Kamal Laksiri reports on methods used during construction of Sri Lanka's Kukule Ganga hydro power project

KUKULE GANGA hydro power project is a run-of-river type power plant designed to harness the tributary Kukule Ganga of Kalu river, one of the five main rivers in Sri Lanka. The project is located 90km south east of Colombo, lying in a 10km long stretch of the river in which river drops by about 190m while on its way from the upper reaches to lower most levels. It has a catchment area of 312km2 at the weir site. The project area records an average rainfall of 3750mm resulting in a mean annual discharge of 30.4m3/sec at the weir site.

The installed capacity of the power plant is 70MW with an estimated average annual energy generation of 317GWh. The project is funded by the Japan Bank for International Cooperation (JBIC) through a loan of US$177M. Main construction works of the project commenced in July 1999 and at present it is in the final stage progressing towards commissioning scheduled to middle of this year. With the completion, Kukule Ganga will become the second underground power plant in Sri Lanka.

With regards to layout, the head and tail ends of the project are connected to the existing highway by two newly built access roads each approximately 9km in length. A diversion weir built across the river at the head works end diverts the water into the headrace system, comprising the head race tunnel, vertical pressure shaft and penstock tunnels and feeds the two turbines installed in an underground cavern sitting deep in massive Gneissic rock. Water passing through the turbines enter the tail waterway comprising draft tube tunnels and tailrace tunnel followed by an open canal and rejoin the river at a point 9km downstream of the weir site. The vertical axis Francis type turbines running under a gross head of 185m are directly coupled to the generators producing 70MW of electric power. The voltage of the electricity produced in the generators is stepped up underground before transmitting to the national grid. For this purpose, two 46MVA transformers are installed in a cavern, adjacent to the main power cavern and the high voltage cables from the transformers are brought out to the out door switchyard through the 320m long cable and ventilation tunnel.

The outdoor switchyard is connected to the nearest grid substation by a 27km long 132kV double circuit line spanning over steel pylons. The improvements and extensions required at this grid substation are also implemented under this project.

Permanent access to the power and transformer caverns is through the 550m long inverted D shape main access tunnel running at a 6.25% slope. An air exhaust plant installed at the end of the cable and ventilation tunnel provides the permanent ventilation to the caverns.

The headworks comprises of a 110m long, 16m high concrete diversion weir incorporating the water intake, the sand trap and the flushing tunnels. The riverbed at the weir site consists of Gneissic rock and the weir ogee elevation (197el) is about 3m above the riverbed. Thus gates form a substantial part of the reservoir capacity.

The reservoir or the headpond as termed, created by the weir, has a storage capacity of 1.6Mm3 with a surface area of 88Ha.

The main weir structure has been designed in six individual blocks connected with vertical shear keys. While four of the blocks (Blocks 1 to 4) form the spillway, the other two (Blocks 5 & 6) are designed as solid blocks and are provided with openings to store spillway stoplogs. In the design stage the hydraulic behaviour of the weir was verified by modal studies carried out in a hydraulic laboratory in Colombo. The weir is equipped with four radial gates of size 9.3m high and 12m wide and designed for 10,000 year return period flood of 2365m3/sec. These radial gates known as GIBB automatic crest gates were originally developed for Victoria dam in Sri Lanka. These radial gates will open in a predetermined sequence and in steps to match flood discharge requirements. An advantage of these gates is that they will open the correct amount without the need for either an operator or any form of power supply to move or to initiate movement of the gate.

The gate towards the left end of the weir is equipped with a 2m high flap gate, which is for the purpose of fine regulation of water level. The weir is also equipped with a sluice gate 3m x 5m (height/width) for sediment flushing. All these gates are provided with stoplogs to be used in maintenance and repair works.

A special feature in this weir is the provision of a sand trap for the purpose of sediment removal. The sand trap consisting of two large tanks each 90m long and 13m wide is designed to trap sediment particles of sizes bigger than 0.3mm. The V shaped tank bottom is provided with a hydraulically operated flushing device (by Bieri) to flush the sediments down to the flushing tunnels which are running right underneath the tanks. The flushing arrangement is designed in such a way that when the sediment height at the tank bottom reaches a pre set value, the operating system automatically opens the flushing gates and flushes the sediments. It is also possible to flush one tank while the power plant is in operation through the second tank. At the entrance to the tanks, stilling racks are provided to slow down the water velocity and hence to enhance the sediment deposition.

The Forebay is located immediately downstream of the sandtrap and the headrace tunnel starts from the bottom of this.

Waterway system

The upstream part of the waterway system consists of a 5710m long headrace tunnel, a 120m deep pressure shaft, a 60m long steel lined high pressure tunnel (penstock) bifurcating in to two conduits leading to the power house.

The 120m deep vertical pressure shaft has extended upward above tunnel level to act as the upstream surge facility. The surge shaft runs to a height of 90m above tunnel level to safely accommodate the calculated maximum surge elevation of 233el and hence both surge and pressure shafts act as a 210m deep single vertical shaft. The diameter of the pressure shaft is 4.8m and it is increased to 7.5m in the surge shaft area and finally in the top most part at ground level the diameter is 10.5m. The whole shaft is lined with reinforced concrete.

The headrace tunnel has a slope of 1% and is designed to convey the full load discharge requirement of 47.5m3/sec

In the tailwater side, two draft tube tunnels 55m in length starting from the turbines unifies and joins the 1600m long tailrace tunnel followed by 115m long open canal. Originally, for the tailrace side a vertical shaft was designed as the surge facility for the 1.6km long tailrace tunnel. However, during detailed design stage, it was replaced by a tunnel running at an 11% slope and connecting to the power cavern main access tunnel. The advantage being it could be used as an access tunnel for tailrace tunnel and draft tube tunnel during excavation.

Taking the advantage of the excellent rock conditions prevalent in the area, both the headrace and tailrace tunnels have been designed as unlined. The tunnel cross section is horseshoe with an internal diameter of 6.4m. In areas where the rock conditions are found to be not favourable, a concrete lining is provided. Thus 750m length in headrace tunnel and 126m length in tailrace tunnel have been concrete lined. The reinforced concrete lining is 5.6m in diameter and the thickness of the lining is 400mm in both tunnels.

Both the headrace and tailrace tunnels invert is paved with concrete placed on a layer of compacted muck in the unlined areas.

At the end of the headrace tunnel a rock trap, 100m in length, is provided for the purpose of collecting loose rock fragments detached from unlined surfaces. The size of the rock trap has been designed according to Snowy Mountains Criterion of 1.25m3 per 100m2 of upstream unlined surface area.

The headrace tunnel is also provided with a permanent access door of 2.3m diameter, steel construction, in construction adit No. 2. This is to be used in future inspections and maintenance works.

Underground cavern complex

The main power cavern housing electromechanical equipment is located 220m deep in a massive Gneissic rock mass. This cavern is 52m long, 15.9m wide and has a maximum height of 31.3m above the drainage sump. Adjacent to the power cavern lies the transformer cavern housing two unit transformers. Originally, one larger cavern for all equipment including transformers was designed, and later considering the geological conditions as well as fire safety aspects two separate caverns, as at present, were selected. The main access tunnel and the cable and pipe galleries connect both caverns.

In the design of the cavern complex both two-dimensional finite element method (FEM) as well as three dimensional boundary element method (BEM) was performed. With the excellent rock conditions prevailing, and also as confirmed in the design, a 150mm thick shotcrete layer with a wire mesh and pattern bolting were found to be adequate as the permanent rock support. Later during construction the 150mm thick shotcrete layer was replaced by a 90mm thick fibre reinforced shotcrete layer.

Further, a number of instruments including multiplepoint and single point extensometers, load cells, piezometers and convergent measurement points were installed inside the caverns for monitoring the rock mass behaviour. The monitoring of these instruments is continuing and the observations confirm the rock mass behaviour as predicted in the design.

Construction aspects

For the production of concrete at site, two modern batching plants, one at the headworks end and the other at the power house end, were used. Each capable of producing concrete at an average rate of 25m3/hour, they were associated with chill water plants to produce ice required in temperature controlling. The aggregates required were produced in the crusher plants established at site. At the power house end, suitable rock obtained from underground excavation was used for aggregate production while at the weir site a quarry was operated. In fine aggregate requirements, mainly crushed sand obtained by crushing rock was used wherever possible and the use of river sand kept to a minimum.

Each batching plant was supported with a fleet of 5m3 capacity truck mixes for concrete delivery within the site. Day to day testing of materials was carried out at two site laboratories equipped with standard material testing facilities.

The river diversion during construction was designed for the 10 year flood of 650m3/sec and implemented in two stages. In the first stage river flow was confined towards the left half of the river and construction of weir blocks 2 to 6 and sand traps were completed. In the second stage, the rest of the weir and sand trap structures was constructed, while the river was diverted through the completed parts of the weir i.e. blocks 2 to 4. The cofferdams used were of composite type consisting of steel pipe pile walls and earth fills supported with Gabions. Pre-cast pre-tensioned concrete beams were used in the deck of the bridge with 4.5m wide roadway spanning over the weir. Use of cement grouting was required in the weir foundation as consolidation grouting and in the 10m deep grout curtain running parallel to weir axis and penetrating into the abutments. Application of shotcrete was used in slope stabilisation both as a temporary and permanent support.

Underground works

Excavation of the power and transformer caverns and all the tunnels and shafts were done by drill and blast method. Drilling for blast holes were done using electrically powered rubber tyred drill Jumbos equipped with two and three booms. These Jumbos fixed with a working bucket were used in charging of holes and in the rock supporting exercises. Explosives used included Dynamite and Anfo with Nonel detonators. In an average round, 4.4m long blast holes are drilled and result in an advance of 4m full face.

Main power cavern excavation was started through the 320m long test adit, which had been driven to the cavern area for testing of rock mass during feasibility studies. Finally this test adit is incorporated as the cable and air exhaust tunnel after necessary modifications. Thus the main cavern excavation benched down through the test adit until the main access tunnel excavation reaches the cavern level. At this stage the cavern excavations continued through the main access tunnel and at the last stage, cavern bottom part excavation implemented through draft tube tunnels and the access tunnel to the high pressure tunnel.

In the excavation of the headrace tunnel two construction adits, adit No. 1 and 2 of length 425m and 490m respectively, were used. Later these adits are closed with concrete plugs. Further a valve for tunnel dewatering and a permanent access door have been incorporated in these plugs in adit 1 and 2 respectively.

In the original tender stage designs, the size of these adits were smaller to suit the size of the muck trains. However at the Contractor’s request, larger size adits were designed later so that rubber tyred excavation equipment could be used without any additional cost to the Employer.

All underground excavation works were conducted round the clock with two 12 hour shifts and with five and six days a week alternatively. The net advance in tunnel excavation in headrace and tailrace tunnels was 4m in a 12 hour shift.

The 210m deep surge and pressure shaft excavation was done in two steps by Alimak method. In one step, the pressure shaft from horizontal high pressure tunnel level to headrace tunnel level and in the second step surge shaft section from headrace tunnel level to ground level was excavated. In both steps initially a pilot hole of 2m x 2m was excavated upward. Then the reaming of the pilot hole to the final diameter was executed by slashing down from top to bottom.

Concrete lining of the entire pressure and surge shaft was done using a 1.2m high slip form shutter moving at an average concreting rate of 0.3m/hour. Similarly, in the concreting of the selected sections of headrace and tailrace tunnels, two 6m long telescopic shutters were used.

In the concrete lined areas in the headrace tunnel, consolidation grouting to a depth of 10m around the tunnel and contact grouting to seal the rock concrete interface were carried out. In the tailrace tunnel only the contact grouting was done.

Hydromechanical Equipment

The supply and erection of all Hydromechanical equipment in Kukule are under one contract lot (Lot No. 3) and includes the following gates and other devices.

Spillway radial gates

The weir spillway is equipped with four radial gates and three of them are 12m wide and 9.3m high. The other gate is combined with a 2m high flap gate which is for the purpose of reservoir water level regulation during low inflow (less than 50m3/sec) periods. One set of stoplogs, grappling beam and a gantry crane are also provided, to be used in the spillway gate maintenance and repair works. The stoplog is in four interchangeable elements, which are normally stored in the two pits provided in the weir structure (in Blocks 5 and 6).

Trash rack and raking machines

Two complete sets of fixed trashracks of 5m height and 7.5m width are provided at the intake upstream of the sand traps. They are fixed to the intake mouth concrete surface, which is at 70° inclination. The rack bars at 50mm spacing are designed for 1m head difference. Two raking machines with long arms able to extend down to trashrack bottom for cleaning are provided along with a belt conveyor and a trailer on tow for disposing the collected debris. These raking machines could be operated automatically.

Sluice channel tainter gate

This gate and a radial gate of 5m width and 3m height is provided for the purpose of sediment flushing. The gate is operated by hydraulic servomotors and is also provided with a stoplog to be used in the maintenance and repair works. A flap gate of 2m height intended for removing floating materials such as timber logs is provided in the same bay (sluice channel) above the sluice channel gate

Sandtrap equipment

This includes the Bieri flushing devices located at the bottom of the sand trap tanks, sandtrap flushing gates and the stilling racks. The Bieri flushing devices are hydraulically operated and consist of four elements in each tank. The stilling racks, in four rows in each tank are installed at the entrance of the sand trap for the purpose of reducing the velocity of water entering, and hence to enhance the sediment deposition in the tanks. Two sand trap flushing gates of 1.2m width and 1.8m height are provided at the end of the flushing channels. These gates are operated hydraulically and controlled by the sand trap flushing device control unit.

In addition to the above equipment two types of stoplogs are provided at the entrance and the exit of the sand trap tanks. At the entrance two sets of steel sliding type gates of 3m width and 4.25m height with motor driven hoists are provided, while at the other end light weight stoplogs made of Aluminum and manually handled are used. Both stoplog types are to be used during maintenance works. The idea of the latter type is to prevent the back flow of water into one sand trap tank from the forebay while the power plant is in operation through the second tank.

Headrace intake gate

A main intake gate, vertical sliding type and hydraulically controlled, is provided in the tunnel intake shaft located at the downstream side of the forebay. This gate with fixed wheel in either side and of 3.5m width and 4.8m height is always in the open position and closed only in an emergency or in a tunnel dewatering. This gate could be remotely operated from the main control room in the power house end.

Tailrace tunnel outfall stoplog

One stoplog operated by a mechanical hoist is provided at the 1.6km long tailrace tunnel exit. This stoplog will be used in dewatering the tailrace tunnel for maintenance and inspection purposes. It has a width of 3.7m and a height of 4.7m.

Permanent access door to the main tunnel

A steel water tight door, 2.3m diameter embedded in the concrete plug at adit No. 2 is provided to be used as an access to the headrace tunnel in future inspections and maintenance works.

Tunnel steel liner

The entire high-pressure tunnel starting from the bottom of the vertical pressure shaft is steel lined. The steel lining starting from 27.75el, covers the lower elbow and runs up to two turbine main inlet valves after bifurcating through a 60° Y (Wye) type distribution body. The steel lining is designed for a 6 bar external pressure against buckling.

Electromechanical equipment

The electromechanical equipment selected for Kukule are standard equipment found in a hydro power plant, and include the following main items: two 35.2MW Francis type turbines; two 42MVA generators; two 46MVA transformers and all other auxiliary and ancillary equipment.

The two Francis type turbines designed for Kukule have a rated output of 35.2MW under a net head of 179m and rotating at 500 rpm. At full load generation both turbines consume water at 47.5m3/sec rate. The turbines were model tested in the manufacturers laboratory in Japan for design verification.

The main inlet valves of the turbines are of spherical type and have a diameter of 1.7m. Similarly in the downstream of the turbines, two flap gates of 3.7m width and 2m height are provided in the draft tubes. These gates are to be used in the turbine dewatering.

The 42MVA generators are connected to unit Transformers with generator breakers. Each generator is associated with an auxiliary transformer thus enabling independent starting of the units.

The 46MVA unit transformers are water cooled (OFWF ­oil forced water forced) with the water received from tailrace. An advantage of this transformer cooling arrangement is the double tube construction. This arrangement enables the early detection of a leakage of either oil or water developing due to a tube rupture or corrosion. It also helps to prevent oil contamination with water in such a tube failure.

The 13.8/132kV unit transformers installed in the underground cavern are connected to the outdoor switchyard by 132 kV XLPE cables running along the 320m long cable tunnel. The cable tunnel is divided into several sections by fire barriers. The idea is to minimise the damages by isolating different sections in case of a fire.

Inside the power house an overhead gantry crane with main hook capacity of 90t along with a 5t capacity auxiliary hook is provided for permanent use. Two standby generators of capacities 315kW and 125kW are installed at the power house end and weir site end respectively for emergency use.

The 27km long overhead line, which transmits the power generated at Kukule to the national grid, is a 132 kV double circuit construction. The conductor used is Lynx and the pylons are made of galvanised, standard steel sections. A speciality in this line is the inclusion of an OPGW (Optical fibre conductor) in one of its earth wires.

Environmental considerations

Kukule Ganga is the first hydro power development in Sri Lanka put into execution after the environmental regulations came in to strict practice. As per these regulations, an environmental impact assessment (EIA) was carried out during 1993 and possible impacts were identified. The following are the main issues addressed and the countermeasures adopted.


The number of families required to resettle due to project implementation is 17 and majority of them were in the headworks end. They were provided with financial assistance and alternative lands and also action is being taken to provide them with a new house where appropriate.

Waste water treatment

All waste water released from workshops, yards, tunnel faces etc. are made free of oil and solid particles by using settling ponds, before they are released to the nearby streams.

Other waste handling

All waste from construction sites, offices, yards etc. are collected separately and treated accordingly.

Ground water monitoring

To see the effects of the tunnel construction, ground water levels along the tunnel route were monitored regularly using the bore holes existing at site.

Water quality monitoring

Basic water quality parameters such as pH, BOD, COD, DO, conductivity, etc are monitored continuously at several locations along streams representing the inlets and outlets of main construction sites. Sampling and testing are done in a monthly basis and the results are analysed to discover what changes take place.

Dust and gas monitoring

Dust and different gas levels present in underground sites are monitored regularly. Air samples are collected from critical work places and tested in the laboratory to measure the dust levels. The presence of toxic gases is measured at site using portable measuring equipment. Water spraying is carried out regularly as a means of dust control at construction sites and along the site roads.

Water release to downstream

With the construction of the weir, the river stretch, which is about 9km between the weir and tailrace outfall, goes dry. Thus to keep this stretch of the river bed wet in maintaining the quality of water to sustain aquatic fauna and also to avoid Malaria vector breeding, a residual flow of 0.3m3/sec is released from the reservoir when there is no water discharge from the weir during dry periods. For this purpose a special residual flow valve has been installed in the weir.

Head pond reclamation works

Under headpond reclamation and training works, the lands lying around the reservoir full supply level, which will have shallow water depths, are developed. The idea of this exercise is to preserve as much of the limited land resources as possible and to prevent creation of shallow swamps which would become a potential source for Malaria vector breeding. This is done by excavating parts of such lands to a depth of 1.5m and by raising the level of the rest of those lands by 1.5m above water level using excavated material. Thus while the excavated part of the land contribute to the reservoir storage, the filled area which is safely above the water level, could be used for cultivation or other purposes.

Environmental monitoring

All construction sites including workshops, yards, etc. are inspected regularly to see how the inforced safety and environmental regulations are practiced at site. These inspections are conducted jointly with the participation of representatives from Contractors, Consultant and the Employer. These inspections help to identify any shortcomings and adverse conditions developing with regard to environment and enable the implementation of remedial measures at an early stage.

Difficulties encountered

With regard to the difficulties encountered in the project execution, the weir foundation geological problems could be considered as the most relevant. During the weir foundation excavations it was found that the right bank ground conditions are not as predicted in the investigations. The firm rock levels were about 10m to 15m below the expected levels. This resulted in the redesigning of the right bank excavations and foundation details. All unsuitable materials had to be removed and the weir structure was founded on firm ground. Further a 16m high concrete guide wall which was not in the original design, had to be designed and constructed to protect the right bank downstream against erosion during spilling.

In the execution of underground works, geological problems were encountered but were managed satisfactorily without causing serious impacts to the progress. One such was the poor ground conditions prevailed in the originally selected two adit portals. Both adit portals had to be shifted to suitable locations and resulted in small increases in adit lengths.

Also, during the excavation of the head race tunnel, a few stretches with poor ground conditions coupled with water inflow were discovered. However by grouting ahead of the face along with adequate rock supports such zones, it could be overcome though they slowed down the tunnel advancing.

Similarly in the excavation of the surge shaft, deep weathering was observed and hence the excavation from surface had to be extended by about 20m up to sound rock levels in which shaft excavations by Alimak method could be adopted. In the weathering areas shaft excavation had to be supported with shotcrete, grouted anchors and lattice arches.

Project implementation schedule

The feasibility study of the Kukule Ganga hydro power project was carried out during the period June 1991 to August 1992 with World Bank funding. The final designs of the project and preparation of tender documents for individual contract lots were carried out from September 1992 to March 1993.

After securing funds in 1994 for project implementation followed by appointment of project consultants in 1996, the main Civil Works were commenced in July 1999. Implementation of other contracts followed subsequently with two of them (contract Lots 4.1 and 4.2) facing delays in awarding due to lapses in contract finalisation.

At present the project is in its final stage and the reservoir impounding is scheduled to June this year and commissioning of the first generator will follow immediately. By the end of the latter part of this year both generators are expected to be in full commercial operation.

Successful completion of the Kukule Ganga Hydro power Project will enable to fulfil the envisaged power and energy generation and will contribute to meet the country’s rising energy demand.

Further the mitigatory measures adopted in the project execution to minimise the negative environmental impacts, will enlighten how environmentally friendly the future hydro power projects could be developed.