Note : This cover story was published in CSIR-NISCAIR’s prestigious magazine Science Reporter in June 2020.
The bridges are one of the greatest inventions of mankind. They connect civilisations. They connect two pieces of land. They connect the hearts and souls of people. They have travelled a long and successful journey. From a fallen tree dragged into position to serve as a plank, to forest tendrils intertwined as an elementary suspension bridge or rafts tied together in a pontoon, to the marvellous piece of architectures built today like India’s iconic Chenab bridge. Mankind has evolved the bridge building technology scientifically.
Today bridge building is like fighting a war up high near the sky or down in the valley. When the effort scales such heights we get the masterpiece. One such masterpiece is the Chenab Bridge project being built in Kashmir. You can call it an Engineering milestone, an iconic figure or a dynamic piece of architecture. It is the World’s highest rail bridge being built by Indian Railways. It is a matter of pride for India. Purpose of the bridge is to connect the heaven on earth, Kashmir, to rest of India. Work is in progress and is expected to complete by 2021. Kashmir Valley has also got gift of 200 km road under Chenab Bridge project connecting various interior villages to National Highways and main roads.
Workers are on a humungous mission at a place where geography is not very kind to them. They are busy building the next wonder of the world. This is like a war for them, where they are fighting with the weather conditions, chilled and snowy winters, the terrain, the height, the slope, the distance, the technicalities, etc. But they are standing tall and rigid changing the trend of the wind.
Result is the iconic structure of Chenab Bridge, which is a part of the 272 km railway route from Udhampur to Baramulla in Kashmir. It is also known as Udhampur-Srinagar-Baramulla Rail Link project (USBRL), constructing which comes with a package of significant engineering challenges. The project involves 200 km of access roads, 27 tunnels and 37 bridges.
Out of this 272 km railway route, 161 Km length is divided in two stretches. One stretch is of 136 Km from Banihal to Baramulla and second stretch is of 25 Km from Katra to Udhampur. This segment of the stretch has already been constructed & commissioned in different phases from year 2009 to 2014. The intervening portion of 111 km long from Katra to Banihal is more challenging being treacherous and rugged. The alignment crosses deep gorge of Chenab valley necessitating the construction of High Bridge on steep slope.
The bridge is a steel and concrete arch which consists of 2 parts. First part is the arch which helps in bridging the river. Second part of the bridge is Viaduct, an approach span having piers of Reinforced Cement Concrete (RCC). Here, to increase the tensile strength, mild steel bars are used in cement concrete thereby reinforcing it.
The arch is a large steel framework made of 24,370 tonnes of steel. The chords are sealed steel boxes which are internally stiffened and filled with concrete to assist in controlling wind-induced forces on the bridge. Another advantage of concrete filling is that internal painting will not be required. The number of bearings has been minimised, as it reduces the maintenance and inspection efforts, and improves the riding quality. Chief Engineer Mr. B S Tomar who is on the mission of Chenab Bridge Project in Kashmir shocked us by saying that in one of the foundation of main Arch, approximately 3.5 Lakh cement bags have been consumed. The design of major arch rail bridges requires considerations of a number of additional parameters, such as fatigue, global stability, second order effects, composite action, etc. The bridge has anti-terror features. The bridge is designed in such a way that even if one of the 17 piers if blown up, it would not lead to collapsing of the bridge.
Most stunning part of the whole structure is the level difference between Katra & Banihal. This difference is approximately 890 meters. One side of the bridge is higher than the other side, thus making a bridge parallel to ground was next to impossible. Then enters the idea of meeting this high level difference. The height of the bridge was supposed to be increased above all expected levels.
Principal Executive Director (Bridge), Railway Board Mr. A K Singhal explained that, “Off course engineers had the option to construct bridge across river Chenab at lower elevation. But in that case for meeting the level difference, the gradient between Chenab Bridge and Banihal station would have been much steeper than what was planned. This would have restricted the haulage capacity of moving trains. Also, due to steeper gradient, number of slip and catch sidings also would necessarily be constructed making the operations of trains quite tedious, risky and also at higher cost as compared to present condition. So, there was no option but to have a high bridge across Chenab River.”
The supreme idea of building such a masterpiece bridge brings into consideration the level of upgradation in the technology used. When started, there was no facility, no roads to travel, no place to relax. But, today approximately 200 km of road has been developed around the area, joining Jammu and Kashmir.
These types of special bridges are always technology intensive, which facilitates engineers in designing, planning and construction. To erect such a massive structure at such a height, using a cable crane was a project in itself.
Design of the bridge is unique as it would be arch shaped, standing tall on two legs far apart. These legs are actually the steel pillars on both sides of the river bank. The pylons on either side of the river are about 130 m and 100 m high and two auxiliary self-propelled cable cranes were used to tow temporary auxiliary ropes across these pylons. The ropes support the partly finished arch parts. After arch completion, the trusses will be added, finally the girder will be constructed as a horizontal sliding type platform.
The arch is being supported by piers and trusses. This type Arch Bridge is being constructed first time in Indian Railway. It is beyond doubt that technology has made the construction possible at such height by facilitating to insure the safety, strength and stability of the structure with the consultation of renowned national/ International consultants for designing and final execution.
The bridges are designed considering the topography, environmental and geotechnical aspects apart from various loads acting on the bridge components. Mountainous terrains, deep gorges, tall piers, unequal height of piers, poor soil conditions, floods in rivers, snowfalls, heavy rainfall, seasonal and diurnal temperature changes high wind and seismicity are some of the challenges with respect to design of bridges. Design loads are considered as per Indian Railway standards for Railway bridges and Indian Roads Congress Standards for highway bridges.
Overall depending upon the purpose of use i.e. Pedestrian, Road vehicular traffic or Railway Movement the loading characteristics are assumed and design is finalized. Again, based on life span of bridge, selection of material is decided i.e. Steel Bridge/PSC Bridge/Composite etc, which in turn again depends on local availability of material and cost factor. Geotechnical and environmental condition plays important role in Bridge design as it may be affected by Weak Soil/Winds/Earthquake/ corrosive environment etc. Further availability of machinery and experienced manpower together with funding of Bridge Cost, and life cycle analysis may lead to varying designs.
Mr. Parag Verma, Chief General Manager of Business development at IRCON International Limited, a specialised construction organisation providing services in the infrastructure sector with core competence areas of Railways and highways construction highlighted the above do’s and dont’s of designing in terms of such bridges.
Material plays an important role for ensuring technical capabilities to the Chenab Bridge. Such unique structure is built with the help of some unique material. These special material include :
- High quality steel of Grade E250C being used for fabrication of deck superstructure. The C grade steel has special toughness properties at subzero temperature making the steel enough ductile in chilly cold weather conditions. Z grade steel used in fabrication of Arch has special thickness properties preventing lamellar failure of steel plates.
- Double Corrosion protected Dywidag bars which is pre-stressed high tensile steel threaded bar used for slope stabilization having the ultimate Tensile strength.
- Cable Anchors are used for stabilization of more critical slopes such as below main Arch foundations.
- For steel to steel interfaces, multi metal grout with pressure has been used for ensuring the 100% contact in between, so as to prevent any point loading causing rupture etc.
- For maintenance, regular painting of large bridges is an intimidating task. Hence, a painting scheme was developed, having renewal of over 15 years, compared to approx. 5 to 7 years in most other Indian railway bridges.
By virtue of Chenab bridge’s location, terrain and uniqueness of structure, the desired shape, design and layout are on the verge of completion with application of specialised methodology for construction of Chenab Bridge.
Chief Engineer of Chenab Bridge Project Mr. B S Tomar who is on the mission in Kashmir, explained in detail the difference in construction methodology of this bridge from other bridges. “To start with, major challenge was the jointed, weathered and weak Himalayan rocks in the area. So to fill up the fissures between rock joints, consolidation grouting or hardening of the founding areas have been done for their strengthening before casting of foundations.”
Tomar added that two World’s longest cable crane of 915 m span were used for erection of components and segments of Arch, Piers & Trestles. Two Cable cranes of individual load carrying capacity of 20 MT & in combined capacity of 37 MT are used for erection of steel arch, pier & trestles. The cable cranes used for the project is of the length of 915 meter, which is the largest in the world. Quite interestingly, they are using Self Compacting concrete for filling in hollow steel ribs of main arch to increase weight and enhance lateral stability of structure against high wind speed loading. Incremental launching of plate girder of deck superstructure form, ends in curve of 2.74 degrees with transition having total curved length of 268 m done for the first time on Indian Railways.
More than 85% of Steel fabrication is carried out at site workshops with world class latest welding machines and methods such as customised automatic Twin arm SAW machine, Flux core Arc welding (FCAW) and NDT by Phased Array ultrasonic testing (PAUT) instead of conventional Radiography testing (RT). The steel used in the bridge is blast proof and Railways is also installing an online monitoring and warning system to ensure swift response if the need arises.
Chenab site lab has been accredited with National Accreditation Board for Laboratories (NABL) certification for testing of welded elements for first time on Indian Railways. First time continuous welded plate girder has been used on Railway Bridge. Bridge has been designed to bear earthquake forces of zone V, which is the highest intensity seismic zone in country. The height of steel pier at one of the main Arch foundation is 130.855 M, which is quite higher than Qutub Minar of 73m indicating the mammothness of the structure.
List of Challenges
A question comes in mind, Why is Chenab Bridge a Cynosure for all ? The answer lies in the Unique/Peculiar Construction Style of Chenab Bridge.
When the concept of developing World’s highest bridge was put forward, challenge word sounded small in front of this mega structure. The most difficult part of this railway project is crossing the deep gorge of the Chenab river located at the Salal Hydro Power Dam. Another challenge of the project is the large number of tunnels with total length of 63 km, as well as bridges of 7.5 km. These have to be constructed on the highly rugged and mountain terrain of the Himalayas.
Secondly, stabilisation of foundation and erection of the bridge were other major challenges in hand. For stabilising arch, it is needed to control the deflection by anchoring the arch by numbers of forestay and backstay cables at Nodal points. When wind speed exceeds 50 KMPH, the erection work has to be stopped from safety point of view. Every work at such great height, whether it is a permanent or temporary one is quite risky and needs to be done with utmost care and skill. The work at height is carried out after fixing wire net underside so as to arrest fall of any object or material etc.
Special workers having used to working at great heights such as thermal power plants, refineries & oil industry, ship building etc. have been hired for tedious & cumbersome erection work. These workers are tested daily for their critical medical parameters and only healthy workers are then allowed to work. During work (working with cable crane, Tightening of HSFG bolts, installing temporary platforms etc.), these workers have necessarily to wear safety jackets all time for preventing their fall and ensuring safety.
Some other challenges include availability of high strength materials like stay cables, equipments required for bridge erection, trained manpower for precision welding and painting etc. The challenges do not stop with design and construction but maintaining the bridges for a design life of more than 100 years is another major challenge.
Central Road Research Institute popularly known as CRRI, a constituent laboratory under Council for Scientific and Industrial Research has contributed to the design and proof checking of many bridges. With respect to Railway bridges, when asked about the latest technology used by CRRI in building bridges, CRRI Director Mr. Satish Chandra mentioned that in railway yards, a number of cable stayed bridges have been built, for example Krishnarajapuram bridges, Burdwan bridge and so on. Therefore, construction of cables stayed bridges is gaining momentum in both road and railway sector. Although erection of bridges over railway yards without stopping train traffic is a major challenge.
Apart from technology, skilled manpower is another constraint. CRRI Director Mr. Satish Chandra emphasised on periodical refresher courses and customized training programs under CSIR-integrated skill initiative to upgrade the skill of practicing engineers with latest advancements in bridge design and construction.
This skilled manpower is the need of the hour. For any structure as unique and as wide as Chenab bridge, only machines can’t work, rather machines and skilled labour complement each other. This is all about the technologically advanced machines and manpower. Chenab bridge will surely get space in historical documentation and would be remembered over centuries.
In the end, let’s also give tribute to the hundreds of bridges being built by local people in remote areas without any steel or crane. These people are not skilled but the challenges they faced like issues of travelling, schooling, etc. motivated them to invent their own techniques and build bridges with a difference. Because, where everything seem to stop, bridges make the way ahead. Bridges are fascinating architectural pathways, allowing us to move to unexplored places.