Long wanted and far longer needed, construction is finally getting underway for the all- weather Patnitop road tunnel on the NH1A highway, on the Chenani-Nashri section of the route between Udhampur and Banihal, in Jammu & Kashmir, in northwest India.

The 8980m long tunnel with its approach roads will offer a bypass of approximately 10km in length to the existing, winding and exposed 41km long stretch of the NH1A. Further, as one of the longest road tunnels to be built in Asia, not only will it provide significant time savings on each trip it will also, by being a shorter, sheltered route, enable the highway to remain open throughout the year.

Presently, due to severe weather conditions, the roads in the area can be closed for up to about 40 days each year. Accessing the road requires an ascent and then descent of 1000m. The new tunnel is to be at an elevation of approximately 1200m.

Leighton Contractors India was awarded a contract to build the approximately 14m (13.3m internal) wide, horseshoe-shaped tunnel, which will carry the bi-directional, 2-lane road through the Patnitop mountain range. The main tube will be shadowed by a 5m (internal) wide escape tunnel to which it will be linked by both pedestrian and vehicular cross passages.

Plans and designs are being finalised to enable excavation to start in the near future.

Contract award
The Engineering, Procurement and Construction (EPC) contract is valued at approximately US$570M on a lump sum, fixed price basis, and was awarded to Leighton by part of the India-based group IL&FS Transportation Networks Ltd (ITNL).

In May, the National Highways Authority of India (NHAI), part of the national Ministry of Road Transport & Highways, issued a Letter of Award for the concession to upgrade the 41km long road between Chenani and Nashri. The concession agreement was signed at the end of June with ITNL’s special purpose vehicle for the project, Chenani Nashri Tunnelway Ltd (CNTL). The concessionaire will receive payment for the infrastructure facility and services through a semi-annual annuity.

ITNL is itself part of the Infrastructure Leasing & Financial Services (IL&FS) group.

Leighton announced the contract award in July. Then, in a statement, Leighton India’s managing director, Russell Waugh, described the road tunnel project for the Australia-based international group as a ‘fantastic opportunity’.

He added: ‘The award of this project recognises Leighton’s position as a preeminent contractor in India and our ability to deliver technically challenging engineering and infrastructure projects.’

The need for a tunnel
The NH1A runs from Srinagar to south India. In the Himalayan foothills there is very complicated terrain to negotiate where it follows part of an ancient route. In 1954, some decades after being converted to a single-lane, the road was formally declared a national highway.

From the late 1960s, work was undertaken to widen the highway in stages with the horizontal and vertical alignments, but economic growth in the country demands that the capacity of the strategic transport axis is further increased to four lanes as a two-lane dual carriageway.

The local landscape, which can experience intense seasonal rainfall, is among the steepest anywhere, notes Leighton. The highway in this area has blind summits, steep gradients and sharp hairpin bends. It is very deficient in horizontal curves, adds Leighton.

For the upgrade scheme between Udhampur and Banihal, there are eight tunnels required (including Patnitop – by far the longest). The others have lengths of 888m, 870m, 610m, 585m, 385m, 322m and 195m. These are two-lane, single direction tubes, some for traffic to flow north and the others south, unlike the contraflow arrangement for the far larger Patnitop tunnel.

The excavated height for the short tunnel would be a standard 8.7m and the finished profile would give 5m clearance above the 7.5m wide carriageway and 1m wide pavements.

At Patnitop, the existing road runs on steep slopes through areas of frequent landslides and crosses mountain ridges en route to the Kashmir valley. In a move that would alter the alignment but deliver significant benefits to ease of travel and time saving, it was proposed that a long, two-lane road tunnel be constructed to pass through the Patnitop range. Ths would establish an all-weather route for the Government’s highway improvement plan.

Early Planning
Czech firm D2 Consult undertook an investigation of alternatives for the Patnitop road tunnel over 2006-7 when working as a sub-consultant for Louis Berger Group, which was assisting NHAI with its feasibility studies for the highway upgrades.

Five tunnel options were investigated during feasibility studies, ranging in length from 1.2km to 9.2km.

A 9020m long single tube will have a mined length of only 40m less and the maximum overburden was foreseen to be 1050m. The excavation method would be NATM. As some sections of the alignment were anticipated to experience relatively large convergence during tunnelling, there was some work done by D2 Consult on the concept of yielding elements, or lining stress controllers.

The firm’s services looked at design of primary and permanent lining, M&E equipment and preparation for the procurement phase.

In addition, D2 Consult also completed both initial and tender documentation for the other seven much shorter tunnels on the 122km long Udhampur-Banihal section of the highway. With the studies examining the proposed tunnels in light of the latest technology and practices in road tunnelling, not then familiar in the area. It said there was need to both explain and justify the approaches to the client.

Geology
Geology along the alignment of Patnitop tunnel is dominated by the sedimentary strata of the Muree Formation – composed of sandstone, marlstone, claystone, siltstone, shale, conglomerates and some small concretions in clay/silt zones. The strata alternate from weak to hard layers.

The sandstones are variably weathered and joints are often filled with clay. The claystone and clayey shales are weak. The consultant noted that the fundamental influence of the thickness of the clay layers, and possible presence of groundwater, were the main factors related to face stability.

Bounded by the Nuree thrust and the main boundary fault in the south, the tunnel alignment – varying in elevation, says Leighton, between 1170m-1195m above sea level – passes through the Kud Syncline.

Historically, the area is seismically active. The consultants said that this factor plus terrain morphology and information available on faults led to the assumption that horizontal stress would be quite low perpendicular to the tunnel axis. It added that at the area of maximum cover the vertical component of rock stress is expected to correspond to the full overburden.

From the limited information from surveys and limited experience of such tunnelling in the area, there is great uncertainty over geotechnical types and expectations for excavation support. To the north, beyond the tunnel corridor, there are the igneous and metamorphic rocks of the Panjal Formation.

Tunnel Concept
The concept design from early on was for parallel tubes with the main road tunnel and the escape gallery linked by a series of cross passages at 300m centres.

The tunnel’s vertical alignment was originally envisioned to Slope up 0.5% from either portal to tangent into a peak with a radius of curvature of 20,000m. The gentle slopes would allow for gravity drainage. Now, Leighton’s design has a constant slope of 0.43% northwards between the portals.

At a length of 8930m, the escape tunnel will be 50m shorter than the main bore and linked to it by 29 cross passages. Of these, 15 are for pedestrian use and seven wider tubes are solely for vehicle movement. The remainder have two-step widths and a centre wall for an electrical transformer room.

The early design concept during feasibility studies and procurement preparations suggested that the lead faces of the tunnelling works could be in the escape gallery, which could act not only as an exploratory hole ahead for the main tube but also to provide drainage and transport support.

The range of overburden is 200m-1000m.

Undertaking the works by traditional NATM, the tunnels will be constructed with both primary and secondary linings.

Construction begins
Following CNTL signing the 20-year concession with NHAI to upgrade the Chenani-Nashri road, including construction of Patnitop tunnel through a design, build, finance, operate and transfer agreement, it quickly awarded the EPC contract to Leighton. The construction period runs to mid-2015.

Leighton is currently mobilizing resources and procurement for the EPC contract and has already retained a specialist consultancy to work with its inhouse team on early planning and design work. Further details were not immediately available.

The mountainous terrain and remote location presents a wide range of interesting engineering, logistic and management problems to be solved, says Leighton.

For instance, when excavation gets underway and is proceeding from the ends of the tube, the firm says that one of the prime challenges will be sequencing the activities across multiple work fronts in the tubes and cross passages.

It adds that effectiveness of the ventilation design is integral to the firing times in drill and blast operations. Efforts to advance the multiple drives also need to be coordinated with other activities, such as concrete lining works. The contractor adds that further challenges are expected to come from the wet season and also meltwater runoff.

Primary lining is to include shotcrete, mesh, rock anchors, lattice girders and spiles for weaker areas, depending on the support classification determined by probing ahead as the heading and bench excavations advance.

Plant and progress
Leighton is to draw upon a number of Atlas Copco jumbos for the excavations – four fully computerised E3Cs and three E2Cs.

The contractor described its intended excavation sequence for Patnitop tunnel as follows:
• Preparation of the Portal Heading Platform & Benching to receive the Atlas Copco E-3 Jumbo Boom.
• Utilising the Jumbo Drilling Machine. A number of horizontal drill probes 100m in length will extract geology samples for analysis. Support classification can then be determined.
• Technical data is fed into the automatic computerised multi-head Jumbo Drilling Machine. For example, drilling pattern, area, length and hole diameter.
• Drilling to heading tunnel face.
• Charging the face with Bulk Emulsion, non electric detonation initiation explosives.
• Blasting.
• Blast emissions, ventilation through 2.4m dia ducting.
• Mucking shifting and scaling using CAT 980 Loaders.
• First pass of shotcrete with steel fibre, using Normet Robotic Shotcrete Machine.
• Rock Bolts of approximately 5m in length, using the Jumbo (Atlas Copoco)
• Second pass of shotcrete
• Clear tunnel face.
• Drilling to face heading.

Leighton anticipates advancing each face of the Patnitop tubes by approximately 2.7m per day.

Four NATM excavation and support classes, A-D, were specified in the tender ocuments. The excavated area would range from 147m2-175m2, depending of ground conditions and support needed. Progressing from anticipated best class to poorest, the excavation support would change from the heading and bench being up to 100m apart with 200mm thick sprayed shotcrete sprayed on mesh and 6m or 8m long rock anchors, to the heading and bench being very close and 400mm thick shotcrete used as well as rockbolts of 8m and 12m lengths.

For the seven smaller tunnels, the feasibility study and initial design by D2 Consult described three basic NATM classes, A-C. These ranging in advance from 3m down to 1m steps for ground changing from stable to weak, squeezing rock. That progression also anticipated shotcrete thickness and rockbolt lengths doubling to 300mm and 6m, respectively. The excavated area would range from 83m2-101m2.

At Patnitop, the other major plant and equipment that Leighton plans to use includes:
5 x Shotcrete Units
4 x Tunnel Mobile Work Platforms
2 x Haggloader
2 x Tunnel Excavator
10 x Dump Truck- Cat ADT 730/Volvo A30
2 x Grout Pump
4 x Concrete Pump
6 x Telehandler
4 x Loader- 980/966
16 x Genset- 750/500/250/160/125 Kva
Mobile Crusher- 100 TPH- 2 Nos

In total, the contractor expects to excavate 1.7M m3 of spoil, requiring 220,000m3 of shotcrete and call for approximately 900,000 linear metres of rock bolts.

The volume of concrete needed for the permanent lining is estimated to be 250,000m3.

As the EPC contractor refines its plans and own detailed design further, and excavation commences some months from now, the Patnitop road tunnel will be a key project to follow to see how the international approach to NATM is expanding the practice of sequential excavation in this area of India and Asia.


Location of Patnitop road tunnel in J&K, Northwest India Patnitop road tunnel will have a main bore with a smaller escape bore running parallel, connected by cross passages for pedestrians or vehicles Early works underway onsite to preparing to excavate the almost 9km long Patnitop road tunnel The Government of India is increasing the capacity of the NH-1A road between Jammu and Srinagar to four lanes The Patnitop project is a 54-month programme to construct the 14m wide main tunnel, escape tunnel and cross passages, and work has just got underway