A majority of the hydropower projects are in the Himalayan mountainous regions, where conveyance of water from dam to power house is done through HRT. In the process, normally, a sizeable head is created, which is crucial for maximizing power generation. The Himalayas being comparatively younger mountains, the rock formations are unstable and fraught with surprises. Because of the difficulties in forecasting geological formation along deep and long tunnels, particularly in a complex geological environment, tunneling difficulties such as chimney formations, squeezing ground conditions, heavy water inrush etc, abound.
Main structures of Sewa Hydroelectric Project
Sewa-II Hydroelectric Project is situated in a lush green valley of the Sewa River, a tributary of the mighty Ravi River. The Head Race Tunnel starts from Gati village and ends at Maska village of the Kathua District of J&K. Main structure of the project includes the 53m high concrete gravity dam near village Gatti to divert the water into powerhouse through a water carrier system consisting of:
- 10.02km long Head Race tunnel of 3.30m dia horseshoe including underground pressure tunnel (3nos) with 2.4m in dia, 380m long and vertical pressure shaft 622m with surface penstock length of 175m.
- A diversion tunnel 6m dia and 298mm long already stands constructed on the right bank to facilitate construction of Diversion Dam. Diversion tunnel was designed to take even the flood discharge.
- Surface powerhouse aims at generating 120MW power with three turbines of 40MW each and 100.5mx22mx49m in size.
- A typical orifice type spillway with 4bays of 7mx10 and 46m width.
- Intake tunnel 3m dia D-shaped, (2nos) 153m and 130m long with desilting chamber (2nos).
- Du-four type 95m long, width 8m and depth varying from 10.7m to 11.2m.
Layout of Head Race Tunnel and Adits
The most striking feature of this project is its head race tunnel, which is awarded to Gammon India Limited in Lot SW-1 works by the National Hydroelectric Power Corporation (NHPC), Faridabad. (Layout of the project is shown in figure-1).
Head Race Tunnel 10.02 km long 3.3 m dia horse shaped was planned in the right bank of the river Sewa. To facilitate the construction of HRT, four Adits were proposed. The 122 m long Adit-1 downstream of nallah, 226m long Adit-II on the left bank of nallah, 418m long Adit-3 on left bank of nallah and 255 m long Adit-4 on the road of surge shaft will meet the Head race tunnel, respectively at RD 347m. RD 2717m, RD 5678m, and RD 9960m.
Geology along Head Race Tunnel alignment
In order to facilitate the excavation of HRT, four Adits have been provided with 8 faces of varying lengths.
HRT crosses from high grade metamorphic rock to medium and low-grade metamorphic rock. The upstream part of HRT (of about ±5115 m chainage) is traversed by augen/granite gneisses. The contact of Dalhousie granite and Tanawal group is thrusted, but the contact is sharp (encountered at RD 493m U/S of Adit-III, Face-V). The remaining part of HRT lies in the highly tectonised sequence of Tanawal formation comprising Carbonaceous Phyllite, Slate, Phyllitic assemblage, Limestone and Quartzite.
Tunnel Excavation and Rock Support
The scope covers the boring, overt and invert lining and construction of intake structure for carrying the water of Sewa River to the Power House. This was a very fast track structure, nevertheless the job was completed within the time schedule by achieving the required progress, despite several difficulties.
The Head Race Tunnel with inner diameter of 3.3m and a revised length of 10.84km was constructed by using the drill and blast method. The local progressive loosening was limited by employing careful excavation methods and timely installation of support elements such as shotcrete, welded wire mesh, rock bolts, steel ribs, fore poling, and precast lagging. The adequacy of the support elements can be assessed by 3-D logging.
Permanent rock supporting system adopted was tensioned with the help of rock bolt/anchors (2m long) and shotcreting of varying thickness from 50mm to 100mm. Overt and invert concrete provided of 225mm thickness. Tunnel boring was done through Adit-1, Adit-2, Adit-3 & Adit-4. Since the strata of the tunnel in most part was Class-II&III and self-supported rock, emphasis was given on drilling and blasting pattern for smooth profile.
A pull of 2.5–3.5m was largely achieved during heading excavations. As excavation of considerable length had to be done from each face, a number of refuges were provided at intervals by Gammon for maneuvering and loading of the dumper and other equipment in the tunnel. Here, shear zone consisting of altered clayey gouge and crushed rock material prone to occasional rock mass failure was to be encountered, and therefore, steel ribs support was provided. Rock support during excavation comprised the following five types:
Class-I: The excavated section remains basically unsupported and spot rock anchors were installed.
Class-II: The support system consists of 50mm thick shotcrete along with spot rock-anchoring.
Class-III: The rock support system consisted of 100mm shotcrete in two or more layers with welded wire mesh and 2m length 25mm dia. pattern rock anchors @1.5m c/c.
Class-IV: Steel ribs of ISMB 125x75mm at a spacing of 0.75m c/c along with rock anchoring, wire mesh, shotcreting 100mm and backfill concreting were provided.
Class-V: For class-V rock, in addition to Class-IV rock supports, the steel ribs were extended in invert also.
|Charging Explosives & Blasting||2:30hrs||2:30hrs||2:30hrs||2:30hrs||2:30hrs|
|Steel Supports (with lagging)||8:00hrs||12:00hrs|
|Advance in meters||2.75||2.50||2.25||2.00||0.5 to 1|
Road inside the tunnel is required to be reasonably good to facilitate rapid movement of muck hauling dumpers, concrete transit mixers and other construction equipment, as well as to minimize frequent breakdown of equipment.
Construction Planning and Equipment
To achieve a high rate of excavation, modern construction equipment such as Scheff Loader ITC-312, low bed tippers and scoop tram were used for the small size tunnel, as shown below.
The conventional drill and blast method using 5 Atlas Copco single boom Rocket Boomers were adopted at all the 4 Adits and the method was found more suitable for excavation.
In order to minimize hydraulic losses and to protect the surrounding rock, 30 to 55 cm thick concrete lining of the tunnel was carried out in three phases: Kerb beam, Invert and Overt. After laying the Kerb-Beam, the Overt was laid, followed by the Invert. Total 9 telescoping concreting gantries, each 9m long and 3.3m in dia, and horseshoe in shape, were used for Overt concreting, and 24 (2 cum) transit mixers were used for transporting concrete directly into the hopper of the concrete pump.
Mix Design Used
Grade of Concrete: M25A40
Water Cement ratio: 0.5
D. Slump: 110 ± 10
Type of cement used: OPC
FA: CA: 45:55
CA Ratio: 20:25:55
The gantries were telescopic collapsible folding type, suitable for continuous concrete lining. Compatible concrete pump and batching plant transit mixer combination ensured continuous feeding of concrete. The shutters were electrically controlled for movement on rails and with hydraulic power packs and hydraulic jacks. Construction of kerbs helped in easy fixing of shutter. Before starting concreting, dummy trial of shutters was done, so as to check that there are no undercuts. Invert concreting was poured in-situ.
Arrangement was made with equipment manufacturers to provide service base at each Adit itself, so that any equipment breakdown could be attended to immediately by experienced service personnel. This ensured 75% availability of most of the equipment.
Major Problems Encountered During Excavation and the Remedial Measures
Initially, delays occurred due to misconception in the minds of local population regarding tunneling activities like blasting, which they feared would cause vibration in the adjoining areas. However, after undertaking blasting trials inside the tunnel and checks carried out along with representative of local administration and local panchayats for any ground vibration on the surface, the apprehensions were dismissed. In certain reaches of low rock cover, crossing under nallas and poor geological strata, problems of cavities, chimney formations, collapses, and heavy water inrush, were faced.
Cavities encountered in HRT with large over-breaks and solutions adopted
At certain locations, formation of chimney or cavity or large over breaks with heights ranging from 6-18m have occurred. At such locations the faces were sealed with muck filled bags, followed by shotcreting and fixing bulkhead. Small spaces, such as manholes through these bulkheads were created and reinforcement steel was provided both in longitudinal and transverse directions.
The backfilling of concrete was done using concrete pumps. The rock strata on either side were grouted and allowed to stabilize. Later on, the muck was removed gradually in multi-drifting fashion, followed by the erection of segmental ribs. After erection of a complete rib section, 5m long anchors were provided.
Adit-wise details of cavities and solutions
Adit-II and HRT (Face-III &IV)
Adit-II portal is situated near a seasonal nallah known as Kembli. The exposed rock mass, i.e., gneiss is highly sheared and shattered near the Adit portal. The initial ±60m of Adit was excavated through poor rock mass falling in Class-IV condition. It was observed that due to the sheared and shattered nature of the rock mass with clayey filling material along joints, the rock strata were very unstable and a huge cavity was formed during excavation. Since rock fall during excavation is a regular phenomenon, instant support was used for safe excavation. Excavation proceeded mainly with steel ribs erection and back filling.
Excavation of HRT through Face-III proceeded in a weak and weathered rock mass. The superincumbent cover over HRT was less as a result, rock mass was found to be weathered, filled with clayey/crushed rock material resulting from the impact of physical agencies. The weak tunneling media rock mass was supported with Class-IV treatment. At some portions, chimney formation created huge void and excess muck. Moreover, on 16.06.06 at RD ±506m, the tunnel got almost day lighted due to low cover zone along a seasonal nallah. Stretch between Face-II & III was resurveyed, and it was decided to detour the tunnel as further stretch to avoid the low cover zone.
Area between Face-II and III, especially before Gulesta nalla (from Face-III), due to sheared and weathered nature of rock mass, poor tunneling media was encountered during tunneling. Excavation in detoured tunnel also experienced hostile conditions and huge cavities were negotiated mainly from RD 481-501& 602-608m. Heavy water inflow conditions were experienced in this reach. Cavity treatment consumed a lot of time, as difficulty was faced in backfilling in the area due to heavy water inflow. Detouring of HRT between Face-II & III increased the tunnel length by ±43m. Moreover, ±60m of length from Face-III was abandoned for purpose of design, while from Face-II, 27m of length was abandoned.
The new total excavated length from Face-111 is 689m and tunneling media for this Face remained mainly in weathered gneiss rock, and some intrusive rock bands were also negotiated. Excavation from Face-IV was negotiated in Gneiss rock which offered good rock mass condition up to RD ±775m. On 15.04.06 at RD ±775m, a shear zone was encountered in this reach as a huge amount of slush entered from the shear zone and plugged the tunnel many times with pulverized material and water.
Various methodologies like rib erection, fore poling, grouting, core drilling and DRESS methodology were adopted to treat shear zone, but nothing succeeded. On 25.07.06, 02.08.06 and 08.08.06, huge amounts of slush flowed form the shear zone. Once again on 06.11.06, tremendous amount of slush entered from the shear zone and plugged the tunnel for a length of 400-500m with pulverized material and water. Removal of slush consumed a lot of time as it was very difficult to haul the slush in flowing condition. It took almost eight long months to treat the cavity with different methods and to remove the slush.
Eventually, it was decided to detour the tunnel, and the first blast was taken on 04.01.07. In the detoured part also extension of shear zone was encountered at RD ±775m on 26.01.08 which continued up to RD ±770m. In this patch, the shear zone was successfully treated by providing supports like fore poling, grouting and rib erection with back filling. Detouring in Face-II, III, & IV caused increase in the length of HRT. Moreover, 40m length in U/S and 55m in D/S face was abandoned for proper detouring. After rerouting Face-1V, the rock mass negotiated for the tunneling media was gneiss and it stretched up to RD 1293m (Breakthrough RD of Face 1V).
Adit-III and HRT Face-V &VI
This rock was very low in strength and could not provide enough standup time. As a result, the excavated portion collapsed at the junction of HRT. The restoration work consumed time, as heavy rib support was provided to open up the junction portion. Carbonaceous phyllite rock continued in Face-V up to RD 50m, while it ended in Face-VI at RD 71m. As a result of poor tunneling media, both the Faces fell in Class-IV rock mass condition and were supported with steel ribs at 0.5m c/c spacing.
At RD 493m Jutogh thrust was encountered at Face-V. Huge cavity formation took place in this reach which was supported with steel ribs. Tunneling media after RD 493m onward remained in the gneiss rock and this face has been excavated up to RD 1586m. (Breakthrough RD of Face-V)
At Adit–III Face-VI, heavy seepage conditions were encountered (with ±7800 lit/min max discharge measured on 16.08.08). To tackle the seepage problem, three dewatering pumps of 60HP (one spare), one 25 HP, two 16 HP and one 10HP were deployed. In the initial stage, around the junction area of Adit-3, carbonaceous rock mass was negotiated, and this rock mass crossed the HRT Face-V and Face-V1 at RD 54m and RD 71m, respectively. This paved the way for poor rock mass tunneling media, and for safe tunneling media ribs of 50cm c/c spacing were provided. At Face-V1, the rock media remained mainly in carbonaceous phyllite, siliceous limestone, limestone, and a very considerable stretch remained in the slate rock mass. Slate was observed till the breakthrough RD i.e., 2139m (From Face-V1).
Adit- IV and HRT Face-VII &VIII
The excavated length from Face-VIII is 56m and the tunneling media for VIII mainly remained in quartzite rock mass. This rock mass was observed up till the RD 223m of Face-VII. Thereafter, tunneling media for this Face remained mainly in carbonaceous phyllite, phyllitic limestone, and slate, sheared limestone, and weathered slate rock types.
In intermediate reaches, carbonaceous rock mass led to the poor tunneling media and ribs were provided at regular interval from this Face to move ahead for better and safe tunneling. Heavy ingress of water from RD 430 to 580m was a cause of concern. Proper drainage holes of 15m long each were provided at the crown portion to relieve the excess hydrostatic pressure. Very poor tunneling media was also negotiated from RD 1535m onward where sheared carbonaceous phyllite rock mass along with heavy ingress of seepage water was observed from the Face.
To proceed further, this reach was treated with fore poling, injection of grouting, and dry mix was fixed behind the rib portions. The segmented support system was adopted throughout this reach. The last stretch of this Face was excavated in the weathered slate rock mass. The breakthrough from Face-VII was made at RD 2278m RD.
Additional curves were provided from Face-V1 and Face-VII to avoid the low cover zone and relocate the tunnel in adequate rock cover zone. For this reason, the tunnel from Face-V1 was detoured at RD 1562m after providing a curve. Another curve was provided at RD 1989m. Face-VII was also detoured at RD 1723m.The total increased length between Face-V1 and Face-VII is 84m.
Ventilation was critical owing to the longer lead and low diameter of the tunnel. To tackle this, one 75 kva ventilation fan at portal, one 45 kva fan, and one 20kva fan inside the tunnel are installed at Face-VI & Face-VII, respectively.
Dewatering and ventilation are critical activities in the excavation cycle; they can frequently hamper the work progress, which consequently increases the cycle time of other tunneling construction activities.
Size of tunnel, steep slope, storage of seepage and rainwater at Face
Due to the small size and deep gradient of the tunnel, it was difficult to plant the equipment to be used inside the tunnel. After going through plant directories, 7ton dumpers and Scheff loader ITC-312/Scoop tram and Ajax Fiori/Thwaites combination/JCB and 7ton Tipper combination were selected for working inside the tunnel for removing the muck. Even these equipment could not perform well on the down gradient on the expected scale.
Since the tunnel was bored in down gradient from four Faces, there were serious problems for dewatering. Seepage also increased at different RD’s due to heavy rainfall. Dewatering was done by using higher capacity pumps in different stages, deploying specialized agencies. In the down gradient, deep sumps were made at 250m intervals and dewatering pumps were installed, as shown in figure.
The construction of Head Race Tunnel at Sewa H.E. Project is unique where meticulous planning for boring and concrete lining was done well in advance. As a result, concrete lining wok was started immediately after day lighting of each face of the tunnel by installing telescoping gantries and successfully completing the lining of a major portion of the tunnel.
Proper construction planning of each activity paved the way for faster construction. Teamwork with total coordination and cooperation, and the dedication of the Gammon and NHPC engineers were key to the successful completion of the project.