Post on 24-May-2020
Geotechnical Investigations in Adverse Geological Occurrences in
HRT of Tala H.E. at Kalikhola, Bhutan
Dixit, Mahabir Babbar, S.K. Kumar, Bibhas1
Senior Research Officer Joint Director Chief Engineer
e-mail: mdixit@nic.in e-mail: skbabbar@nic.in e-mail: bibhas_kumar2003@yahoo.co.in
Central Soil and Materials Research Station, Hauz Khas New Delhi1Central water Commission, R.K. Puram, New Delhi
ABSTRACT
Tala Hydroelectric Project is a run of the river scheme in South West Bhutan in Eastern Himalayas, located
3 km downstream of the existing 336 MW Chukha Hydroelectric Project on river Wangchu. The project
consists of 92 m high concrete gravity dam. The construction of HRT near the upstream face of Kalikhola
was very problematic due to very poor rock conditions. The area was full of shear seams and pockets of
water bodies. From RD 1008 m onwards to RD 1088 m there were large over breaks and squeezing rock
conditions. Forepoling was used to construct the tunnel. The designs of forpoles required initial and secant
modulus values of the affect area. In the present study, in order to evaluate strength deformation
characteristics of the material undisturbed samples have been collected from the affected locations and were
subjected to Gradation analysis, Atterberg limits, Insitu density, Specific gravity tests, Triaxial shear under
unconsolidated undrained condition and Unconfined compressive strength tests. Initial Modulus values were
determined as per Kondner (1963) and Duncan & Chang (1970) hyperbolic model and secant modulus were
determined from stress strain curve of unconsolidated undrained triaxial tests. It is found that initial and
secant modulus values increases with increase of confining pressure. Secant modulus values decreases with
increase in deviator stress.
Indian Geotechnical Conference – 2010, GEOtrendz
December 16–18, 2010
IGS Mumbai Chapter & IIT Bombay
1. INTRODUCTION
Tala Hydroelectric Project is a run of the river scheme in
South West Bhutan in Eastern Himalayas, located 3 km
downstream of the existing 336 MW Chukha hydroelectric
project on river Wangchu. The project consists of 92 m
high concrete gravity dam, 3 desilting chambers each of
250 m x 13.9 m x 18.5 m size for removal of suspended
sediments of 0.2 mm and above size coming with the river
water diverted through the intake structure, a modified
horse shoe tunnel of 6.8 m diameter and 23 km in length
to carry the water to underground powerhouse (206 m x 20
m x 44.5 m) for utilizing a gross fall of 861.5 m. A simple
horse shoe type tail race tunnel of 3.1 km length and 7.75
m diameter discharges the water back into river Wangchu.
The installed capacity of powerhouse is 1042 MW (6 x
170).
The construction of HRT near the upstream face of
Kalikhola was problematic area due to very poor rock
conditions. The area was full of shear seams and pockets
of water bodies. During excavation of HRT from Kalikhola
U/s face, the rock mass conditions started deteriorating from
RD 662 m onwards with unstable crown and face, large
over breaks, flowing and squeezing rock mass conditions.
Tunnel finally collapsed at RD 709 m on 16th July 2002
infilling 70 m already excavated HRT with about 3500 m3
of fallen muck. It was, thereafter, decided to divert HRT
from RD 607 m at 450 from the original alignment and
110m inside the hill abandoning 102 m of already excavated
HRT. Similar adverse geological occurrence (AGO) was
encountered from RD 1008.25m in Kalikhola U/s face
during March 03. The HRT reach of 337 m was excavated
in extremely weak rock mass conditions. The soil in the
affected reaches was squeezing out during excavation.
Under similar conditions the tunnel collapsed at Rd 662.
Therefore, tunnel alignment was changed to 450 from
original alignment. The forepoling was used to construct
the tunnel. The designs of forpoles required initial and
secant modulus values of the affect area.
1016 Mahabir Dixit, S.K. Babbar and Bibhas Kumar
2. TESTING OF SHEAR ZONE MATERIAL
In order to study Strength deformation behaviour and to
find Initial and secant modulus values, of the affected area,
2 undisturbed samples in core cutters were collected from
the face of HRT Kalikhola u/s at RD 1088 m. The following
laboratory investigations were carried out on the soil
samples as per the Bureau of Indian standards IS 2720
(Part III), (Part IV), (Part V), (Part XI), (Part XXIX), and
(Part XL).
• Mechanical analysis
• Atterbergs limits
• Insitu density and moisture content
• Specific gravity
• Unconfined compression tests
• Triaxial shear tests under unconsolidated undrained
condition
Classification of the samples was done as per IS: 1498
Results of Mechanical analysis and Atterbergs limits are
presented in Fig.1A The results of Insitu density, Moisture
content and Unconfined compression tests are presented
in Tables 1-3. Initial modulus were determined as per
Kondner (1963) and Duncan & Chang (1970) hyperbolic
model and Secant modulus were obtained from stress strain
curve of unconsolidated undrained triaxial Shear tests.
These values are presented tin Figs. 1-8
3. DISCUSSIONS OF TEST RESULTS
It is seen from Fig.1a that the particle size larger than 4.75
mm are in the range of 23.8 to 29.7 %, however, few stray
boulders of 300 mm size were also found in the excavated
material. The material was classified as Silty Sand (SM)
based on Indian standard classifications.
Fig. 1: A Mechanical Analysis and Atterberg Limits
Table 1: Results of Insitu Density, Moisture Content
and Specific Gravity
Field No. Insitu Wet
Density
γwet
( gm / cc )
Insitu
Moisture
Content
( % )
Insitu
Dry
Density
gm/cc
Core cutter-I
1.65 7.64 1.53
Core cutter-II
1.67 8.07 1.55
Results of Insitu density, Moisture Content and
Unconfined compressive strength (UCC) of the undisturbed
samples are presented in table 1-2. Insitu dry density of
the samples was found of the order of 1.50 gm/cc at Insitu
moisture content closer to 8 %.
UCC of the samples was found to vary from 0.141 kg/
cm2 to 0.303 kg/cm2. As per International Society for Rock
Mechanics (ISRM) suggested method
(ISRM 1981), UC below 1.00 MPa i.e. about 10 kg/
cm2 should be considered as soil.
Table 2: Results of Unconfined Compressive Strength
Field No.
Unconfined Compressive
strength
Kg/cm2
Core cutter-I 0.303
Core cutter-II 0.141
At site also, the rock mass was classified as Adverse
Geological Occurrence (AGO) which is beyond rock mass
class VI as per Barton’s ‘Q’ classification system.
Variation of deviator stress and axial strain and
hyperbolic representation of stress strain curve as per
Kondner (1963) and Duncan & Chang (1970) hyperbolic
model is presented in Figures 1-8 for two undisturbed
samples at different confining pressures.
Initial modulii were determined as per Kondner (1963)
and Duncan & Chang (1970) hyperbolic model and secant
modulus were obtained from stress strain curve of
unconsolidated undrained triaxial tests. Values of initial
modulus at different confining pressures and secant
modulus at 1/3 and 2/3 of maximum deviator stress at
different confining pressure are presented in table 3. It is
apparent from the results that initial and secant modulus
values increases with increase in confining pressures.
Secant modulus values decreases with increase in deviator
stress.
Table 3: Test Results of Initial & Secant Modulus
Secant Modulus, E Field
No.
Confining
Pressure
kg/cm2
Initial
Modulii
kg/cm2
1/3 of max
Deviator
Stress
kg/cm2
2/3of
Max
Deviator
Stress
kg/cm2
2 66.7* – 73.3 Core
cutter-I 4 83.3 95.7 58.8
2 43.5* 91.7 41.5 Core cutter-II 6 111 121.4 73.5
* Does not fall in to a pattern. Values should be close to values
at 1/3 of deviator stress
Geotechnical Investigations in Adverse Geological Occurrences in HRT... 1017
Figs. (1-8): Variation of Deviator Stress and Axial Strain and Hyperbolic Representation of Stress Strain Curve as Per Kondner
(1963) and Duncan & Chang (1970) Hyperbolic Model
1018 Mahabir Dixit, S.K. Babbar and Bibhas Kumar
4. CONCLUSIONS
Particle size larger than 4.75 mm was fond in the range of
23.8 to 29.7 %, however, few stray boulders of 300 mm
size were also found in the excavated material.
The unconfined compressive strength of undisturbed
s a m p l e s v a r i e d f r o m 0 . 1 4 1 k g / c m2 to 0.303 kg/cm2. As per
International Society for Rock Mechanics (ISRM) suggested
method (ISRM 1981), the unconfined strength below 1.00
MPa i.e. about 10 kg/cm2 should be considered as soil.
The material was classified as SM per BIS
Classifications. At site also, the rock mass was classified
as Adverse Geological Occurrence (AGO) which is beyond
rock mass class VI as per Barton’s ‘Q’ classification system.
Initial and secant modulus values increases with
increase in confining pressures. Secant modulus values
decreases with increase in deviator stress.
ACKNOWLEDGMENTS
Authors are thankful to Director CSMRS for giving
encouragement for writing this paper and permitting the
use of CSMRS data. The authors are also thankful to staff
of the soil discipline due to their significant contribution
in testing of the samples.
REFERENCES
Duncan, J.M. and Chang, C.Y. (1970) “Non linear analysis
of stress and strain in soils”, ASCE, Journal of the Soil
Mech. and Foundation Engineering Div.Vol. 96 pp
1629-1653.
Kondner, R.L. (1963) “Hyperbolic stress-strain response
of cohesive soils”, Journal of the Soil Mech. and
Foundation Engineering, ASCE, Balkema, Rotterdam.