Design of Auger highwall mining

DESIGN OF AUGER HIGHWALL MINING: A CASE
STUDY AT MAE TAN COAL MINE, THAILAND
Sophea Boeut1
and Pipat Laowattanabandit2
1
Mining and Petroleum Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand,
Tel. +66944152833, Fax. +6622186920, Email: boeurtsophea@gmail.com
2
Mining and Petroleum Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand,
Tel. 66 2 218 6926, Fax. +66 2 218 6920, Email: Pipat.L@chula.ac.th
Abstract
Most coal mines adopted surface mining method have been stopped after reaching the ultimate pit
limit. However, in some mines still exists some significant amount of coal in the highwall which
can be extracted using other mining methods. Highwall mining method, which is considered as the
semi surface-underground mining operation, was originally developed in the early 1940s in the US.
It is suitable to excavate this remaining coal. Technically, there are two types of highwall mining:
continuous and auger highwall mining with the rectangular and circular-hole opening respectively.
The highwall miners are used to drill the residual to extend the mining life or recovery ratio.
In this study, few illustrious empirical approaches were compared to the 3D modeling of
FLAC3D to design the pillar system in highwall mining special case to Mae Tan Coal Mine. Based
on this case study, the stress and strength of the pillars were estimated from both types of theories.
Several criteria were proposed to estimate one optimum condition design for this coal mine. The
result from numerical analysis provided almost the same as empirical distributary theories of stress.
Moreover, the strength from numerical modeling also similar to the average empirical strengths.
Hence, the recommendation of average result of empirical equations was suggested to use for Mae
Tan coal. The result, at the safety factor of 1.6 for web pillar and 1.3 for barrier pillar, showed that
the quantities of coal increased when the penetration length was made deeper, although the percent
recovery of coal decreased. According to this pillar design, at 21% of recovery ratio, pillar width of
2 m and barrier pillar of 3 m were the final judgment for the design. But in terms of the economy,
this recovery ratio does not benefit really high enough to mine. So to extend this recovery as well
as the tonnage of coal, further detailed study on double passes and multi-pass on the upper coal
seam layer should be included.
Keywords: Auger, barrier pillar, empirical analysis, highwall mining system, numerical analysis,
web pillar
Introduction
Mae Tan Coal Mine belongs to SGC group, located in northern part of Thailand, about 600
kilometers from Bangkok Capital of Thailand (Fig. 1). Mae Tan's coal ranges from lignite
to sub-bituminous [1] and is operated as open pit mining. This coal production is supplied
to cement plants. Nowadays, final pit of this mine is almost reached. However, there is
some coal still remaining in the pits.
To extend the mining life and coal production, Auger Higwall Mining (AHM) plays a
potential mining technique for Mae Tan coal mine. The auger highwall system is
horizontally drilled direction into the coal seams with the circular opening (Fig. 2). The
mineable dip angle is less than 15 degrees. The new highwall miner machine can be used
with 23 degrees of dip angle in Australia and 16◦
-23◦
dip in Indonesia [2]. Auger mining
consists of cutting head, auger flight, and latching mechanism. The working process of
auger miner is very common. Firstly, the auger cutting head drives directly from the auger

machine into the coal seam and additional flights are added as the hole deepens. Once each
flight is in place, a tractive effort is provided by hydraulic rams. Each auger flight is
connected via a latch pin assembly that operates by remote control from the operator’s air-
conditioned cabin [3]. The remote control of auger miner lets people work from a distance
by commanding the auger miner. This helps to avoid any dangers such as coal self-
combustion, coal dust or other gases to people who are working in the mine. The
requirement for working force is about 3 to 4 persons per working shift or equal to be 12
people per 3-working shifts.
Fig.1: Research Location
Fig. 2: Auger Highwall Mining (CRONSHAW, 2009)
Regional Geology and Rock Properties
Observing on the stratigraphy of this mine, there are five types of rock excluding coal such
as top soil red clay, sandstone, shale, ball clay, carbonaceous shale and Rhyolitic Tuff. The
whole lithography of rock is provided in Fig.3. The elevation at the top is about 288-meter
height varied to about 120 meters-height at the bottom. According to the lithography,
sandstones are the thickest layer and then the top soil. Two coal layers, the upper one is
about 2.5 meters thick and the second layer is about 4 meters thick. The detail of rock

properties of Mae Tan mine is provided in Table 1 for further design both empirical and
numerical analyses.
Fig. 3: Lithology of Mae Tan Mine
Auger Highwall System
The pillar system in this mining technique is separated into web pillars or pillar supports
and barrier pillars or highwall supports. The barrier pillar is designed not only self-support,
but also for regional support. It should be designed to stiffen the system to be permanently
stable and function to ensure that the collapse of any web pillars will not be longer
propagated into adjacent panels. The design concept of auger mining consists of three
categories as the single pass, double passes and multiple passes as shown in Fig. 4 [4]. The
single pass is confirmed to have only one row of hole. Secondarily, the double passes are
the technique of application two rows of auger holes, which has the lower first hole and the
upper second holes. The last one is multiple passes which is the application of auger miner
into different coal seams (Fig. 4(c)). However, this research is mainly studied exclusively
on the operation of auger mining on the single pass.
Table 1: Rock Properties of Mae Tan Mine
Rock Types Density σC Cohesion ϕ v E
g/cm MPa MPa ◦C GPa
Top soil 2 1 0.2 15 0.25 0.80
Sandstone 2.3 5 2.8 20 0.25 1.73
Shale 2.3 7.5 2 20 0.25 3.75
Ball Clay 2.2 6.5 2.3 18 0.25 2.86
Carbonaceous shale 2.2 6.9 0.1 10 0.25 3.20
Coal 1.25 4.0 1.8 20 0.25 1.13
Rhyolic Tuff 2.7 30 10 25 0.25 53.01

(a) (b) (c)
Fig. 4: The Auger Highwall Mining layout, (a) Single pass, (b) Double passes,
(c) Multi-passes
Empirical Analysis
Pillar Strength:
Same as the other underground mining, AHM design is based mainly on stress-strength
concept in order to estimate the stability of the pillars and highwall system. Since there is
no detail guidance for coal mine in Thailand and the strength of coal is also weak, both
web and barrier pillar were designed based on some well-known empirical analyses (Table
1) and then numerical analysis of FLAC3D was taken to verify the criteria selected for
Mae Tan mine. Finally, the optimum layout was chosen for this mine. The Strength of
pillar tends to increase in strength relating to the ratio of width to height of the pillar
increases [5]. Equation (1), (2) and 3 are expressed the pillar strength according to Mark-
Bieniawski empirical theory [6], Salamon-Munro of South Africa [7] and UNSW of
Australia [8]:
Table 2: Pillar Strength Estimation
Strength formulae Original
Mark-Bieniawski C= (0.64+0.54 )
W
S
h

USA (1999) Eq. 1
Salamon Munro
0.46
C 0.66
=
W
S
h
 South Africa (1967) Eq. 2
UNSW
0.52
C 0.84
=
W
S
h
 Australia (1996) Eq. 3
Where S is pillar strength, σc is the UCS strength of coal, W represents both web and
barrier pillar width and h is pillar height as well as the diameter of augering flight for the
single pass of auger mining. The second aspect is the stress estimation. Commonly, stress
distribution on pillars is dependent on overburden depth, rock density and pillar width as
well as room width. Then the stress of the pillar is indicated in equations below:
Web Pillar Stress:
p
P v
p
+
( )
W D
W
   Eq. 4
v H  Eq. 5
Where σv is the vertical stress, which is influenced by vertical depth of overburden (H) and
unit weight of rock (γ), Wp is pillar width and D is the auger diameter.

Barrier Pillar Stress:
The barrier pillar is designed in supposing there is no web pillars in the panel. The stress
subjected to the panel was transferred to the barrier pillar as expressed in the next equation:
PN BP
BP v
BP
+
( )
W W
W
   Eq. 6
σBP is stress on barrier pillar and WBP is the width of barrier pillar. Particularly, WPN is
panel width. If there are the number of web pillar (N) in a panel, though the panel width is:
PN P( )W N W D D   Eq. 7
Factor of Safety:
Generally, the ratio of strength to stress is the factor of safety. Usually, safety factor of
web pillar is ranged from 1.3 to 2.0 depending upon the regional geology and further
usage of the surface. Thirty percent of mine plants have applied the value of web stability
factor between 1.3 to 1.6 and the value of the factor of safety greater than 1.6 is about
forty five percent of mine plants, [9, 10]. However, according to NSW DEPARTMENT
OF PRIMARY INDUSTRIES of Australia, for any coal mines which have no detail
guidance, the minimum safety factor 1.6 is suggested. For the highwall pillar, since it was
already assumed that the web pillars were collapse, its safety factor can be taken as low as
1.0 [9, 10].
Pillar Strength
Factor of Safety=
Pillar Stress
Eq. 8
Coal Production:
In this production section, the author focused on coal recovery ratio, and the amount of
producing coal. The recovery ratio is one factor measuring the economic effectiveness.
However, it was not enough to study only one factor, so that the tonnage of coal was
included in mine. The recovery ratio (%R), and coal production (T) per drilling hole are
expressed in the following equations.
2
p c
Area of hole
% or % / ( )
Area of effective block of coal 4
D
R R D W h

      Eq.9
cT A l    Eq.10
Where A, hc ,γc and l are the extracted area, coal seam height, specific gravity of coal and
effected drilling length.
Design Criteria for Web Pillar:
Table 3 below represents the design criteria in this research. The maximum excavated
length was 100 meters along coal seam. The auger diameters were selected according to
the types of commercially available machines. Moreover, the designed width of web
pillars was between 0.5 to 4.0 meters with the interval of 0.5 m. The stress loaded on the

pillars was investigated along pillars from the shallowest to the designed deepest length.
Stress distribution on the pillars was truly depended on the overburden rock and pillar
widths as well as the diameter (D) of the auger machine.
Table 3: Criteria Design for Web Pillar:
Criteria Input:
Web Pillar widths (m): 0.5 1 1.5 2 2.5 3 3.5 4
Diameter (m): 1.5 1.9 2.1
Minimum Safety Factor of web pillar: 1.6
Numerical Analysis
The computer modeling is an important feature to demonstrate the stress distribution on
the highwall system and FISH function is coded to generate the pillar strength. From 3D
modeling for highwall mining, the stress distribution on the highwall is varied due to the
depth variation. The deepest depth of overburden generates greater stress concentration on
the pillar. So FLAC3D modeling performs the exact feature of the stress-strength analysis.
Later on the factor of safety of both web and barrier pillars can be calculated where it is
next used to compare with the empirical modeling. The analysis was carried out under
these initial conditions:
- Grid generation.
- Initial boundary condition response was the roller along the sides and the bottom of
the model.
- Elasto-Plastic material was selected for stress simulation and strength-softening
material was used to estimate the pillar strength.
- Gravity and body load were applied with the gravity acceleration of 9.81 m/s2
.
The convenience of FLAC3D modeling allows creating various geometries of the model.
(a) (b)
Fig. 5: 3D-geometry analysis of Auger highwall mining; (a) Stress on the system before
excavation, (b) Stress concentrating on web pillar after excavation
15
0
m
174 m
Drilling hole 100 m

0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 20 40 60 80 100
Stress(MPa)
Length (m)
Numerical Pillar Stress of
D=2.1 m
Wp=0.5
Wp=1
WP=1.5
Wp=2
Wp=2.5
Wp=3
Wp=3.5
Wp=4
(a) (b)
Fig. 6: (a) The average stress on the sections of the pillar, (b) Stress-strain curve of the
pillar with different pillar widths.
The analysis process was done until the unbalance condition reached. There is the change
of stress loaded on the system before and after the excavation finished. The greatest stress
located at the deepest point in the initial state, however, the stress state becomes bigger on
the pillars after the hole was excavated (Fig. 5). Then the average stress on the pillar were
estimated (Fig. 6 (a)).
To estimate the pillar strength, the model of the pillar was created as demonstrated in
Fig. 6 (b) with the mechanical properties of the bedrock, coal and upper-bed rock. FISH
function was created for the strength estimation for each pillar and the load was applied to
reach the maximum peak strength. After that, the stress-strain curve was obtained.
Result and Discussion
Comparison of Empirical and Numerical
Web Pillar Stress:
(a) (b)
Fig. 7: Result of Stress analysis along the length 100 meters and auger diameter of 2.1 m
from: (a) Empirical Stress, (b) Numerical simulation for vertical stress.
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
Empirical pillar stress of
D=2.1 m
0.0
2.0
4.0
6.0
8.0
10.0
12.0
1050 3050 5050 7050
StressMPa
Steps
Pillar stress of D=2.1m and Wp=0.5m
at L=100m
at L=90m
at L=80m
at L=70m
at L=60m
at L=50m
at L=40m
at L=30m
at L=20m
at L=10m 0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 0.001 0.002 0.003 0.004 0.005
PillarStress(MPa)
Strain (%)
Numerical Strength of D=2.1 m
Wp=4.0
Wp=3.5
Wp=3.0
Wp=2.5
Wp=2.0
Wp=1.5
Wp=1.0
Wp=0.5

Web Pillar Strength:
Fig. 8: Comparison of the results of numerical modelling with empirical pillar strength
equations.
In this study, the web pillar simulation was done only on a case of diameter of 2.1 m to
compare the stress, strength and safety factor from empirical analysis. After getting the
results from both analyses, the smallest one was proposed to apply for design.
Corresponding to the simulation results, the stress concentrated on the pillars was quite
similar to the stress from distributary theories. Yet, most results from numerical analyses
were still lower than that from the theories. Strength from numerical is in the middle of the
three empirical theories. For the same word, it is very closed to the average empirical
strengths. Thus, in this research, the empirical analysis with the average of strength is
taken to design.
Web Pillar Stability Factor:
Fig. 9 below is the demonstration of factors of safety from the empirical (a) and numerical
(b). From both graphs, the value of stability factors was purely closed to each other. But
still the computed results from the numerical were slightly less than from the calculation.
As the consequence of this explanation, Fig. 10 (a) and (b) are the result of safety factors
of the auger machine diameter of 1.5-m and 1.9 m of the average strength of empirical
methods. The safety factor baseline of 1.6 was set up as this design (following the
recommendation from Technical Reference Mine Safety CTR-001 of Australia “For a
Notification of Highwall Mining and Auger Mining (As a High-Risk Activity”) [4].
(a) (b)
Fig 9: Safety factor of web pillar; (a) Safety factor from empirical analysis, (b) Safety
factor of web pillar from numerical analysis.
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
SafetyFactor
Length, (m)
D=2.1 m
Empirical Safety factor of web pillar
SF = 1.6
0.0
2.0
4.0
6.0
8.0
0 0.5 1 1.5 2 2.5 3 3.5 4
Strength,(MPa)
Pillar Width, (m)
Numerical and Empirical Strength of D = 2.1 m
Mark-Bieniawski
Salamon-Munro
UNSW
Average of Empirical
Numerical Analysis

(a) (b)
Fig. 10: Average empirical factor of safety of (a) Auger diameter of 1.9 m, (b) Auger
diameter of 1.5 m.
Coal Production:
From this baseline value, the safe entry length can be estimated according to each pillar
width. From Fig. 9 (a), Fig. 10, the maximum safe excavated length can be established as
presented in Fig. 11 (a) and the x-direction shows the pillar width and the y-direction as
the possible drilling length. The safe drilling length can be increased longer if the web
pillar widths are wider. The auger diameter of 1.9 m and 2.1 m required the pillar width of
4 m to drill until 100 m while the auger diameter of 1.5 m can be accessed to 100 m with
the pillar width of 3 m.
The coal recovery ratio and tonnage of coal per excavated hole were investigated. The
coal recovery ratio is higher at the shallow extracted length and get lower on the deeper
extraction length. It is opposite from coal tonnage which provides a huge amount of coal if
the drilling length increased. T1, %R1, T2, %R2 and T3, %R3 refer to the recovery ratio, and
coal tonnage of auger diameter of 1.5 m, 1.9 m and 2.1 m respectively.
(a) (b)
Fig. 11. (a) The safe drilling length and (b) Coal recovery versus tonnage with different
suggested web pillar widths.
Fig. 11 points out the relationship between the recovery ratio and the amount of coal
producing per safe drilling hole. The graph in Fig. 11 (b) demonstrates the contrary
0
20
40
60
80
100
0.5 1 1.5 2 2.5 3 3.5 4
SafeDrillingLength,(m)
Pillar Width, (m)
Safe Drilling Length (L)
D=2.1 D=1.9 D=1.5
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
SafetyFactor
Length (m)
D = 1.9 m
0
100
200
300
400
500
0%
10%
20%
30%
40%
50%
0.5 1 1.5 2 2.5 3 3.5 4
RecoveryRatio,(%)
Pillar Width, (m)
% Recovery Ratio vs Tonnage
%R1
%R2
%R3
T1
T2
T3
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
SafetyFactor
Length (m)
D = 1.5 m
Wp=4.0
Wp=3.5
Wp=3.0
Wp=2.5
Wp=2.0
Wp=1.5
Wp=1.0
Wp=0.5
SF=1.6

direction of both recovery ratio and tonnage of coal. The increasing of recovery ratio
occurred if the web pillar width was small, while the tonnage of coal increase if the drilling
length increase. The graph in Fig. 11(a) is showing the parallel connection of pillar width
to the safe entry length. The bigger web pillar width, the longer safe auger hole can be
drilled. So for the absolute decision for choosing one good web pillar in this design, it
must be clearly examined between the recovery ratio and tonnage of coal. By plotting the
safe entry length (in Y-direction) to the pillar width (in x-direction), auger diameters of 1.9
m and 2.1 m required the pillar width of 4 m to reach 100 meters of the safe entry length.
On the other hand, the auger diameter of 1.5 m required 3 m of the pillar width to extend
the length to 100 meters. This is followed the safety factor baseline of 1.6 (Fig. 9 (a) and
Fig. 10 (a) and (b)). Therefore, it can be determined that the smaller pillar width gave the
better recovery ratio, though the shorter safe length can be mined and the less amount of
coal tonnage it gets. It can be said that the highest tonnage, the longer drilling length it is.
Based on equation 10, the tonnage of coal is really related to the safe entry length and hole
diameter. Thus:
 The greatest recovery ratio is 33% which is gained to the auger diameter of 2.1 m
and pillar width of 0.5 m, corresponding to 52 tons of coal obtained. After that the
recovery ratio was continued to decrease to 21% at the pillar width of 2 m. This
time, the safe entry length was 52 meters with the tonnage of 225 tons a hole.
 Looking at the auger diameter of 1.9 m, the maximum recovery ratio was 30% at
the pillar width of 0.5 m but the tonnage has been just only 50 tons. When the
recovery ratio decreased to 20% at the pillar width of 1.5 m, the tonnage has been
increased to 161 tons.
 For the auger diameter of 1.5 m, the maximum recovery ratio was 22% at the pillar
width of 0.5 m but the tons of coal were just 52 tons.
Based on the description above, the better condition was the auger diameter of 2.1 m
with the pillar width of 2 m as well as the safe entry length about 60 m. The production
of coal from this was the recovery of 21% and the amount of coal mined about 225
tons per hole.
Barrier Pillar Stress:
In this section, the barrier pillar was studied. The relationship of the barrier pillar to the
number of web pillars in a panel is the main factor to design. From the earlier result of web
pillar design, the optimum criteria are the auger diameter of 2.1 m and web pillar width of
2 m with the safe drilling length of about 60 m. Then the different number web pillar in a
panel of 1, 2 and 3 were proposed to design. The same concepts of web pillar design,
empirical and numerical analyses were suggested in this design. Nine cases of empirical
and numerical analysis were investigated to find out one best barrier pillar and the number
of web pillar per panel. The results are demonstrated as below:

0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
WBP = 5 m
Empirical Stress
(a) (b)
(c) (d)
(e) (f)
Fig. 12: Stress of barrier pillar; (a), (c); (e) for WBP = 3, 4 and 5 m respectively for
empirical analyses, (b), (d) and (f) WBP = 3, 4 and 5 m respectively for empirical analyses.
Barrier Pillar Strength:
The barrier pillar strength was observed from the numerical and empirical as shown in Fig.
13 below. The barrier pillar widths of 3 m, 4 m and 5 m were conducted for this analyses.
0.0
5.0
10.0
15.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
WBP = 3 m
Numerical Stress
N=1
N=2
N=3
0.0
5.0
10.0
15.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
WBP = 3 m
Empirical Stress
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
WBP = 4 m
Empirical Stress
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
WBP = 4 m
Numerical Stress
N=3
N=2
N=1
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
Stress,(MPa)
Length, (m)
WBP=5 m
Numerical Stress
N=3
N=2
N=1

Fig. 13: Comparison of barrier pillar strength from empirical and numerical analyses
Stability of Barrier Pillar:
The result of stress and strength of the barrier pillar were the sources for estimation the
stability of the pillars. The ultimate safety factor of 1.3 was proposed for the design of
barrier pillar and from the different criteria input, different stability factor values were
shown (Fig. 14). Regarding to Fig. 12, both stresses from computer modeling and
empirical calculation provided similar value. In most cases, the value of stress from the
numerical modeling were less than that from the equation. Therefore, the numerical
strength curve was a little bit greater than the predicted average strength of the theories
(Fig. 13). From the stress-strength relationship, the safety factor can be carried out, in
addition, the factor of safety from average empirical analysis can be used directly to apply
for Mae Tan coal.
Panel Design:
There is the relationship between the amount of coal mined and the number of web pillar
in a panel and barrier pillar width. Every coal mine has its exact volume of the reserve. For
Mae Tan mine, the total coal length of the reserve is 600 meters. After Fig. 14, the
increasing of the number of web pillar intended to reduce safety factor. Be noted that the
safety factor baseline of the barrier pillar was 1.3. So from that, the observation was:
 Fig. 14 (a): This is a case of WBP=3m. From this figure, it can be investigated that
in order to reach a safe length of 60 m, only N=1 which its safety factor was on the
baseline.
 Fig. 14 (c): This is the case of WBP=4m. From this, to reach safe length of 60 m,
N=1 and N=2 can be used since their safety factors were located on the baseline.
 Fig. 14 (e): The case of WBP=5m shows that N=1, N=2 and N=3 provided the
safety factor which were larger than the baseline in order to drill until 60m.
One more factor was included in the decision of panel design. It was the relationship of
tonnage to the number of web pillar. Fig. 15 below clarifies this relationship. The
increasing of N leaded to increase the amount of coal could be mined. There was the
limitation of N to the design as already described. After that the summary Table 4 was
demonstrated the relationship of safe criteria to the amount of coal tonnage for each
specific number of web pillar in a panel design. Finally, the optimum criteria for panel
design was Wp =2 m, WBP = 3 m with N=1.
2.0
4.0
6.0
8.0
2.0 4.0 6.0
Strength(MPa)
Barrier Pillar Widths
Numerical Barrier Pillar Strength of D = 2.1 m
Mark-Bineiawski
Salamon-munro
UNSW
Average of Empirical
Numerical

0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
Safetyfactor
Length
WBP = 5 m
Safety factor from Empirical
N =1
N=2
N= 3
SF=1.3
(a) (b)
(c) (d)
(e) (f)
Fig. 14: Factor of safety of barrier pillar; (a), (c); (e) for WBP = 3, 4 and 5 m respectively
for empirical analysis, (b), (d) and (f) WBP = 3, 4 and 5 m respectively for empirical
analysis.
0.0
2.0
4.0
6.0
8.0
0 20 40 60 80 100
Safetyfactor
Length
WBP=3m
Safety factor from Numerical
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
Safetyfactor
Length
WBP = 4 m
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
Safetyfactor
Length
WBP = 5 m
-1.0
1.0
3.0
5.0
7.0
0 20 40 60 80 100
Safetyfactor
Length
WBP = 3 m
N = 1
N=2
N= 3
SF=1.
3
0.0
2.0
4.0
6.0
8.0
10.0
0 20 40 60 80 100
Safetyfactor
Length
WBP = 4 m
N =1
N=2
N=3
SF=1.3

Fig 15: Relationship between tonnage (T), number of web pillar (N)
and barrier pillar width (WBP).
Table 4: Summarized table of WBP, N and T.
Conclusion
The numerical stress and strength have been compared with the empirical stress and
strength in this paper. The numerical strength was smaller than Mark-Bieniawski strength,
but it was still larger than Salamon-Munro and UNSW. Furthermore, it is almost the same
as the average empirical strength. The output of stresses from numerical and empirical on
both web and barrier pillar were very similar to each other. So the verification of empirical
analyses by numerical analysis demonstrated that the average empirical equations of Mark-
Bieniawski, Salamon-Munro and UNSW should be applied at Mae Tan coal.
Safety factor of 1.6 was used to design the web pillar. Then the result showed that the
auger diameter of 2.1 m, web pillar of 2 m and the minimum safe entry length about 60 m
were the best selection of web pillar design with the barrier pillar of 3 m. Finally, the panel
width was 6.2 m with 2 holes in a panel. The recovery ratio was obtained 21%. This is the
optimum criteria for Mae Tan coal mine for a single pass design as shown in Fig. 16.
As the recommendation, the single pass design for Mae Tan mine does not provide the
best recovery and coal extraction ratio in term of economic. To increase its production, the
design should be further studied in more detail and the design should be included the
complicated design system, as the double and multi-pass design.
WBP Number of Web Pillar Tonnage (tons)
3 1 29348
4 2 28322
5 3 27835
22
24
26
28
30
32
1 2 3
Tonnage
(10^3)tons
Number of web pillar (N)
Rlationship of Tonnage VS Number of Web pillar
Wbp = 3 m
Wbp = 4 m
Wbp = 5 m

3 m2 m
6.2 m
2.1 m
Fig. 16: Highwall system in Mae Tan coal mine
Acknowledgment
The author would like to highly respect to ASEAN scholarship for this master’s degree.
The author also thank to SCG-Mae Tan Coal Mine for allowing the author to do this
research.
References
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Philippines: Electrical Generation Authority of Thailand, 2005.
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[4] NSW_DPI, "Technical Reference Mine Safety CTR-001_For A Notification Of Highwall Mining
and Auger Mining (As a High-Risk Activity)," NSW Department of Primary Industries, 2008.
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the 26th international conference on ground control in Mining, 2007, pp. 1-18.
[7] J. Van der Merwe, "New pillar strength formula for South African coal," Journal of the South
African Institute of Mining and Metallurgy, vol. 103, pp. 281-292, 2003.
[8] J. M. Galvin, B. K. Hebblewhite, and M. D. Salamon, "University of New South Wales coal pillar
strength determinations for Australian and South African mining conditions," in Proceedings of the
second international workshop on coal pillar mechanics and design, National Institute for
Occupational Safety and Health, Report IC, 1999, pp. 63-71.
[9] D. Newman and R. K. Zipf, "Analysis of highwall mining stability-the effect of multiple seams and
prior auger mining on design," in Proceedings of the 24th international conference on ground
control in mining. Morgantown, 2005.
[10] R. K. Zipf, "Ground Control Design For Highwall Mining " Transactions-Society For Mining
Metallurgy And Exploration Incorporated, vol. 318, p. 226, , 2005.

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