Hindustan Construction Company (HCC) has added another feather in its cap with the construction of Pir Panjal Tunnel, slated to be amongst India's longest railway tunnel.
The Pir Panjal Railway tunnel is part of the 202-km Udhampur-Srinagar-Baramulla Rail Link project undertaken by the Northern Railways. It was awarded to IRCON and Konkan Railways for development. The stretch between Quazigund and Dharam - under IRCON's ambit - was divided into six zones and the contracts of Zone IV and V were awarded to HCC. Zone V consisted of the 10.96-km Pir Panjal tunnel.
Constructed in the rugged terrains of Pir Panjal pass in Jammu and Kashmir, the tunnel passes about 440 m below the existing Jawahar Road Tunnel and its alignment crosses the NH-1A at three locations. This tunnel is slated to be amongst India's longest railway tunnel. It is 100 per cent waterproof with provisions for ventilation, fire fighting and safety monitoring access across its entire length. Besides, a 3 m-wide concrete road has been constructed adjacent to the railway track throughout its length (10.96 km) for maintenance and emergency purposes.
New-age construction
HCC adopted the New Austrian Tunnelling Method (NATM) where the geological stress from the surrounding rock was used to stabilise the tunnel hole along with the use of the latest 3-D monitoring system. It also adopted the Austrian standard for rock classification system for geotechnical design and determination of rock supports and excavation class.
The alignment of Pir Panjal tunnel comprises soft ground, medium and very hard strata. The Pir Panjal range is dominated by folding, leading to different dipping directions of the strata. Based on the geological investigation, the rocks between the tunnel portions were classified into eight types and the support system was designed accordingly. The primary support measures adopted in the tunnel were wire mesh, primary inner lining shotcrete, lattice girder, rock bolts and fore poles. The excavation area of the main tunnel varies from 67 sq m to 78 sq m based on the rock condition and the final finished cross sectional area is 48 sq m.
Adits and shafts were provided to shorten the critical length of drive and isolating difficult ground near portals. By providing an adit (south side) and a shaft (north side), the critical length was reduced to 7,600 m, as against the total length of 10.96 km. The total length of the adit is 772 m. The shaft was connected to the main tunnel with a cross passage of 38 m. The cross section of the adit and shaft allows for convenience and ventilation during construction and operation.
The methodology
The construction methodology adopted in Pir Panjal tunnel could be broadly divided into six parts that involved excavation of top heading along with the support installations; excavation of benching and invert along with the support installations, casting of invert and kicker; fixing of waterproofing membrane and geotextiles; and casting of overt.
The inert excavation is applicable only for rock class V and above. Based on geological requirements, there were three kinds of excavation: tunnel excavation, road header, drill and blast method. In the hard rock strata, the drill and blast method was the best option. The blast holes were drilled in the face with a two-boom hydraulic jumbo (L2C and L2D) and then the explosives were loaded in the drilled holes. The explosives used in the tunnel were power gel, long delay detonators (LDD), non-electric detonators (NEDs), Dcord, and 40- and 32-mm diameter explosives for effective blasting. The NEDs were very useful as loading could take place parallel to other activities.
The concrete work for the tunnel lining was carried out after completion of excavation and primary support works. The lining of the main tunnel was carried out in three stages to a length of 12.5 m (called one block), leaving construction joints behind that are all blocked by water stoppers. Concreting the invert, i.e. bottom portion of the tunnel, was the most challenging part as it leaves no space for the traffic and equipment movement required for carrying out the face activities. A parallel invert bridge was used to enable concrete casting. The concrete casted in the invert is a RCC concrete of M 30 grade. The reinforcement used in the invert is of Fe 500 grade 10 mm diameter.
The invert lining was cast with the help of a concrete pump. Transit mixers were used to convey concrete from the batching plant to the invert under construction.
Compaction and vibration of the concrete was cast in layers in the invert over the reinforcements and manually done by needle vibrators.
The conveying pipe was added or removed as per the requirement. The drain thus left behind was later closed from the top by precast concrete slabs (780 + 600 + 150 mm) separately cast in the casting yard. The kicker casting work was similar to the invert casting work and taken up subsequent to the invert casting.
For the overt lining, RCC lining was carried out in rock class VII and VIII; for the remaining rock classes, PCC lining 300 mm-450 mm thickness was used. In RCC lining the reinforcement bars of 10 mm and 16 mm diameter were used to prepare the mesh for the overt structure. A separate gantry jumbo was used to manually place, bind and align the steel mesh. The mesh was further reinforced by Pantex lattice girders placed at equal intervals of 1m c/c. The gantry shutters consist of collapsible shutters that need to be realigned after placing them into a newly reinforced block. The gantry is a hydraulic formwork and operable from a single control unit.
For the installation of waterproofing membrane and geotextiles, specialised work was carried out simultaneously across the block. They were installed in the walls of the tunnel to enable the ground water to get channelled through the waterproofing membrane to the perforated PVC pipes. These pipes placed in the corner of the kicker, embedded in the no fine porous concrete, drain out the water collected through the diversion of the waterproofing membrane to the main drainage gutters.
Braving all odds
The project employed the use of road headers for the first time in India for railway tunnelling. For soft ground terrain, road headers were deployed to work in class IV and above strata. Owing to mixed rock conditions at the tunnel, the optimum cutter speed could not be achieved. The team planned to use the road header to excavate 3.12 km of the 4.22 km, but only 9 per cent of the total excavation was achieved using road headers.
During underground excavation, the team unexpectedly came across a heavily populated village barely 20 m above the tunnel alignment. Blasting activity was avoided in the tunnel and to avoid noises of vibration, a major portion was excavated using tunnel excavator or breakers.
During soft ground tunnelling, the project team also encountered heavy ingress of water, wherein water seeped in from all sides at the rate of 70-180 l/sec that ultimately impacted production rate. To tackle this, drainage holes of diameter 114/76 mm with a length of 3 m to 25 m were drilled to drain out the water.
Owing to poor geology and variation in characteristics of rock classes, rocks burst and fell in some locations, demanding additional support and restoration work that caused unforeseen delays. Moreover, 1,500 workers braved the freezing winter from December to March when the temperatures dipped from -3° C to -10° C.
The access area to project sites would often be covered in snow and HCC together with the Border Roads Organisation deployed machines to clear the snow to ensure round-the-clock work. HCC commenced the tunnel excavation in November 2005.
The initial breakthrough of the 2,750 m length tunnel between Banihal and Tethar was achieved on July 2009 and the overall breakthrough between Banihal to Qazigund was achieved in October 2011. The tunnel was recently opened for a trial run in December 2012.
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