – Size distribution of groundfalls.

Large > 1 CY

12 incidents

7 reportable

3 fatalities

2 injuries (DL)

Medium > 1 

CF

16 incidents

13 injuries 

(DL)

3 NDL

Small < 1 CF

42 incidents

37 injuries 

(DL)

5 NDL
information that requires consideration in any proposed scheme to 

detect or eliminate rock fall hazards from highwalls. 

 From 1998 to 2002, one fatality occurred during highwall 

mining. Based on estimates of the number of operating highwall 

miners and the approximate crew sizes, it appears that average 

highwall mining employment during this period is about 1000 

persons. With one fatality over a 5 year period among 1000 

workers, the fatality rate for highwall mining appears to be 0.02 

fatalities per 200,000 employee-hours worked. This rate is 

comparable to the average fatality rate of 0.02 for surface coal 

mining in the U.S. and similar to the average fatality rate of 0.024 

for surface coal mines in Kentucky and West Virginia during this 

same time period. 

 This analysis suggests that as a whole, highwall mining harbors 

the same fatality risk as ordinary surface coal mining, which agrees 

with expectations. Based on the distribution of fatal accident 

classifications shown in figure 4, ground control and in particular 

highwall stability, along with powered haulage and machinery 

accidents posed the major risks in this very safe modern mining 

method. 

GROUND CONTROL SAFETY CONCERNS IN HIGHWALL 

MINING 

 By far, the overriding ground control related safety concern in 

highwall mining is highwall stability (4, 13, 14). As discussed in 

the prior section, 3 of the 9 fatalities connected with auger and 

highwall mining in the last 20 years were caused by highwall 

collapse, and the only fatality that occurred in the last five years at 

highwall mining operations was again due to highwall collapse. 

Thus insuring highwall stability through proper ground control 

engineering is of paramount importance to safe highwall mining 

operations. 

 In the central Appalachians, where the majority of highwall 

mining occurs in the U.S., hillseams are the most prominent 

geologic structures that affect highwall stability. Hillseams (or 

mountain cracks) are near vertical fractures in the rock that are 

formed in response to natural weathering and erosion of hillsides 

(19). They extend from the surface down to several hundred feet. 

Their orientation is roughly parallel to the hillside, but they can 

also run across narrow points or ridge lines. They are often 

accompanied by a secondary set of fractures at right angles to the 

dominant fracture. Hillseams may cause vertical wedges or long 

rectangular slabs to separate from the highwall. Figure 6 shows a 

highwall containing hillseams. A highwall stability safety hazard 

arises when rock slabs that form along the hillseams detach and fall 

away from the highwall face. The resulting rock falls can range in 

size from blocks less than 1 yd3 to large slabs of more 1,000 yd.3

 

Many highwall mining operations will skip a hole where a hillseam 

enters the highwall. Where a hillseam is known to run parallel to 

the highwall face, the entire area between the entry and exit points 

may be skipped. 

 The second concern related to highwall ground control is the 

stability of web and barrier pillars (4, 13, 14, 16, 17). Figure 7 

shows a typical set of highwall mining holes, the web pillar in 

between holes and a barrier pillar (skip block or skip hole) left 

between panels. Proper ground control engineering is required for 

sizing the web and barrier pillars for stability. Layout of these 

pillars has a profound effect on coal recovery and the project 

economics; however, equally important is its effect on highwall 

stability from a safety standpoint. Adequate stability of the web 

and barrier pillars, especially at shallow depth near the surface, is 

essential for highwall stability. Web pillar failure and the 

subsequent subsidence of the overlying rock can destabilize the 

highwall face. Figure 8 shows an area where web pillar failure led 

to large rock falls from the highwall. Figure 9 shows an even more 

spectacular failure (16). In this case, 30 to 50 web pillars failed 

suddenly, which caused substantial rock fall from the highwall. 

The rock fall was sufficient to completely bury a 110 ton coal 

haulage truck. Fortunately no one was in the pit when this failure 

occurred. 

 An additional ground control related safety concern with 

highwall mining is a “stuck” or trapped miner and the ensuing 

retrieval or recovery operation. Anecdotal evidence suggests that 

many trapped miners result from a ground control problem such as: 

- roof fall 

- web pillar failure (ride, squeeze) 

- floor failure in multiple lift mining 

- excessive span due to cross holes 

 Roof rock quality is frequently the cause of a roof fall that 

results in a trapped miner, and it also influences the success in 

retrieving them. The operator is more likely to pull out a trapped 

miner under shale roof than under a strong sandstone roof. Weak 

shale breaks up more easily than strong sandstone during the pull 

out. 

 Many trapped miners also result from rolls. Undulations of the 

coal seam or a change in seam pitch may cause tight spots that can 

trap the highwall miner during withdrawal. 

 When a highwall miner gets trapped, several options exist as 

listed below in order of increasing difficulty and frequency used. 

There are safety hazards to be aware of with each. 

- surface retrieval (pull it out) 

- surface excavation 

- underground recovery 

 Surface retrieval is by far the least complicated option. Many 

operators have built special devices to hook onto separated 

equipment in the hole. The operator pulls on the trapped highwall 

mining equipment with anything available such as the launch 

vehicle, dozers, loaders or haul trucks. The major hazards 

associated with surface retrieval are the tight cables and connectors 

used during the pull. 

 Excavating from the surface may be the safest option since the 

major hazard is again highwall stability. However, removing at 

least 100,000 cubic yards of rock is not uncommon. Furthermore, 

during excavation, the trapped equipment is likely to become 

damaged due to nearby blasting. 

 Underground recovery is arguably the most hazardous and 

essentially requires the set up of a small underground coal mining 

operation. MSHA requires the operator to submit a recovery plan 

to the District Manager that must be reviewed and approved prior 

to beginning the underground recovery. 

 Interviews with numerous MSHA roof control specialists 

suggest that about 10 to 15 highwall mining systems became 

seriously trapped during 2003 and required a substantial retrieval 

effort such as underground recovery, surface excavation or a major 

surface retrieval. If there were about 60 highwall miners operating 

in 2003 then the odds are that about 1 in 4 will become trapped 

during any given year and require a major recovery/retrieval effort. 
 

Figure 6 – Hillseams indicated by arrows in contour mine highwall. Note that weathering along hillseams can extend several 

hundred feet or more below the surface. 

Figure 7 – Typical highwall miner holes and web pillars. Note barrier pillar or “skip hole” indicated by arrow.
Figure 8. Web pillar failure occurred along highwall to the right. Note that several large rock wedges were dislodged as 

indicated by arrows. Undamaged highwall is to the left. 

Figure 9. Site of massive web pillar collapse resulting in highwall slope failure. Photograph is taken from 

adjacent spoil pile. Highwall is about 150 feet high. A 110 ton coal haulage truck is buried in rockfall debris. 
ANALYSIS OF MSHA HIGHWALL MINING GROUND 

CONTROL PLANS 

 MSHA recognizes the ground-control-related safety concerns 

associated with highwall mining and has required each portable 

auger or highwall mining operation to develop and follow “an 

appropriate highwall ground control plan, which addresses the web 

spacing and other measures necessary to safely conduct the high 

rates of recovery.” (12) Various MSHA Coal Mine Safety and 

Health Districts provided NIOSH-PRL researchers with 40 

highwall mining ground control plans. Most of the plans (80%) 

came from the central Appalachians in Kentucky and West 

Virginia, and most (again 80%) were dated 2002 through 2004. As 

expected, about half the plans specified use of a Superior Highwall 

Miner and the other half planned to use an Addcar system. The 

number of plans from MSHA is somewhat lower than the number 

of highwall miners in operation as estimated in table 1; however, 

this minor shortfall does not detract from our conclusions. 

 From these 40 plans, 51 distinct cases were compiled from 

which to evaluate highwall mining designs. Figure 10 shows the 

distribution of maximum seam thicknesses and maximum cover 

depths considered in the plans. In three-quarters of the planned 

highwall mining, maximum seam thickness is between 3 to 6 feet. 

Relatively few highwall miners (less than 12%) have planned 

mining heights greater than 7 feet. Most of these thicker seam 

operations are in the western U.S. in MSHA District 9. In most 

operations (about 82%), maximum depth of cover is less than 300 

feet. The rest have a planned maximum depth of cover in the range 

300 to 500 feet. At this time no one appears to be operating under 

more than 500 feet of cover, although this could change soon. 

 Maximum seam thickness and maximum depth of cover are the 

main inputs for geotechnical design of web pillar width. In about 

15% of the plans examined, the ARMPS program (22) was the 

analysis method. Over 25% used another form of a tributary area 

method for analysis. Past experience was the basis for many 

designs, but unfortunately, the analysis method could not be 

identified in most of the highwall mining ground control plans 

examined. As shown in figure 11a, the minimum web width 

specified in the plans ranges from 3 to 7 feet in over 82% of the 

cases. More important for stability is the width-to-height (W/H) 

ratio of these web pillars. Figure 11b shows that W/H is in the 1 to 

1.25 range for about 50% of the cases, while it is between 0.5 and 1 

in 25% and more than 1.25 in the remaining 25% of cases. In 

general, keeping the web pillar W/H ratio above 1 is desirable to 

maintain better web pillar integrity. Designs with W/H ratio less 

than 0.5 were not encountered. 

 Finally, figure 11c shows estimates of the web pillar stability 

factor based on data provided in the ground control plans. The 

estimates use tributary area method to calculate pillar stress and the 

Mark-Bieniawski formula (18) to calculate strength of a strip pillar 

assuming coal strength of 900 psi. In 45 % of the cases, it appears 

that stability factor exceeds 1.6; while in 31%, the stability factor 

appeared to range from 1.3 to 1.6. Thus, in over three-quarters of 

the cases examined, a satisfactory web pillar stability factor most 

likely exists. However, in a few circumstances (about 8%), 

stability factor may be in the 1 to 1.3 range, while in another 8% 

stability factor was apparently slightly less than 1. These stability 

factor estimates from the ground control plans are estimates only, 

and judgment of individual plans is not implied. 

 Figure 12a provides data on minimum barrier pillar width found 

in the plans. Most barriers appear to range in width from 10 to 25 

feet, but in almost half the cases a firm dimension on barrier pillar 

width was not specified. In most cases, the reasoning behind the 

barrier pillar width was unknown. Experience-based design rules 

were employed in some cases. For example, about 15% of the 

plans sized barrier pillars as 1 web-pillar-width plus 1 hole-width, 

while another 15% used 2 web-pillar-widths plus 1 hole-width. In 

about 10% of the cases, barrier pillar widths were designed using 

tributary area method with a stability factor of one and the 

assumption that all web pillars in a panel have failed. 

 Figure 12b shows data on the W/H ratio of highwall mining 

barrier pillars. Unfortunately there was no information in about 

one-third of the cases considered. However, when data was 

available, the W/H ratio was 3 or more in about two-thirds of the 

cases and less than 3 in the remaining third. For stability reasons, a 

barrier pillar with a W/H ratio above 3 has sound geomechanicsbased advantages (16). 

 Finally, figure 12c presents data on the number of highwall 

miner holes between barrier pillars. When information is available, 

it appears that about 37% of the plans specify no more than 20 

holes between barrier pillars; 44% specify 10 holes, and 15% 

require as few as 5 holes between barrier pillars. Comment on the 

number of holes between barrier pillars is reserved for later 

discussion. 

Figure 10. Maximum seam thickness (top) and maximum 

depth of cover (bottom) distribution from MSHA highwall 

mining ground control plans.

0

5

10

15

20

25

0 to 100 100 to 200 200 to 300 300 to 400 400 to 500 Unknown

Maximum depth of cover - ft.

Number of cases

0

2

4

6

8

10

12

14

16

3 to 4 4 to 5 5 to 6 6 to 7 more than

7

Unknown

Maximum seam thickness - ft.

Number of cases
0

2

4

6

8

10

12

14

3 to 4

4 to 5

5 to 6

6 to 7

7 to 8

8 to 9

9 to 10

more than 10

Unknown

Minimum web width - ft.

Number of cases

0

5

10

15

20

25

30

less than

0.5

0.5 to

0.75

0.75 to

1.0

1.0 to

1.25

1.25 to

1.5

more

than 1.5

Unknown

Minimum W/H ratio of web

Number of cases

0

5

10

15

20

25

less than 1.0 1.0 to 1.3 1.3 to 1.6 more than 1.6 Unknown

Approximate web pillar stability factor

Number of cases

Figure 11. Minimum web pillar width (top), minimum widthto-height (W/H) ratio (middle) and approximate web pillar 

stability factor (bottom) from MSHA highwall mining ground 

control plans. 

0

5

10

15

20

25

less than

10

10 to 15 15 to 20 20 to 25 more than

25

Unknown

Minimum barrier pillar width - ft.

Number of cases

0

2

4

6

8

10

12

14

16

18

< 2

2 to 3

3 to 4

4 to 5

5 to 6

6 to 7

> 7

Unknown

Minimum W/H ratio of barrier pillar

Number of cases

0

2

4

6

8

10

12

14

"5" "10" "15" "20" Unknown

Number of holes between barrier pillars

Number of cases

Figure 12. Minimum barrier pillar width (top), minimum 

width-to-height (W/H) ratio (middle) and number of holes 

between barrier pillars (bottom) from MSHA highwall mining 

ground control plans. 
PERFORMANCE ANALYSIS OF HIGHWALL MINING 

 As part of their safety research effort in highwall mining, 

Pittsburgh Research Laboratory personnel analyzed highwall 

mining performance data from several highwall mining operations. 

For planning purposes, most operators develop maps showing the 

location, orientation and length of their highwall mining holes. 

Upon completion of mining, the maps are updated to show the 

actual hole depth mined and the reason for early pull out, if 

applicable. The analyses conducted herein sought to understand 

the reasons for early pull out from highwall mining holes in order 

to avoid trapped miners, improve highwall stability and improve 

safety. 

 The company maps covered a three-year period from late 2000 

through early 2003 during which time, 5,289 holes were mined for 

a total completed footage of about 2,560,000 feet of highwall 

mining hole. Total planned footage was estimated at about 

3,192,000 feet; therefore, 632,000 feet or about 20% of planned 

footage was lost due to early pull out. 

Hole Completion Analysis 

 This analysis examined the various reasons for early pullout 

from the highwall mining holes. After examining the maps 

containing the highwall mining performance data, eight “loss” 

categories were created to summarize the hole completion notes for 

each hole. The eight categories are: 

1. Full depth – No explanation is necessary. 

2. Mechanical/electrical – due to broken hydraulic hose, low 

oil pressure, an overheated motor, tripped breaker and so 

forth. 

3. Guidance – due to crossed holes. 

4. Slope stability – due to a bad highwall. 

5. Water – due to the hole flooding or severe mud causing the 

hole to become “gobbed out.” 

6. Geology – due to a pinching coal seam, a bad roll or hard 

cutting due to a rock parting. 

7. Rock fall – due to bad ground or bad ribs. 

8. Barrier/skip hole – No explanation necessary. 

 Figure 13a shows the number of successfully completed holes 

(full depth) and the number of holes pulled out of for one of the 

above reasons. Some findings are: 

1. only 35% of holes reached the full depth planned 

2. approximately 20% were short due to rock falls 

3. approximately 15% were short due to water problems 

4. approximately 11% were short because of adverse geology 

5. approximately 10% were short due to mechanical/ electrical 

problems 

6. guidance problems and slope stability accounted for the 

remaining 9% of shortfalls. 

 Further analyses estimated the coal losses due to early pull out 

associated with each of the above loss categories. Figure 13b 

shows that about 31% of the coal losses are due to rock fall, about 

20% of the losses were due to water and “gobbed out” and another 

19% are due to adverse geology. Slope stability and 

mechanical/electrical problems with the highwall miner each 

accounted for about 11% of the losses, and guidance (crossed 

holes) caused the remaining 8% of the losses. A hole completion 

analysis was completed for each property and each seam. Different 

loss categories became more or less important depending on mining 

property or coal seam, but generally, rock fall and water (“gobbed 

out”) remained the dominant reasons for early pull out. 

Stability Factor Analysis 

 For each panel or logical group of highwall mining holes, web 

pillar thickness was measured and a maximum depth of cover was 

estimated. Tributary area theory was used to estimate average 

pillar stress. These pillar stress estimates did not account for the 

presence of barrier pillars so the computed stress is likely 

conservative. Pillar strength was computed using the MarkBieniawski strength formula for strip pillars assuming in-situ 

strength of 900 psi for the coal. 

 The stability factor for over 3000 individual highwall mining 

holes was then estimated from the average stability factor for the 

corresponding panel. About 75% of the stability factors were in the 

range from 1.3 to 2.2 and averaged about 1.6 as expected. About 

5% of the stability factors were in the 1.0 to 1.3 range and the 

remaining 20% were above 2.2. No stability factors were estimated 

below 1.0. 

 Having a stability factor estimate for each highwall mining 

hole, an attempt was made to correlate this stability factor to the 

loss category and some performance measure such as depth of hole 

achieved or coal losses due to early pull out. The working 

hypothesis was that lower design stability factor should result in 

more coal losses due to geotechnical problems such as rock falls or 

possibly slope instability. 

 Four groups of stability factors were considered, namely 1.00 to 

1.30, 1.31 to 1.60, 1.61 to 2.00 and more than 2.01. In each of 

these stability factor groups, the relative losses were evaluated for 

each loss category. Figure 14 shows the result. The 

mechanical/electrical, guidance, slope stability, water (gobbed out) 

loss categories showed no discernable dependence on stability 

factor. The rock fall category does show a clear downward trend 

with higher stability factor; however, it may not be statistically 

significant. For stability factors in the 1.0 to 1.3 range, rockfalls 

0

5

10

15

20

25

30

35

Full Depth

Rock Fall

Water (gobbed out)

Geology

Mech/Elec

Guidance

Slope Stability

Barrier/Skip

Relative Loss %

0

200

400

600

800

1000

1200

1400

1600

1800

Full Depth

Rock Fall

Water (gobbed out)

Geology

Mech/Elec

Guidance

Slope Stability

Barrier/Skip

Number of Holes Mined

Figure 13. Loss category analysis of 5,289 highwall mining 

holes (top) and relative losses versus loss categories-3,192,000 

planned ft. and 632,000loss ft (bottom). 
were the dominant reason for early pullout from the holes. With 

stability factors above 1.6, reasons other than rockfalls become the 

main reason for short holes. Planned stability factor for most of the 

highwall mining holes was about 1.3 and use of this relatively high 

stability factor minimizes losses due to rock falls and other 

geotechnical problems. No statistically significant correlation was 

found between web pillar stability factor and losses due to rock 

falls. The primary recommendation from the stability factor 

analysis is to continue designing web pillars with a minimum 

stability factor of 1.3. 

OPERATING PRACTICES TO AVOID TRAPPED MINERS 

 Up to this point, geotechnical engineering and planning factors 

have been discussed to improve highwall stability and minimize the 

risk of a trapped highwall miner. However, certain operating 

practices can also help decrease this risk. Four practices are 

summarized that can help highwall mining operators avoid 

entrapment of valuable mining machinery in the hole as a result of 

adverse geotechnical conditions. 

Avoid Mining Into The “Head Of Hollow” 

 Stream valleys frequently follow a weakness in the geologic 

strata. In underground coal mining, it is well know that roof 

conditions may become very poor under stream valleys or other 

major surface drainages. The same idea applies to highwall 

mining. Highly fractured zones under or near the stream may lead 

to inflow of water and very poor roof conditions for highwall 

mining and increased risk of a trapped miner. Careful geologic 

mapping should note the orientation of these weakness zones for 

future mine planning purposes. Despite the considerable coal 

reserves that are lost, highwall mining in a head of hollow should 

be avoided. 

Avoid Mining Near Outside Corners 

 Ridge points, outside corners, cut-throughs or other abrupt 

changes in direction of a contour mining highwall are areas with 

increased risk of highwall stability problems. These areas are 

prone to contain hillseams, that daylight from the highwall. In 

addition, outside corners may be more highly fractured due to 

blasting on two sides, especially near cut-throughs. Whenever 

possible, mining should begin a safe distance from the outside 

corner and proceed away from the corner. For unknown reasons, 

pillar stability problems resulting in a trapped highwall miner have 

occurred when mining towards an outside corner. 

Carefully Align Each Highwall Miner Hole 

 Proper hole spacing and alignment at the beginning of each 

hole is essential to maintain design web width for the entire length 

of the hole. Maintenance of proper web width is essential for 

avoiding web failure, crossed-holes and the risk of a trapped 

highwall miner. Careful alignment is especially important in 

multiple lift mining where highwall miner holes are stacked, and 

interburden thickness is less than one hole width. Any 

misalignment can result in collapse of the septum between layers of 

holes and a trapped miner. 

Consider Using An Onboard Guidance System 

 Initial alignment of a highwall miner hole is especially critical 

since it is generally not possible to steer the highwall miner head 

much with either the Superior or Addcar highwall mining system 

once it is in the hole. Local geologic anomalies such as rolls can 

deflect the miner slightly with potentially adverse effects. And as 

Schafer (7) and Arrowsmith (8) point out, once you are in the hole, 

the operator has no idea where the miner really is, thus there is little 

control and verification of actual web width. An onboard guidance 

system provides location of the current hole, relative to the 

previously mining holes. Again, although steering the miner head 

in itself is generally limited, a guidance system is likely to prevent 

many trapped miners due to crossed holes, web pillars too small 

and roof falls caused by poor guidance (17). 

CONCLUSIONS AND RECOMMENDATIONS 

 Under most circumstances, design of web and barrier pillars to 

maintain adequate highwall stability during highwall mining is a 

straightforward and well-understood endeavor. Tributary area 

method (16), ARMPS (18) and numerical models (14, 17, 19) are 

available to estimate pillar stresses. Coal pillar strength is also well 

understood and can be estimated by various coal pillar strength 

formulas such as Mark-Bieniawski formula (18) or others (19, 20). 

In applying these strength formulas, coal strength of 900 psi is 

normally used, unless well-justified field experience dictates 

otherwise. Experience has shown that laboratory tests do not 

necessarily provide a reliable estimate of pillar strength. 

 Several issues pertaining to highwall mining web pillars have 

emerged, namely previous auger mining holes in web pillars and 

the presence of nearby underground mining. Many highwall 

miners are re-working highwalls that were auger mined. The old 

auger miners may have penetrated only 100 to 200 feet into the 

highwall with circular holes anywhere from 12 to 36 inches in 

diameter; whereas today’s highwall miners can penetrate over 1000 

feet, if they are able to negotiate past the old auger holes. 

Examination of MSHA highwall mining ground control plans 

showed that old auger holes were expected in at least 11 out of 51 

cases or about 20%. The ground control unknown is the strength of 

a highwall mining web pillar that contains several auger holes. The 

issue is especially critical since this part of the web pillar is right 

under the highwall where stability is most important. One logical 

approach to the problem is to decrease the coal strength depending 

on the size and spacing of the old auger holes, and thus 

substantially increase the width of the web pillars. NIOSH is 

currently examining this problem to of provide simple, practical 

guidelines for the strength of web pillars containing auger holes. 

 Another related issue is the presence of nearby underground 

mining, either in the same seam or seams very close above or 

below. In addition to ground control issues, old workings may 

present gas and water inundation hazards. Again, examination of 

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

Full Depth

Mech/Elec

Guidance

Slope Stability

Water (gobbed out)

Geology

Rock Fall

Barrier

Relative Loss %

SF 1.00 to 1.30

SF 1.31 to 1.60

SF 1.61 to 2.00

SF 2.01+

Figure 14. Relative losses as function of loss category and 

stability factor. 
MSHA highwall mining ground control plans showed that nearby 

mining was expected in at least 20% of the cases. In no cases were 

the nearby mines active, but their operating status was not always 

clear. Many operators choose to stay away from old workings by 

at least 50 feet. Others have attempted to recover pillars in old 

workings now accessible with highwall mining equipment. Results 

of such endeavors are unknown. Again, the ground control 

unknown is the strength and stability of the remaining “pillar” in 

the old underground mine. Success of such operations will depend 

on accurate and reliable maps of the old workings coupled with 

accurate guidance and control of the highwall miner. 

 When seams split, many operations conduct “multiple-seam” 

highwall mining. The choice of sequence, top then bottom split or 

vice versa, depends on operational requirements. Again, accurate 

guidance and control of the highwall miner is essential to carefully 

stack web pillars and to assure that the pillars remain stacked 

deeper in the hole. The multiple-seam issue is really only a 

consideration if the seams are less than one hole-width apart and 

the seam thickness is less than the hole width. If the separation 

distance is sufficient, the highwall mining layers are independent of 

one another and can be designed separately. However, if the seams 

are close, the highwall mining layers will interact and the strength 

of the two-layer system is less than the strength of either layer. The 

ground control unknown is the strength of this layered system. 

Simple practical guidelines for this issue are forthcoming. 

 A related problem to multiple-seam highwall mining is 

multiple-lift highwall mining as applied to thick seams. First, a top 

cut is mined followed by a bottom or floor cut. This situation is 

more common in the western U.S. and also Australia, where thicker 

seams prevail. The major ground control problem is the strength of 

the taller web pillar which will usually have a W/H ratio less than 

one. Web pillars with low W/H ratios are inherently weaker, and it 

becomes imperative to maintain proper web width through accurate 

guidance and control of the highwall miner. In thick-seam, 

multiple-lift highwall mining operations, many operators find it 

prudent to employ a guidance system such as HORTA for better 

web pillar width control. 

 A final ground control issue with highwall mining concerns 

barrier pillars where the major design question is their width and 

the number of highwall miner holes between barrier pillars (14, 16, 

17). Barrier pillars serve several important safety functions. First, 

they act to stiffen and stabilize the highwall. Coal removal during 

highwall mining softens the base of the highwall causing it to move 

downward. This motion can destabilize the highwall and lead to 

potential failure. Periodic barrier pillars reinforce the highwall 

against potential slope failure. Second, barrier pillars will increase 

the overall stability of the system, preventing a domino-type pillar 

failure (pillar ride, squeeze or cascading pillar failure). Figure 9 

shows the consequences of not having barrier pillars (16). Once a 

web pillar failure starts; it is difficult to stop, and the web pillar 

failure can induce a catastrophic highwall slope failure. Barrier 

pillars behave like bulkheads on a ship by compartmentalizing the 

mine layout and limiting failure consequences to a smaller area. 

 In addition to safety, the use of barrier pillars also provides 

economic benefits. Barrier pillars appear to cost companies money 

because of the coal resources left in the ground. The only real 

economic cost of a barrier pillar comes from consuming exposed 

highwall 5 to 10 % faster depending on whether barrier pillars are 

left every 10 holes or 20. However, productivity in tons per unit 

time (day, month or year) remains constant and may actually 

increase. The presence of a barrier pillar is likely to prevent a ride 

or a roof fall, thereby promoting better mining conditions and 

possibly preventing a seriously trapped highwall miner. The use of 

barrier pillars is thus likely to prevent months of lost production 

revenues, which are very real economic costs. NIOSH is 

examining the safety aspects of barrier pillars and potential new 

highwall mining layouts that may provide greater highwall stability 

without decreasing coal recovery. 

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