KOYNA HYDRO ELECRIC
PROJECT STAGE IV EXTENSION OF HRT
order to have optimum utilisation of available water for Koyna
Reservoir for eastward irrigation, Government of Maharashtra,
Irrigation Department has planned No. of lift Irrigation scheme
namely Takari, Tembhu, Mhaisal etc along the Krishna river,
downstream of Krishna-Koyna confluence. Due to these Irrigation
Projects, the irrigation water demand on eastward side increased
substantially. Koyna Reservoir being main source of water
for all these schemes, it was necessary to ensure availability
of water from the Koyna Reservoir for meeting these irrigation
Tables prepared during formulation of Project Report
for Stage- IV, assumed the minimum draw down level as
KRL 2067.5 ft (630.174m) as that was the observed MDDL
for previous 25 years. Accordingly, Government of Maharashtra
approved KHEP Stage IV project with lake tapping at
KRL 2027.55 ft(618.0m) MDDL being 2067.5 (630.174m)
ft. KRL. The water distribution of Koyna Reservoir till
then was this a) 67.5 TMC for westward diversion for
power generation, 30 TMC for eastward releases for irrigation,
industry and drinking purposes, inclusive of evaporation
losses, and around 25 TMC as carryover or dead.
After formulation of MKVDC in 1996, eastward demand
from Koyna reservoir increased to 50 TMC inclusive of
evaporation losses. Thus there was a need to increase
the allocation by 20 TMC. In 2003, 5 ft high flaps were
attached to the radial gates of the Koyna Dam thereby
increasing the reservoir capacity by 6.47 TMC. Remaining
14 TMC had to be released from water below KRL 630 m.
14 TMC water if drawn from the dead storage reduces
the water to KRL 618. That means now we had to plan
a new Lake Tap at KRL 606 m. To reach at place where
ground level is 606 m, the Head Race Tunnel had to be
extended by 4.5 Km. Accordingly, Extension of
HRT of KHEP Stage IV work was taken in hand
Project Components: In preliminary
planning of project, approach tunnels were introduced
for excavation for HRT, but this was not possible, because
at one end, there was reservoir and at the other end
there was either forest or high ground. After discussion
during 24th Panel of Experts meeting, it was decided
to do the excavation through 2 or 3 shafts. Junction
Service Gate Shaft cum construction shaft and one or
two construction shafts at suitable location.
A) Junction Service Gate Shaft: Junction
Service Gate Shaft of size 15m X 6.5m is at 130 m from
the junction point of old and new HRT. Junction point
is at ch.920 m of old HRT of Stage KHEP IV. Two service
gates of size 4m X 9m operated by hydraulic hoists at
the top are housed in this shaft.
B) Head Race Tunnel: HRT is of 9.5m
standard horse shoe type confirming to IS-4880 (Part-II)
1968. It has a negative gentle slope from Intake Structure
to Construction Shaft No.2 and positive gentle slope
from Construction Shaft No.1 up to junction. The total
length of HRT is about 4572 m. Entire length of HRT
is lined either with P.C.C or R.C.C. About 35% of length
of HRT passes below the reservoir.
C) Construction Shafts No 1 & 2:
Two construction shafts each of size 14m X 6.5m at ch
1929 m and ch 3460 m from the junction are constructed.
Size of shaft is kept sufficient in view to have movement
of excavators for mucking operation and simultaneous
movement of two mucking buckets and passenger trolleys
Intake Structure: Intake structure proposed
is very similar to that of KHEP Stage IV which houses-
• A Stop Log Gate / Emergency Gate in 2 shafts, at the
end of each limb of intake tunnel with hoisting structure
at the top.
• Silt pit about 4.5 m deep immediately following the
junction of the limb.
• R.C.C structures for housing removable trash racks
with hosting arrangement at top of the control shaft.
• A bell mouth entrance with two Service Gate of size
4 m X 9 m operated by hydraulic hoists, at the top of control
Itake Tunnel: Two circular R.C.C lined intake tunnels
each of 6.5 m die. are proposed. The lngth of each tunnel
is about 205m.
Lake Tap Arrangement : Double Lake Tap is similar
to KHEP Stage IV. It included transition from circular to
'D' shape at muckpit. proper inclined tunnel at 45 degree,
plug proper of about 6.0m depth. and over ground protection
work. The final rock plug blast willbe taken after completion
of all the work d/s of muckpit and lowering of Service Gate
and Trash RackPannel and Emergency Gates.
Additional Surge Gallery: In order to supplement
the existing, surge, an additional gallery of size 75m(L)X205m(W)X35m/h)
with an Airvent of size 14mX6.5m ad orifice of about 22.73Sqm.
circuler at the junctionof gallery and connecting tunnel is
proposed. The invert of this gallery is slopping Connecting
tunnel joins the HRT of KHEP Stage IV at about 65m.u/s of
surge shaft. The air vent opens at KRL 706m by the road side
of surge shaft.
When we plan
a Hydro project, we construct the water outlet in the body of
the dam or a separate Head Race Tunnel is excavated from the reservoir
side. So long as there is no impoundment of the water in the reservoir,
excavation of Head Race Tunnel can be done in conventional ways.
After the reservoir is impounded and we are planning for a new
powerhouse for which Head Race Tunnel is to be excavated, we cannot
excavate it from reservoir side, because there is water. In such
situation, we choose a suitable location on the bank of the lake,
above the high flood level and from there we start excavating
either a shaft or an approach tunnel. After reaching the required
level of Head Race Tunnel, we start excavating one limb of the
Head Race Tunnel towards the lake, and the other limb towards
powerhouse. The limb which is being excavated towards Lake is
not connected finally to the Lake. A suitable rock plug with calculated
thickness is left unexcavated at the mouth of this limb and the
water is kept at bay. At the other end the construction of the
powerhouse is carried out uninterrupted. Once the construction
of powerhouse is completed and the powerhouse is on the verge
of commissioning, the rock plug at the bottom of the lake and
at the mouth of the Head Race Tunnel is blasted and water is allowed
to flow towards the powerhouse. This process of finally blasting
the rock plug at the bottom of the lake with underwater procedures
is known as Lake tap. This procedure is extensively used in Norway
to connect Lakes at different levels for either power production
of for supply of water.
Since the HRT of Stage
IV was introduced after impoundment of Shivaji Sagar Lake, Koyna
project had to resort to this technique and accordingly first
Lake Tap to connect the HRT of Stage IV PH, was done on 13 March
1999. Lake tap mouth was planned at KRL 618 m.
SELECTION OF SITE FOR UNDER WATER LAKE TAPPING
Geology at Lake Tap Location. Compact, Joint
less Rock with low permeability is preferable.
Location of Gate Shaft. : Above normal Flood level
Length of Intake Tunnel :
Minimum length is preferable particularly in open Lake Tap (For
Koyna it is 220m.)
Air Pocket required to provide Cushion for blast shock:
Based on model studies air cushion kept was 13m.
Muck Pit: Muck pit is provided at the bottom of the
plug to collect the fragmented rock of the plug. As a thumb
rule, the volume of the Muck Pit should be 3 times the loose
volume of the rock plug.
Thickness: Thickness of the plug should be equal to
the diameter of the intake tunnel or if the intake tunnel is
rectangular, it shall be equal to the short side of the rectangle.
In 1995, water level in lake depleted so low that the ground where
the Lake Tap was planned, was exposed and engineers got opportunity
to inspect the nature of ground, overburden, and other features
of the land. Staining walls were built around the lake tap plugs
and surrounding slopes were stabilized and rock pitching was done.
Plugs were rock bolted to achieve strength and extra stability.
Excess overburden was also dredged. Thus the final lake tap activity
was smooth and without any surprises of natural or technical vagaries.
The plug had a thickness of 6 to 6.5 m. It was inclined at 450 to
vertical. Consequently the drilled holes for charge had a length
of 8 to 8.5 m.
Specific charge was 100% higher than the normal tunneling
requirements. Because the plug was inclined to 450, total
volume of plug was 290 m3. As the specific charge was as high
as 4.5 kg/ m3, explosives used per plug are as high as 1320
kg. At the time of final blast, pressure rise against the
emergency gates was observed to be 33% of the static head.
The first Lake tap created history in Koyna Project and the
event facilitated import of new technique in India.
of KHEP- St 4 and 4(B)
this technique was to be used again in Koyna Project for connecting
the extended HRT to Lake.
The extended HRT was not connected to the old HRT because
the old HRT was live and charged with water. The newly constructed
expansion chamber near the surge was also not connected to
the old HRT for the same reason. Before the second Lake Tap
could be executed, it was necessary that the Extended HRT
should first be connected the old HRT and similarly the Expansion
Chamber at the surge should also be connected to the old HRT.
The junction of expansion chamber at surge well, to the old
HRT was a crucial activity, because water velocities in the
tunnel there, reach 6 to 7 m per second during starting and
closing of the Power House. Therefore this junction was designed
as steel lined junction and cutting these steel plates, bending
them to the necessary shapes, lowering them in the HRT and
transporting them to the location with a lead of 4 kilometers,
welding them in place, and then concreting the annular space
was initially thought to be a lengthy process. The detailed
planning of these works initially showed that a time of 200
days was necessary to complete these works. This meant that
the old HRT had to be evacuated for 200 days and powerhouse
of Stage IV would be off the grid for that much time. Considering
accurate position of the power supply in the grid, Energy
Department was reluctant to allow the work. So this work was
practically abandoned for a period of 4 years. In 2010 the
then Chief Engineer D. N. Modak accepted a challenge of completing
this Herculean task. As per the suggestions of POE member
Mr. Kadkade, the Steel lining was replaced with Steel Fiber
Reinforced Shotcrete (SFRS), which was expected to reach strength
of more than 55 NM2, which will be more than the requirement.
On this new concept the total construction program was updated
and it was found that the work can be completed within a span
of 120 days. The chief engineer then presented this program
to the Minister Of Water Resources And Power, and to the secretaries
of Energy Department, and finally energy Department consented
to carry out the work. Energy Department insisted that this
work shall be carried out in the rainy season instead of the
conventional dry season, because the energy requirement in
the rainy season is very less and energy Department can afford
to close the powerhouse of stage IV for 120 days in the rainy
an outage was declared from 16th of July 2010 to 31st
of October 2010. Engineers of koyna project worked relentlessly
not caring for the vagaries of the nature, risks in
underwater execution of the work, domestic problems
and the work was actually completed in 100 days. The
stage IV power house was again commenced on 1 November
2010, to supply power to the people for Diwali festival.
ASG and Old HRT Connection
plug excavation by controlled drilling and blasting.
inch diameter rock anchors.
m long – 378 Nos.
3.0 m long – 789 Nos.
3.4 m long - 390 Nos.
inch diameter pre-tensioned type rock bolts.
m long – 179 Nos.
3.5 m long – 495 Nos.
4.85 m long - 483 Nos.
Fibre Reinforced Shotcrete (SFRS)
spraying at ASG and old HRT Connection
and old HRT Connection
work of Lake Tapping
After completing the connection work of old HRT and
Extension of HRT the planning of Lake Tap work was
done and accordingly Lake Tap work was started from
August 2011. Some of the important activities of the
Lake Tapping are as under;
Prior to start of the work, arrangement of sufficient
dewatering system by deploying high capacity submersible
and centrifugal pumps was made to pump out the leaking
To meet out any emergency situation of power failure,
sufficient capacity diesel generators were also
Erection work of Service Gates and Emergency Gates
was completed along with the hoist structures.
work of Lake Tapping:
• After completing the connection work of Joining old
HRT and Expansion Chamber to the old HRT, preparation for
Lake Tap work was started from August 2011. Some of the important
activities of the Lake Tapping are as under;
• Prior to start of the work, arrangement of sufficient
dewatering system by deploying high capacity submersible and
centrifugal pumps was made to pump out the leaking water.
• To meet out any emergency situation of power failure,
sufficient capacity diesel generators were also deployed.
• Erection work of Service Gates and Emergency Gates
was completed along with
thickness of 5 and 5.5 m. for Left side rock plug and right
side rock plug were finalized by probe drilling piercing up
to the lake.
• Rock plugs were trimmed off to the final thickness
of 5 to 5.5 m. by controlled blasting.
• Final rock plugs were strengthened by rock bolting
in the peripheral ring 1 m. wide. Rock plugs were cement grouted
and finally shotcreted
• Final probe drilling of 5 holes was carried out piercing
up to the lake and based upon the cores observation, drilling
and blasting pattern was decided.
• There are total 119 holes drilled in each plug out
of which 110 holes are 51 mm diameter charge holes and 9 holes
are dummy (No charge) holes having diameter 89 mm and 102
mm for Left side rock plug and Right side rock plug respectively.
NSP-711 water resistant, Plastic explosives from Orica, Denmark
and NONEL detonators from Czech Republic are used. Pipe charges
are prepared by putting explosives in PVC pipes and put in
each hole. NONEL detonator’s connections were made and
finally connected to firing cable which was extended up to
the ground level at a safe place.
After final connection, both intake tunnels are evacuated,
emergency gates were then put down
Both the intake tunnels then filled with water up to the designed
level to create compressed air pocket below the plug. Once
desired air pressure is reached the final blast is done by
pressing buttons from the safe place at the top.
Pattern of Rock Plug
the blast, the fragmented rock rolled down and
collected in the muck pit. Once the water settled,
flow of water in the HRT was started by opening
the Emergency Gates.
Drilling in Rock Plug
Loaded and Ready for Blast
Gate with Hoisting Arrangement