It would take a crystal ball, according to Peter Reichenspurner, Strabag technical area manager, to see when the ambitious Rohtang Pass project will end now. Poor rock, harsh winter conditions and even communist union activities have caused severe time delays to the Himalayan project.
The 8.8km single bore road tunnel is located in the north of India, in the Jammu & Kashmir region, at over 3,000m above sea level. When complete it will be the longest tunnel above an altitude of 2,500m in the world.
"We are very much behind schedule," says Reichenspurner. "We are 2,100m in from the south portal, and 1,400m in from the north. The bid was submitted in January 2008, with the letter of acceptance given on 24 September 2009. Then we waited for the winter to pass for preparatory works to begin in the spring of 2010. Excavation began in early autumn 2010 and a total 63 month contract duration was specifi ed. Completion was to be in 2015."
Harsh conditions
Local support for the project is not lacking. The Rohtang Pass is closed for the long Himalayan winter periods, sometimes up to half a year. The Leh-Manali Highway is the only access to the communities north of the pass, and at 4,000m above sea level. When it is cut off, so are the communities. Project client, and military pioneers the Border Roads Organisation (BRO) is responsible for keeping the route clear, when it can, but the diffi culty of this task led to the ‘all-weather’ tunnel solution. The project is the fi rst major infrastructure scheme commissioned by BRO, which has traditionally focused on bridge building/repair, maintaining the roads, and abutments.
This can’t be ignored for the tunnel sites. "On the south side we have a responsibility for maintaining our access road, unless a bridge is damaged," says Reichenspurner. "The road branches off from the main public highway, and runs for around 10 to 11km to the southern portal site. There is no helicopter access to site, and it is impossible for us as the contractor to maintain the remaining 60-100km of public road. That is the client’s task, as works will not proceed without clear access to a hospital.
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By GlobalData"On the north side, there is no access road in the contract and it is all on the client. We have had nothing to complain about – a bridge was knocked out at one point, and the client rebuilt it in two weeks, it was incredibly efficient."
The short straw
The sites are very different. The south portal works are near Manali, which is actually a popular Indian tourist spot, so conditions are not too bad for the workers. The north side is an austere contrast. "There is nothing there," says Reichenspurner. "You have to get guys in who like the mountains and won’t have a psychological breakdown. It’s a long way [c.2,000km] from Mumbai to either site."
While continuous excavation throughout the year from the south was the plan, the north is necessarily shut down during the winter snows.
The winters have been more severe than the teams imagined. In the fi rst chill, the north was shut down from early December 2010 through to July 2011. And on the south side, the contract expectation was for maybe 10 days suspended works per year. In reality, between 30 and 40 have been experienced in a season. According to Reichenspurner, there has been too much snow to get trucks up the road, and there is just too much road to realistically clear.
The Rohtang Pass tunnel is being bored by NATM. A tunnelling solution with a shielded TBM was dismissed, primarily because of the anticipated signifi cant sections of squeezing rock with high deformations. The NATM approach offered greater fi exibility in adjusting the excavation works and support systems, according to Strabag, as has been necessary.
During excavation, the ground is probed up to 100m ahead of the face. Then, in preparing for each advance of the face, the typical number of drill holes is 180. Stroke lengths are between 1.5m and 3m typically, but up to 4m for the best conditions. For rock classes one, two and three, 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, fi ve and six.
Each face was to advance with two headings, two benches and an invert. The contractor planned on achieving a total advance rate of about 14m/day, split equally between the drives. The lowest daily advance rate for either drive was expected to be 2m, not below 1m as is currently being achieved at the southern portal.
The TBM route was mooted for the Rohtang project. "It was more than just a discussion," says Reichenspurner, "it was an option. It would have been of the order of 11.5m diameter. We had a serious look at this idea, but if you are going for a 9km drive, you need a certain degree of continuity in the rock. We have had rock classes one to seven so far, with some squeezing rock too. There’s a 250 to 300mm maximum convergence before TBMs get stuck.
This, and more, happens every year in the Himalayas. Which is about what we are experiencing.
"The other thing to consider is the machine transportation. The main bearing would be approximately 70t for example. You face the challenge of getting this all the way from the port in Mumbai, over bridges with a 36t capacity, to the worksite."
As far as the JV knows, BRO received no tenders with a TBM solution. "And we were right," adds Reichenspurner, "the NATM solution was the right way to go."
Rocky progress
The quality of the rock from the southern drive has been atrocious. "It is extremely deteriorated rock," says Reichenspurner. "It is like mud or wet concrete and not at all stable. It can’t be considered rock; you could extract it by hand if you wanted to. We haven’t even used explosives on the southern drive since January, though the northern side is continuing with regular drill and blast work. Some poor ground was expected, but not to this extent.
"You can only really guess at the geology from aerial photos in this mountainous situation. But the best guess of the client’s geologists now is that this poor ground will continue for another 500 to 600m. And with us down to less than 1m per day on this [8,800m] drive, the time repercussion should be clear to see."
The ground is not the only concern. At the current 300m of overburden at the face, ice cold surface water is seeping down to the excavation. "It’s like an underground river, not concentrated to a single location but coming from everywhere. Approximately 30 to 40 litres per second ingress has to be pumped out. If the water takes out even small particles of ground, we get a loss of stability."
To manage the conditions as much as is possible, the team is taking small sections of the face out at a time then shotcreting it back up. Other measures such as a double pipe roof and chemical injections, which were not originally specifi ed, have been employed. These changes of design are the domain of the designer, Australia-based Snowy Mountains Engineering Corporation (SMEC), which was contracted separately to the construction side of the job. SMEC declined the opportunity to comment.
"We’ve also put in an almost circular invert, just to stop the tunnel collapsing," adds Reichenspurner.
Red mist
Further delays to the project have been caused by ‘industrial’ activity. Although the local support for this project is very high – the Rohtang Pass Tunnel has been on the table as an idea for 20 years – there has been resistance from other areas, says Reichenspurner. "We have at times had extreme union problems. These are not even really over pay and working conditions for local labour, which is good on site, it is a political movement."
The problem group in question is a Maoist Union, which is attempting to fi ex its muscles and show it holds some power over the project. The client, as a government military organisation, is understandably reluctant to get involved in political matters, though this complicates things for the contracting JV, which is not ideally placed to deal with the unrest. Reichenspurner says that two to three weeks have been lost in the last year to strike action. The impact is severe, as the site itself does not have to be targeted – the thin lifeline that is the public highway merely needs to be blocked for the site to be shut down.
Although such unions were illegal at the start of the project, they have since been legalised.
Moving forward
Current ground conditions and the unknown situation in the drive ahead are the biggest challenges to be faced. If conditions in the south suggest further extensive delays, then the response will be to accelerate progress from the north in the sound rock. It might be possible to store supplies in the completed excavation to allow the team to "move forward as safely and as quickly as possible".
Reichenspurner concludes, "It’s been an extreme project for all of us; engineers, management, labourers, the client. We hope it gets better soon and we can get into good rock. It’s a very ambitious and incredible project"
Lining intentions
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.
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.
