There is no cause for concern over the safety of pillars supporting the Kochi metro viaduct through the city’s slushy terrain, though the basement of a high-rise building under construction near the viaduct collapsed at Kaloor and the road near it developed a slant and unevenness on Friday, E. Sreedharan, principal advisor to DMRC, has said.
The 85-year-old Metro Man who visited Kochi to oversee the metro work, spoke to The Hindu on how the metro agency was not at all worried about the structural safety of the viaduct which has been built to last 100 years.
The metro corridor from Aluva is set to be extended up to Thripunithura, covering 26 km, passing through a city which has a few dozen unstable buildings. Many of them precariously slant to one side owing to adverse soil conditions or shoddy construction.
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Thus, constructing the 18-km Aluva-Maharaja’s College viaduct using around 4.50 lakh cubic metres of concrete and massive amounts of steel, while also ensuring safety and durability, is by no means a simple feat. Mr. Sreedharan said all pillars were designed with a factor of safety of ‘three’, meaning they can support three times the design load. “Their design is to safely support two fully-loaded trains [each with 900 commuters] and operating parallel to each other at the top speed of 90 km per hour,” he added.
On the specifications adopted for piling for the Kochi metro, Mr. Sreedharan said each pillar [pier] stood atop four piles. Each pile has a diameter of one metre and is as deep as 40 to 45 metres in places like Kaloor which are slushy and have a soil comprising a mix of marine clay and sand. In such terrain, a steel pipe lining up to a depth of around 8 m is given, so that the piles do not collapse. The diameter of each pile at stations [which are cantilevered structures] is 1.50 m. Thus, the foundation will be stable even if there is a crack on the road surface. In non-slushy areas like Muttom, piles are anchored atop rocks, the Metro Man said.
The extra depth for piling often caused nominal delay in completing civil work at some construction sites. The metro agency’s Kochi project director Dani Thomas spoke of how Kochi’s narrow roads posed a major challenge to completing civil work in a time-bound manner. “We overcame this challenge by substituting U-girders with I-girders, especially in the corridor from Edappally to the city hub. This is because launching pre-cast U-girders needs more space than is needed to launch I-girders. Cranes with up to 350-tonne capacity are needed to launch U-girders, whereas 150-tonne ones are enough for I-girders,” Mr. Thomas said.
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Even after launch, I-girders have to be topped with a deck slab casting, a parapet erect, and stitch concrete, making it time consuming and labour intensive. As for piling, reliance was on hydraulic rigs which came at a higher rent than tripod rigs, but which saved considerable time and subsequently less traffic disruption, he added.