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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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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
85 Vol. 64, No. 1, Jan. - Mar., 2007 Special Issue on Tehri Dam Project
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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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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
86 Vol. 64, No. 1, Jan. - Mar., 2007 Special Issue on 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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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
87 Vol. 64, No. 1, Jan. - Mar., 2007 Special Issue on Tehri Dam Project
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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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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
88 Vol. 64, No. 1, Jan. - Mar., 2007 Special Issue on Tehri Dam Project
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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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