GROUND CONTROL DESIGN FOR HIGHWALL MINING 

R. Karl Zipf, Jr., NIOSH, Pittsburgh, PA 

Abstract 

Highwall mining is an important surface coal mining 

method, and it may account for approximately 4% of total 

U.S. coal production. Highwall stability is the major 

ground control related safety concern. Ground control 

plans for highwall mining should specify hole width, web 

pillar width, barrier pillar width and number of holes 

between barriers. This paper offers simple design charts 

for these parameters aimed at providing “ball park” 

checks. Web pillars containing pre-existing auger holes 

are analyzed and a design chart for estimating their 

minimum width is also presented. Finally, close 

proximity multiple seam highwall mining, which may 

have caused several serious highwall failures, is analyzed. 

Web and barrier pillar recommendations for this special 

situation are presented. 

Introduction 

Auger and highwall mining continues to grow in 

importance as a coal production method from surface 

mines. Currently, there may be up to 60 highwall mining 

systems and as many as 150 auger mining systems 

operating in the nation’s surface coal mines. Recent 

estimates suggest that upwards of 45 million clean tons 

representing about 4% of total U.S. coal production 

comes from these methods (1). 

Volkwein, et al., (2) review the evolution of auger 

and highwall mining systems in the U.S. including the 

earliest augers dating from the mid 1940s, early highwall 

mining concepts such as the “Carbide Miner”, the “Pushbutton Miner”, the “Edna Miner” and the "Metec Miner" 

and finally several continuous haulage concepts such as 

Consol’s “Tramveyor” and Arch Coal’s “Archveyor.” 

Articles in World Coal (3) and Coal Age (4) discuss 

recent developments in the highwall mining technique. 

Two manufacturers now dominate the market for 

highwall mining systems with each having about 30 

systems in operation. The Superior Highwall Mining 

Company (5) developed the Superior Highwall Miner 

shown in figure 1, while Mining Technologies Inc. (6) 

developed the Addcar system shown in figure 2. 

Figure 1. Superior Highwall Miner under 

construction. Note thin seam cutter-head, control cab 

and cable reel. 

Figure 2. Addcar Highwall Miner in operation. Note 

Addcar in launch vehicle, control cab and discharge 

conveyor. 

By far, the overriding ground-control-related safety 

concern in highwall mining is highwall stability (1, 7, 8). 

Studies have shown that 3 of the 9 fatalities associated 

with auger and highwall mining in the last 20 years were 

caused by highwall collapses, including the only fatality 

that occurred in the last 5 years of highwall mining 

operations (1). 
 

Two major factors affect highwall stability 

during highwall mining, namely geologic structure and 

pillar stability. Hillseams (or mountain cracks) are the 

predominant geologic structures that affect highwall 

stability in the eastern U.S. Failure along these near 

vertical fractures in the rock can lead to large rockfalls 

from the highwall. Hillseams were an important factor in 

the highwall collapse shown in figure 3 that resulted in 

the fatality mentioned above.

Figure 3 – Highwall collapse resulting in fatality. 

 Web pillar failure and the 

induced subsidence of the overlying rock can also 

destabilize the highwall. As documented in earlier studies 

(1, 8, 9, 10), numerous highwall failures have originated 

from web pillar failure. 

This study focuses on design of stable web and 

barrier pillars for maintaining highwall stability. 

Tributary area method and the ARMPS program (11) are 

summarized briefly along with recommended input 

parameters. Simple design charts to estimate web and 

barrier pillar width are presented. These simple charts are 

useful for estimating the stability of web and barrier 

pillars observed in the field. 

Prior work on highwall mining ground control 

identified several issues requiring 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 (1). This study 

provides solutions to two of those issues. A simple 

design chart is presented for estimating web pillar width 

when the web pillar contains auger holes in various 

configurations. The issue of close proximity multiple 

seam highwall mining, which has been the source of 

several large highwall failures, is also examined in detail. 

Design recommendations are presented to maintain 

highwall stability when this situation occurs. 

Ground Control Analysis of 

Highwall Mining Layouts 

When designing a highwall mining layout, the 

mining engineer must specify 1) web pillar width, 2) 

number of web pillars between barrier pillars and 3) 

barrier pillar width. The design parameters are 

determined by the highwall miner hole width, the mining 

height and the overburden depth. In addition, the mine 

planner must estimate the pillar strength, the applied 

stress on pillars and the pillar stability factor. 

Coal Pillar Strength 

Numerous empirical formulas are available to 

predict coal pillar strength; however, the MarkBieniawski formula applies best for web pillars, which are 

very long, narrow rectangular pillars. For long pillars 

whose length is much greater than their width, the MarkBieniawski formula (11) reduces to

 SP = SI [ 0.64 + 0.54 W / H ] (1) 

Where: Sp = web or barrier pillar strength

 SI = in situ coal strength 

W = web or barrier pillar width

 H = mining height 

In situ coal strength is normally taken as 6.2 MPa 

(900 psi). Mining height can be equal to the seam 

thickness, but it may be greater if some rock is mined 

with the coal. 

Coal Pillar Stress 

Tributary area method is useful to estimate vertical 

stress on web and barrier pillars. Average vertical stress 

on a web pillar is 

SWP = SV (WWP + WE) / WWP (2)

Where: SV = in situ vertical stress 

WWP = web pillar width

 WE = highwall miner hole width. 

The highwall mining equipment dictates the hole 

width which varies from 2.7 to 3.6 m (9 to 12 ft). In situ 

vertical stress depends on the overlying rock density and 

overburden depth. Vertical stress gradient is typically 

0.025 MPa/m (1.1 psi/ft). Overburden depth may be 

taken as the maximum overburden depth on a highwall 

mining web pillar, which is very conservative, or 

alternatively as a high average value computed as 

DDesign = 0.75 * DMAX + 0.25 * DMIN (3) 
 

Where: DMAX = maximum overburden depth 

DMIN = minimum overburden depth. 

Finally, the stability factor for web pillars against 

strength failure is simply 

SFWP = web pillar strength / web pillar stress (SWP) (4) 

For design purposes, the stability factor for web 

pillars typically ranges from 1.3 to 1.6. Based on data in 

MSHA highwall mining ground control plans, studies (1) 

found that stability factor for web pillars in practice 

ranged from 1.3 to 1.6 in about 30% of the plans and 

exceeded 1.6 in 45%. These stability factor estimates 

from the ground control plans were based on the 

information provided, and their adequacy is not implied. 

This survey also found that the width-to-height (W/H) 

ratio of web pillars exceeded 1.0 in 75% of the cases 

examined. In general, keeping the web pillar W/H ratio 

above 1 is desirable to maintain better web pillar integrity. 

If the number of web pillars in a panel is selected as 

“N”, then the panel width is given by

 WPN = N (WWP + WE) + WE (5)

Neglecting the stress carried by the web pillars (i.e. 

assuming that they have all failed), the average vertical 

stress on a barrier pillar is 

SBP = SV (WPN + WBP) / WBP (6)

Where: WPN = panel width 

WBP = barrier pillar width 

Similarly, the stability factor for barrier pillars 

against strength failure is simply 

SFBP = barrier pillar strength / 

barrier pillar stress (SBP) (7)

Because the stress carried by web pillars within a 

panel is neglected, the stability factor for barrier pillars 

can be as low as 1. Studies (1) found that the width of 

barrier pillars exceeded 5 m (16 ft) in more than half the 

cases examined and more important, the W/H ratio for 

barrier pillars exceeded 3 in 2/3 of the cases. Barrier 

pillars with a W/H ratio greater than 3 are superior for 

sound geomechanics reasons. 

The ARMPS program (11) applies similar relations to 

the above for estimating the stability factor of web and 

barrier pillar combinations. When using ARMPS to 

analyze highwall mining layouts, the mining engineer 

should consider all the web pillars plus one barrier pillar 

in the analysis. The loading condition is normally 

development loading (option 1); however, if old 

underground workings are nearby, alternative loading 

conditions such as a front gob (option 2) may be 

necessary. 

Web and Barrier Pillar Design Charts 

and Design Examples 

The above equations for web and barrier pillar 

analysis can be implemented into a spreadsheet (9) or 

programmable calculator. In lieu of either, figures 4 and 

5 are design charts for web pillars while figure 6 provides 

design guidance for barrier pillars. Figure 4 applies to a 

2.7-m-wide (9ft) highwall miner hole, while figure 5 

applies to a 3.6-m-wide (12 ft) hole. In figures 4 and 5, 

options a and b apply to stability factors of 1.3 and 1.6, 

respectively. In figure 6, options a, b and c apply to panel 

widths of 30.5, 61 and 122 m (100, 200 and 400 ft), 

respectively. Note that this design chart assumes a barrier 

pillar stability factor of 1.0 and it neglects any load 

carrying capacity of the web pillars within a panel. 

Compared to ARMPS, these charts always give wider 

web and panel widths and are therefore conservative. 

 

0 

1 

2 

3 

4 

5 

6 

20 40 60 80 100 120 140 160 180 200 

Depth of cover - m 

Web pillar width - m 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 

Figure 4A. Suggested web pillar width with stability 

factor of 1.3, coal strength of 6.2 MPa and 2.75-mwide hole. 
 

 

4 

5 

6 

7 

8 

20 40 60 80 100 120 140 160 180 200 

Depth of cover - m 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 

Web pillar width - m 3
 

2
 

1
 

0
 

Figure 4B. Suggested web pillar width with stability 

factor of 1.6, coal strength of 6.2 MPa and 2.75-mwide hole. 

 

6 

7 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 

20 40 60 80 100 120 140 160 180 200
 

Depth of cover - m 

5

Web pillar width - m 4
 

3
 

2
 

1
 

0 

Figure 5A. Suggested web pillar width with stability 

factor of 1.3, coal strength of 6.2 MPa and 3.66-mwide hole. 

 

9
 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 

20 40 60 80 100 120 140 160 180 200
 

Depth of cover - m 

8
 

7
 

6
 

5
 

4
 

3
 

2
 

Web pillar width - m 1
 

0
 

Figure 5B. Suggested web pillar width with stability 

factor of 1.6, coal strength of 6.2 MPa and 3.66-mwide hole. 

 

14 

12
 

20 40 60 80 100 120 140 160 180 200 

Depth of cover - m 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 

Barrier pillar width - m 10
 

8
 

6

4
 

2
 

0
 

Figure 6A. Suggested barrier pillar width for 30.5-mwide panel assuming coal strength of 6.2 MPa and 

stability factor of 1.0. 

20
 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 

20 40 60 80 100 120 140 160 180 200
 

Depth of cover - m 

18
 

16
 

14
 

12
 

10
 

8
 

Barrier pillar width - m 

6

4
 

2
 

0
 

Figure 6B. Suggested barrier pillar width for 61.0-mwide panel assuming coal strength of 6.2 MPa and 

stability factor of 1.0. 

 

25
 

Mining height = 0.61 m 

Mining height = 1.22 m 

Mining height = 2.44 m 

Mining height = 3.66 m 20
 

Barrier pillar width - m 15
 

10
 

5
 

0 

20 40 60 80 100 120 140 160 180 200
 

Depth of cover - m 

Figure 6C. Suggested barrier pillar width for 122-mwide panel assuming coal strength of 6.2 MPa and 

stability factor of 1.0. 
 

To use figures 4, 5 or 6, the user begins with the 

design depth on the x axis, moves up vertically to the 

applicable mining height and then moves left horizontally 

to the y axis where the suggested web (or barrier) pillar 

width is read. Several examples below illustrate the use 

of these design charts. 

Table 1 describes conditions for the first example. 

The operator prefers to leave a barrier pillar after every 10 

highwall miner holes. Minimum acceptable stability 

factor for the web pillars is 1.3, and at the highwall itself 

near the start of the holes, stability factor must exceed 1.6. 

Table 1. Design example 1. 

Coal seam thickness 1.8 m (6 ft) 

Highwall miner width 3.66 m (12 ft) 

Maximum depth of cover 122 m (400 ft) 

Highwall height 30.5 m (100 ft) 

Design depth of cover (equation 3) 99 m (325 ft) 

Web width at 99 m for stability factor = 1.3 2.4 m (8 ft) 

Web width at 30.5 m for stability factor = 1.6 0.9 m (3 ft) 

Panel width (equation 5) 58.2 m (192 ft) 

Barrier pillar width at 99 m 8.5 m (28 ft) 

To estimate the web pillar size, use figure 5A for a 

hole width of 3.66 m (12 ft) and a stability factor of 1.3. 

Using the design depth of cover and a mining height of 

1.8 m (6 ft), the suggested web pillar width is about 2.4 m 

(8 ft). This web pillar has a desirable W/H ratio of 1.33 

since it is more than 1. 

Using figure 5B, the requirement that stability factor 

exceed 1.6 directly under the highwall is checked. For a 

depth of cover of 30.5 m (100 ft) and a mining height of 

1.8 m (6 ft), the minimum web width with a stability 

factor of 1.6 is about 0.9 m (3 ft). Therefore, the 2.4-mwide (8 ft) web pillar must have a stability factor much 

greater than 1.6. 

The operator plans to leave barrier pillars after every 

10 holes, so based on equation 5, the panel width is 58.2 

m (192 ft). From figure 6B for a 61-m-wide (200 ft) 

panel with a design depth of cover of 99 m (325 ft) and a 

mining height of 1.8 m (6 ft), the suggested barrier pillar 

width is about 8.5 m (28 ft). The barrier pillar has a 

desirable width-to-height ratio of 4.66 since it is more 

than 3. This barrier pillar width happens to equal exactly 

the common rule-of-thumb for barrier pillar width which 

is 1 hole width plus 2 web pillar widths. 

As a check, situation 1 was analyzed with ARMPS. 

The web pillars alone had a stability factor (on 

development) of 1.32. Adding the 8.5-m-wide (28 ft) 

barrier raised the overall stability factor to 2.21. 

Table 2 describes conditions for the second example. 

The operator prefers to leave a barrier pillar after every 10 

highwall miner holes. Minimum acceptable stability 

factor for the web pillars is 1.3, and at the highwall itself 

near the start of the holes, stability factor must exceed 1.6. 

Table 2. Design example 2. 

Coal seam thickness 0.9 m (3 ft) 

Highwall miner width 2.75 m (9 ft) 

Maximum depth of cover 76.2 m (250) 

Highwall height 15.2 m (50 ft) 

Design depth of cover (equation 3) 61 m (200 ft) 

Web width at 61 m for stability factor = 1.3 0.9 m (3 ft) 

Web width at 15.2 m for stability factor = 1.6 0.6 m (2 ft) 

Panel width (equation 5) 35.6 m (117 ft) 

Barrier pillar width at 61 m 3.0 m (10 ft) 

To estimate the web pillar size, use figure 4A for a 

hole width of 2.75 m (9 ft) and a stability factor of 1.3. 

Using the design depth of cover and a mining height of 

0.9 m (3 ft), the suggested web pillar width is about 0.9 m 

(3 ft). This web pillar has a desirable W/H ratio of 1.0. 

Using figure 4B, the requirement that stability factor 

exceed 1.6 directly under the highwall is checked. For a 

depth of cover of 15.2 m (50 ft) and a mining height of 

the 0.9 m (3 ft), the minimum web width with a stability 

factor of 1.6 is less than 0.6 m (2 ft). Therefore, the 0.9­

m-wide (3 ft) web pillar must have a stability factor much 

greater than 1.6. 

The operator plans to leave barrier pillars after every 

10 holes, so based on equation 5, the panel width is 35.6 

m (117 ft). From figure 6A for a 30.5-m-wide (100 ft) 

panel with a design depth of cover of 61 m (200 ft) and a 

mining height of 0.9 m (3 ft), the suggested barrier pillar 

width is about 3 m (10 ft). The barrier pillar has a 

desirable width-to-height ratio of 3.33 since it is more 

than 3. This barrier pillar width is 35% less than the ruleof-thumb (1 hole width plus 2 web pillar widths) for 

barrier pillar width of 4.6 m (15 ft). 

Situation 2 was also analyzed with ARMPS as a 

check. The 0.9-m-wide (3 ft) web pillars alone had a 

stability factor (on development) of 1.17; however, 1.0-mwide (3.25 ft) web pillars had a stability factor of 1.29. 

Adding the 3-m-wide (10 ft) barrier raised the overall 

stability factor to 1.89. 

The last example shows how the design charts can be 

used as a spot check of highwall mining web pillars in the 

field. Table 3 summarizes “as-mined” field conditions. 

The operator prefers a stability factor for the web pillars of 

at least 1.3. The pit foreman notices that the crew has been 

using 1.2-m-wide (4 ft) web pillars in a particular area. 

Could this practice have unacceptable consequences? 

Table 3 – Design example 3. 

Coal seam thickness 1.2 m (4 ft) 

Highwall miner width 3.66 m (12 ft) 

Maximum depth of cover 122 m (400) 

Highwall height 30.5 m (100) 

Design depth of cover (equation 3) 99 m (325 ft) 

Web width at 99 m for stability factor = 1.3 1.8 m (6 ft) 

Web width observed in field 1.2 m (4 ft) 
 

To estimate the web pillar size, use figure 5A for a 

hole width of 3.66 m (12 ft) and a stability factor of 1.3. 

Using the design depth of cover and a mining height of 

1.2 m (4 ft), the suggested web pillar width is about 1.8 m 

(6 ft) to achieve a stability factor of 1.3. The 1.2-m-wide 

(4 ft) web pillar must have a stability factor much less 

than 1.3, and a web pillar failure with potential highwall 

stability consequences is entirely possible. Using 

ARMPS to check the graphical results showed that the 

stability factor was 0.72 or much less than 1.3 as 

expected. 

Highwall Mining Through Old Auger Holes 

Many highwall miners are re-working highwalls that 

were previously auger mined. Review of MSHA 

highwall mining ground control plans indicates that at 

least 20% of the highwall mining operations expect to 

encounter old auger holes somewhere on a property (1). 

Figure 7 shows typical highwall mining web and barrier 

pillars containing pre-existing auger holes.

Web Pillar Barrier 

Pillar 

Figure 7. Highwall miner web and barrier pillars with 

old auger holes. 

 From a 

ground control standpoint, the critical issue is the strength 

of a highwall mining web pillar that contains a row of 

auger holes. As mentioned earlier, maintaining stability 

of web pillars is crucial for maintaining stability of the 

highwall above the active mining operation. 

Conventional coal pillar strength formulas do not apply 

directly to this situation. 

Analysis 

To address this issue, NIOSH researchers constructed 

numerical models of coal pillars using FLAC2D (12). 

The first phase of this modeling effort replicated the 

empirical pillar strength predictions from the MarkBieniawski formula for strip pillars. Having calibrated 

the numerical model to this empirical formula enabled 

further strength investigations of web pillars containing 

auger holes. The numerical models computed the stressstrain behavior of the web pillars over a range of widthto-height (W/H) ratio from 0.5 through 9, first for solid 

web pillars, and then for web pillars with auger holes. 

Figure 8 shows a typical model for the study, in 

this case, a pillar with W/H ratio of 3. 

Figure 8. Typical coal pillar FLAC model. 

 Element size is 0.1 

m, and this particular model contains 140 elements 

vertically and 120 elements horizontally. Three layers 

comprise the model, namely a 5-m-thick layer of floor 

rock, a 2-m-thick coal seam and a 7-m-thick layer of roof 

rock. Each layer follows the strain-softening, ubiquitous 

joint constitutive model in FLAC2D where a horizontal 

weakness plane represents bedding. Table 4 summarizes 

the essential input parameters used in this model. 

Table 4. Input parameters for strain-softening 

ubiquitous joint constitutive model in FLAC2D. 

Coal Rock

Property Bedding Matrix Bedding Plane Matrix Plane 

Modulus 2.5 GPa - 7.0 GPa -

Poisson’s ratio 0.25 - 0.25 -

Cohesion 1.9 MPa 1.1 MPa 6.0 MPa 4.5 MPa 

Friction angle 31° 27° 26° 25° 

Tensile strength 0.6 MPa 0.3 MPa 1.9 MPa 1.4 MPa 

 To 

obtain strain-softening behavior, the cohesion values 

decrease to 10% of their peak value over a failure strain 

of 0.005 (0.5%). 

Load is generated in the model via a constant 

downward displacement applied to the top row of 

elements. Finally, average vertical stress and strain are 

computed across the pillar’s midpoint as load on the 

model increases. 
 

Figure 9 shows the computed stress-strain curves for 

a series of solid coal pillars with W/H ratio ranging from 

0.5 to 9.

 

4.50E+07 

W/H = 1 

W/H = 1/2 

W/H = 9 

W/H = 7 

W/H = 2 

W/H = 5 

W/H = 4 

W/H = 3 

4.00E+07 

3.50E+07 

3.00E+07 

Stress - MPa 2.50E+07 

2.00E+07 

1.50E+07 

1.00E+07 

5.00E+06 

0.00E+00 

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 

Strain 

Figure 9. Computed stress-strain curves for solid coal 

pillars. 

 Below a W/H ratio of 2, the pillars exhibit a 

high degree of strain-softening material behavior. Upon 

reaching peak strength, the pillar loses much of its load 

bearing capacity and its residual strength is but a fraction 

of its peak. Beyond a W/H ratio of 3, the pillar exhibits 

an increasing degree of strain-hardening behavior, that is 

its load bearing capacity continues to increase as 

deformation occurs, but at a lower rate. For pillars with a 

W/H ratio in the 3 to 5 range, the computed stress-strain 

curves exhibit distinct peak strength at some strain value. 

For pillars with a W/H ratio beyond 7, this distinct peak 

strength is not seen, and peak strength is recorded at 0.025 

strain or 2.5%. 

Figure 10 plots peak strength computed with the 

numerical model versus W/H ratio (the data points) along 

with a line showing pillar strength versus W/H ratio as 

predicted by the Mark-Bieniawski formula for strip 

pillars. 

0 

5 

10 

15 

20 

25 

30 

35 

40 

Strip pillar strength - MPa

Strength (FLAC) 

Mark-Bieniawski Strength 

0 1 2 3 4 5 6 7 8 9 10 

W/H ratio 

Figure 10. Numerical-model-computed pillar strength 

compared to pillar strength from Mark-Bieniawski 

formula. 

 As seen by inspection, the agreement is excellent. 

Furthermore, the form of these numerical stress-strain 

curves matches that of laboratory tests performed by Das 

(13) on coal specimens of varying W/H ratio. Based on 

this calibration, the numerical model can be used with 

confidence to evaluate the strength of highwall mining 

web pillars containing auger holes. 

Three auger hole configurations are considered, 

namely, 0.6-, 0.7- and 0.8-m-diameter holes, all on 1 m 

centers. The last configuration follows the common rule 

of thumb in auger mining that recommends an auger web 

width of 1⁄4 the auger hole diameter. 

Figure 11 shows typical computed stress-strain 

curves for highwall mining web pillars with a W/H ratio 

of 3 containing auger holes with the above three 

geometries. A conservative value near the peak of these 

stress-strain curves was selected as the web pillar 

strength. These numerical studies found that the peak 

strength of a highwall mining web pillar containing auger 

holes is significantly less than the strength of a solid web 

pillar and nearly independent of the highwall mining web 

pillar’s W/H ratio. Table 5 summarizes the auger hole 

geometries considered and the computed highwall mining 

web pillar strength. By the Mark-Bieniawski formula, the 

strength of a solid highwall mining web pillar with a W/H 

ratio of 3 is about 14 MPa (2034 psi). These calculations 

for a range of practical auger mining geometries indicate 

that the strength of a highwall mining web pillar 

containing auger holes is 25% to as little as 15% of the 

solid web pillar strength. 
 

0.00E+00 

5.00E+05 

1.00E+06 

1.50E+06 

2.00E+06 

2.50E+06 

3.00E+06 

3.50E+06 

4.00E+06 

4.50E+06 

5.00E+06 

Stress - MPa 

Web/diam. Ratio = 0.25 

Web/diam. Ratio = 0.43 

Web/diam. Ratio = 0.66 

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 

Strain 

Figure 11. Computed stress-strain curves for web 

pillars with auger holes and W/H ratio = 3. 

As noted earlier, the numerical calculations showed 

that the strength of a highwall mining web pillar 

containing auger holes is independent of its W/H ratio. 

This observation is to be expected for closely spaced 

auger holes where the strength of the auger hole webs 

determines the strength of the overall highwall mining 

web pillar. In most auger mining, the auger web pillars 

are usually closely spaced and somewhere within the 

range considered in table 5. 

Table 5. Auger hole geometry and highwall miner 

web pillar strength. 

Auger 

hole 

diameter 

(m) 

Auger 

hole ctrto-ctr 

spacing 

(m) 

Auger 

web 

width (m) 

Auger web 

width to 

hole 

diameter 

ratio 

Auger % 

extraction 

Highwall 

miner web 

pillar 

strength 

(MPa) 

0.6 1.0 0.4 0.66 60 3.50 

0.7 1.0 0.3 0.43 70 2.75 

0.8 1.0 0.2 0.25 80 2.00 

If the auger holes are widely 

spaced with an auger-web-width-to hole-diameter ratio 

much greater than 1, then the presence of the auger holes 

may not affect the highwall mining web pillar as much 

and that web pillar will increase in strength as its W/H 

ratio increases. 

Solutions and Recommendations 

Based on these reduced highwall miner web pillar 

strengths that are independent of W/H ratio, required web 

pillar width is computed assuming a safety factor of 1.3. 

The analyses use simple tributary area method to calculate 

pillar stress, and the pillar strength is as above in Table 5. 

In most practical situations, the horizontal depth of auger 

holes is less than 61 m (200 ft) where the depth of cover 

rarely exceeds 46 m (150 ft). Figure 12 presents a simple 

chart for estimating the minimum width of a highwall 

mining web pillar containing auger holes of various 

configurations. 

 

 

 

 

2 

4 

6 

8 

10 

12 

14 

Web pillar width - m 

Auger web/diameter ratio = 0.25 

Auger web/diameter ratio = 0.43 

Auger web/diameter ratio = 0.66 

0 10 20 30 40 50 

Depth of cover - m 

Figure 12. Design chart for web pillars with auger 

holes of various density. 

60

This chart assumes a highwall miner hole 

width of 3.66 m (12 ft). The user should size these webs 

to contain at least three intact auger mining webs hence 

the design chart begins at a web width of 2 m (6.6 ft). 

This chart also neglects many other factors that may 

adversely affect the strength of a highwall mining web 

pillar containing pre-existing auger holes such as the age 

of the auger holes, presence of water, low coal strength 

and other factors. The chart is also based on results of a 

numerical stress analysis that while of good quality may 

not consider all the relevant factors in any given situation. 

For these reasons, the engineer must use this design chart 

with caution. 

By way of example, a highwall miner operator 

encounters a section of highwall containing 0.75-mdiameter (2.5 ft) auger holes spaced on average about 

1.05 m (3.5 ft) apart. The auger holes are 61-m-deep (200 

ft) where the maximum depth of cover is 45 m (150 ft). 

The auger web is therefore about 0.30-m-wide (1.0 ft) on 

average. The auger web width-to-diameter ratio is 

0.30/0.75 = 0.40. On figure 12, the suggested web pillar 

width is about 4.5 m (15 ft). This web pillar would 

contain 4 or 5 auger hole webs. 

Close-Proximity Multiple Seam 

Highwall Mining 

Many highwall mining operations recover multiple 

seams in very close proximity to one another. In the 

eastern U.S., this situation arises frequently when a thick 

seam splits into thinner seams. In western U.S. mines, 

certain very thick seams can exceed the working height of 

the highwall miner, and a multiple seam mining approach 

may be utilized (14). Multiple seam mining becomes 

most problematic when the interburden thickness between 

seams decreases to less than about one highwall miner 

hole width (4m or 12 ft). While firm data on the number 

of highwall mining operations engaged in such multiple 
 

 

seam mining is not available, anecdotal evidence suggests 

that 20 to 40% of the highwall mining operations will 

encounter such mining conditions somewhere on a 

property. 

Close proximity multiple seam highwall mining 

appears to have caused several extensive highwall failures 

of the type that may pose a ground control danger to the 

working crews. Figure 13 shows one example where a 

1.5-m-thick (5 ft) lower seam was mined first followed by 

a 0.9-m-thick (3 ft) upper seam.

Figure 13. Highwall collapse in multiple seam mining 

area.

 A weak, laminated 

interburden ranging in thickness from 1.2 to 3 m (4 to 10 

ft) separated the two seams. Catastrophic collapse (or 

domino failure) of the web pillars occurred that resulted 

in this extensive highwall failure. Figure 14 shows 

another example where close proximity multiple seam 

highwall mining resulted in web pillar collapse and 

highwall failure. 

Figure 14. Highwall collapse in multiple seam mining 

area. 

 This particular failure also trapped the 

highwall miner which was finally recovered by surface 

excavation after several months time. 

Again, while firm data on the number of highwall 

failures resulting from collapse of close proximity 

multiple seam highwall miner workings is not available, 

the success rate for the practice appears low. At one 

particular operation, such mining was conducted in four 

separate areas of their property. Web pillar collapse and 

extensive highwall failure resulted in three of the four 

areas. In each area, the length of highwall affected by 

failure as shown in figure 13 was on the order of 300 m 

(1,000 ft). Eyewitnesses stated that the failures happened 

suddenly and without warning and that crew members 

evacuated the area quickly. The highwall failure areas 

became quiet again after a day, enabling crews to recover 

equipment, move the highwall mining machine to a new 

area and resume mining. 

Preventing web pillar collapse and the ensuing 

possibility of highwall failure is imperative for ground 

control safety in close proximity multiple seam highwall 

mining. Unfortunately, the conventional pillar design 

methods do not apply well to closely spaced seams less 

than about one highwall miner hole width apart. For 

example, with the four failures mentioned above, the 

ground control plan for both seams required 0.9- to 1.5­

m-wide (3 to 5 ft) web pillars and 3.6- to 4.6-m-wide (12 

to 15 ft) barrier pillars spaced every 5 highwall miner 

holes. Based on tributary area method and the ARMPS 

program, the stability factor for individual seams far 

exceeded 1.3. Nevertheless, failure occurred. 

Analysis 

Figure 15 shows a likely failure mechanism that leads 

to web pillar collapse in closely spaced seams. 

Figure 15. Pillar-beam failure mechanism. 

 When 

web pillars are not stacked, they will load the interburden 

beam and induce tensile failure in its lower outer fibers. 

The strength of this pillar-beam system is much less than 

the strength of an ordinary pillar on a solid rock 

foundation. 

To calculate the approximate strength of this pillarbeam system, a 1.2-m-thick (4 ft) and 3.6-m-wide (12 ft) 

interburden beam loaded by a 1.2-m-wide (4 ft) web pillar 

is considered as shown in figure 16. Roark (15) provides 

analytic solutions for the maximum bending moment in 
 

 

beams for various load geometries and end conditions. 

For purposes here, it is sufficient to say that 

MMAX = f (EC, LG, BG) (8) 

where MMAX is the maximum bending moment at the 

beam’s midspan and f is a function that depends on 1) end 

conditions EC that are either fixed or free, 2) load 

geometry LG that is either concentrated or distributed and 

3) beam geometry BG that includes beam width, beam 

thickness and load placement. Note that for this 

simplistic analysis to apply, the beam thickness must be 

much less than the beam width. 

Figure 16 – Analysis of pillar-beam failure 

mechanism. 

The load on the beam derives from the upper web 

pillar stress σP, and its magnitude is σP * W for both the 

concentrated and distributed load geometry where W is 

the web pillar width. The maximum tensile stress in this 

beam occurs in the lower outer fibers and is given by 

σT = (MMAX c) / I (9) 

where c is the half-thickness of the beam (i.e., 0.6 m or 2 

ft) and I is the beam’s moment of inertia. 

By combining these relations, it can be shown that 

the maximum allowable web pillar load σPMAX depends on 

the tensile strength of the rock σTrock in the lower outer 

fibers of the beam. Table 6 provides approximate values 

for σPMAX as a function of σTrock for various load 

geometries and beam end conditions. 

Table 6. Approximate maximum pillar load for a 

particular pillar-beam geometry. 

End Condition Load Geometry 

Concentrated load Distributed load 

Free ends σTrock / 4.50 σTrock / 3.75 

Fixed ends σTrock / 2.25 σTrock / 1.583 

For the particular geometry considered as shown in 

figure 16, the approximate strength of the pillar-beam 

system is less than the tensile strength of the rock by a 

factor of 2 to 4! The compressive strength of a typical 

highwall mining web pillar is on the order of 8 MPa. The 

tensile strength of rock may be on the order of 2 MPa. 

According to this simplistic analysis, the strength of a 

pillar-beam system is on the order of 1 MPa, and is 

therefore much less than the typical web pillar strength. 

Although this analysis is very approximate, it strongly 

supports the reasonability of the failure mechanism shown 

in figure 15. The effective strength of a web pillar located 

over an underlying highwall miner hole is much less than 

might be expected, and that low strength can lead to 

catastrophic web pillar collapse and highwall failure in 

areas of close proximity multiple seam highwall mining. 

This analysis assumed that the overlying web pillar 

was located at midspan of the underlying highwall miner 

hole, and is therefore a worst case scenario. Moving the 

overlying pillar away from midspan dramatically 

increases the strength of the pillar-beam system, but does 

not change the viability of the failure mechanism or the 

conclusions from the analysis. Strong countermeasures 

are required to prevent this failure mechanism from 

occurring and inducing a possible highwall failure. 

Solutions and Recommendations 

The obvious solution to prevent this failure 

mechanism from occurring is to carefully stack upper and 

lower seam web and barrier pillars. Proper stacking at the 

start of a hole is easy to accomplish if the operator is 

mining the seams from the top down. The upper holes are 

plainly visible and positioning the highwall miner directly 

below is no problem. Unfortunately, in most close 

proximity multiple seam highwall mining, the seams are 

mined from the bottom up for operational considerations. 

After coal is removed from the surface bench, the 

highwall miner extracts the lower seam. Next, the pit is 

partially backfilled, and the highwall miner recovers the 

upper seam. This mining sequence requires a reliable 

record of the actual placement of the lower holes. Careful 

surveying is one option, and another is to simply paint 

lower hole locations on the upper seam. 

Attaining proper stacking is simple at the start of a 

hole, but there is no guarantee that stacking will be 

maintained deep within the holes. A previous study (1) 
 

found that crossed holes occurred in about 3% of all 

highwall miner holes attempted. Therefore in any 

segment of highwall with one hundred holes, web pillars 

and holes will have drifted from their intended position to 

create the failure scenario shown in figure 15. Improper 

stacking appears to have figured prominently in both web 

pillar collapses and highwall failures shown in figures 13 

and 14. 

While maintaining proper stacking of web pillars 

along the entire hole depth is difficult to achieve, 

maintaining proper stacking of barrier pillars is more 

practical owing to their greater width. In conjunction 

with carefully aligned barrier pillars, limiting the number 

of highwall miner holes to about 5 will also lessen the 

possibility of web pillar collapse and highwall failure in 

these close proximity multiple seam highwall mining 

situations. 

Even if web pillars are perfectly stacked, yet another 

failure mechanism can lead to premature failure of close 

proximity multiple seam web pillars. As shown in figure 

17, failure of weak interburden rock between closely 

spaced seams results in a taller web pillar with a lower 

width-to-height ratio that is necessarily weaker.

Figure 17. Tall pillar failure mechanism. 

 If the 

strength of the upper or lower pillar is 1, calculations 

show that the combined height pillar strength is about 20 

to 30% less. This strength decrease may be enough to 

trigger web pillar collapse and highwall failure in certain 

situations. In close proximity multiple seam highwall 

mining, suggested practice is to design the web pillars 

based on a combined height of both seams plus the 

interburden thickness. 

Summary and Conclusions 

Highwall stability remains a major concern during 

highwall mining. Geologic structure (hillseams) and 

pillar stability are the two major factors affecting highwall 

stability. 

This study summarizes the essential relations 

necessary to calculate the applied stress and strength of 

web and barrier pillars used in highwall mining. These 

relations are the basis for simple design charts for 

selecting web and barrier pillar widths. 

One set of charts provides suggested web pillar width 

given the overburden depth, mining height and highwall 

miner hole width. The various charts assume stability 

factors of 1.3 and 1.6. An initial suggested web pillar 

stability factor is about 1.3. In addition to web design 

during the planning phase, these charts allow the user to 

estimate web pillar stability while in the field. 

Another set of charts provide suggested barrier pillar 

width given the overburden depth, mining height and 

panel width between barrier pillars. These charts neglect 

the strength of web pillars within the panel and assume a 

stability factor of 1.0 for the barrier pillar. In checking 

the suggested web and barrier pillar widths from these 

charts against the ARMPS program, the charts always 

provide a conservative (high) suggestion for pillar width 

and a low estimate for the stability factor of a web and 

barrier pillar system. 

This study also examined the strength of highwall 

web pillars that contain old auger holes. Numerical 

modeling studies found that the strength of a highwall 

mining web pillar containing auger holes is 25% to as 

little as 15% of the solid web pillar strength. The strength 

decrease depends on the ratio of auger web width to auger 

hole diameter. A smaller ratio results in a greater strength 

reduction. Furthermore, the numerical model studies 

found that the strength of a highwall mining web pillar 

containing auger holes does not increase as the width-toheight ratio of that pillar increases. The reduced strength 

of that highwall mining web pillar remains low and is 

independent of its W/H ratio. Results of this study are 

summarized in a design chart for estimating web pillar 

width when auger holes are present. 

Finally, this study examined the issue of close 

proximity multiple seam highwall mining. This practice 

appears to have caused several extensive highwall failures 

that may have posed a ground control danger to the 

working crews. The likely mechanism for these multiple 

seam web pillar collapses and the induced highwall 

failures arises from adverse stacking of the web pillars in 

conjunction with a thin interburden (less than one 

highwall miner hole width) between seams. Poor 

surveying or drift of the highwall miner holes often cause 

the upper web pillars to lay over the middle of the lower 

holes. Simple estimates for the strength of this pillarbeam system demonstrate that its strength is much less 

than the strength of the ordinary pillar. This drastic 

underestimate of highwall miner pillar strength led to web 

pillar collapse and highwall failure in these close 

proximity multiple seam highwall mining situations. 

One possible solution that prevents this failure 

mechanism from occurring is to stack barrier pillars and 

decrease the number of highwall miner holes between 

barrier pillars. Due to their greater width, maintaining 

proper stacking of barrier pillars is practical even though 
the highwall miner holes will almost certainly deviate 

from their planned trajectory. Limiting the number of 

highwall miner holes to about 5 decreases the possibility 

of serious web pillar collapse due to adverse stacking and 

subsequent highwall failure. 

This study addresses the web pillar stability issue in 

highwall mining including general design, web pillars 

with old auger holes and close proximity multiple seam 

highwall mining. Maintaining proper web pillar stability 

is an essential component in the achievement of stable 

highwalls and ground control safety in highwall mining. 

Numerous highwall failures trace their origin to an 

underlying web pillar failure. However, the other factor 

affecting highwall stability is geologic structure and in 

particular hillseams. Toppling or plane shear failure 

along hillseams can lead to extensive highwall failure 

directly. Proper web pillar design may have little or no 

influence controlling the safety hazards wrought by 

failure along adverse pre-existing geologic structures. 

New monitoring technology could detect highwall 

movement and provide warning of instability in the near 

future. NIOSH ground control research is presently 

exploring these possibilities. 

References 

1.	 Zipf, R.K. and S.K. Bhatt, “Analysis of Practical 

Ground Control Issues in Highwall Mining,” 

published in Proceedings of 23rd Conference on 

Ground Control in Mining, West Virginia University, 

2004, pp. 210-219. 

2.	 Volkwein, J.C., T.P. Mucho and S.K. Bhatt, “Safety, 

technological, and productivity potentials of highwall 

mining,” U.S. Dept. of Labor, MSHA, Holmes Safety 

Association Bulletin, January 1995, pp. 4-11. 

3.	 Arrowsmith, D., “Highwall evolution,” World Coal, 

Vol. 12, no. 9, Sept. 2003, pp. 15-18. 

4.	 Fiscor, S., “Contour Mining Puts Highwall System to 

the Test,” Coal Age, July 2002, pp. 20-22. 

5.	 Superior Highwall Miners, L.P., Beckley, West 

Virginia, http://www.shm.net, 2004. 

6.	 Mining Technologies, Inc., Ashland, Kentucky, 

http://www.addcarsystem.com, 2004. 

7.	 Gardner, G. and Kelvin Wu, “Overview of safety 

considerations with highwall mining operations,” 

published in Proceedings of 21st Conference on 

Ground Control in Mining, West Virginia University, 

2002, pp. 236-241. 

8.	 Duncan-Fama, M.E., B. Shen, M.S. Craig, M. Kelly, 

I.L. Follington, and B.E. Leisemann, “Layout design 

and case study for highwall mining of coal,” 

published in Proceedings: International Congress on 

 

Rock Mechanics, eds. G. Vouille and P. Berest, 1999, 

Vol. 1, pp. 265-268. 

9.	 Zipf, R.K., “Catastrophic collapse of highwall web 

pillars and preventative design measures,” published 

in Proceedings of 18th Conference on Ground Control 

in Mining, West Virginia University, 1999, pp. 18­

28. 

10.	 Adhikary, D.P., B. Shen and M.E. Duncan Fama, “A 

study of highwall mining panel stability,” Intl. J. 

Rock Mech. & Mining Sci., V. 39, 2002, pp. 643­

659. 

11. Mark, C., F.E. Chase and A.A. Campoli, “Analysis of 

retreat mining pillar stability,” published in 

Proceedings of 14th Conference on Ground Control in 

Mining, West Virginia University, 1995, pp. 49-59. 

12.	 FLAC2D, Fast Lagrangian Analysis of Continua, 

Itasca Consulting Group, Inc., Minneapolis, MN. 

13.	 Das, M.N., “Influence of width/height ratio on 

postfailure behavior of coal,” Int. J. of Mining and 

Geol. Eng., V. 4, 1986, pp. 79-87. 

14.	 Ross,T., T.L. Vandergrift and S.H. Choi, “Highwall 

augering in ultra-thick western coal reserves: unique 

geotechnical and operational challenges,” published 

in Proceedings of 18th Conference on Ground Control 

in Mining, West Virginia University, 1999, pp. 202­

209. 

15.	 Roark, R.J., “Formulas for Stress and Strain,” 

McGraw Hill Book Co., New York, 1965, 432 pp.