The Teesta hydroelectric project is one of the most ambitious projects ever undertaken by India's National Hydroelectric Power Corporation. Variation in rock mass condition is a peculiarity of Himalayan geology and offers yet another challenge for tunnelling engineers to plan and complete construction within the time schedule. M M Madan describes the geology, planning and current construction progress of the project
TEESTA stage V is one of six stages planned as part of a cascade development on the river Teesta in Sikkim, India. The project’s main feature, a 17.67km long headrace tunnel, offers the biggest challenge to engineers. The topography of the area is rugged, the alignment passing through a high mountainous region. Geological conditions are poor, inrushes and large quantities of seepage water are expected, especially in low cover zones occupied by perennial drainages.
This US$424M run-of-river scheme envisages a concrete gravity dam, 95m in height, the headrace tunnel, and a vertical pressure shaft utilising a gross head of 216m for generation of 510MW of power. Two diversion tunnels of 12.2m diameter, 473m and 610m long, are to be constructed on the right bank of the Teesta river to divert the water during dam construction.
Three 6.5m diameter vertical intake structures are located on the left bank of the river which lead to three 315m long, Dufour type, desilting chambers. The desilting chambers are provided with 4.5m diameter silt flushing tunnels to discharge the silt back into the Teesta river. The 9.5m diameter horseshoe shaped headrace tunnel will carry water from the intakes to the power house. The alignment includes four kinks, required for construction of four access adits. A 92m high and 30m diameter surge shaft is located at the end of the headrace tunnel.
At the surge shaft, the water conductor system trifurcates into three steel lined vertical pressure shafts of 4.7m diameter. Two adits, one at the top and another at the bottom are planned for the construction of pressure shafts/penstocks. An underground power house cavern will house three Francis turbines, each 170MW. The transformer cavern will also be located about 30m from the main power house cavern. Three D-shaped tailrace tunnels, 165m long and 6m diameter, are proposed to discharge the water back into the river.
Topography and regional geology
The Teesta river flows to the southwest in an area bound by east-west trending ridges ranging in height from 1300-2600m. The rock types present in the area are a low grade metamorphic sequence of phyllitic quartzite, quartzitic phyllite, quartzite, and phyllites belonging to the Daling group of the Lesser Himalayas. The Main Boundary Fault in the south separates the formation from the Siwalik rocks. This is continuous and well defined throughout the Himalayas. The Daling group is overlain by a sequence of high grade metamorphics of the Darjeeling Group represented by Kyanite, sillimanite, garnetiferous-biotite gneiss, biotite schist etc. Further north, the Main Central Thrust separates the higher Himalayas.
Around the diversion tunnels, the governing rock slope facing the river is 60? to 65?. Along the slope, separation of rock blocks is apparent, due to local wedge formations. In general, the rock mass is medium strong to strong (RMR values), with slight to moderate weathering along the joint surface. Foliation is the most prominent joint set and exhibits variable orientation due to folding within the rock mass.
The desilting chambers will be excavated under a cover varying between ± 75m (initially) to ± 170m. The entire rock mass is traversed by occasionally sheared quartz veins and lenses. The rock is mainly class III and partly class IV rock.
The headrace tunnel lies in an alternating, sequence of rock, also foliated and jointed, with folding in many places. This has resulted in the formation of anticline and syncline structures of varying magnitude. Shear zones filled with clay and rock fragments are foreseen, as well as moderate to heavy water seepage.
Adits and diversion tunnels
The project is scheduled for completion in seven years (including two years for mobilisation). For execution of the underground works, five D-shaped adits/access tunnels were planned with lengths varying from 200-400m. Since the equipment engaged in the excavation of the adits is also being used for the headrace, power house, transformer gallery and tail race, the availability of equipment is determined by construction of the larger works.
Construction of the twin diversion tunnels, 12.2m in diameter and 610m and 473m in length, commenced in September 2000. The tunnels have been excavated using a top heading and bench sequence. Excavation of the top heading was carried out from both inlet and outlet ends, creating four faces. The heading had a 85-90m2 area with a 7.7m height to accommodate a twin-boom hydraulic drilling jumbo.
In poor rock conditions, where extensive jointing was observed, the tunnel heading section was reduced to 64m2. The total excavated diameter of the diversion tunnel is around 13.5m with a face area of around 150m2. Total cycle time including preparation, drilling, charging, blasting, defuming, scaling, mucking, shotcreting, rockbolting, permanent support (where required) and other miscellaneous works was around 20hrs. With this cycle time, completion of 1083m of heading was achieved in September 2001. Bench operations began immediately.
During the excavation of diversion tunnel I, the principal rock types negotiated were phyllitic quartzite with bands of quartzites and phyllites. Near the outlet portal at 80-100m (from the outlet side) rock fall occurred in the conventional left side crown, due to the presence of an interfolial shear and crushed rock mass. Along the shear zone loosening of the rock mass with separation of rock blocks and water seepage was observed. In diversion tunnel II the principle rock types encountered were phyllitic quartzites with bands of quartzites, phyllites and schist. The major portion of this tunnel falls in good to fair rock class, however near the outlet portal, a fair to poor rock mass was negotiated.
The breakthrough of diversion tunnel II (473m) was achieved on 28 June 2001, three months ahead of schedule. Break through of tunnel I was achieved on the 28 August 2001 one month ahead of schedule. Concrete lining commenced in November 2001 and is now well under way, with two 10m long and one 4m long gantry shutters and a smaller gantry shutter for the curved portion of tunnel. The concreting is planned in invert and overt portions for a period of four to five months. Grouting and cleaning will be a concurrent activity. The intake structure and gate erection, with overlapping, will take approximately five months.
The entire works for the diversion tunnel are planned for completion in September 2002, with the river being diverted 24 months later. As soon as the diversion tunnel is completed, the dam will be constructed. The depth of overburden in the riverbed is in the order of 45-50m. Therefore, jet grouting in the areas of the upstream and downstream cofferdams has been planned, to reduce the permeability of the riverine material. Riverbed excavation will commence in the dam area at the beginning of the working season, down to the bedrock at 40-45m depths, for the placing of foundation concrete. Concreting of the main dam is planned for the 2003 working season.
Excavation of the three 6.5m diameter D-shaped intake tunnels, varying from 60m to 150m in length, started in November 2001. The tunnels are designed to carry 117m3 of water and will join three desilting chambers, 20m x 24.5m in size, with a total length of 354m including transitions. The desilting chambers will be excavated through adit-I, as well as the intake tunnels. An additional adit, connecting the mid points of the desilting chambers is also planned, which will join the chambers just above the hopper location. The crown will be excavated from the intake tunnels and adit-I as a pilot with widening carried out to the full width. The desilting chambers have three separate gate operation and inspection chambers, advanced from an access leading off adit-I. Excavation of the desilting chambers has been planned for a period of 18 months with concreting to follow. Excavation of the 4.5m diameter silt flushing tunnel will be carried out through its outlet as well as from underneath the hoppers of the desilting chambers.
Head race tunnel
The 9.5m diameter, 17.67km long headrace runnel is being excavated from five adits of 296m (adit I), 375m (adit II), 411m (adit III), 227m (adit IV) and 201m (adit V) in length. The excavation of the headrace tunnel was awarded in March 2001. The contracts were split and awarded to: M/S Jai Prakash Industries for the initial 1.6km of the headrace tunnel and adit 1 (TT-2) and the final 2.2km of the headrace tunnel and adit V (TT-4); the main 13.8km long headrace tunnel and adits II, III, IV was awarded to M/S Gammon India (TT-3). After initial mobilisation and infrastructure development, work commenced on all adit portals. Main excavation of adits I and III were started in June 2001, while the excavation of the other adits started in August 2001.
The geology encountered in the portal area of adit-I consisted of highly jointed phyllitic quartzites and thinly foliated and weathered bands of phyllites/schist. The rock mass falls into the poor category. After progressing about 5m, a sheared band of schist was encountered. Further water seepage from the face loosened the rock mass resulting in overbreak. The initial section was supported by steel ribs and back-filled with concrete. Further in, the rock improved and the tunnel was supported using Swellex rock bolts and shotcrete. At adit-V the bedrock is covered with slope wash material. The entire adit length is being supported with steel sets due to the poor rock strata.
A total of eight faces of the headrace tunnel from these 7m x 6.5m size D-shaped adit tunnels, are being advanced. Tunnelling is being carried out in fairly good to very poor rock strata in differing stretches along the tunnel alignment, the critical period of the headrace tunnel drive is between adit-II and adit-III for a controlling length of about 2.5km. Tunnelling in this poor to very poor rock will be carried out using a pilot heading or a multi drift method. Alternatively hydraulic hammers are also available for use. Extensive shotcreting, wire mesh and rock bolting will be involved.
The excavation of the headrace tunnel started in February 2002 and is also being executed by a top heading and bench method. The excavated diameter of the tunnel has been kept to around 10.6m with a face area of 95m2. The heading is 65-70m2 and approximately 7.25m height. Tamrock twin-boom drilling jumbos have been deployed on each face to drill 4m long holes. A cycle time consisting of preparation, drilling, charging, blasting, defuming, mucking, scaling, shotcreting, rock bolting and support arrangements is again around 20hrs. An effective progress of 6m/day/face is required to meet the schedule (except in poor conditions). The entire heading is planned for completion within in a period of 800 days. Benching (30-35m2 area, 3.5m in height) will be carried out in 10m lengths from both faces using air tracks and wagon drills, over a period of 320 days.
Concrete lining in the headrace tunnel has been planned for all eight faces concurrently with 12m long collapsible shutters used to achieve an average progress of 12m/24hrs. An average concrete quantity of 235m3/shutter will be required. With the above planned progress, lining will be completed in a period of 500 days. Invert concreting is planned as concurrent activity with grouting and cleaning. Total tunnel construction is planned for a period of 2040 days.
Open excavation work for the surge shaft commenced, after the construction of the approach, in August 2001. A 30m diameter, 92m deep, surge shaft will be excavated with of a raise borer. A 200mm diameter hole will be drilled, top-down, to the headrace tunnel crown. Meanwhile, the approach to the bottom of the surge shaft will be through adit-V. A 1.8m diameter hole will then be reamed (bottom-upwards) over a period of two months. After the completion of the pilot hole, widening will be carried out in stages of 3m depths. Concrete lining is planned to be taken up in 2.5m high shutters, the whole surge shaft is scheduled for completion in August 2005.
Excavation of three, 175m deep, pressure shafts of 4.7m diameter is to be carried out through adits joining the pressure shafts at the top and bottom. After excavating the 350m long bottom adit, the horizontal portions of all three pressure shafts are being excavated. The top 50m long adit has been excavated alongside the top horizontal portions of the pressure shafts. The vertical portions will be excavated with the help of a raise borer (as with the surge shaft). Concrete lining of the pressure shaft is planned after the erection of steel liners. The complete activity is planned for completion by August 2004.
Power house and transformer galleries
Excavation of the 100m x 22m x 47.5m underground power house commenced in November 2001 after the completion of the ventilation and main access tunnels. The 310m long ventilation tunnel, which serves the power house cavern and transformer gallery, also acts as a cable tunnel.
The main access tunnel which bifurcates at its portal, leads to the power house and transformer galleries, for lengths of 171m and 149m respectively. The principle rock types encountered around the portal and the initial section of the main access tunnel is traversed by three sets of joints, but the rock mass is rated as good.
Rock bolts, chain link-wire mesh and shotcrete have been used to support the exterior rock face around the main access tunnel portal. The rock mass in this area (classified as fair to poor) is closely jointed, moderately weathered quartzite with bands of phyllitic quartzite. Initially the area was covered with slope wash material, after the removal of overburden and weathered rock the portal was established with the help of five steel ribs backfilled with concrete. Rock slopes facing the river Teesta are being supported by 3m long rock bolts, chain-link and shotcrete.
The power house cavern is being excavated in three sections namely central, left and right. Thereafter slashing to full width and benching down to the service bay level by July 2002 is envisaged using twin-boom drilling jumbos. Extensive shotcreting, wire mesh, rock bolting and grouting are being carried to stabilise the roof arches of the cavern. Benching is planned to be finished for October 2002. Concreting of the power house is planned to commence in December 2002 and will continue up until August 2005. Excavation of the 85m x 14.5m x 10.7m transformer gallery has also started, using a similar method.
It is planned to excavate three 6m diameter, 165m long, D-shaped tunnels from the outlet end concurrently. The tailrace outlet is near the river, therefore massive river protection works have been undertaken. Open rock cut excavation has already started.
Construction activities are now in full swing with all of the construction adits completed and headrace tunnel drill and blast excavation under way. The final lining of the diversion tunnels is nearing completion and excavation of the power house cavern and transformer caverns has started.
Preparatory work for the dam construction has begun with the first set of jet grouting trials under way and the construction team is on course to finish work by February 2007, the scheduled date of commissioning.