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.