Drill and blast excavation started last month at the 8800m long road tunnel at Rohtang Pass in Himachal Pradesh state, in northwest India. Dywidag International is undertaking the tunnelling works on the construction project for a 60/40 joint venture between its parent, Strabag, and local group Afcons. The JV’s contract with the national Border Roads Organisation (BRO) calls for the single tube scheme to be completed by the end of 2015.

Like the transport problems encountered in many other high mountain regions, the experience of the travel over the Rohtang Pass, at almost 4000m a.s.l., is of severe snow and high winds that can block the road for long periods in winter. The loss of full availability of this sole road, the Manali-Leh highway, cuts-off every community to the north of the pass for at least four months, and sometimes even up to half of the year.

The national Government wants the barrier of Rohtang Pass overcome to allow all-weather passage from the north of the state to the south, linking to Manali’s active tourist-buoyed economy just beyond the mountain hurdle and farther – and to have such a strategic link in place to help ensure constant communication throughout the state.

BRO elected a tunnel solution for the all weather route, which will shorten the travelling distance by 45km with commensurate benefits to safety and time over the present dangerous and slow road. The tunnel is being built at an altitude of approximately 3,100m a.s.l.

Geology
Geology in the area consists of metamorphosed sequences of schists, phyllite and migmatite with gneiss and granitoid bodies. There are three fault zones and areas of high squeezing rock.

The first 2.2km of the alignment from the south portal is expected to be schist (UCS 60MPa-80MPa), predominantly followed by about 1km of migmatite/fault zones (UCS 30MPa-50MPa and also 80MPa-100MPa) then schist again for about 1.2km.

The remainder of the tunnel—the northern half—is expected to be dominated by migmatite/minor schists and shattered zones (UCS 60MPa-120MPa).

In terms of rock mass classes, along the alignment of the tunnel approximately 41 per cent of the geology is expected to have favourable condition with classes 1, 2 and 3, says Dywidag.

Just over a quarter—27 per cent of the route—is expected to have geology with rockburst conditions and rock mass classes of 3M, 4M and 4S.

The remaining third, or 32 per cent, of the geology is anticipated to have squeezing rock conditions with rock mass classes of 5, 6 and 7.

Overburden along the tunnel alignment is calculated to range from 30m at the portals up to 1,850m almost at the end of the Slope, up from the north portal and at midpoint of the tunnel.

In terms of location, the squeezing conditions are anticipated in part of the migmatite/fault zone at the southern end of the tunnel alignment, in the middle of the northern migmatite/minor schist and also in the northern section of the central stretch of schist, according to the tender information. For much of the remainder of the northern half of the alignment’s passage through migmatite/minor schist there are potentially rockburst conditions.

The head of groundwater over the tunnel is estimated to range from 2MPa to almost 19MPa near the top of the vertical alignment of the tunnel and around the stretch of maximum overburden, according to tender documents.

In terms of groundwater inflows, the anticipated general conditions range from dripping and wet in the schist near the south portal, though that rock type near the middle of the tunnel alignment is believed will be dry. The migmatite/fault zone in the south is expected to be dripping-to-wet, and in the northern half of the alignment range from dry to wet. In the fault and shattered zones there are high probabilities of sudden or high inflows.

The ground has only recently been opened with excavations underway at the south and north portals since September.

Concept and procurement
The Rohtang Pass tunnel is being built as a single bore with a boxed recess in the invert for emergency egress. The horseshoeshaped tube will be excavated to 12.1m wide and 13m high. It is to have a finished width of 10m to carry two lanes on the carriageway (8m wide) and 1m wide footpaths on either side. The headroom is to be 8.3m in the finished tunnel.

The road pavement slops at 0.5 per cent upwards from each portal.

An emergency escape box tunnel will run the full length of the tube, in the base below the carriageway and is to be 3.6m wide and 2.25m deep.

The design was undertaken by Australia’s Snowy Mountains Engineering Corporation (SMEC), its SMEC International unit working on the task and tender preparations in 2007.

Early procurement plans had envisaged contract award happening to allow excavation to start by October 2008 but the Rohtang tunnel and road construction job was not awarded to the Strabag-Afcons JV until September 2009 with a total contract period of just over six years. Construction work is envisaged to take about five and a half years, says Dywidag, which is executing the underground section – Lot 1 of the scheme.

Of the entire construction period, the tunnel excavation works are expected to take four years to complete by mid-2015. The contract bid value for the tunnel, including the emergency box in the invert and road pavement construction, is approximately EUR 250M (USD 345M).

Preparatory work was underway for the initial months and the contractor had to wait until a few months ago for the deep snow to melt sufficiently and allow access to the areas that will become the tunnel portals, and the main construction work to commence.

Additional works in the Lot 1 contract include construction of avalanche and rock protection galleries at the north and south portals.

The client is responsible for undertaking road works and maintenance of bridges on the main highway and connections and the roads on the project needed for access and site logistics.

Excavation
The Rohtang Pass tunnel is being bored by NATM. A tunnelling solution with a shielded TBM was dismissed, primarily because of the anticipated significant sections of squeezing rock with high deformations. The NATM approach offers greater flexibility in adjusting the excavation works and support systems, should that be necessary, says Dywidag.

In addition, there would be fewer transportation problems for drill and blast equipment on the terrain and winding roads compared to moving parts of a TBM, especially over the Pass to the north portal.

Drill and blast is to be done in rock classes 1-4 and in loosened or disturbed ground of classes 5, 6 and 7, or soft strata, the contractor would look to possibly using mechanical excavators.

Excavation and shotcreting equipment to be used:
• Atlas Copco WE3C boomers, L2D Boomers
• Liebherr L944T crawler excavators
• Liebherr L576T wheel loaders
• Meyco Potenza shotcreting units

During excavation, the ground is to be probed up to 100m ahead of the face. Then, in preparing for each advance of the face, the typical number of drill holes is to be 180, says Dywidag.

Stroke lengths are expected between 1.5m-3m, typically, but up to 4m for the best conditions. For rock classes 1, 2 and 3 the stroke lengths would reduce from 4m to 3m to 2m – and this latter advance would be used for the heading in Classes 3M, 4M, 5 and 6.

The shortest stroke lengths, 1.5m, would be in rock classes 4S and 7. Stroke lengths of 1m to 3.5m would be used where there are rockburst conditions, at classes 3M, 4M and 4S. In squeezing rock at classes 5, 6 and 7, stroke lengths of 1.5m-2m would be used.

Each face is to advance with two headings, two benches and an invert. The contractor plans on achieving a total advance rate of about 14m/day, split equally between the drives when both portals are in service.

Dywidag estimates that the daily advance rate for a drive could range from more than 8m down to approximately 2m over classes 1-7, with at least 6m/day foreseen as possible for classes 1-3.

As access to the north portal, at the other side of the pass will suffer from the same restrictions that have generated the project – inaccessibility for some months in long winters. However, the initial month or so of each winter could see tunnelling work continuing from the north portal using stored materials and achieving a high proportion of planned advance rates.

Primary support
The primary lining consists mainly of fibre-reinforced shotcrete and friction bolts of 4m to 10m lengths.

For headings and benches, shotcrete layers will vary in thickness from 50mm for rock class 1 and 100mm for classes 2, 3 and 3M to 150mm for classes 4M and 4S.

Rockbolts for those classes will be: 4m long at 2.3m grid for class 1; 6m long in a 1.9m grid for class 2; 8m long in a 1.5m grids for classes 3 and 3M; and, 10m long in a 1.5m grid for class 4M and a 1m grid for class 4S.

In rockbrust conditions the bolts would be fully grouted for 3.5m in classes 3M and 4M.

Unless otherwise directed, no primary support would be provided at invert level where the rock is classes 1-4.

Steel ribs or lattice girders will be used where there is a need for profile maintenance before shotcrete is applied, such as at sections of weaker or squeezing ground. The main support in such deep tunnel conditions remains the long rockbolts while the profile support allows for the NATM approach of controlled deformation and rock pressure reduction, the convergence being monitored and measured by 3-D point geometry at cross-sections.

If ground is weaker still, then support at or ahead of the face can be installed using spiling bars or tubes and grouted dowels.

In weak, difficult ground the supplementary measures could include improving stability by constructing a temporary invert slab in the top heading or provisional underlining arch support.

For rock class 5 the primary support of the heading is steel sets at 1.5m centres, 300m thick shotcrete and 32mm spiling bars at 0.45m centres. The benches and invert are to have 400mm slotted fibre-reinforced shotcrete support with 4m long, 25mm dia. grouted dowels in line with the steel sets.

In rock class 6, the contractor plans to have steel sets at 1.5m centres with 350mm thick shotcrete and 76mm spiling tubes at 0.3m centres. For the benches and invert, the support is to be 400mm shotcrete or as directed, underpinning of arch support as directed and 4m long, 25mm diameter grouted dowels.

The same thickness of shotcrete and array of spiling tubes is planned for the heading support in rock class 7 although the steel sets are to be more closely stepped, at 1m centres. The sets are to continue into the benches and invert, otherwise the support is as for class 6.

To achieve early ring closure, there is a consequent requirement for invert support to follow close behind the heading and spoil removal, which could potentially impede logistics of the upper level works. Therefore, the contractor has elected to draw upon the Alpine tunnelling experience of approaching the excavation as a complete linear construction site by using a suspended platform deck, which would allow the upper level works to keep pushing ahead.

The suspended deck is to be a 270m long, high-performance Rowa Hanging Platform that will have its rails suspended at existing rockbolts. The platform, some 50m behind the face, is key to achieving the construction period called for in the tender, which would not otherwise be feasible, says the contractor.

The tunnel drives will have blowing ventilation with Korfmann AL17-2000 and – 2500 main axial flow fans with a flat duct to the bench/invert areas, and smaller duct with Korfmann AL12-450 secondary axial flow fans to the headings.

More than 800,000m3 of rock is to be excavated during construction of the road tunnel. Spoil will be moved by conveyor to a mobile crusher behind the suspended platforms deck.

Final lining
The road tunnel is to have a secondary, final lining of 550mm thick cast insitu concrete, and which is partly reinforced. However, its construction follows that of the escape (“egress”) tunnel where ground conditions did not require an invert arch to be built.

The escape tunnel itself will be built after the drainage pipe. It will have a base slab that is cast insitu, and where there is no invert arch, such as support in rock classes 1-4, a binding layer will be first be laid. The concrete walls and roof slap of the escape tunnel are to be precast as single, U-shaped, 1.5m long sections. A crane below the suspended platform deck will be used to place the sections.

Construction of the escape tunnel, the refill and compacting around the box, and then secondary lining will follow a few hundred metres behind the deck.

For the inner arch lining, steel reinforcement will be installed to form kickers and a formwork traveller will be moved on rails to cast the concrete shell in 12m long sections.

For the smoke duct, the roof slab will be supported by roof corbels, which are constructed either with or after the inner lining arch, on the left and right side of the tunnel. Each section of roof slab is to be 12m long and will be installed by a dedicated formwork traveler.


Location of Rohtang Pass road tunnel in Himachal Pradesh, northwest India The 8.8km long Rohtang road tunnel will provide an all-weather bypass to the exposed road over the pass that is blocked for months each winter Rohtang Pass road tunnel is a single tube, two-lane bore with an escape box tunnel running along the invert Cast insitu concrete for the secondary lining and roof slab of Rohtang road tunnel will follow construction of the escape tunnel in the invert At the opposite side of the winter-blocked Rohtang Pass, the tunneling activities from the north portal will suffer restricted access