Paper 4

JTM Vol. XVI No. 2/2009
109
ESTIMATION OF ROCK SUPPORT TYPE FOR
THE SOUTH RAMP DOWN B DEVELOPMENT
AT PONGKOR UNDERGROUND GOLD MINE
Budi Sulistianto1
, Ridho K. Wattimena1
, Suseno Kramadibrata1
,
Tyas A. Rabudianto1
, Ahmad Ardianto2
Sari
Pada tambang bawah tanah, akses jalan menuju cebakan bijih mempunyai fungsi yang sangat vital. Oleh karena
itu, jika kondisi massa batuan masuk kategori tidak menguntungkan, maka system penyangga yang diterapkan di
jalan akses tersebut menjadi kebutuhan penting. Penyelidikan geoteknik harus dilakukan untuk memenuhi
kebutuhan data yang diperlukan dalam desain sistem penyangga. Tambang Ciurug adalah salah satu lokasi di
Tambang Emas Bawah Tanah di Pongkor dimana kegiatan penambangan sedang aktif, dan terkonsentrasi di level
500m sampai level 700m. Dalam rangka untuk menunjang keberlangsungan tambang, pengembangan tambang
dilakukan untuk mencapai level 400m.Ramp down sebagai akses untuk mencapai level ini dibuat. Penyelidikan
geoteknik untuk memahami kondisi massa batuan di sepanjang jalur ramp down dilakukan. Berdasarkan data
yang diperoleh dari pemetaan geologi struktur dan pengamatan inti bor, orientasi dari joint utama relative sejajar
dengan urat bijih. Dengan menggunakan RMR, segmen-segmen di sepanjang jalur ramp down akan berada di
batuan yang dikategorikan masuk kedalam kelas II dan III. Berdasarkan kelas tersebut dan juga
mempertimbangkan system penyangga yang sudah digunakan di tambang ini, maka jenis penyangga yang
disarankan adalah friction bolt dikombinasikan dengan wiremesh serta baja/kayu dalam bentuk three piece sets
bila dijumpai batuan dalam kondisi buruk. Penagkat lunak yang berbasis Metode Elemen Hingga (FEM)
kemudian digunakan untuk memverifikasi jenis penyangga yang disarankan.
Kata kunci: tambang bawah tanah, rump down, desain penyangga, RMR, FEM.
Abstract
In the underground mine, access road to the ore deposit is vital due to its function. Rock support applied in the
access road, therefore, becomes necessary if the rock mass condition is unfavourable. Geotechnical investigation
is very important to satisfy the need of data in the design of the rock support. Ciurug Mine is one of the locations
in Pongkor Underground Gold Mine where mining activity is running, and concentrated at level 500m to level
700m. In order to continuing the mine life, mine development is carried out to reach level 400m. The ramp down
as access to reach this level is constructed. Geotechnical investigation for understanding the rock mass condition
along the ramp down line is then carried out. The orientation of major joint revealed from the structural mapping
and core observation, is relatively parallel to the vein. Using RMR along segments of the ramp down line, it is
obtained that the rock mass was vary from class II and III. Based on this class and also considering the support
system used in this mine, the design of the support type is friction bolt combined with wiremesh as well as
steel/timber in three pieces set type in the worse condition. Finite Element Methods (FEM) based software is then
used for verifying the suggested support.
Keywords : underground mine, ramp down, support design, rock mass rating (RMR), FEM.
1)
Department of Mining Engineering, ITB, Bandung 40132, Indonesia. Email : bst@mining.itb.ac.id
2)
PT. ANTAM, Tbk., Jakarta, Indonesia
I. BACKGROUND
Ciurug Mine is one of active underground
mine in Pongkor Underground Gold Mine
which is located in Bogor of West Java
Province, Indonesia. The gold ore at this area
is mined by fully mechanized cut and fill
stoping method. Current mining activity is
concentrated at level 500m to level 700m.
In order to continuing the mine life, the mine
development is expanded down to Level 400m.
As mine access to the level 400m, a ramp
down is excavated. For this reason, a
geotechnical investigation in this area was
carried out emphasizing in comprehending the
rock mass properties and subsequently putting
forward the required roof support arrangement
along the ramp down. The investigation is still
required after part of the ramp down has been
excavated for understanding the geological
structure and the rock mass condition.
II. GEOTECHNICAL INVESTIGATION
2.1.Geological Structures Mapping
Scan line of geological structure mapping was
carried out at Xcut 8, Xcut 9, Xcut 10 and
footwall drift in the South Ciurug Mine of
Level 500m, and the result is shown in Figure
1. Having obtained the mapping, the major
orientation of weakness planes was assumed
continue so that the general picture of the
weakness plane existing in the proposed ramp
down is understood and predicted. The
geological structure of the weakness plane is
considered continue because the height

Budi Sulistianto, Ridho K. Wattimena, Suseno Kramadibrata, Tyas A. Rabudianto, Ahmad Ardianto
110
difference between Xcut and the ramp down B
was less than 100 m. The values of the
predominant orientation and average joint
spacing of joints for each cluster are given in
Table 1 (Sulistianto et.al., 2008).
The vein at Ciurug mine has orientation
(strike/dip) about 060°/70° and the data of
geological structure presented in Table 1
indicates that the predominant orientation of
the weakness planes is relatively in line with
that of vein (71o
).
2.2.Rock Drilling and Laboratory Testing
The geotechnical drilling was carried out from
the footwall drift of Level 500 through the
rock mass nearby the proposed ramp down.
Figure 3 shows the positions of drill holes
which are named GTRD 01 and GTRD 03.
RQD from each borehole was also determined.
The rock samples for laboratory testing
obtained from GTRD 01 and GTRD 03 were
then tested to determine their physical and
mechanical properties, and the results were
given in Table 2.
III. ROCK SUPPORT ESTIMATION
3.1. Evaluation of the Geotechnical
Condition of the Ramp Down
Based on the UCS values (see Table 2), the
intact rock nearby the proposed ramp down
can be classified into medium high strength
rock (Bieniawski, 1973). As revealed from
field observation, the condition of the rock
mass nearby the proposed ramp down can be
said as massive and strong rock although the
rock mass is intersected by a number of
veinlets and joints, except at rock mass close to
the vein which is categorized as alteration
zone.
Following the ramp down design and
considering the analysis purposes, the ramp
down is divided into 3 segments such as S, E
and N (see Figure 4). This division is made
concerning with the fact that the major
orientation of the geological structure in this
area is corresponding with that of vein and
direction of tunnel excavation. The excavation
starts at segment S, at which the joint
orientation at the front view of its cross section
incline of 72o
CCW of the horizontal axis (see
Table 2). When the excavation reaches the
segment E, the position of the geological
structure would appear horizontal at the front
view cross section. Likewise, when the
excavation hits segment N, the position of the
geological structure would appear opposite to
the presentation of the geological structure that
is seen at segment S.
3.2. Determination of Rock Mass Condition
Using RMR
Having observed the rock mass conditions in
the ramp down, the following are the input
parameters for the RMR.
1. Compressive strength value obtained from
drill-core sample tested in laboratory
(Table 2). The strength of intact rock in
ramp down B location level 490 obtained
from GTRD 01 (55-56.35 m depth), that is
57.92 MPa and GTRD 03 (in 35.75 –
36.75 m depth) that is 70.09 MPa.
2. RQD value obtained from GTRD 01 and
GTRD 03 (Table 2), RQD value for level
490 is 90% - 98%
3. The average of joint spacing determined
by measurement of discontinuities from
nearest location is 1.24 m (table 1).
4. The thickness of filling material is about
1-10 mm, and the type of gouges are
strongth material, that is quartz.
5. The water condition of the rock mass can
be generally deemed wet.
6. The degree of weathering of the rock mass
indicated moderately weathered.
By using all the parameters mentioned the
basic RMR could then be determined, and this
was subsequently corrected with the presence
of the main orientation of the weakness planes
against the direction of the excavation. The
final RMR or the corrected RMR suggests that
the rock mass condition can be said as fair rock
mass which is given in Table 3.
3.3.Supporting System
Based on the result of the rock mass
classification method mentioned above, rock
support used in the mine which is friction-bolt
combined with wire-mesh but sometimes
installed also steel/timber in three pieces set
type if facing the worse rock condition.
Referring the support pattern in the hard rock
(Hoek et.al, 1995), for the good rock class,
only friction bolt at roof with distance between
bolt is 1.5-2m, spacing of 1m, is required. For
the fair rock class, the pattern of support is
friction bolt at roof and side-wall, distance of
bolt is 1.5-2m, spacing of 1m, and wire-mesh
at the roof, and inclination of bolt should be
modified considering the discontinuities
orientation.
3.4. Analysis of Supporting System Using
Numerical Method
FEM based software named Phase 2.V.50 from
Rockscience was used for analysing the
proposed supporting system at the ramp down.
Modelling was exercised using two approaches
such as tunnel without support and with
support. The geometry of the opening was set
as the real condition which was height of 4 m

Estimation of Rock Support Type for TheSouth Ramp Down B Development
at Pongkor Underground Gold Mine
111
and width of 4 m. The input parameters for this
modelling used the data given in Table 2.
In order to get an understanding of how to
install a rock bolt properly, modelling was run
applying breccia rock and polimic breccia with
direction of the weakness plane as it is in
segment N (Figure 4). The major and minor
principal stresses were 4.076 MPa and 0.4.
MPa respectively and the counter clockwise
angle between σ1 to the horizontal plane was
76o
(Sulistianto et al, 2003). The modelling
suggested that the breccia rock mass of the
opening without support was stable but it
become weaker when the rock mass is polimic
breccia. The displacement increased at
parimeter of the opening (Figure 5). Thus, rock
bolt should then be installed in order to prevent
the opening collapses.
IV. ROCK SUPPORT DESIGN
VERIFICATION
Currently, there are part of segment at ramp
down B which have been constructed.
Observation in order to verify the rock support
design is conducted at ramp down B (metre 0 –
90), including structure mapping to know the
actual orientation and condition of
discontinuities, ground water condition and
intact rock strength using point load test.
4.1. Mapping of Discontinuities
Result of geological structure mapping
obtained the major discontinuities which is still
in line with the discontinuity estimated at the
previous geotechnical investigation. The result
of this mapping can be seen at Table 5 and
Figure 6. The visualisation of joint mapping
for the rock mass in ramp down at distance of
30 – 90 m can be seen in Figures 7 and 8.
Based on the result of stuctures mapping at the
location can be seen that dominant orientation
of discontinuities (057°/73°) having dip
direction which is parallel relative with vein
(060°/70°). From the result of this observation
giving information that existing structure is
same as determined estimate before. So that
the previous support recommendation can be
used for the next excavation.
4.2. Evaluation of RMR Value
The orientation of major structure used as
correction factor of RMR system regarding to
the tunnel direction. The result of structures
mapping and their influence to the tunneling is
like seen at Table 6.
RMR parametres which have been measured at
location of ramp down B can be explained as
follows :
1. UCS value obtained from point load test
51.43 MPa untill 87.47 MPa rock strength
at ramp down B as the result of point load
test. This rock strength can be classified as
medium strength rock (Bieniawski, 1973).
2. By Priest and Hudson equation, RQD
value can be determined. λ (frequency of
discontinuity per metre) as input of this
equation ( 1)(0.1λ100RQD 0.1λe
  ),
average RQD value is 99.31 %.
3. Avarage joint spacing is 0.82(Table 5)
which is revealed by direct structures
mapping at the side wall of Ramp down.
4. Discontinuities condition of ramp down
have rough surface. It means that good for
underground stability. Joint aperture is 1 –
10 mm, so it is classified into opened
joint, however, it is found also joint filled
by clay and calcite (weak materials).
5. The water condition of the rock mass can
be generally deemed wet.
6. The degree of weathering of the rock mass
indicated average weathered.
RMR is then calculated in the some segments
of Ramp down B with 10 metre distance. This
segmentation is conducted with consideration
of existence of different parameter value
especially at parameters of UCS, RQD and
joint spacing. By using all the parameters
mentioned above, the basic RMR could then
be determined, and this is subsequently
corrected with the presence of the main
orientation of the weakness planes against the
direction of the excavation. The final RMR or
the corrected RMR suggests that the rock mass
condition can be said as good rock to fair rock
mass (shown in Table 7).
V. CONCLUSSION
1. Rock mass condition based on observation
at excavated area of ramp down B is
classified into class II (good rock mass)
untill class III (fair rock mass).
2. Therefore, the support system which is
friction-bolt combined with wire-mesh but
sometimes replaced with steel/timber in
three pieces set type if facing the worse
rock condition are still able to be applied.
ACKNOWLEDGEMENT
The authors would like to thank to Pongkor Gold
Mining Business Unit of PT. Antam, Tbk. for the
possibility and opportunity to conduct this study.

112
REFERENCES
1. Bieniawski, Z.T., 1989, Engineering
Rock Mass Classifications, John Wiley
& Sons Inc., Canada.
2. Hoek, E., and Brown, E.T., 1980,
Underground Excavation in Rock, The
Institution of Mining and Metallurgy.,
London.
3. Hoek, E., Kaiser, P.K., and Bawden,
W.F., 1995, Support of Underground
Excavations in Hard Rock, A.A
Balkema., Rotterdam.
4. Sulistianto, B., Rai, M.A.,
Kramadibrata, S., Nakagawa.H.,
SetiawanID., Janu, E., Risono, 2003,
Determination of In situ Stress Using
Hydraulic Fracturing Method at
Pongkor Underground Gold Mine, West
Java Indonesia. Proc. of the 3rd Int.
Symp. on Rock Stress RS Kumamoto
’03, Kumamoto, Japan., A.A. Balkema
Publ.pp 383-388.
5. Sulistianto, B., Wattimena, R.K.,
Kramadibrata, S., Rabudianto, T.A.,
Ardianto, A., 2008, The Role of
Geotechnical Investigation on The
Ramp Down Development in Pongkor
Underground Gold Mine. Proc. of Int.
Symp. on Earth Sci. And Tech. 2008,
Fukuoka, Japan., CINEST,pp 291-298.
Table 1. Scan line result conducted in the Cross Cut (Sulistianto, 2008)
No Location
Stereonet
Dip
Direction/Dip
Joint Spacing
(M)
1
Footwall
Drift
Ciurug
(N161O
E)
056°/76 1.69
331°/87° 1.80
2
Xcut 8
Ciurug
(N79O
E)
073°/72° 0.43
3
Xcut 9
Ciurug
(N76O
E)
075°/74° 0.31
4
Xcut 10
Ciurug
(N77O
E)
080°/69° 0.41

113
Table 2. Recapitulation of laboratory testing
Table 3. RMR of the ramp down B
Location RMR Condition Segment
60 Fair S11
RAMP 63 Good S13
DOWN
B
71
Good E13
51 Fair N11
56 Fair E11
Table 4. Specification of the proposed rock bolt
Table 5. Result of structures mapping at Ramp Down B
Table 6. Discontinuity Orientation of Ramp Down B
Location Tunnel
The number
of
Orientation
of
Effect of discontinuities to the tunnelling.
Direction Joint set Discontinuity
RAMP N 353° E Dip direction of discontinuities almost
DOWN
B N 308° E
2
057°/73°
303°/79°
perpendicular to tunnel direction,
this condition is not good
UCS Test Direct Shear Test Triaxial TestBore hole Depth
from collar
(m)
Lithology RQD

(MPa)
E
(MPa)
 Cpeak
(MPa)
Cresidual
(MPa)
peak
( o
)
residual
( o
)
C
(MPa)

( o
)
Density
(gr/cm3
)
GTRD-01 25.50-24.00 Breccia 98.00 59.72 17631.90 0.21 0.17 0.09 38.6 27.1 12.58 47.40 2.54
GTRD-01 55.00-56.35 Breccia 90.00 57.92 12841.20 0.22 0.28 0.16 35.7 25.2 6.94 54.11 2.53
GTRD-03 08.95-11.25 Andes.Breccia 95.00 59.13 12470.45 0.27 0.35 0.19 41.4 26.7 15.29 47.55 3.16
GTRD-03 35.75-36.75 Breccia 98.00 70.09 16302.17 0.25 0.38 0.23 45.2 26.8 23.26 28.77 3.33
345 Mpa 120 kN 445 Mpa 160 kN
460 Mpa 165 kN 510 Mpa 180 kN
2.79 kg
Yield Strength
Ultimate Tensile Strength of Tube
Friction Bolt Diameter
Hole Diameter
SPESIFICATION
Mass per metre
Minimum Typical
47 mm
43 mm (min) / 45.5 mm (max)
Length 2.4 m / 3 m
Location Joint Set Average Joint
SET
Dip Dir/
Dip
Spacing (m)
RAMP
DOWN
B
2
057°/73°
303°/79°
0.82

114
Table 7. Rock Mass Condition of
Ramp Down B
Location RMR Condition Meter
RAMP 68.97 Good rock 0 – 10
61.29 Good rock 10 – 30
58.94 Fair rock 40 – 60
DOWN B 60.94 Fair rock 60 – 70
62.43 Good rock 70 – 80
61.47 Good rock 80 – 90
Figure 1. Location of investigation

115
Figure 2. Excavated segment of Ramp Down

116
Figure 3. Location of geotechnical drilling
Figure 3. Location of geotechnical drilling
Figure 4. Main segment at the ramp down excavation
Figure 6. Stereonet of discontinuities orientation at ramp down B.
Direction of
Excavation
S
E
N
Tampak
Depan
Tampak
Samping
S S
E
N
E
N
Front Side
t

117
Input model Before support installation After support installation
Breccia :
E : 12.841 MPa
 : 0.22
C : 6.94
 : 54.11 °
Joint :
C : 0.16 MPa
 : 25.2 °
Andes. Breccia:
E : 12.470 MPa
 : 0.27
C : 15.29
 : 47.55°
Joint :
C : 0.16 MPa
 : 25.2 °
Figure 5. Recommendation of support system using numerical method
3D’s visualization 2D’s visualization
Tunnel direction : N 353° E ; Discontinuities : 057°/73°, 303°/79°
Figure 8. Discontinuities Visualization of RAMP DOWN B meter 0-30

118
3D’s visualization 2D’s visualization
Tunnel direction : N 308° E ; Discontinuities : 057°/73°, 303°/79°
Figure 9. Discontinuities Visualization of RAMP DOWN B meter 60 – 90

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