The Pir Panjal railway tunnel is a key element of the new Udhampur-Srinagar-Baramulla broad-gauge railway through Jammu & Kashmir. It is not only the longest railway tunnel in India (beating the 6.5kmlong Karbude Tunnel of the Konkan Railway) and the second longest in Asia, but is also being constructed under very difficult conditons of both geology and access to the site.

The early stages of the project and geology were the subject of an article by Geoconsult project director Dr Friedrich Prinzl. This article concentrates on recent work to complete the tunnel, which also has the project designation T-80.

Geology
It should be emphasised that the geology is quite unpredictable, including squeezing ground (see Figure 2, below). This makes the flexibility of the adopted NATM suitable, within which the necessary tunnel support design can be employed according to the ground conditions encountered.

Due to the uncertainty contractor HCC drilled probe holes, 64 or 76mm in diameter and 25-55m long with a 5-10-m overlap, throughout the drive. The number of holes in each session was between one and six depending on the ground encountered.

Contracts
Tunnel construction was split into four contracts, of which HCC was awarded the great majority (see Figure 3). Contract VA also includes slope stabilisation, slope protection and portal construction at the south portal, and an 85m-long connection to the access adit. Similarly contract VB includes construction of the north portal.

Environment
As described by Prinzl, the site logistics were very difficult, particularly in the early stages. The climate includes a winter season with snowfall from November to March. Between December and March temperatures are down to -10oC with frequent snowfalls covering access tracks. Work sometimes had to be halted although the aim is round-the-clock operation.

The environment inside the long tunnel is also important, particularly in ventilation terms. The capacity of the force-fan ventilation systems for all faces took into account requirements for personnel (4.25m3/min each, diesel drive (3.5m3/min for each kW of power) and a general return air velocity requirement of 0.3m/s across the fully excavated section.

Alignment
The tunnel is aligned approximately north-south on a straight route for its entire length through the Pir Panjal pass that connects the Banihal valley at the south end with the Kashmir valley at the north end. Vertically the base tunnel passes under mountainous terrain with a maximum overburden of 1,100m. It also passes 440m under the existing Jawahar road tunnel.

Railway standards require the maximum deviation, both vertically and horizontally, to be 10mm/km. Maximum offsets of up to 60mm are allowed on excavation breakthrough. HCC says that it is not possible to achieve such accuracy by conventional tunnelling surveying, so net surveys were carried out regularly.

Construction method
Ircon International is in overall charge of the railway construction between Dharam and Baramulla with Geoconsult-RITES and HCC providing the special tunnelling expertise. Ircon has already completed the 199-km Quazigund to Baramulla stretch, which was put into operation in September 2009. Ircon divided the 56.4-km Dharam to Quazigund stretch into six zones to expedite the work, of which zones IV and V comprise the Panjal Tunnel, contracted to HCC, plus 13 other tunnels.

The whole tunnel has been constructed on NATM principles, with the majority of excavation by drill and blast, but with roadheader (an Indian railway first) and hydraulic excavator excavation in softer ground. This has been the first major application of NATM in India. In addition to the tunnel itself, HCC was required to provide an access adit and shaft to facilitate construction and to allow maintenance, provide ventilation and for emergency egress.

Excavation of the main tunnel was launched from three faces:
1) South portal northwards;
2) From the access adit northwards and southwards;
3) North portal southwards.

The 2750-m stretch between the south portal and adit includes 650m of soft ground (rock class VII and VIII), which was excavated by sequential excavation in advances of 1.0-1.5m using a Korean Sambo ST-280 Tunnel Excavator boom on a Doosan crawler-tracked carrier. The rotating boom was very convenient for work in restricted sections, reports HCC.

In this stretch mucking out was by a Liugong side-tipping loader and (Atlas Copco) Wagner 20-t articulated dumptrucks. Ground control was particularly important due to the heavily populated village of Cheril over the tunnel with 2-40m of overburden. The ground was mainly clay but with variable hard strata interlying. Firstly a crown heading was dug and supported, and then the two side headings. With temporary walls and crown support installed the central core was excavated. Breakthrough in this stretch was achieved in July 2009. Boulders were encountered in glacial deposits from 380m into the tunnel. These were dug up and broken up, if necessary, by hydraulic breaker, avoiding blasting.

With the increased ground instability and the overlying village, the excavation sequences was modified into eight smaller portions rather than the more usual four (two top headings, two side headings, a core drive and three drives in the bench.

Another stretch of soft ground, 600m long, was also excavated in the same way from the north portal. Before commencing the soft ground excavation the portal faces were reinforced with 9m-long face bolts, and the surrounding slopes stabilised with two layers of reinforced sprayed concrete. To prepare for initial excavation the portal face was further reinforced with 150×150-mm steel girder forepoling, 76mm-diameter pipe roofing, and self-drilling anchors.

Some 521m into the tunnel the miners met shattered ground with heavy water inflow. This necessitated pipe roofing with 76 and 114mm-diameter pipes, each 12m long, the latter requiring the use of a Casagrande forepoling rig. Regular drain holes were drilled to release groundwater, and sometimes in the face to reduce water pressure in advance of excavation. Water flows were up to 180 litres/s compared to the predicted maximum of 70 litres/s.

The adit and shaft allowed difficult ground at the portals to be isolated so that it did not hold up excavation of the main tunnel drives over the remaining 7,600m. The sequence of construction from the two faces in this main stretch was:
• Excavation of full-width top heading, followed by temporary support;
• Excavation of the bench and invert followed by support (invert excavated only for rock classes V and above);
• Casting of concrete invert lining;
• Casting of concrete ‘kicker’;
• Fixing of waterproof membrane over geotextile;
• Casting of main lining.

A Wirth T3.20 Q roadheader with transverse cutterhead was employed from the north portal and was found by HCC to be effective as long as the UCS of the ground rock was less than 100MPa. However, a number of difficulties were experienced, mainly because the heterogeneous geology expected through the length of contract VB (length 3.12km out of 4.22km) was much more mixed. Optimum cutterhead speed could not be achieved says HCC. With supplier assistance, the roadheader was put to work again from 711m into the tunnel.

Rock encountered was often very hard. Pick consumption varied between 0.025 to 0.3 per cubic metre excavated depending on the actual rock UCS. Further problems resulted from a shear/fault zone encountered at 677m containing clay and limestone boulders. Mixed ground was encountered in two further zones comprising fine-grained limestone with calcite and clay infilling. There was also a water flow of 35-40 litres/s here, which created mud that was difficult to remove with the roadheader’s mucking out system.

Spoil was removed by dumpers loaded by the roadheader’s own conveyor. However, as most rocks encountered in the Pir Panjal Tunnel are above 100MPa UCS, usage of the roadheader was limited to nine per cent of total excavation.

In other (hard rock) stretches, drilling and blasting was carried out with two Atlas Copco Boomer twin-boom drill rigs (L2C and L2D). For blasting HCC used nonelectric detonators with long delays and ‘power gel’ explosive in 40-mm and 32-mm cartridges. HCC says that non-electric detonators were found to be particularly advantageous due to their inherent safety, allowing other activities to be carried out in parallel with charging. Mucking out was as in soft ground excavation.

In the stretches where water-bearing structures had been identified, 76mmdiameter probe holes were drilled to 20-30m. Depending on the results, drainage holes of 76 or 114mm diameter were then drilled, 3-25m in length, in successive rounds. This enabled sufficiently dry conditions for successful excavation.

In the young ‘mountain building’ ground conditions, shear zones were identified in advance along faults and contact areas by probe holes and geotechnical monitoring. Various measures were taken to successfully negotiate these. In drill-and-blast excavation the round lengths were reduced and the number of forepoles increases to 30-40 per round. The length was also increased to 6-9m, plus four to six face rockbolts, 9m in length. Grout was injected through the forepoles. Rockbolting of the periphery was also increased.

Each new face excavation was sealed with sprayed concrete 50-200mm thick, and this was increased on the periphery (walls, crown, etc) to 300mm, including two layers of wire mesh reinforcement. A hydraulic excavator was used if drill-and-blast was not appropriate in sequential excavation, as in the soft ground.

In order to facilitate emergency and maintenance access HCC has laid a 3mwide concrete road adjacent to the railway alignment. A gravity slope to both portals from a high mid-point facilitates drainage of this base tunnel.

Support
In accordance with NATM, ground instrumentation has been employed not only for routine monitoring of ground movements but also to assess stress redistribution and stabilisation before applying the final lining. The readings are input to a 3D visual monitoring system.

Based on geological investigation the tunnel alignment was classified into eight different conditions for NATM support purposes. Since lining thicknesses therefore varied, the excavated section varied between 67 and 78m2, with a final finished sectional area of 48m2.

The primary support, according to the design for each of the eight rock classes, consisted of a combination of wire mesh, sprayed concrete, lattice girders, rock bolts and forepoling as necessary.

Firstly a face-sealing layer of sprayed concrete, 50-100mm thick was applied after every round, chiefly for safety. This was followed by installation of Fe500 grade wire mesh sheets of 150 by 150mm and 6mm thick, on the periphery of the fresh concrete to reinforce the primary lining. No wire mesh is required for rock classes I, II and II in hard round, but one layer is used for class IV and two for class V and above.

The main sprayed concrete lining, grade M25, is then applied by wet process to a thickness of 100-300mm depending on the rock class. The mix includes alkali-free accelerator such as BASF Meyco SA-160 or Sika Sigunit L54 AF. The spray equipment used is a robotic Cifa machine.

Another common element of NATM-type support, particularly in poor ground, is lattice girders. They enable triaxial loads to be distributed around the tunnel section with better uniformity. Two sizes are used in the Pir Panjal are type 70 and type 90 with 16 by 25mm bar lattice. The reinforcement bar is 25, 16 or 10mm diameter.

Rock bolting played a major part in stabilising poor ground. In total around 15,000m of rock bolts were installed using the twin-boom Atlas Copco jumbos.

In bad ground, where excavation may cause a roof collapse, forepoling was employed in the crown to introduce pre-excavation support elements, thus shortening the unsupported excavated surface. Three types were used:
• MS pipe of 40mm diameter
• Self-drilling (SDR) bolts of 32mm diameter
• Reinforcement bar (rebar) of 32mm.

In particularly bad areas 76mm-diameter pipe roofing was also employed in the heading and bench drives during sequential excavation. Each element was installed using a Casagrande forepoling rig or the
blasthole jumbos.

In addition, in the soft ground excavated sequentially, the temporary invert of the top heading was lined with sprayed concrete and wire mesh to ‘close the ring’ to NATM principles in order to redistribute the surround ground stresses for equilibrium.

Permanent concrete lining
All concrete used for final lining is of M30 standard. Once excavation and primary support had been completed in a stretch of tunnel lining can be carried out in parallel with other activities further along the tunnel. It is normally carried out in four stages in 12.5m lenghts, known as a ‘block’, as:
• Casting of invert (if required),
• Casting of ‘kicker’ portion,
• Fixing of waterproofing membrane,
• Casting of ‘overt’ i.e. walls and crown.

The invert was for stretches in rock classes V, VII and VIII. Use of a special cantilever ‘invert bridge’ allowed excavation to carry on further forward at the same time as invert casting beneath the bridge. First the invert was cleaned of loose material and fill laid to enable face access before shifting the 25m-long bridge into position. Excavator, manual and jet cleaning are used. In position, one end of the bridge rests on the already cast invert block and the other on the cleaned invert space. Vehicles can travel over it as reinforcement and concrete is placed. Reinforcement and shuttering can then be installed underneath followed by concreting.

Reinforcement, erected manually, is made of Fe500 grade steel of 10mm diameter. Longitudinal shuttering includes the formation of a drain channel.

Concrete is carried in lining via transit mixers from the batching plant and then pumped to the casting area, with needle vibrators used for compaction. The drain is closed over with flat pre-cast slabs.

Waterproofing
The whole tunnel is being waterproofed with Sika pvc membrane overlying geotextile fleece and beneath the final cast-in- place concrete lining. Installation is a specialist activity. Leaking groundwater is
channelled through the geotextile whilst prevented from entering the main tunnel by the membrane. Water is collected by perforated PVC pipes embedded in ‘no fines’ porous concrete in the corner of the
kicker and channelled to the main drain. The procedure is:
• Installation of 200mm-diameter pvc pipes embedded in ‘no fines’ concrete, with manholes at every 50m,
• Application of smoothing sprayed concrete layer, 20mm thick, to form a suitable surface for the membrane,
• Installation of geotextile and membrane from a gantry to facilitate handling. Roundels and pins fix the geotextile with the membrane at 1.5-m spacing. Each membrane sheet is double-seam welded to the next with a 100-mm overlap. Hot air welding is used at the roundels and then for each sheet overlap.

The main concrete lining is of two types: reinforced (RCC) and unreinforced Portland (PCC). RCC is used within stretches of extreme rock support classification VII and VIII, whilst the remainder was lined in PCC, 300-450mm thick. The RCC reinforcement mesh uses bar of 10 and 16mm diameter. The resultant mesh is further reinforced with Pantex lattice girders at one-metre intervals. A separate gantry is used to install the reinforcement and then another Cifa formwork gantry is moved into place.

The formwork is hydraulically operated and controlled from a single console for the collapsible shuttering, with a check on accurate alignment with each block to be concreted. The shuttering has ‘windows’ in strategic positions, and its own pipework, for easier concreting. Vibrators for concrete compaction are also integral to the gantry. Concrete production and delivery is as for the invert and kickers.

Challenging conclusions
Some of the challenges that have been faced were already described by Prinzl (2010). For its part, HCC list its main challenges as:
• Changes of geological conditions compared to anticipated rock class, plus rock bursting;
– Excavation of soft ground;
• Blasting restriction due to proximity of villages over tunnel alignment;
• Excavation through heavy water inflows;
• Excavation in shear (fault) zones;
• Casting of invert lining;
• Lower performance of roadheader.

According to HCC there is a ‘huge variation’ between the predicted rock classes and characteristics compared to the geological interpretative report presented to Ircon at the contract tender stage. The differences are shown in Table 1.

Progress
With the completion of excavation, the final heading breakthrough occurring six and a half years after the start of tunnelling. Completion of the permanent lining is now under way, with HCC reporting only 850m of the total 10,960m of the excavated tunnel left to be completed.

However, Dr Prinzl reports that in a few sections of shaley ground with high ground pressures, since the beginning of 2012 the lining has been affected by creep displacements of 5-10mm per month, mainly horizontally. These were observed under an overburden of approximately 800-900m.

"The deformation could be controlled only after installation of extensive bolting with 12m-long rockbolts in the side walls," says Prinzl.

HCC says that the length affected is 638m in shaley ground. In addition to ther invert, the thickness of the rest of the concrete lining has also been increased to 450mm and the shale grouted throughout, in addition to rockbolting.

The concreting of the railway base for ballast-less track is also under way. Trackfixing and M&E were scheduled to start last month (May).