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[Dr. N. Subramanian]

Elevated Cast-in-situ Balanced Cantilever Bridge for Kochi Metro nearing completion

The construction of a semi-circular box-girder cantilever bridge, which is coming up between the Kadavanthra and South stations of Kochi Metro will be completed by First week of Nov.2018. This is the frst time ever that a complicated structure is being constructed on a Metro track in India.


Turning point:A view of the 90-metre-long balanced cantilever viaduct of the Kochi metro over railway tracks at the Ernakulam Junction railway station.H. VibhuH_Vibhu  

The 90-metre-long balanced cantilever span of the Kochi metro’s viaduct that hovers over rail tracks at the Ernakulam Junction railway station is set to become an iconic landmark in Kerala. The cantilever bridge is being constructed on the lines of lightweight, cost effective, and versatile form-traveller technique, which enables easy movement of construction materials on rails. Both the ends of the bridge will have 65 m extensions, to ensure the balance of the bridge. Thus, the total stretch of the bridge is 210 m. Total length is 471.6 m-Spans: 65m+90m+65m + 251.606m viaduct. Deck width is 9.24 m.

Structure of bridge across the tracks, between the pillars is undergoing construction. Though there are other metros in India that run on Balanced Cantilever structure, the one at Kochi stands out with its curved design, spanning over 152 meters. This bridge is constructed by SP Singla Constructions Pvt.Ltd (# 47, Sector – 9, Panchkula – 134113. Haryana. India).

The span towers over the ground at 16 metres, which is more than the height of a five-storey building. Tunnel-like box segments, which are about the height of a room, support the massive span whose weight is balanced by two 65-metre-long fixed spans on either side. Each segment is three metres long.

The total cost of the cantilever span and supporting girders, totalling 220 metres, is Rs. 58 crore.
Aquamarine-blue colour
The span will be painted aquamarine-blue (epoxy paint) that will enliven the structure for up to 20 years. The structure is ready, except for a three-metre portion at the centre of the span which will be readied in a week. It will be followed by laying of tracks and other equipment for the metro’s extension up to Thykoodam.

110-km-long cables

A total of 120 pre-stressed steel cables, which will be 110 km long if put together, link the cantilever span with the girders.

“It is more or less like a hanging bridge supported by steel cables—only that the cables are concealed here beneath huge chunks of concrete. Two tower cranes, each with a capacity of five and six tonnes and capable of reaching up to a distance of 50 metres, were deployed to put the segments in place.

The structure, whose work began in January 2016, took 34 months to complete. It was mainly due to the delay in acquiring land from the Southern Railway, and the delay in getting clearance from the Commissioner for Railway Safety.

Workers had to be doubly cautious since a few hundred passenger and goods trains criss-cross the rail lines beneath. The presence of 25-kV overhead electric lines too was considered. DMRC engineers were posted all through to ensure quality of work and work-site safety, sources said.

The precast girders beyond the 220-m structure, on the railway station side and beside Karshaka Road, will be in place by December. Unlike in the rest of the metro viaduct, the cantilever structure will have steel parapets on the side, flanked by polycarbonate sheets.

“Readying the lengthy span was the most complicated task along the metro-rail corridor. Both the ends had to meet at the centre of the alignment. The quality of high-strength concrete and pre-stressing was of utmost importance,” said DMRC sources

100 m span Balanced Cantilever bridge for Delhi Metro

It is interesting to note that the Delhi Metro has constructed a bridge with a 100 m long span at Okhla over the Indian Railways tracks on the Central Secretariat – Badarpur corridor.


The bridge is a major engineering landmark as it has been constructed without hampering the railway traffic below. Earlier, the Delhi Metro had constructed a bridge over the Indian Railways lines near Pragati Maidan, where the length of the main span was 93 m.

The DMRC had to use such a long span over the Northern Railways track as four Indian Railways lines are passing below the bridge. The bridge, which is located between the Okhla and Jasola Metro stations, has a total length of 250 m and is divided into three spans. While the middle span is of 100 m, the other two spans on the sides are of 75 m each.

• The Delhi Metro has employed the ‘Balanced Cantilever Construction methodology (BCLC)’ for its construction so that the rail traffic below the Metro corridor is not hampered. This technique employs the use of a bridge builder that constructs the bridge segment by segment at a time.
  
• This bridge has been built using M-60 grade concrete and about 130 workers were engaged in the construction of the bridge on a daily basis.
  
• The construction of this bridge was difficult since the bridge was built on a very sharp curve with a 300 m radius and the installation of the individual segments of the bridge on the curve was difficult as the rail traffic moving below could not be obstructed during the construction.
  
• The heavy works such as gantry movement were done under traffic blocks on railway tracks. Since the bridge balance was always in a see-saw condition, engineers had to maintain strict construction sequence. Additional temporary supports were designed for added safety.
  
• Engineers monitored the stresses and strains in these temporary supports as well as inclination and tilt in the super structure of the bridge at every stage of construction.
  
• Visual electronic indicators were also installed with the help of Japanese scientists and the Japan International Cooperation Agency. Japanese consultants on DMRC works interpreted all monitoring data regularly and kept designers in the loop. Strict survey control and monitoring could ensure a safe construction with perfect alignment.
  
• The existing railway tracks below this bridge link New Delhi with Agra, Mathura on Mumbai main line and then to other parts of the country and therefore is a very important railway stretch.

Read more at:
https://www.thehindu.com/todays-paper/tp-national/tp-kerala/metros-cantilever-span-getting-ready/article25365949.ece
http://www.delhimetrorail.com/press_reldetails.aspx?id=QEsaDPYJZWQlld