In early August 2004, the Indian Railways embarked on one of its most ambitious projects. The project, declared a national project, was the construction of a new railway line from the town of Udhampur to Baramulla district in Jammu, India. The project involves constructing a number of tunnels and bridges, culminating in the construction of a new steel arch bridge over the deep gorge of the river Chenab. Once completed, the Chenab Bridge, at a height of 360m above the river bed, would be the world’s highest railway bridge, a title currently held by the Biepanjiang Bridge in Guizhou province of China.
The undertaking comprises of a multifaceted group of scientists and researchers from around the country. Experts from institutes like IISc, Bangalore, IIT Delhi, National Institute of Rock Mechanics, Research Design Standards Organization of the Indian Railways as well as experts from outside the country have been roped in for the project.
The bridge stands over the river Chenab between Bakkal and Kauri villages in the Reasi district in Jammu and Kashmir. The region is deemed as a Seismic Zone 5, an area with a high threat of dangerous earthquakes. Apart from the earthquakes, the bridge also needs to endure wind speeds going up to 260kmph, the force of a cyclonic storm.
Prof. Madhavi Latha, an Associate Professor at the Department of Civil Engineering and working in the field of Earthquake Geotechnical Engineering at IISc, has spent the last decade working on the project as a geotechnical consultant. “My role in this project is to assess the stability of slopes under various critical conditions of earthquakes and design rock anchor systems to make the slopes resistant to earthquake forces” she explains.
The 1315m long bridge rests on the slopes of the two mountains on either side of the river. “Selecting the slope profile was a big task. Many profiles were considered and final profile was selected based on the stability as well as optimization of natural rock excavation required” Prof. Madhavi said.
The slopes themselves, comprised of heavily jointed rocks, could pose a threat during earthquakes. According to Prof. Madhavi “These joints pose various types of failures including formation of rock wedges which can slip off during earthquakes, rock falls and toppling and rock slides. It was a challenge to understand the possible failures and to develop stabilizing mechanisms. Stabilization is being done using rock bolts, which pass through few meters into the slope and stitch the joints together along with improving the load capacity of rock”. The slopes are also under constant surveillance. A GPS station constantly monitors coordinates of all the points on the slope to avert disaster.
The steel arch portion of the bridge is 465m long, making it also the largest single span railway arch bridge in the world. The single steel arch design was chosen not only for its practicality but also the aesthetic merits it added. “Erection of arch elements is a big challenge because of the height of the arch and because there is no ground support for the erection. For erecting the arch, world's largest cable crane is commissioned at the site” she said.
The arch rests on steel piers going as high as 120m.”Foundations for bridge piers are huge and are resting on the slopes. Hence the load bearing capacity of rock below these foundations should be improved substantially. This is done by pumping cement slurry or grout into the jointed rock below the foundations” Prof. Madhavi explained.
In her lab at IISc, which houses some of the most modern earthquake simulators in the country, Prof. Madhavi and her team analyzed the slopes for stability under various earthquake scenarios. The team also designed the bolts used to stitch the joints for better slope stability.
Prof. Madhavi expressed excitement when saying “The bridge construction is in full swing now. One of the slope profiles is complete and the foundations are being constructed. The other side slope profile is being excavated”. Hopefully by the end of 2016, the country will be in the record books once again.
About the expert
Gali Madhavi Latha is an Associate Professor at the Department of Civil Engineering and works in the field of Earthquake Geotechnical Engineering at IISc.