In 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 needs.


1. Working Tables prepared during formulation of Project Report for Stage- IV, indicated that the minimum draw down level would be substantially higher than the MDDL of Stage I & II i.e 2000 ft.(609.6 m) and accordingly. Government of Maharashtra approved KHEP Stage IV project with lake tapping at KRL 2027.55 ft(618.0m) MDDL being 2067.5 ft.(630.174m) KRL. In order to have optimum utilisation of available water for Koyna Rservoir for eastward irrigation, Government of Maharashtra, Irrigation Department has planned No. of lift Irrigation scheme viz Takari, Tembhu, Mhaisal etc. along the Krishna river downstream of Krishna-Koyna confluence. Due to this Irrigation Projects, the irrigation water demand on eastward side has increased substantially. Koyna Reservoir being main source of water for all these schemes, it is necessary to ffacilitate available of water from the Koyna Reservoir for meeting these irrigation needs. Accordingly, Extension of HRT of KHEP Stage IV works is tplannedo enable KHEP Stage IV work at lower MDDL of Stage IV Extension of HRT of KHEP Stage IV Scheme envisages construction of 4.5 KM. long HRT on the upstream of existing tunnel with a lake tap at lower elevation, thereby lowering the existing MDDL from KRL 630.174m to KRL 618.00m. This will permit utilisation of additional 15 TMC of water for irrigation use on the eastern side, so as to have the optimum use of existing storage. Project Components: In priliminary planning of project, there was an approach tunnel introduced. After discussion during 24th Panel of Experts meeting, it wa decided to provided Junction Service Gate Shaft cum construction shaft and one construction shaft in lieu of Approach Tunnel. A) Junction Service Gate Shaft: Junction Service Gate Shaft of size 15mX6.5m is at 130m 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 4mX9m. operated by hydraulic hoists at the top are housed in this shaft.

B) Head Race Tunnel: HRT is of 9.5m standerd hourse shoe type confirming to IS-4880(Part-II)1968. It has a nigative gentie slope from Intake Structure to Construction Shaft No.2 and positive gentie slope from Constuction Shaft No.1 up to junction. The total length of HRT is about 4572m. Entire length of HRT is lined either with P.C.C or R.C.C about 35% of length of HRT is passing below the reservoir.

C) Construction Shaft No 1 & 2: Two construction shafts each of size 14mX6.5m at ch1929m and ch.3460m from the junction are proposed. 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 trollely.

D) Itake 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.
  • A silt pit about 4.5 m deep immediately following the junction of the limb.
  • R.C.C structures for housing removable trash rack with hosting arrangement at top of the control shaft.
  • A bell mouth entrance with two Service Gate of size 4mX9m operated by hydraulic hoists. at the top of control shaft

E) 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.

F) 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.

G) 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.

On 25th April 2012 at 10.57 AM extended head race tunnel of Koyna Hydro Electric Project Stage IV connected to reservoir by Lake Taping technique by the Auspicious hands of Hon. Chief Minister, Maharashtra State Shri. Prithviraj Chaven and under Chairmanship Hon. Dy. Chief Minister, Maharashtra State Shri. Ajit Pawar in main presence of Minister, Water Resources except MKVDC Shri. Sunil Tatakare, Minister Water Resources (MKVDC) & Guardian Minister, Satara Shri. Ramraje Naike Nimbalkar and in presence of
Koyna Hydro Electric Project is constructed during the period from 1956 to 1962 mainly for Hydro Electric Power Generation. The gross storage capacity of Koyna Dam is 105.25 TMC. The total installed capacity of Koyna Project through Stage I to IV and Koyna Dam Foot Power House is 1960 MW. Koyna Stage I was commissioned in 1962, Koyna Stage II in 1967 and Koyna Stage III was commissioned in 1975. Then during 1985, in view to meet the increased peak electricity demand of the State, it was decided to convert Koyna Project into peaking station. Hence next Stage i.e. Koyna Stage IV of capacity 1000 MW parallel to Koyna Stage I to II was planned. Koyna Stage IV was commissioned in 1999.
Necessity of Extension of HRT:
At the time of planning of Koyna Stage IV, use of water of the Koyna reservoir and necessity to complete the Koyna Stage IV at the earliest were kept in mind. Accordingly maximum draw down level (MDDL) of Koyna Stage IV was decided as KRL 630.00 m. and Lake Tap intake level was kept as KRL 618.00 m. with 12 m. water cushion. The gross storage capacity of the Koyna Dam at the time of planning of Koyna Stage IV was 98.78 TMC with eastward irrigation water use of 23 TMC. After formation of Maharashtra Krishna Valley Development Corporation (MKVDC) in 1996, Tembhu, Takari and Maisal Lift Irrigation Schemes were planned and for these schemes additional demand of 20 TMC was raised for eastward release. To meet out this demand, 5 feet flaps to the radial gates of Koyna Dam were provided in 2003 thereby increasing the FRL by 5 feet. Thus storage capacity of the Koyna Dam was increased by 6.47 TMC and gross storage capacity of the dam becomes 105.25 TMC. The balance 14 TMC of water use was required to be met out from the storage below KRL 630.0 m. Hence it was decided to keep the MDDL of Koyna Stage IV to be KRL 618.00 m. This has necessitated to lower down the Lake Tap level at KRL 606.0 m. Therefore, the Head Race Tunnel (HRT) of Koyna Stage IV was planned to extend further so as to have Lake Tap at lower location of KRL 606.0 m. This work of Extension of HRT of Koyna Stage IV was started in 2001. This shall make available additional 15.128 TMC water for eastward utilization.

Lake Tapping:
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.

Important Aspects involved.
• 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.
• 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.

The 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.
Layout of KHEP- St 4 and 4(B)
Now 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 season.
The 1000 MW capacity Stage IV Powerhouse was again started for generation. Along with the old and new HRT connection, the work of connection of Additional Surge Gallery with the old HRT was also completed in the same time limit. The scope of this work of both the connection i.e. connection of new and old HRT and connection of Additional Surge Gallery with old HRT is as follows;
Finished ASG and Old HRT Connection
1. Rock plug excavation by controlled drilling and blasting. 3811 cum.
2. 1 inch diameter rock anchors. 1.9 m long – 378 Nos.
3.0 m long – 789 Nos.
3.4 m long - 390 Nos.
3. 1 inch diameter pre-tensioned type rock bolts. 3.1 m long – 179 Nos.
3.5 m long – 495 Nos.
4.85 m long - 483 Nos.
4. Steel Fibre Reinforced Shotcrete (SFRS) 1194 cum.
SFRS spraying at ASG and old HRT Connection
New and old HRT Connection

Final 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 unde;

  • Erection work of Service Gates and Emergency Gates was completed along with the hoist structures.
  • 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 leakage water. To meet out any emergency situation of power failure, sufficient capacity diesel generators were also deployed.
  • Final plug thickness of 5 and 5.5 m. for Left side rock plug and right side rock plug was decided by probe drilling piercing the lake.
  • Rock plugs were trimmed off to the final thickness of 5 to 5.5 m. by controlled blasting.
  • Final rock plugs were strengthen by spilling rock bolting in the peripheral ring 1 m. wide. Rock plugs were cement grouted and finally shotcreted.
Rock Plug
  • Final probe drilling was carried out piercing the lake and based upon the results, 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 burn holes having diameter 89 mm and 102 mm for Left side rock plug and Right side rock plug respectively.
Drilling Pattern of Rock Plug
  • Bonogel 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 upto the ground level at groundlevel at the safe place. After final connection, both intake tunnels are evacuated, emergency gates were then put down
Muckpit at Intake Tunnel
  • Both the intake tunnels then filled with water upto 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.
Final Drilling in Rock Plug
  • After the blast, the blasted rock plug material falls and collected in the muck pit. After stabilization of water, flow of water in the HRT will be started by opening the Emergency Gates.
Explosive Loaded and Ready for Blast
Emergency Gate with Hoisting Arrangement
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