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Construction : Focus | April 2016 | Source : Infrastructure Today

Tunnelling into the future

How can tunnels be built underneath layers of hard rock or in ground sometimes as soft as toothpaste? What is underground already that might get in the way?

Those gargantuan machines painstakingly threading their way beneath hills, roadways, mountainous terrain and beneath the ground in many of India's cities over the last few years have accomplished much. The underground is home to a tangled tapestry of tunnels, sewers, foundations, power lines, abandoned stations, solid rock and loose strata. All this makes it even more remarkable that engineers have managed to construct a number of landmark and strategic tunnel projects. The country's longest rail tunnel, the 11-km Pir Panjal Tunnel in Jammu & Kashmir became operational in 2013. India's (and south-east Asia's) longest road tunnel, the 9-km Chenani-Nashri Tunnel, is also nearing completion. Moreover, this will be the first tunnel in India to be equipped with an integrated tunnel control system through which ventilation, fire control and signals will be automatically actuated. The Road Transport and Highways Ministry is also expected to invite bids for the Zojila pass road soon. At 14.08 km in length, this tunnel will surpass even the aforementioned Chennai-Nashri to become the longest road tunnel in south-Asia when completed. Zojila pass is situated at an altitude of 11,578-ft on the Srinagar-Kargil-Leh National Highway which remains closed during winters (December to April) due to heavy snowfall and avalanches, cutting off the Leh-Ladakh region from Kashmir. At an altitude of 10,000 feet, the 8.8 km Rohtang tunnel is still under construction. (Incidentally, this tunnel will also be surpassed by the Zojila pass tunnel). The difficulties of construction in mountainous terrain with heavy snowfall, heavy ingress of water, avalanches, flash floods and other difficulties have delayed the Rohtang project. However, the challenging portion of the tunnel has reportedly been dug and work is now progressing at a faster pace.

The world's longest tunnel boring machine (TBM)-driven tunnel without intermediate access (adits) û the 43.5 km Alimineti Madhava Reddy (AMR) water tunnel û is under construction in the state of Andhra Pradesh. This tunnel will transfer flood-water from the Krishna river to the arid regions of Andhra Pradesh, providing irrigation to 400,000 acres of farmland and clean, drinking water to 516 villages. TBM manufacturer Robbins Tunneling and Trenchless Technology (I) has currently completed boring over 25 km of this 10-m diameter and 43.5 km-long tunnel. Boring through with two TBMs on either side, the company has achieved progress rates of over 500 m a month on this project.

However, the company's best progress rate was during the construction of the Mumbai water tunnel. Driven out of a 109 m deep shaft, at 8.3 km long and 6.25 m diameter, the company achieved a rate of 870 m in a single month, using a hard rock main beam machine. This remains the record for all TBM tunnelling in India. TBMs have been successfully used for the Kishanganga Hydro Power Project in Jammu & Kashmir for the construction of a 14.6 km long tunnel with an impressive monthly tunnelling progress of 816 metres. Delhi Metro Rail Corporation (DMRC) used the New Austrian Tunnelling Method (NATM) technology, also known as sequential excavation method (SEM) for the first time to construct a tunnel in Phase III of the Delhi metro project.

Tunnel Boring Machines (TBM)
Nonetheless, the history of using TBMs in India until the current times has been rather limited and not a successful one at all. The TBM drives for the Dulhasti and Parbati hydro schemes remain painful memories. It is only recently that tunnelling technologies have begun to move forward.

Today, the action is all with respect to the metro rail projects. Urban congestion has become a serious problem in many cities in India so the creation of underground space, and in particular, the development of underground transport is environmentally essential. Recognising this, metro projects were given a boost in May last year by the Urban Development Ministry which proposed to implement metro rail systems in 50 cities. This was approved by the Prime Minister. Therefore, in addition to the 12-odd existing metro systems, metro rail projects have been proposed, are in the planning stage or are already ongoing in several cities across the country. With the increasing numbers of projects, tunnelling methods have also evolved. Especially for metro projects, worldwide, the vast majority of projects are constructed using TBMs. In some areas, where metro stations are deeper underground, the drill and blast method may be used. However, because of the problem with human response to blast vibration, this is becoming a smaller percentage of tunnelling methodology.

The Indian market today is second only to China in usage of TBMs across projects, whether metro rail, hydro power or water transfer projects. In fact, projects have also started using crossover machines, such as in the ongoing 10-m diametre Sleemanabad Carrier Canal project in the state of Madhya Pradesh. James Clark, Project Manager, Robbins India, says, 'A crossover could be a mix of EPB and hard rock. If we expect more soft ground, the emphasis would be on the EPB capabilities of the TBM. If we expect more hard rock, the emphasis on the design of the machine would be on the hard rock capabilities. This way, we have one machine that can operate in different modes depending on the conditions.'

The general rule of thumb, Clark adds, is for TBMs to be deployed when a tunnel is longer than three km. This is because TBMs are usually custom-made for each project, and the mobilisation for each TBM is usually around 12-14 months. In that time, the drill and blast method can complete about half the tunnel length over a three km stretch at progress rates of 100-150 m a month. 'When you factor in the cost of the TBM, it becomes a lot more expensive than drill and blast,' says Clark. Tunnelling activity in India is bound to gather more momentum as urban metro systems, water and waste-water projects as part of the Smart Cities mission and a host of other government programmes provide a fillip to tunnelling activity. Renewable energy has also become important and the ruling government is aiming to include hydro power as well within its ambit. With increased experience on such projects and new technologies, new hydro projects can be designed and completed in two-three years, even those requiring excavation up to 50 km by TBM method. However, the method needs precise planning. The ground the TBM passes through has to be meticulously probed and mapped with advanced laser guiding systems used to help ensure the machine stays on track. A high level of preparedness is necessary since in case of unforeseen and unfavourable conditions, the adverse effects of TBM in terms of time and cost are far greater than conventional methods, putting huge investments at risk. In the past, most projects have faced delays due to long-standing issues. These pertain to geological surprises, weak risk identification and assessment, contractual disputes, old contracting practices, outdated tunnel design practices and construction methods, and inadequate safety mechanisms.

Contractors need to begin using advanced technologies and methodologies to enhance productivity and ensure timely delivery of projects. In addition, deployment of modern systems for monitoring, lighting, ventilation, safety and security also needs to be explored.

CONCLUSION
While excavations using TBM have been successful in the case of Delhi Metro, AMR Water Project and Mumbai's water tunnel project and Malabar Hill tunnel, to name a few, the Himalayas remain a major challenge. Experience suggests once can overcome problems with sufficient and advance information. Often, time and cost constraints have compromised a detailed investigation at the planning stages resulting in delays in the project during the execution stage. No doubt it is challenging, especially in projects with a high overburden, with the mountains seemingly soaring right up to the sky above the tunnel alignment. However, insufficient geological information in the DPR and in the geological baseline report effectively amounts to handing the risk over to the contractor. Hence, it is essential detailed exploration is done not only before the start of a project but also continuously ahead of the tunnel face. Probe holes of at least 75 - 100 mm diameters with perforated pipes at the crown level ahead of a tunnel face and the use of geophysical exploration techniques could be used to ascertain the presence of groundwater. Advanced geophysical methods like tomographic analysis and radar, though expensive, can be cost-effective in the longer run, particularly in the Himalayas. Additionally, crossover machines can begin to be a good solution here to deal with both hard rock and the ingress of water in the snowy regions. Last but not least, the issue of safety has also begun to be taken seriously with tender documents mandating safety consultants as part of the project team. While projects in the remoter areas are probably yet to catch up in this regard, the situation is vastly improved from over a decade ago.

Perhaps the last word should go to Clark, the expatriate project manager who has been in India for the last decade. 'India's improved by leaps and bounds since I've been here,' he says, 'and in all aspects. When I first arrived, the Indian tunnelling industry was just starting to adopt TBMs and the knowledge available in the country was pretty restricted due to lack of experience. Now, there's a huge supply of experienced local guys.

A good example is the fact that the expatriate contingency on projects is getting smaller every year. For instance, on our AMR project, we have almost 100 technical guys and less than 10 per cent of that workforce is expats. At the beginning of this project, five or six years ago, we had close to 20 per cent.ö

Chenani-Nashri tunel fact file
It will be the first tunnel in India that will be equipped with an integrated tunnel control system through which ventilation, fire control and signals will be automatically actuated.
It will save fuel to the tune of `27 lakh per day between Chenani and Nashri.
The tunnel will provide safe and all-weather route to commuters.
There is also a parallel escape tunnel (6 m diameter) for evacuation of commuters in case of eventualities.
The two tubes (main and escape tunnels) are internally connected through 29 cross-passages, each after a gap of 300 m.

Did you know?
Notable

FredhSlls road tunnel was opened in 1966, in Stockholm, Sweden, and the New Elbe road tunnel was opened in 1975 in Hamburg, Germany. Both tunnels handle around 150,000 vehicles a day, making them two of the most trafficked tunnels in the world.
Eiksund Tunnel on national road Rv 653 in Norway is the world's deepest sub-sea road tunnel, measuring 7.776 km long. The deepest point is at -287 m below sea level. It opened in February 2008.
Chicago Deep Tunnel Project is a network of 175 km of tunnels meant to reduce flooding in the Chicago area. Begun in the mid-1970s, the project is due to be completed in 2019.
The La Linea Tunnel in Colombia, will be (2016) the longest (8.58 km) mountain tunnel in South America. It crosses beneath a mountain at 2,500 m above sea level with six traffic lanes and a parallel emergency tunnel. The tunnel is subject to serious groundwater pressure.
Source: thefreedictionary.com

Did you know?
Longest

The Thirlmere Aqueduct in North West England, United Kingdom, is the longest tunnel, of any type, in the world at 154 km.
The Gotthard Base Tunnel will be the longest rail tunnel in the world at 57 km. It will be totally completed in 2017.
The Seikan Tunnel in Japan is the longest undersea rail tunnel in the world at 53.9 km, of which 23.3 km is under the sea.
The Channel Tunnel between France and the United Kingdom under the English Channel is the second-longest, with a total length of 50 km, of which 39 km is under the sea.
The Ltschberg Base Tunnel opened in June 2007 in Switzerland and was the longest land rail tunnel, with a total of 34.5 km.
The Lµrdal Tunnel in Norway from Lµrdal to Aurland is the world's longest road tunnel, intended for cars and similar vehicles, at 24.5 km.
The Zhongnanshan Tunnel in People's Republic of China opened in January 2007 and is the world's second longest highway tunnel and the longest road tunnel in Asia, at 18 km.

Source: thefreedictionary.com

 
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