Punatsangchhu II Hydro Electric Project 1020MW in Bhutan. First Highway Tunnel 1.5km long constructed for traffic.

1. INDOROCK 2013: Fourth Indian Rock Conference 29 – 31 May 2013
Intrepid Approaches in the Successful Construction of First Highway
Tunnel in Bhutan
Santosh K. Sati*, R. N. Khazanchi** and K. K. Sati***
* Geologist, Punatsangchhu H. E. Projects -I&II, P.O-1267, Wangdue, Bhutan.
**Managing Director, Punatsangchhu Hydroelectric Project Authority, Thimphu, Bhutan.
***Dy. GM, Jaiprakash Associates Limited, Bhutan.
(e-mailsantoshsocialarmy@gmail.com)
ABSTRACT: Punatsangchhu Hydroelectric Project-II, a Run-of-River Scheme in Wangdue-Phodrang district,
is located 115km east of Thimphu, the Capital of Bhutan. A part of National Highway on right bank shall be
inundated due to the reservoir submergence. To reroute the NH on the right bank the field investigation was
carried out at site. However, on the basis of site geological & topographical conditions it was not feasible to
construct road. The area was again re-examined and found suitable to reroute the National Highway through a
12m (W) x 8.48m (H), 1.5km long a Tunnel. A detailed Geological investigation carried out all along the
highway tunnel for suitable layout planning and designing. In view to suit the tunnel diameter most suitable
equipments i.e. Atlas copco make E2C drill jumbo and Komatsu make PC400 excavator were deployed at site to
excavate the full section in one go. An intermediate adit was also provided to accelerate the construction. The
tunnel has been excavated by conventional drill and blast method smooth drilling and blasting pattern was
designed, considering different rock categories/classes to achieve target and minimized the overbreak. The
Barton’s ‘Q’ system was adopted for rock classification and providing the support system.
Key words: High Way Tunnel, Construction Practice, 3D Log, Face Log.
1 INTRODUCTION
The Punatsangchhu-II Hydroelectric Project (PHEP-II) in kingdom of Bhutan lies in the Eastern Himalayas
between latitude 26.70 N and 28.40 N and longitudes 88.70 E and 92.20 E. (Fig.1) Bhutan with geographical
area of 38,394 km2
.The catchment area (up to dam site) 6390km2
extends between latitude 270
15’ and 280
30’ N
and longitude 890
15’ E and 900
30’ E. It is completely mountainous, significance part of which is covered by
snow. The PHEP-II contemplates construction of a 86 m high (from the deepest foundation level) Concrete
Gravity Dam to divert 477cumecs of water of Punatsangchhu River through a 8.6 km long, 11 m diameter
Headrace Tunnel to an Underground Powerhouse for generation of 1020 MW. Due to reservoir submergence a
part of National Highway shall be inundated. So it became necessary to reroute the National Highway through a
Highway Tunnel. A 1.5km long, 12m (W) x 8.48m (H) first Highway in Bhutan proposed at right bank of the
Punatsangchhu River to divert the traffic. The layout plan of Highway Tunnel is depicted in Fig.2.
Fig 1 Project Location Map

2. INDOROCK 2013: Fourth Indian Rock Conference 29 – 31 May 2013
Fig 3 Geological L- Section through Highway Tunnel
2 GEOLOGICAL SETTINGS
Regionally the PHEP-II area is located within the Tethyan Belt of Bhutan Himalayas and at the proposed dam
site; rocks of Sure/ Shumar Formation of Thimphu Group of Precambrian age are exposed. The rocks of
Thimphu Group in general are characterized by coarse – grained quartzo-feldspathicbiotite-muscovite gneiss
with bands of mica schist, quartzite and concordant veins of foliated leucogranitoid, migmatites with minor
bands of amphibolites and marbles. Garnet porphyroblasts are also seen within these gneisses.
3 GOTECHNICAL CONSIDERATION AND LAYOUT PLANNING
For the purpose of diverting traffic due to the reservoir submergence and stripping, excavation and treatment of
right abutment of 86 m high concrete dam, a highway tunnel was provided. Geological and geotechnical
evaluation of Highway Tunnel has been made based on the analysis of surface data and 3D Logging and Face
logging during construction and geological data collected during the course of the investigations.
During planning of tunnel layout and its portal vertical and horizontal cover criteria were considered and
accordingly the highway tunnel alignment was proposed to have sufficient vertical and as well as lateral cover.
The minimum vertical cover is ˜10m and the maximum is about ˜150m (Fig 3). Lateral cover was low at
Northern portal is about 5m in initial reach. During designing of highway tunnel it is kept in mind that the
alignment should be straight with smooth curve from Northern portal, the alignment of highway tunnel crossing
the four power intake with vertical cover of 21m were considered. The slope of tunnel is 1:54.36 from Northern
portal at RD 0 to RD 1000m after that it is 1:23.50 from RD 1100 to 1500m at Southern Portal. During
Fig 2 Layout Plan Of Highway Tunnel

3. INDOROCK 2013: Fourth Indian Rock Conference 29 – 31 May 2013
Fig 4 Percentage of Expected and Actual Rock Class Encounter in Highway Tunnel
construction it is felt that from 2 faces it is difficult to excavate the highway tunnel within scheduled time frame.
An intermediate adit was proposed to facilitate and accelerating the excavation of the highway tunnel.
4 ANTICIPATED ROCK MASS QUALITY AND ROCK ALTERATION
The expected / anticipating rock mass quality during investigation and planning of Highway Tunnel, is based on
the surface mapping and rock outcrop exposed in the area and it vary due to geological surprises ie.
Shear/fractured, faults, ground water inflow during construction.
The rock mass along the alignment of Highway Tunnel has been subjected to three type of alteration that is
generally developed along shear/fractured zones and influenced the strength of rock mass. The alterations also
influence the strength and abrasivity of rock mass. The percentage of anticipated and actual rock mass condition
encountered during construction is shown in Fig 4.
5 ANTICIPATED ROCK TYPE ITS CATEGORY AND EXCAVATION CONDITION
The rock type along the tunnel alignment broadly divided in to three units:
1) Biotite Gneiss
2) Leucogranite
3) Pegmatite with Biotite Gneiss
On the basis of litho tectonic characteristic and other influencing factor viz- excavation direction and orientation
of discontinuities, ingress of ground water and overburden cover, the rock mass behavior evaluate the effects of
influencing factors. The tunneling media have been classified in different units as given in Table 1.
Table 1 Summary Different Units Encounter during Construction.
Rock Mass Lithilogical Description Rock mass Behavior Deformation
1. Thinly bedded Biotite gneiss Spalling -
2. Biotite, hard and Massive Less jointed
(Fresh)
High overbreak, forming wedge In few cm
3. Leucigranite. Strong to very strong. Stable -
4. Biotite gneiss with intrusion of pegmatite. 2-3 m overbreak, forming
wedges, rock fall.
In few cm
5. Pegmatite cursed and fractured with bends
of biotite gneiss
High overbreak -do-
The rock mass quality of the bedrocks for the entire tunnel alignment was anticipated on the basis of Q (Barton
et al 1974, 1983, 1994) and RMR (Bieniawski et al 1978, 1989, Deere et al 1988) systems for the assessment
The rock conditions expected during tunneling was Very good (5%) Fair (65%) Poor (30%) and actual
encounter during construction is Fair (78%) and poor (22%) categories.

4. INDOROCK 2013: Fourth Indian Rock Conference 29 – 31 May 2013
6 GEOLOGY ALONG TUNNEL ALIGNMENT
The tunnel Geological mapping was carried out by using face and 3-D geological log sheets at every advance.
The rock strata encountered during tunnel alignment is mainly qurtzo-feldspathic gneiss with subordinate biotite
schist bands and occasional patches of intrusive pegmatite. At the portal face, moderately jointed hard and
massive quartzofelsphathic gnessies is exposed forming scarp face. The general trends of foliation vary from
N700
E-S70W to N800E-S800
W with 200
-250
dip towards SE. On the basis of Barton’s Q system the rock strata
has been categorized into Fair (78%) and poor (22%) categories. The important geological features observed
during excavation and their characteristics are given in Table 2.
Table 2 Characteristics of Important joints encountered in Highway Tunnel.
S.No. Joint
Set
Strike Dip and
Direction
Spacing
(in m)
Persistence
or
Continuity
Aperture/
Filling (mm)
Feature
1. J1 N700
E-S70W to
N800
E-S800
W
200
-250
: SE 0.2 to
0.5.m.
> 20m. Tight to clay filled,
2 to 50mm open.
Foliatio
n joint
2. J2 N400
W-S400
E to
N N600
W-S600
E
700
: N E 2 to 5m. 5m to 15m. Granular to clay
filled.
Joint
3. J3 N400
W-S400
E 600
-750
: SW 2 to 10 5m to 12m Generally 2mm,
granular/crushed.
Joint
4. J4 N100
W-S100
E 400
-800
:
N800
E
1to 3m. 10 to 20m. Open upto 30mm
Granular
Joint
5 J5 E -W 300
to 600
:N 1to 2m. >10m Tight, sometime
clay filled.
Joint
6. J6 N600
E-S600
W to
N800
E -S800
W
400
-600
: SE 5 to 50m >8m Clay/crushed Shear
7 EXCAVATION AND SUPPORT SYSTEM
The excavation of Tunnel was started from outlet and Inlet end in January 2012, by heading (7.5m) and and
benching (1.0m), methods. In view to accelerate the construction schedule tunneling methodology was reviewed
and the excavation in full section was carried out by deploying Atlas copco make E2C boomer which is designed
to drill horizontal hole upto 8.5m height. An intermediate adit was also made to speed up construction. In all
respect the tunnel excavation was completed within 09 month in September 2012. The quantities of work
executed during excavation are given in following Table 3.
Table 3 Quantities of Works For Highway Tunnel.
Quantities of Work
Excavation 150118 m3
Shotcrete 4563.53m3
Rock Bolts 18720 RM
Steel Ribs 280.80MT
8 DRILLING AND CHARGING PATTERN
The major governing factors for any tunnel excavation and its stability are encountered Geological
discontinuities drilling pattern and rock support system at every advance. During excavation of Highway Tunnel
burn cut pattern of drilling was implemented. Special attention was given in drilling and charging of periphery
holes, aligned horizontally @ 0.3m c/c spacing and charged alternatively with 25mm dia cartridge for control
over brake. The drilling depth was considered on the basis of encountered rock categories i.e. 3.5m depth in fair
to good categories and 2.0m depth in poor categories(fig.5 &6). In totality the average powder factor achieved at
site was < 1.4 kg/m3 and the over brake was under 11%.

5. INDOROCK 2013: Fourth Indian Rock Conference 29 – 31 May 2013
X- SECTION OF COMMUNICATION TUNNEL
FIGURE - 6a
FOR CONTROL OVERBREAK PERIPHERY HOLES
SHOULD BE CHARGED WITH ONLY PRIMERY
CARTRIDEGE OF 40MM & REMANING CARTRIDGE
OF 25MM/32MM. THROUGH DECKING SYSTEM
MINIMUM EXCAVATION LINE
UNCHARGED HOLE
0.80
0.80
0.80
0.30
0.30
0.30
0.60
0.85
0.85
0.85
PRIMER 40MM
STEMMING
CARTRIDGE
25MM
DECKING(0.3M)
N.E.D
DECKING(0.3M)
CARTRIDGE
25MM
DECKING(0.3M)
DECKING(0.3M)
CARTRIDGE
25MM
CARTRIDGE
25MM
CARTRIDGE
25MM
1.002.50
0.85
3
2 1
0
44
4
4
R6.10
4
4
4
4
3
3
3
2
22
1
1
1
0.85
1.00 1.00 1.00
0.80
0.80
0.50
1.001.001.001.001.001.000.94 0.94
0.30
0.30
R6.10
8.48
6
6
6
6
6
6
6
6
6
6
6
6
6 5
5
5
5
55
5
5
5
5 7
7
8
7
7
7
7
8
8
88
8
8
8
8
9
9
9
9
9
9
10
10
10
10
10
1010
10
10
10
10
10
11
12
11
11
11
11
11
11
11
12
12
12
12
121212
12
12
12
12
12
15 15 15 15 15 15 15 15 15 15 1513 13
13
13
13
14
14
14
14
14
14
14
14
14
14
14
141414141414
14
14
14
14
14
14
14
14
14
14
14
14
13
13
13
1.00
11.23
0.55
0.98
1.00
17
2.38
0.50 13
0.50
13
0.62 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.62
16 16 16 16 16 16 16 16 16 16 1617


UNCHARGED HOLE
FIGURE - 6c
Face Area = 86.70 M
2
Expected Pull = 3.2 M. (average)
Charge Factor = 1.39 Kg / M
3
DRILLING & CHARGING PATTERN OF CUT HOLES
FIGURE - 6b
0.860.18
0.25
0.86
2.00




2.00
2 1
0
0.25
4
3
4
4
4
4
4
4
4
3
3
3
2
22
11
1
Fig 6a-c Drilling & Charging Pattern Adopted in Fair Rock Category
X- SECTION OF COMMUNICATION TUNNEL
FIGURE - 5a
FIGURE - 5c
0.7001.300
PRIMER
32MM
STEMMING
CARTRIDGE
25MM
N.E.D
DECKING
(0.3M)
Face Area = 86.70 M
2
Expected Pull = 1.70 M. (average)
Charge Factor = 1.27 Kg / M
3


UNCHARGED HOLE
DRILLING & CHARGING PATTERN OF CUT HOLES
FIGURE - 5b
0.860.18
0.25
0.86
2.00




2.00
2 1
0
0.25
4
3
0.30
0.30
3
2 1
0
44
4
4
R6.10
4
4
4
3
3
3
2
22
1
1
1
1.00 1.00 1.00
0.50
1.001.001.001.001.001.000.94 0.94
0.30
0.30
R6.10
8.48
5
5
5
5
55
5
5
5
5 13
14
1.00
11.23
0.55
0.98
1.00
16
2.38
0.50
0.50
0.62 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.62
15 15 15 15 15 15 15 15 15 15 1516
0.50
0.75
0.75
0.75
0.75
0.50
MINIMUM EXCAVATION LINE
0.30
6
6
6
6
6
6 6
6
6
6
6
67 7
7
7 7
7
8
8
8
88
8
8
8
9
9
9
9
9
9
10
10
10
10
101010
10
10
10
10
11 11
11
11
11
11
12
12
12
12
12
12
12 12
12
12
12
12
12
12
13
13
13
13
13
13
13
13
1313
13
13
13
13
13
13
13
13
13
14 14 14 14 14 14 14 14 14 14
FOR CONTROL OVERBREAK IInd ROW HOLES
SHOULD BE CHARGED WITH ONLY PRIMARY
CARTRIDEGE OF 32MM & REMAINING CARTRIDGE
OF 25MM.THROUGH DECKING SYSTEM
UNCHARGED HOLE
0.50
1.00
0.85
0.85
0.85
0.85 0.85
TO AVOID OVERBREAK & CAVITY THE HOLES WITH IN
WEAK FEATURE/SHEAR SHALL BE LEFT UNCHARGED
WHILE THE HARD PATCHES SHALL BE ALTERNATIVELY
CHARGED AND REMOVED BY ONLY PRIMARY CARTRIDEGE.
0.30
4
4
4
4
4
4
4
3
3
3
2
22
11
1
4
Fig 5a-c Drilling & Charging Pattern Adopted in Poor Rock Category

6. INDOROCK 2013: Fourth Indian Rock Conference 29 – 31 May 2013
9 ROCK SUPPORT
On the basis of encountered rock strata the support system was timely provided concurrent to excavation The
immediate support consists of a steel fiber reinforced support (SFRS) shotcrete lining, 32mm diameter resin
grouted rock bolts. The thickness of shotcrete is 75mm in rockmass II, 100mm in rockmass III, and 125mm in
rockmass IV. The length of rockbolt is 6m in all rock mass class. The spacing of rockbolts is 1750*1750
staggered in rockmass II, 1500*1500 staggered in rockmass III and 1500*1000 staggered in rock mass IV. Steel
rib installed in only rock mass class IV where Q value is >2, where required at a spacing of 500cm to 1000cm
and in some overbrak reaches where overbreak is about 3 to 4m above crown. The final lining will be constituted
of unreinforced concrete 500 mm thick. The excavation method and support system were selected accordingly to
rock mass quality.
The problem faced during excavation is mainly due to presence of low angle long continuity biotite schist bands
associated damp condition. Such reaches were excavated by low charging getting short pull and supported with
ISMB-250 Steel ribs with backfill concrete.
10 CONCLUSION
After the Geological and Geotechnical investigation of the area and the results of site investigation indicate the
rock mass condition have different characteristics. Complex geological and tectonic framework of the terrain
required challenging excavation and construction procedures depending on site condition, according the layout
of Highway Tunnel were finalized. During construction it is felt that from 2 faces it is difficult to excavate the
highway tunnel within scheduled time frame. An intermediate adit was proposed to facilitate and accelerating the
excavation of the highway tunnel. Drilling and Blasting pattern for Highway Tunnel were examine after every
advanced and according rock mass class the pattern review and modified for better outcomes. On the basis of
encountered rock strata the support system was timely provided concurrent to excavation and support system
were installed as per rock mass class and site condition.
ACKNOWLEDGEMENTS
The Authors gratefully acknowledge the encouragement of the Engineers, Designers and those who are involved
in preparing and presenting this paper.
REFERENCES
Barton N. (1983) “Application of Q-System and Index Test to Estimate Shear Strength and Deformability of
Rock Masses”. Int. Symp Eng Geol and Und Const. Lisbon. Vol. II, pp II.51 – II.70.
Barton, N., R. Lien R. & J. Lunde (1974) “Engineering Classification of Jointed Rock Masses for the Design of
Tunnel Support”. Rock Mechanics, 6, p. 189-236.
Barton, N. & E. Grimstad (1994) “The Q-System Following Twenty Years of Application in NMT Support
Selection”. Felsbau 12 Nr. 6, pp. 428-436.
Bieniawski Z.T. (1978) “Determining Rock Mass Deformability. Experience from Case Histories”. Int. J. Of
Rock Mech and Min. Sci. Vol. 15 pp 237-242.
Bieniawski Z.T. (1989) “Engineering Rock Mass Classifications”. Ed WILEY. New York, 252 pp.