infrastructure: nuclear plants...2003/10/22 · bharatiya nabhikiya vidyut nigam limited formed as...
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Infrastructure: Nuclear Plants
India has an ambitious target of power production
by Nuclear Power Reactors to meet future energy
needs of the country. Two Indian companies,
Nuclear Power Corporation of India Limited (NPCIL)
and Bharatiya Nabhikiya Vidyut Nigam Limited
(BHAVINI) are responsible organization to construct
thermal reactors and fast breeder reactors respectively
in the country. NPCIL is currently operating
seventeen nuclear reactors and constructing five
reactors. Many more reactors are at anvil. Bharatiya
Nabhikiya Vidyut Nigam Limited formed as a
company and registered under Companies Act, 1956
on 22nd October 2003 under the administrative
control of Department of Atomic Energy is presently
involved in construction and commissioning of 500
MWe Prototype Fast Breeder Reactor (PFBR) at
Kalpakkam. Kalpakkam is an important nuclear
establishment of Department of Atomic Energy of
India and this coastal site is situated 70Km south of
Chennai. The PFBR is the forerunner for the future
Fast Breeder Reactors to be constructed in various
parts of our country including two more reactors at
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Infrastructure: Nuclear Plants
Kalpakkam to meet the future energy needs of India.
BHAVINI is constructing Mega Project PFBR and
the reactor is now in advanced stage of construction.
Preface:
PFBR is situated on the south of existing twin
units of Madras Atomic Power Station (MAPS). The
centre lines of MAPS unit 2 and PFBR are only 500
meter apart. PFBR and MAPS locations on the beach
of Bay of Bengal, is shown in Figure-1.
The entire PFBR plant is divided into nuclear and
power islands. The reactor location with respect to
MAPS is governed by minimum recirculation of
water discharge from condenser to sea. The PFBR
plant located on the shore takes condenser cooling
water from sea. Sand transportation and littoral drift
are large at sandy beach profile and at sea bed near
PFBR. The intake structure in PFBR was therefore
required to be engineered to avoid the sand entering
the pump house and clogging the intake passage to
condenser cooling water. The intake structure was
Fig 2(a) Integrated layout of shore protection &PFBR Intake &Outfall
Fig-1: Location of PFBR and MAPS at the beach of Bay of Bengal
designed to draw sea water from off-shore location
above sea bed, where depth of water is approximately
10 metres. Central Water and Power Research
Station (CWPRS) has developed the scheme of
drawing condenser cooling water, the length of
intake submarine tunnel, position of intake shaft, the
depth at which the water should enter the intake
shaft, and has finalized the hydraulic parameters of
intake. CWPRS finalised these parameters based on
extensive study of several factors including the height
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of tide, Highest Water Level (HWL) and Lowest
Water Level (LWL), wind velocity and sea current in
different months of the year, effect of outfall water on
the temperature of intake water for Madras Atomic
Power Station and for the intake water temperature
of PFBR. The integrated layout of the Intake and out
fall structures of MAPS and PFBR is given in Figure-
2(a) and 2(b).
General Features of Intake Structures:
The PFBR Intake structure consists of:
Ø Outlet Shaft on the shore
Ø Intake Shaft off the shore
Ø Submarine Tunnel
Ø Approach Jetty for the Offshore intake shaft
surface.
The intake is designed to draw 29m³/sec sea water
for condenser cooling for the 500 MWe PFBR. This
has been computed from the _T of 7° C across
condenser of PFBR. The approach jetty is provided to
facilitate approach to intake shaft.
For coastal sites, the Ministry of Environment and
Forest has the following guidelines:
"Temperature Limit for Discharge of Condenser
Cooling Water from Thermal Power Plant:
New projects in coastal areas using sea water.
The thermal power plants using sea water should
adopt suitable system to reduce water temperature at
the final discharge point so that the resultant rise in
the temperature of receiving water does not exceed
7oC over and above the ambient temperature of the
receiving water bodies.
Existing thermal power plants.
Rise in temperature of condenser cooling water
from inlet to the outlet of condenser shall not be
more than 100C.
Guidelines for discharge point:
The discharge point shall preferably be located at
the bottom of the water body at mid-term for proper
dispersion of thermal discharge.
In case of discharge of cooling water into sea,
proper marine outfall shall be designed to achieve the
prescribed standards. The point of discharge may be
selected in consultation with concerned State
Authorities/NIO.
No cooling water discharge shall be permitted in
estuaries or near ecologically sensitive areas such as
mangroves, coral reefs/spawning and breeding
grounds of aquatic flora and fauna".
(Source: Ministry of Environment & Forest,
Notification, New Delhi dated 22nd December
1998)
Since MAPS is the old unit, the system is
maintained in such a way that the resultant water
temperature at the final discharge point for the
Infrastructure: Nuclear Plants
Fig 2(b): Intake / Outfall Arrangement for MAPS and PFBR
Fig 3: General layout of sea water intake system
Fig 3 above shows general layout of sea water
intake structure. The off shore intake shaft is of
4.25m dia, Tunnel is horse shoe shaped submarine
tunnel of 3.6m dia and on the shore out let shaft is of
6.0m diameter. The horse shoe shape tunnel size has
been arrived based on the adequacy of cross section
even after 40 years of barnacle growth on the tunnel
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combination of MAPS and BHAVINI outfall is
maintained at 10° C.
Outlet Shaft on the shore:
This is 6m diameter vertical shaft at the shore
which is 55m deep.
Intake Submarine Tunnel
Exploratory bore holes were drilled along the
centre line alignment of the proposed submarine
tunnel well before the tunnel construction was taken
Infrastructure: Nuclear Plants
Fig 5: Sectional elevation of intake structure
Fig 6a: Submarine tunnel
Fig 6b: Top view of the tunnel
Fig 6c: Inner view of the tunnel
Fig 4: outlet shaft
Intake Shaft off the shore:
This is 4.2m diameter which has a depth of 50m.
Submarine Tunnel
The submarine tunnel has a length of 556m and a
diameter of 3.6m
Approach Jetty for the Offshore intake shaft
Approach jetty has the length of 567m width is
3.52m and diameter 7.1m
Geotechnical Investigation along the length of
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up. Geotechnical Investigations were carried out on
core samples from these bore holes taken along the
central line of the proposed tunnel alignment by
drilling of the 13 numbers of boreholes of 76mm dia
(NX) at 50m intervals. Boreholes were drilled upto a
depth of 65m below the sea bed for fixing the tunnel
Invert level for safe tunneling. Further, tests were
also carried out for finalizing the design of tunnel
supports, lining thickness etc. These 13 numbers of
bore holes were drilled from fore bay location to
offshore intake location (600m length). Out of 13
core samples collected along the length of the tunnel,
five boreholes namely TBH-1 to TBH-5 were on
shore boreholes and eight boreholes namely TBH-6
to TBH-13 are off shore boreholes. The intake well is
located at the bore hole No.13. The bore holes drilled
at 50m intervals indicate that the hard rock levels
closely follow the sea bed profile (expect bore hole
No.7).
Apart from these, 13 bore holes for geotechnical
evaluation, two numbers of receiver boreholes were
also drilled which are on shore bore holes, for cross
hole tests.
All the fifteen boreholes including on shore and
offshore were plugged using grout material
consisting of cement and bentonite in 1:1 proportion.
Infrastructure: Nuclear Plants
Fig 8: Original layout sea water intake structure, approach jetty, seal pit
& outfall structure
Fig 7a: Construction of approach jetty
Fig 7b: overall view of Construction of approach jetty
Fig 7c: View of construction of approach jetty from shore side
The jetty runs parallel to the submarine tunnel
and is located 15m towards north of the tunnel. Thus
the jetty axis is 15 meters north of the 13 number of
borehole alignment.
The jetty is supported on 36 sets of piles located
at 15m intervals. The piles have been taken into the
hard rock upto 2m depth for socketing. The profile of
rock encountered along the jetty alignment confirms
generally the profile similar to the bore holes along
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the centre line of the tunnel and this is true even at
TBH-7 where the hard rock was found at much
deeper depth.
M/S Design Group Project Consultants (P)
Limited, Bangalore have provided the entire design
and construction detailing for the submarine tunnel
after analyzing the geotechnical investigation data.
They have also analysed the rocks, produced
geological mapping, decided on rock anchoring,
taken decision on geological issues encountered
during construction and have produced detailed
reports of the incidents.
Geological Characterisation of the Lithological
Units:
All boreholes reveal similar stratification. Four
distinct layers were noted in all the bore holes, these
are Sandy soil, Clay layer, Weathered rock and Hard
rock. Geological characteristics of the lithological
units encountered was analysed by experts and the
detailed description of the lithological units are given
below:
Upper Brown Granular Zone
This is upper most zone which comprises of fine
to medium to coarse grain brown sand, with angular
/ assorted grains of transparent and opaque quartz,
minor specks of mica flakes (biotite) and mafics
(hornblende, etc). There is a strand line close to the
shaft. This indicates that it is an area of regression.
The grain size variation is not uniform. Thickness of
this lithological unit ranges between 5.5 - 10.5 m.
Argillaceous Horizon
This lithological unit has a thickness of 2 to 9.7 m
(approx.) and its color is greyish / greenish. This clay
is highly sticky and plastic, with rare shell fragments.
This horizon can be taken as a marker horizon. It can
also be considered as an aquiclude and groundwater
below this unit is likely to occur under semiconfined
and confined conditions. This clay occurs like a plug
and its origin is not confirmed as there is no zone of
transition above and below in its spatial distribution.
This shows a break in sedimentation and deposition
environment. Because of its pale green color,
chemical composition study, plasticity, engineering
property etc., were planned to be conducted.
Weathered Rock
This zone is encountered immediately below the
clay horizon. This zone consists of broken core of
garnetiferous charnockite and chloritised charnockite
/ migmatite. Thickness of the weathered zone ranges
from 0m to 15.6 m (approx.).
Hard Basement Rock
This zone occurs immediately below the
weathered rock zone without any transition to fresh
rock. The hard basement rock has been encountered
in all the Bore holes between 10.5 m to 18 m depths
from ground level except for Borehole 7 (TBH-7),
where hard rock is encountered at a depth of 29 m
below ground level. Depth persistence and lateral
prevalence of the hard rock has been established as
seen from the correlation of the sub surface
lithological data.
Hard basement rock encountered in this strata
belongs to the Archean Charnockite group of rocks
and Migmatite complex comprising igneous intrusive
rocks and metamorphic rock. The charnockite group
of rocks is made up of quartz, pyroxene, feldspar, and
garnet. The charnockite group of rocks is also
migmatised to varying degrees resulting in
retrogression and conversion into migmatite complex
comprising different types. (Reference Geological
Survey of India (GSl) map,1998). The migmatite
complex comprises of different types of gneiss, such
as garnetiferous, biotite gneiss, hornblende gneiss,
augen gneiss and garnetiferous quartzo-felspathic
gneiss. The magmatite are generally grey coloured.
In addition to this the mineralogical composition
and its assemblage manifested in the form of micro
joints, slips, shears, slickensides, rock alteration,
fracture filling, confined only to zones of thin
partings, foliation and joints at different depths.
Excepting for these thin weak zones, the host rock /
country rock appears to be homogeneous, medium to
coarse grained, migmatitic at places; as such there is
no major zones showing any effect of intense
shearing. A deeply weathered zone encountered in
TBH-7 is an exception.
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Geotechnical Stratification
All the thirteen boreholes (TBH-1 to TBH-13)
revealed similar stratification but thickness of layers
vary depending on the location. Ground water was
encountered at 2.5 to 9 m below ground level at
different borehole locations during the investigation.
General stratification of the site and its characteristics
area as follows:
Stratum 1: Loose to medium yellowish Sand
This layer is present in all boreholes. This layer
extends upto 2.5 m depth in TBH-1 & 2 to
maximum 7.0 m in TBH-7. In some boreholes this
layer is again encountered at 5.5 m and 6.75 m after
dense to very dense sand layer. SPT values vary 11 to
30. This range of N values shows loose to medium
dense relative density of cohesionless soil. The soil is
classified as SM-SP, SP and grain size distribution
shows gravel 0 to 6%, sand 88 to 98% and silt +
clay 1 to 10%.
Stratum 2: Dense to very dense Sand
This layer is present in all bore holes except TBH-
7. Thickness of the layer varies from 1.25 to 4.8 m.
SPT values varies from 30 to 71. This range of N
values shows dense to very dense relative density of
cohesion less soil. This soil is classified as SP-SM, SM,
SC, SP.
Stratum 3: Yellowish brown Silty Clay of medium
to Stiff
This is present in some bore holes like TBH-2 and
TBH-3 at depth 7.2 and 8.5 m respectively below
ground level. Thickness of the layer is varying from
1.8 to 4.0 m. SPT values vary from 9 to 12. Soil is
classified as CH. Grain sizes are gravel 0 to 4%, sand
1 to 37%, silt 23 to 41%, clay 37 to 64%.
Stratum 4: Very stiff to hard Yellowish brown
Silty Clay
This layer is present except in TBH-8. Thickness
of the layer varies from 1.1m to 9.75m. N value
ranges from 18 to 57. Soil is classified as CH. Grain
sizes are gravel 0 to 1%, sand 2 to 50%, silt 17 to
45%, clay 33 to 66%.
Stratum 5: Highly Weathered rock
This is moderate to highly weathered rock.
Thickness of this stratum varies from one meter to
15.6 m. N-value exceeded 100 and in some cases
rebound of SPT hammer was observed.
Stratum 6 : Moderately Weathered rock
A small layer of moderately weathered rock is
present below highly weathered rock. It varies from
0 to 5.5 m.
Stratum 7 : Charnockite Bedrock
This stratum is medium to coarse grained hard
rock comprising of Charnockite and gneiss with
garnet crystals. Generally between 10 m to 18 m
depths, from ground level the hard basement rock
has been encountered in all the boreholes except
Borehole BH-7 where hard rock is encountered at a
depth of 29 m below ground level. The Rock Quality
Designation in this layer is generally in the range of
40 to 100. Weighted average of RQD in each
borehole varies from 71 to 85. Core recovery varies
81 to 90. The Rock Mass Rating of this bed rock is
63.8 and it is classified as Class II (good) rock as per
Bieniawski 1979 & IS: 12070- 1987 (Table 7).
Compression wave velocity for the rock strata varies
from 3659 m/sec to 4762 m/sec. Value of shear wave
velocity for this layer ranges from 2000 m/sec to
2300 m/ sec. Dry density and bulk density varies
from 2.66 to 2.99 gm/cubic cm and specific density
varies from 2.69 to 2.99.
Outlet Shaft on the Shore:
The excavation of the onshore shaft and the
submarine tunnel was commenced by the M/S
Gammon India Limited at PFBR site in February
2008. 3D geological log of the same was carried out
to confirm the parameters for design of lining and to
decide upon the reach where consolidation grouting
is required. The log indicated that generally the bed
rock met was Charnockite / Garnetiferous
Charnockite Gneiss with joints tight and incipient,
while the prominent joints were continuous for 5 to
10m in length in some places. These joints got
exposed as a result of blasting while excavation.
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Seepage was noticed at the contact of overburden
and excavated rock surface. However, necessary
precautions were taken which included channelizing
the seepage water and monitoring of seepage in the
shaft.
The outlet shaft construction was incidence free
and was completed without major difficulty with all
the temporary supports.
Submarine Tunnel:
The horizontal horse shoe shaped excavation in
rock for submarine tunnel of size varying from 3.6m
to 4.2m is 560 meter long with Chainage 0.00
starting from centerline of vertical outlet shaft.
Conventional blasting using controlled charge was
deployed for the tunnel boring.
not have been closely maintained to submarine
tunnel axis. This conclusion governed decision
making process for further commencement of tunnel
boring.
TBH-4 was encountered during blasting on
previous day of the incidence i.e on 12.01.2009.
Small quantity sand had fallen down into the tunnel
from the hole when the zone of TBH-4 was blasted.
On 13-1-2009 at 4am further blast of 3m length was
taken up. De-mucking operation of excavated rock
was completed around 11am. At 11.15am subsidence
of sandy soil occurred at grade level (GL) exactly
above the bore hole No.TBH-4. People working on
the grade level noted that sinking of ground over
TBH-4 location and formation of a funnel shaped
chimney at the grade level, and people working in
the tunnel informed that certain slush along with
sand is falling through TBH-4 hole. On further
inspection of the tunnel it was observed that 76mm
dia TBH-4 bore has got unplugged of grouted
material (cement and bentonite in 1:1 proportion).
The slurry of grouted material along with about
12meter cube of sand has fallen down from the hole
on to the invert level of tunnel. Slight water too was
found dripping through the hole. A rod of 25mm dia
and about 4m length could be easily penetrated into
the unplugged hole of TBH-4 from inside the tunnel
bore.
Immediate action taken by BHAVINI after the
incidence-1:
Ø As a safety precaution the tunnel rock excavation
work was stopped forthwith.
Ø The matter was also referred to the experts who
arrived at site for assessment within hours of the
incident.
Decision making process following the incidence:
Ø The tunnel site was inspected by various experts.
Several rounds of reviews and discussions were
held. Experts expressed apprehension that minor
water seepage from TBH-1 and unplugged of
grouted material from TBH-4 does not provide
enough confidence that such incidence (seepage
from grout or unplugging of the grout borehole)
will not happened
Infrastructure: Nuclear Plants
Fig 9: Photos of Submarine tunnel
Two incidences were encountered during the
submarine tunnel construction.
Incidence-1:
Observation:
On completion of CH11500 on 13th Jan 2009 it
was noted that the grout in TBH-4 collapsed and
crumbled into the excavated tunnel. It also was noted
that while carrying out the rock excavation for tunnel
only 3 boreholes TBH-1, TBH-2 and TBH-4 have
been encountered within the alignment of tunnel but
not in centerline of the tunnel; rather they were away
from central axis to varying extent; from 0.5 m to
1.5m. TBH-3 could not be traced inside the tunnel.
This lead site to reach a conclusion that the boring
tool might not have taken exact gravity line while
drilling and/or the position of the drilling rig might
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Hence:
Ø Either the tunnel should be diverted in further
span to avoid encountering grouted before bore
holes during the further tunnel construction.
Ø Or the grouting should be improved before
further blasting for tunnel bore.
Ø The bore hole size is only 76mm diameter and the
over burden over the roof of the tunnel is about
50m. In case of unplugging of borehole below the
sea water, the pressure of sea water that could
gush into the bore hole will be 5Kg.cm2, the
velocity of water will be about 25m/sec and the
discharge will be in the order of 7000LPM. If
dewatering pump capacity is augmented to suit
above as well as if the bore holes are re-grouted
from 3m before reaching such boreholes location
by injecting appropriate cement / chemical grouts
like polyurethane through horizontally or upward
inclined holes towards the roof of tunnel, it is
possible to maintain the same alignment of tunnel
in the further construction too.
However this proposal was dropped for the
following reasons:
Ø Out of balance nine bore holes yet to be
encountered during tunnel construction, one is on
the shore edge and eight are below sea water.
Ø Since no casing pipe has been left while drilling
the boreholes, it may be difficult to identify the
location of the drilled boreholes from the top
surface and take measures to grout the area
around the bore hole. Technologies / methods to
identify the borehole in advance where casing pipe
is not left are not well established.
Ø In this case, the location of borehole can be
identified exactly only after blasting and
excavating the underground tunnel.
Ø Even if the grouted bore holes are identified when
the tunnel excavation is approaching the bore hole
location using radar technology, grouting the
already plugged borehole may poses complexity.
Ø The pressure grouting from consolidation of areas
around the bore hole may not be effective as the
grouting is to be done in hard rock strata under
higher pressure than the tunnel consolidation
pressure (grouting prior to excavation of tunnel is
being done for strata to plug fissures, water
leakage etc). Therefore, tackling the situation if
the borehole in the sea location gets unplugged
during blasting for the tunnel excavation is
complex.
Ø The experience of tackling the flooding of sea
water in the tunnel is not readily available. It is
possible that more than one borehole may give in,
during the blasting / de-mucking in which case
dewatering of tunnel may become difficult.
Ø The project does not have cushion of time to face
a situation of flooding of tunnel which will involve
complex remedial measures.
Ø Keeping safety of workmen into as prime
consideration in decision making, it was decided
to take diversion for further course of tunnel.
Deviation of the alignment of the Tunnel:
Factors that were considered for deciding the
extent of deviation of tunnel are as follows:
Ø The deviation of TBH-1, TBH-2, TBH-4 opening
in tunnel from axis of the tunnel and non
detection of TBH-3 suggests that the deviation of
the axis of the tunnel should be large enough to
avoid meeting the TBH-5 to TBH-13 during
further tunnel construction.
Ø The deviation should be as small as possible to
reduce additional length of jetty required to
approach the new intake shaft.
Ø Further for the shifted location of the intake
structure, sea conditions such as littoral drift,
current etc. considered for the study for the
original design has to remain unaltered.
Ø Irrespective of the uncertainty of the bore hole
alignment (deviation from verticality) and
positional tolerance, the distance of the existing
bore hole from the blasted contour of the tunnel
should be minimum two meters (cover rock
between bore and blasted surface should be min
two meters).
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Ø The tunnel has to be deviated to south-east and
after certain distance made parallel to existing
alignment as jetty on north of tunnel prevents
deviation of alignment to north.
Ø The rock profile in deviated contour should be
predictable from already completed geo-technical
studies.
Ø The water pressure drop should have only
marginal increase even after addition of two bends
in the tunnel. The existing sea water pump
supplying cooling sea water to the condenser
should be checked for its capability to cope up
with the increase flow path resistance.
Ø The bio-fouling concern should not enhance due
to the deviation in the flow path of the tunnel.
It was decided to divert the tunnel towards south-
east from the location of borehole No.TBH4 which is
at a distance of 115m towards east from the fore bay
shaft which is located on land. The straight line
lengths of the tunnel upstream and downstream of
the bends were checked for compliance to Bureau of
Indian Standard, IS 2951. Based on the requirement
of straight length between the bends as per standard
IS 2951, it was decided to deviate the alignment of
the tunnel keeping the angle of deviation as 11º from
TBH-4 and incline length to be maintained to 110m.
The tunnel bore will be again diverted by 11º at the
end of 110m diversion to make it parallel to the
original alignment. The southward
shift in the tunnel alignment thus works out to
21m. With the deviation of tunnel from the location
of borehole No.4, involving horizontally shifting the
tunnel by 21m southwards at the end of an inclined
length of 110m, the increase in total tunnel length
will be around 2m in addition to introduction of two
bends. For deviated alignment of submarine tunnel -
plan (general arrangement please refer Annexure - 1).
CWPRS that estimated the head loss due to
shifting of the tunnel by 21m and two angular
deviations of 11º and at two end of an inclined length
of 110m as 0.023mwc (meter of water column). Thus
the head loss due to change in the layout of the
submarine tunnel is insignificant compared to the
total pressure drop computed for the original layout
which is 1.9mwc. Hence, the additional drop in
pressure because of two bends and increase in length
of the tunnel by two meters does not change the
pumping head requirement of the cooling water
pumps. The pressure drop calculations were based on
IS 2951 (Part-II). Since, the pressure drop due to the
deviation in tunnel alignment is insignificant,
increase in head loss does not result in lowering of
water level in the forebay sump. Therefore, the water
level in the forebay would not fall below the designed
minimum water level. Hence, the submergence
required for the pumps is not altered. This was also
confirmed by DCPL who had carried out initial
design of the tunnel.
M/s IGCAR assessed and confirmed that there is
no impact on biofouling due to the proposed change
in the tunnel alignment by deviation.
M/s CWPRS, Pune has confirmed that for the
shifted location of the intake structure, sea conditions
such as littoral drift, current etc. considered for the
study for the original design will remain unaltered.
With the above deviated alignment of the tunnel,
the new axis of the tunnel with perfect drilled TBH-5
would be about 10.5 meter. Even after considering
TBH-5 alignment shift by 5.5 meters towards south,
the northern boundary of the deviated tunnel will be
2.5 meter from TBH-5. Therefore any opening of
TBH-5 in the deviated tunnel path and consequent
grouting of TBH-5 was not envisaged.
The geologists confirmed that the hard rock
profile at PFBR site generally follows the natural
ground profile. The slope is only form west to east
and the rock profile follows the ground profile as
proved by the TBH bore holes. With the decision of
shifting the tunnel by 21 meter towards south
beyond TBH-5, no change is expected in the hard
rock strata or the profile compared to initial
prediction based on geotechnical investigation. It
may be noted that for the original geotechnical
investigation itself the bore holes were taken at
distance of 50m; each borehole representing the
strata over a radial distance of 25m. The new
alignment is adjudged to be safe and the deviated
alignment of tunnel will also have adequate hard
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rock cover. The available rock cover for the tunnel
from the crown is expected to be 4D on south of
TBH-7 as at this location, the hard rock level is
comparatively at a lower level than the other bore
holes. Whereas at the other borehole locations
indicate rock cover of more than 6D.
Internal pressure due to water at submarine
tunnel level is about 5kg/sq.cm. (50m water
column), where as external pressure due to weight of
rock and over burden soil is about 11.4kg/sq.cm. As
per IS standard 4880 Part-IV, maximum rock cover
required is H i.e. 5Kg/Sq.cm. With this 21m shift of
the off-shore Intake structure towards south from the
original location, the jetty length has also to be
increased by another 21m towards south. The
grouting of already exposed boreholes i.e. TBH-2 &
4 located on land was also undertaken and effected
successfully.
From 13.4 m to 25.05 m the rock is highly
weathered. Further from 25.05 m to 30.0 m the rock
is highly weathered to moderately weathered
Charnockite with poor core recovery and nil RQD
had been obtained. From 30 m to the end of the hole
(65 m) slightly weathered Charnockite with good
core recovery and fair to good RQD had been
recorded.
The occurrence of deep weathering in a single
lithologically similar hole is intruguing. In view of
the completely weathered to highly weathered rock
with very poor core recovery, shattered rock and zero
RQD in TBH-7 alone, it was inferred that the reason
for this may not be lithological but structural
infirmity. With only scanty subsurface data available,
the experts took recourse to the regional geology and
also the geotechnical investigation done for Madras
Atomic Power Station (MAPS) tunnel bore holes
which is existing 500meter north of PFBR submarine
tunnel and was constructed around forty year back.
The absence of dolerite in any of the PFBR boreholes
and the occurrence of dolerite in the MAPS tunnel
bore holes was had suggested to the possibility of an
east-west fault between the two tunnels before actual
tunneling work started. Since, the dolerite rock is
now encountered after the shear zone this possibility
is now ruled out.
Possibility-1
Regionally the foliation trend in the gneissic rock
is N25º to 50º ES25º to 50º W with a dip of 60 to
80 degree in easterly direction. N30º E - S30º W
joints (Foliation joints) are dominant. Hence,
probably the shear zone encountered in TBH-7 could
be a foliation shear.
Possibility-2
Dolerite with sheared contact is reported in the
off shore bore holes drilled at MAPS. The dip of the
dyke is estimated to be 70º close to Kalpakkam, at
Punjeri a N.W.-S.E. dyke is traceable for about 1 km.
In the area around Anaikattu about 15 km south
west of Kalpakkam several WNW-ESE dykes are
reported. In MAPS Reactor I pit a N60º W - S60º E
dyke was reported. It could be seen that the dykes in
the area trend WNW-ESE to NW-SE direction with
Infrastructure: Nuclear Plants
Depth (in m)
0.0 – 7.0
7.0 – 13.40
13.40 – 25.05
25.05 – 26.0
26.0 – 29.0
29.0 – 30.0
30.0 – 60.0
Lithological Details
Medium grained yellowish brown sand
Very stiff to hard brown sandy clay
Yellowish grey completely weathered rock
Highly weathered Charnockite. Poor recovery. RQD
Nil.
Highly weathered grey fractured Charnockite. Poor
Recovery. RQD Nil.
Moderately weathered Charnockite. Poor Recovery.
RQD 20%
Slightly weathered Charnockite. Recovery good,
RQD Fair to good.
Incidence-2 (Shear Zone Encountered between
Ch243 and Ch264)
(Rock condition at TBH-7)
From the analysis of borehole log details of 13
numbers of boreholes it was evident that low rock
will be encountered while tunneling at TBH7 and
site will have to take cautious approach during tunnel
excavation between TBH-6 and TBH-8. Rest other
bore log predicted trouble free construction while
advancing blasting for creation of submarine tunnel
bore. The following are the lithological, core recovery
percentages and RQD details of TBH-7 core samples
as prepared by M/S Geotechnics & Constructions
Pvt. Ltd.
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Infrastructure: Nuclear Plants
The Masterbuilder - April 2011 | www.masterbuilder.co.in110
a dip of 65º to 75º towards S30º W. The contacts of
many dykes are sheared; the shear zone trend is also
in the same trend. If the structural infirmity in TBH-
7 could also have the same trend and dipping
towards SW. Strike trend and apparent dip were
projected on to the new alignment. Thus it was
predicted that rock in the shear zone and adjacent
area will be closely jointed and could render the
crown of the tunnel weak where it intercepts.
A horizontal diamond drill hole was planned to be
drilled with double tube core barrel as tunnel
advanced. It was planned in advance that if drilling
data confirms the prognosis, tunneling in this
hazardous zone has to proceed cautiously. The zone
may be under a hydrostatic head. A similar zone in
Naptha Jhakri HEP in Himachal Pradesh (Himalaya
range) was tackled through DRESS Methodology i.e
Drainage, Reinforcement, Excavation and Support.
The method consists of drainage beyond the
heading by drilling holes with simultaneous insertion
of partly perforated steel pipes, improving the
heading by grouting and shotcreting. Before starting
the work supports (as dictated by design
considerations) was planned to be kept ready and
placed as soon as possible taking care to provide
laggings between the supports and crown. The above
details were brought to the notice of the field staff
and they were kept in readiness to face the situation.
To conform this and take precautionary measures,
a horizontal diamond drill hole was drilled with
double tube core barrel as tunnel advanced. Great
precaution and cautious approach was taken from
Ch.250 to Ch.290.
Observation during sub marine tunneling
operation:
As predicted earlier, during the excavation of
tunnel the shear zone was encountered at Ch.245
continued up to Ch.257.5. The material in the shear
zone consists of highly crushed leucocratic
Charnockite. Although most of it is granular and
non-cohesive, in places it is completely clayey. No
water seepage was notice in the shear zone portion.
From Ch: 257.5 onwards and up to the face of the
excavation at Ch.270 Dolerite was encountered. The
dolerite Dyke although hard and fresh was found to
be blocky and seamy. To the left of the crown damp
surface and dripping conditions prevailed.
The absence of dolerite in any of the bore holes (as
per data provided) and the occurrence of dolerite in
the MAPS tunnel bore holes was referred to and the
possibility of an east-west fault between the two
tunnels was predicted earlier, even before start of
tunnel excavation boring. Since, the dolerite rock is
now encountered after the shear zone this possibility
is now ruled out.
In the MAPS tunnel boreholes, the dolerite is
found to be at least 54 m wide. As per bore hole
details, dolerite was not encountered even in TBH-7,
the logs of TBH-8 also indicate only charnockite and
not dolerite. Hence, it is probable that the dolerite
now encountered is less than 50 m wide.
Remedial measures taken in PFBR tunnel in the
shear zone.
Ø The entire excavation was geologically mapped
Ø From Ch. 243m to Ch 264m (in the shear zone
and blocky and seamy dolerite portions), 75 mm
thk. shotcrete of M35 grade with wire mesh was
applied.
Ø Wherever dolerite was found blocky, it was
stitched by 10mm thick plate anchored 3m deep
into the rock using 25mm diameter rebar.
Ø ISMB 600 @ 600 c/c with steel lagging was
provided in this stretch of submarine tunnel. The
entire inner surface (top, sides and bottom
surfaces) of this dolerite region was supported
with the above referred structural members.
Ø After the 3D geological logging of the submarine
tunnel, consolidation grouting was carried out
between Ch.15m to 30m, Ch.75m to Ch.85m and
Ch 240m to Ch 270m.
Ø Before any blasting for the tunnel, probe holes,
6m deep were drilled from the blasted face to
determine the rock strata ahead of tunnel face.
This was done either by diamond drilling or jack
hammer drilling.
The work of tunnel excavation is under progress
and as of 15th September, 2009, 515 meter out of
Infrastructure: Nuclear Plants
The Masterbuilder - April 2011 | www.masterbuilder.co.in112
560 meter of tunnel was already excavated.
Concluding Remarks:
The PFBR intake structure is a design andconstruction marvel. True to the type of activity, the
construction has met several surprises which werequickly addressed with the help of experts withinIndia. The job has progressed well as per schedule
despite the above mentioned difficulties.
Acknowledgement:
This detailed technical paper is prepared after
drawing technical contents from various reportsprepared by experts and organizations engaged byBHAVINI for intake structure design, construction,
trouble shooting and remedial actions. This reporthas also major inputs from the agencies who havecarried out geo-technical investigation, construction
and inspection activities. The authors thankfully
acknowledge them.
The credit of this report goes to:
Ø Dr. S.K. Jain, Chairman and Managing Director,
M/s Nuclear Power Corporation of India Limited
& M/s Bharatiya Nabhikiya Vidyut Nigam
Limited
Ø Dr. Baldev Raj, Distinguished Scientist and
Director, Indira Gandhi Centre for Atomic
Research, Kalpakkam
Ø Shri S.C. Chetal, Director, REG, Indira Gandhi
Centre for Atomic Research, Kalpakkam
Ø M/S IGCAR who have conceptualised and
conceived entire scheme. Carried out bathymetric
studies, analysed the results produced by various
experts
Ø The entire civil team of M/S BHAVINI Ltd
Ø M/S CWPRS, Pune have designed and done
model studies of Intake Structures and finalised
blasting charge
Ø M/S Gammon India Limited who have finalized
construction and inspection schemes and done
field construction of Intake Structures
Ø M/S DGPCL, Bangalore who have provided the
entire design and construction detailing for the
submarine tunnel after analyzing the geotechnical
investigation data. They have also analysed the
rocks, produced geological mapping, decided on
rock anchoring, taken decision on geological
issues encountered during construction and have
produced detailed reports of the incidents
Ø M/S DBM Geotechnics and Constructions Pvt
Ltd., Bombay who carried out Bore Hole drilling
and Geotechnical Investigations
Ø M/S Anna University, Chennai who gave expert
analysis on geotechnical analysis.
Ø M/S NGRI who carried out cross hole tests
Ø M/S Indian Institute of Technology, Chennai
Ø M/S National Institute of Ocean Technology,
NIOT, IIT, Chennai who have done HWL and
LWL studies
Ø Dr. D.N. Seshagiri, an experienced Engineering
Geologist and Dr. S.R. Gandhi, a Senior Geologist
and Professor at IIT Chennai, who have
contributed significantly in preparation of this
paper. Few names of organisations and experts
have been brought out above. The contribution of
those whose names do not appear is also not less
and is thankfully acknowledged.
References:
Ø Geotechnical Investigation Report for Sea Water Intake
Structure at Kalpakkam in Tamilnadu State for FBR-Project,
BHAVINI-DBM Geotechnics and Construction Pvt.Ltd.
Ø Report from Design Group, Bangalore Titled Paper on
Geotechnical Problems faced during execution of Submarine
Tunnel and Remedial measures carried out.
Ø Physical Thermal Model Studies for Locating Intake/ Outfall of
500MWe Fast Breeder Reactor Project (PFBR)-CWPRS
Ø Mathematical Model Studies for Location of Intake/ Outfall of
500MWe Fast Breeder Reactor Project (PFBR)-CWPRS
Ø Flow Model Studies for Intake Structure of Fast Breeder
Reactor Project (PFBR)- CWPRS
Ø "Supplementary Mathematical Model Studies for Littoral Drift
and Thermal Recirculation for Sea Water Intake/ Outfall of
500MWe Fast Breeder Reactor Project (PFBR)-CWPRS"
Ø Physical Wave Model Studies for Sea Water Intake/ Outfall of
500MWe Fast Breeder Reactor Project (PFBR)-CWPRS
Ø Field Data Collection and Analysis for Condenser Cooling Sea
water System (CCWS) of 500MWe Fast Breeder Reactor
Project (PFBR)-CWPRS
Infrastructure: Nuclear Plants