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.


Working 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 in 2001

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

D) 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 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.

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.
Accordingly 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.
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 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 the hoist structures.
Final 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

Final plug 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.

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 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.
Drilling Pattern of Rock Plug

After 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.
Final Drilling in Rock Plug

Explosive Loaded and Ready for Blast
Emergency Gate with Hoisting Arrangement
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