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W A T E R A N D E N E R G Y I N T E R N A T I O N A L

84 Vol. 64, No. 1, Jan. - Mar., 2007 Special Issue on Tehri Dam Project

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ROCK MASS CHARACTERISTICS

OF UNDERGROUND CAVERNS

H.C. KHANDURI HARISH BAHUGUNA P.C. NAWANI

Sr. Geologist Geologist Director

Geological Survey of India, Dehradun

Abstract

Construction of underground powerhouse at Tehri was inevitable

because of non-availability of the space on the surface and the huge

excavation (required to accommodate the powerhouse) of the steep

back slopes. Excavation of the two huge cavities of the powerhouse

complex i.e., machine hall and transformer hall was tackled carefully.

Geotechnical assessment made earlier revealed that both these

cavities are located in the most competent rock mass at Tehri dam

site i.e. phyllitic quartzite massive (PQM) and phyllitic quartzite thinly

bedded (PQT) Grade–I. Considering the geotechnical parameters

including the in-situ stress measurements, the alignment of these

cavities was preferred in N0209 direction. As the alignment was

also against the dip direction, the excavation in these cavities was

free from any major failure. These gigantic caverns were stabilized

by means of rock bolting and shotcreting barring a patch in the

crown of machine hall where a 5m band of deformed rock mass,

associated with a major longitudinal shear, was encountered. The rockcover between the two cavities was insufficient (i.e. less than 2D),

because of which problem of convergence was recorded during the

excavation of the bus ducts in the common wall. Multiple bore hole

extensometers (MPBAX) and load cells were installed to monitor the

rate and extent of convergence, and a number of deep cable anchors

(blind and through) were installed to stabilize the area.

1. Introduction

Tehri project is to be developed

in two stages of 1000 MW each.

The stage I, known as HPP, has

been completed and it consists

of an underground Powerhouse

of 1000 MW (4 x 250 MW). The

three main cavities in stage I viz.,

machine hall, transformer hall, and

expansion chambers of the complex

are located in the available most

competent rocks (PQM). These

cavities run parallel to each other

and are aligned normal to the

strike of rocks. Other cavities in

the complex are draft tubes, bus

ducts, ventilation tunnels, cable

tunnel, drainage galleries, adits

and tail race tunnels etc., beside

water conductor system comprising

head race tunnels, pressure

shafts, butterfly valve chamber,

penstock assembly chamber and

penstocks.

The intakes for drawing the waterfrom the reservoir into the headrace

tunnels (HRTs) have invert at EL

720m. Out of the four HRT’s, the

two of 8.5m dia are meant for

serving the machines of HPP

while the other two will carry the

water into four machines of PSP

(Stage-II). The transformer hall

cavity is common for both stages

HPP and PSP. Two TRTs of 9.0m

dia each will take the water back

to the river.

The surface powerhouse was

not viable because of the huge

excavation that was required to

create space on the existing steep

(450-550) rocky slopes in order

to accommodate the proposed

powerhouse complex (Stage I &

II). Such excavation would have

endangered the stability of the

steep slopes. The choice of an

underground powerhouse thus

became imperative.

2. Geology at the Site

2.1 At the Tehri dam site the

folded metasedimentary rocks

of Chandpur phyllites (Pt 3

Proterozoic-III) having variable

proportions of argillaceous and

arenaceous constituents (Nawani,

1994) are exposed. Considering

the rhythmicity of intercalated band

of arenaceous and argillaceous

materials and varied degree of

tectonisation effects in them, the

phyllites at the dam site have

been classified into mainly four

lithological variants.

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Phyllitic quartzite massive

(PQM)

Phyllitic quartzite thinly bedded

(PQT)

Quartzitic phyllite (QP)

Sheared/schistose phyllite

(SP)

2.2 The primary bedding and

foliation planes are dipping 45-650

towards N195-2400 and 30-420

towards N160-1800 respectively.

Local variations in the dip direction

associated with general warping

have also been noticed. Different

discontinuities identified at the site

were grouped in different orders,

depending on their continuity.

A total of eight joint sets were

delineated at the site. The shear

zones were graded according to the

thick ness of clay gouge and their

relation with the bedding/foliation

joints. The shear zones with their

attitude parallel/sub parallel to

bedding/foliation were identified

as longitudinal (L shears) and

those having attitude diagonal to

bedding/foliation were termed as

diagonal (D shears) shears.

2.3 At the site eleven major L

shears and six major D shears

were delineated. A block tectonic

model was evolved for the Tehri

dam with an aim to understand the

bearing of the major L and D shears

on the state of geomechanical

behavior of the rock mass at the

dam site. The geometry, orientation,

frequency and interplay of major

(IV order) L and D shears

have considerably affected the

geomechanical characteristics of

the rock mass and it provided a

scope of dividing different sites

into different tectonic blocks.

Four different blocks were thenidentified and the boundaries of

these tectonic blocks are defined

by major diagonal shears. The

underground power house is

mainly confined to block number

3.This paper aims at discussing

the rock mass characters of the

Fig. 1 : Perspective View of Tehri Hydro Power Plant, Tehri Dam Project








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crown at El 630.2m and SPL

at El 624.0m, from RD 0.0 to

188.0m. The roof of the cavity

exposed interbedded sequence

of phyllitic quartzite massive

(PQM) and phyllitic quartzite thinlybedded (PQT) and occasional

bands of quartzitic phyllite (QP).

The sheared/shattered phyllite

(SP) occupies the affected zones/

tectonised zones along the major

shear planes (Fig. 1). The primary

bedding joints were dipping at 42°-

55°/N195-235 and were cut across

by the foliation planes dipping at

38°-44°/N160-210. The variation in

the attitudes of the beds is due

to folding pattern, which is very

conspicuous between RDs 51.0m

and 95.0m, on the left half of the

cavity (facing the heading).

The rocks are traversed by two

prominent joint sets, dipping

towards 52°-78°/N315-350 and

38°-80°/N020-060 and one random

joint set dipping at 78°/N140.

Intersections of northwesterly and

northeasterly dipping joints with S0

and S1 joints, which are sometimes

transformed to L-shears, have

caused prominent wedge failures.

Over breaks of maximum 1.5m or

so, due to wedge failures havebeen observed between RDs 69

and 75.0m and RDs 82.0 and

86.0m. The characteristics of the

prominent joint sets are described

in Table 1.

Numerous minor and major

longitudinal (L) shears have been

intercepted at different locations.

Minor shears (clay gouge 2-10cm

thick) dipping at 42°-600 /N150-190,

50°/N180, 51°/N160 and 50°/N200

have been encountered at the

crown level at RD 81.0m, 94.0m,

114.0m and 156.0m respectively.

Major shear zones (clay gouge

more than 10cm thick) dipping

at 38°/N180, 58°/N220 and 62°/

N230 were exposed at the crown

level at RD 29.0m, RD 45.0m

and RD 120.0m respectively. A

highly deformed zone existed at

the crown at RD 115.0m-120.0mconfined between a minor shear

dipping at 50°/N210 and the major

shear dipping of 62°/N230. The

geological map of machine hall

is given in Fig. 2.

3.2 Wall Sections

The mapping revealed different

bands of PQM, interbedded PQM &

PQT, PQT and deformed/tectonised

PQT and sheared phyllite. Theprimary bedding and foliation,

which are the most prominent joints

as well, dip at 48o-65o /N195-220

and 35o-46o /N165-175 respectively.

On the left wall, structural wedge

induced, over break has been

observed at number of places.

two main cavities i.e. machine hall

and transformer hall.

3. Machine Hall

CavityThe machine hall cavern (197m x

67m x 24m) is aligned in N0200

direction he rock cover available

above the roof of the machine hall

cavity is about 350 m (Photo-1).

The excavation of the machine

hall was started, along its axis

at the crown level, by driving

the approach adit 4 (with it is

crown El 628.75m). The crown

of the cavity is at El 630.2m and

the bottom most portion is at El

563m. The transformer hall and

expansion chambers are located in

the upstream are aligned parallel

to the machine hall.

3.1 The Roof Section

The arch portion was widened

to the required width with the

Photo 1 : Panoramic View of Machine Hall








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The rock mass parameters

estimated are Q=10-13, RMR=65-68, GSI=60-63.

3.3 Insitu Stress

Measurements

Insitu stress measurements were

made at a number of locations,

in the underground openings,

in the powerhouse complex

by flat jack tests, geophysicalmethod (by Russian Experts),

and by hydrodynamic fracturing

(by CSMRS).

The analysis of flat jack tests

data, demonstrates that average

values of stresses equal to

Maximum Horizontal Stress (H) (σ1)

= 8.5MPa (in northerly direction) Minimum Horizontal Stress (h) (σ

3) =

2.65 MPa (in southerly direction)

Vertical Stress (V) (σ2) = 5.2MPa

The natural stresses computed in

the powerhouse cavity (machine

hall) by geophysical methods were

found to be

Maximum Horizontal Stress (H) (σ1) =

10-11MPa (in northerly direction)

Minimum Horizontal Stress (h) (σ3)= 6 MPa (in southerly direction)

Vertical Stress (V) (σ2) = 9MPa

Based on hydraulic fracturing tests

conducted at a depth of 370m, i.e.

at the roof level (El 632m) of the

transformer hall cavity, the following

Table 1 : Characteristics of the prominent joint sets in the Machine

hall cavern.

Sl.No.

Orientation(Dip amount/direction)

Spacing(cm)

RoughnessMaximumcontinuity

(strike wise) (m)

1. 42°-58°/N165-235 (S0 joints) 20-50 Smooth 5-20

2. 38°-44°/N160-210 (S1 joints) 5-20 Smooth 5-20

3. 52°-78°/N315-350 20-200 Rough tomoderately smooth

0.5-4.0

4. 38°-80°/N020-060 1.5-100 Quartz veins 0.3-8.0

5. 78°/N140 (random) 7-10 Smooth 1.0-3.0

Fig. 2 : 3-D Geological Map of Machine Hall Cavity (Crown Portion), Tehri Dam Project (From R.D. 0.0 m to R.D. 188.0 m








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stresses have been worked out.

These results were found to be

more realistic.

Maximum Horizontal Stress (H) (σ1)

= 5.26Mpa (orientation N16.20

E,i.e. parallel to cavern

Minimum Horizontal Stress (h) (σ3)

= 3.14 MPa (normal to the major

principal stress)

Vertical Stress (V) (σ2) = 10Mpa

(corresponding to 370m rock

cover)

The following horizontal stress

ratios have been used in

analysis

Major horizontal stress ratio H/V

= 0.52

Minor horizontal stress ratio h/V

= 0.31

Table 2 : Details of the shears exposed in the Machine hall cavity

(right wall)

Orientation

Dipamount

Dipazimuth

LSZ1 58° N195 4 10 Clay crushed rock

LSZ2 55° N215 7-11 20-25 Clay and crushed rock

LSZ3 48° N170 7-15 20-25 Clay and crushed rock andcrushed rock

LSZ4 46°-50° N180-205 3-4 8-12 Clay and crushed rock andcrushed quartz vein

LSZ5 48°-57° N195-200 3-15 5-35 Clay, crushed rock andcrushed quartz vein

LSZ6 43°-50° N180-205 3-12 15-30 Clay and crushed rock andcrushed quartz vein

LSZ7 50°-55° N195-200 <2-9 20-25 Clay and crushed rock

LSZ8 54° N205 2-3 8-10 Clay and crushed rock

LSZ9 50° N190 <2 4-5 Clay, crushed rock

LSZ10 55° N205 5-7 15-20 Clay, crushed rockLSZ11 40° N215 3-5 10-hed

rock

LSZ13 52° N215 <2 3-4 Clay, crushed rock

LSZ14 57° N215 8-12 20-30 Clay, crushed rock andquartz vein

LSZ15 54° N215 2-3 5-7 Clay, crushed rock

LSZ16 52° N195 <2 5-10 Clay and crushed rock

LSZ17 58° N195 <2 2-3 Clay

LSZ18 48° N200 15-20 40-50 Clay, crushed rock andcrushed quartz vein

Table 3 : The strength characteristics of the rock mass and major discontinuities determined in the

longitudinal direction of Power House Cavern.

Sl.No.

Engineering geologicalelement of rock mass

Specificgravitygm/cc

Modulus ofdeformationE (kg.cm2)

Poisson’sratio

Strengthcharacteristics

Orientationof fractures

Parameters of sets of fractures

Ø C(kg.cm2

Strikeazimuthof dip

Spacing(m)

Phyllitic Quartzite Massive(PQM)

2.76 75000 0.22 45° 8 _ _ Phyllitic QuartziteMassive (PQM)

Phyllitic Quartzite thinlybedded (PQT)

2.77 75000 0.22 45° 8 _ _ Phyllitic Quartzite thinlybedded (PQT)

Sets of fractures Sets of fractures

1. Faults/shears of order-IV _ _ _ 22° 0.2 N190° 1. Faults/shears of order-IV ____

2. Tectonic fractures and

joints of order V and VI

_ _ _ 24° 0.4 N220° 2. Tectonic fractures and

joints of order V and VI

35

3. Tectonic fractures and joints of order V and VI

Tectonic fractures and joints of order V and VI

_

_

_

_

_

_

24°

24°

0.4

0.4

N190°

N310°

3. Tectonic fractures and joints of order V

and VITectonic fractures and

joints of order Vand VI

20

____

4. Joints of order VII _ _ _ 35° 1.0 N200° 4. Joints of order VII 2

5. Joints of order VII _ _ _ 35° 1.0 N020° 5. Joints of order VII 2








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this zone. Overbreak of the

order of ± 2m on both the

sides of centre line is clearly

seen.

• Simi lar tectonised rockmass marked by puckering,

silicification multiple shears

and frequent water dripping,

has been recorded on the

left wall (at El 624m) at RD

46-48m, 61-66m, 86-88m and

151-163m. These bands are

downward extension of their

crown counterparts.

• NE dipping joints are very

critical for the left wall whereasNW dipping joints control the

right wall configuration. Frequent

failures along them have

jeopardized the stability of 1m

wide berm for the crane beam

all along the cavern length,

particularly between RD 150

and 170m.

• PQM rocks on the zero wall

resulted in block toppling due

to wedge formation betweenS

0 /S

1 and NE dipping joints.

• Two zones of sheared/shattered

phyllite/deformed rock mass

characterised by Q= >2-2.2,

RMR= 30-35 and GSI= 25-30

were intersected on either wall,

in continuity from the crown

level downwards.

• As the strike of the deformed

rock mass and the bus-ductalignment are sub parallel,

serious stability problems were

recorded in the crown portion

of these openings (bus ducts)

for a considerable length during

excavations. Longer rock bolts

(+ 15m length) followed by steel

rib supports were installed in

these critical zones.

• Convergence measured by tape

extensometer is reported as

nil at El 609m. The measurestaken in the crown sections

between El 618m and El 622m

are reported to be only about

7mm between January and

March 1997. Six horizontal

drill holes were drilled from

the middle drainage gallery

into the sidewalls of the

machine hall and transformer

hall and multiple borehole

extensometers have been

installed in these holes tomonitor the convergence. In this

critical reach, steel rib support

was suggested.

3.5 Stabilisation Measures

• For the deformed zones

(crown RD 115-120m and 159-

164m) charged with seepage,

immediate reinforced shotcreting

preferably with steel fibers, and

rock bolting (resin), normal oroblique (not less than 150-200)

to the bedding planes, was

done at close spacing in a

staggered fashion to stitch the

weak zones properly, with a

provision of drainage holes.

• In general, resin type (fully

grouted with cement capsules)

rocks bolt of 10m and 15m

lengths, dia 32mm, @ 2.0m

horizontally and 1.5m vertically,with wire-mesh shotcreting

of 100mm thickness (in two

layers) and drainage holes

were recommended for the wall

sections.

3.4 Geotechnical Assessment

and the Critical Zones

The Roof

• A highly deformed/shattered/

puckered zone conf ined

between two minor bedding

shears has been encountered

at RD115-120m (crown

level) where frequent failures

of wedges formed due to

intersecting S0 /S

1and NE/NW

joints were obser ved. This

critical shear zone has been

traced downwards on the right

wall (RD102-108, El 616m) and

left wall (RD 96m – 101m, El616m) where it is characterized

by multiplicity of minor shear

seams confined between two

important L shears. Q 1-2,

RMR 17 and support load P

3.5 kg/cm2 (35 tonnes/m2) were

estimated for the reach.

• Intersection of the bedding

shears with the NE/ NW

dipping joints led to formation

of wedges, which caused overbreaks at several locations at

the crown, at RD 12-17m, RD

66-94m and RD156-165m.

• Profuse water dripping was

noticed from the crown at RD

82-85m, RD 113-119m, RD

152-155m, RD 166-169m and

RD 175-177m.

• Another critical reach of

deformed /shattered distressedPQT was recorded at RD 159

– 166m (crown level) and is

continuing down to RD 141-

151m (El 609m) on the left wall.

A fault plane dipping 800 /N160

(clay 3-5cm) with about 1m

displacement also falls within








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• Longer rock bolts (> 15m length)

followed by steel rib supports

were advised in the deformed

zones.

• In the end wall section, spotbolting was advised in addition

to general pattern of rock

bolting and the negative slopes

were made up by concrete

backfilling.

• Controlled blasting was done

to avoid over breaks.

• Scooping/removal of sheared

rock mass for 20cm depth

fo l lowed by backf i l l ing,systematic rock bolting and

wire-mesh shotcreting were

recommended as protective

measures for the wal l

sections.

3.6 Cable Anchoring on the

Right Wall of Machine Hall

Cavity and Bus Duct

The 40 m wide rock - column

between machine hall andtransformer hall cavities was

considered inadequate and in order

to provide additional strength and

stability to, it was emphasised to

install cable anchors. The cable

anchoring in these cavities had

been done on two patterns i.e.

blind hole cable anchoring and

through hole cable anchoring.

3.7 Power House

Instrumentation

In order to record convergence

taking place during or after the

excavation in the power house

complex, instruments like multiple

bore hole extensometer (MPBX)

and load cells were installed in

the machine hall (at ch 45m, 99m

and 155m), transformer hall and

in the bus ducts no. 3 and 4.

The extensometers were installed

from the drainage galleries of thepowerhouse and wall convergence

of different orders was recorded

in them.

4. Transformer Hall

Cavity

The transformer hall cavity aligned

parallel to the machine hall cavity

in N0200 direction measaures161m

x 34m x18.5m. Exacavation for thecavity began from the crown at El

634.0m by widening the section of

the exploratory drift driven earlier

along its alignment.

4.1 The Roof

The cavity driven across the

rock strike, exposes interbedded

sequence of phyllitic quartzite

massive (PQM) and phyllitic

quartzite thinly bedded (PQT).

PQM bands are exposed along

the central line of crown at RD

60.0m-62.0m, RD 78.0m-102.0m

and RD 116.0m-129.0m, and

in rest of the area, PQT and

undifferentiated PQT/PQM rocks

were present (Fig. 3). The PQM

band between RD 78.0m and RD

102.0m manifests characteristic

deformation features marked

by folding, crumpling, shearing

tension gashes, silicification and

water dripping. The PQT and itstectonised variants are mainly

intercepted on the crown at RD

42-46m, 65-72m and 106-117m,

which extend further downwards

on the both walls. (Fig. 2)

The bedding dips at 56°-64°/N205-

220 and foliation at 42°-45°/N170-

180. A wide variation in the attitude

of bedding due to complex folding

was recorded between RD 78.0m

and RD 101.0m. A steep and wavyfault trace (78°/N190) recorded at

RD 76.0m-78.0m is marked with

silicification and intense jointing.

4.2 Geotechnical Assessment

The PQM rocks encountered in this

opening belong to ‘fair’ category

with Q = 5.7 to 13 whereas

highly silicified PQT (Q=7-9) and

the puckered PQT rock mass

have been assessed as ‘poor’ to‘fair’ type with Q varying from 4

to 6.

• The northeasterly dipping

jo ints and bedding/fol iation

are forming structural wedges,

causing over breaks at the

crown at RD 6-8m, 24-25m

and 42-45m.

• Northwesterly dipping joints,

occasionally, caused sliding of

blocks, particularly on the right

SPL and as such, control the

right wall configuration.

4.7 Stabilisation Measures

• Rock bolts 6-8m long with

spacing 2m c/c followed by

shotcrete layer were provided,

to stabilize the roof.

• Steel ribs @ 75cm from RD

81 to 87m and additional rockbolts @ 1.0m staggered from

RD 87-96m were provided.

• Drainage holes, through the

shotcrete layer, in the water

seepage zones were left in order

to check pore pressure.








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5. Conclusions

• The geotechnical assessment

done in the earlier stages

helped in locating the machine

hall and transformer hall cavities

in the most competent strata

(Grade-I) available at Tehri

dam site. The orientation of

the cavities was also favorable

(against the dip direction) and

as such no major geotechnical

problem was encounteredduring the excavation.

• A zone of sheared/deformed

and highly puckered rock mass

was noticed along the crown

of the machine hall between

RD 115m - RD 120m whichwas stabilized by providing

reinforced shotcreting with steel

fibers, and resin type rock

bolts.

• For the wall sections, resin type

rocks bolt of 10m and 15m

lengths, dia 32mm, @ 2.0m

horizontally and 1.5m vertically

and wire-mesh shotcreting of

100mm thickness (in two layers)

along with provision of drainageholes were recommended as the

main stabilisation measures.

• In the crown of transformer

hall cavity steel ribs @ 75cm

from RD 81 to 87m and spot

bolts @ 1.0m staggered fromRD 87-96m were provided.

The rock cover between the two

cavities was 42m, which was less

than 2D (considering the width of

machine hall i.e. 24m) because of

this insufficient rock cover, problem

of wall convergence was noticed

during the excavation of bus

ducts. A maximum convergence

of 21.60mm was recorded by the

multiple borehole extensometers. Tostabilize the common wall between

the two cavities, 73 blind and 27

through cable anchors of 77 tonne

capacity were provided.

Fig. 3 : 3-D Geological Map of Transformer Hall (Crown Portion), Tehri Dam Project




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