October 15, 2011 3:34:28 am
Work is under way at one of the critical sections of Versova-Andheri-Ghatkopar Metro route a cable-stayed bridge on which Metro trains will zip six metres over traffic on the congested Jog flyover
Well past midnight,when the growl of traffic on Western Express Highways Jog flyover in Andheri has mellowed,a group of engineers and nearly a hundred workers get ready for a truck-mounted concrete boom pump.
During the preceding hours,a giant custom-engineered steel frame has been cleaned and buffed,aligned precisely and screwed into place. Steel for reinforcement of the concrete has been placed along the bottom,as well as the steel that will be stressed to hold in place one more concrete segment. Finally,when the robotic arm of the concrete pump pours its material into the framework,one more tiny part of the financial capitals first Metro Rail corridor begins to take shape.
None of this is out of the ordinary where bridge building is concerned. Only,the team of engineers,the steel frame and the mouth of the concrete boom are all positioned 21 metres off the ground,the height of a seven-storey building,over one of Mumbais busiest traffic junctions.
They are building one of the Versova-Andheri-Ghatkopar routes most critical and challenging sections,a cable-stayed bridge on which Metro trains will zip six metres over traffic on the forever-congested Jog flyover. The bridge itself is barely 185 m long from the east end to west end of the Western Express Highway,a fraction of the 11.4-km route,but we are undertaking construction work over a live flyover,ensuring all the time that traffic below us is not disrupted,that no piece of equipment slips off,and that none of these constraints slows down construction, says a representative of Mumbai Metro One Pvt Ltd (MMOPL),the special purpose vehicle that will build and operate the route.
Those constraints have made it incumbent to use some nifty – and expensive – engineering techniques. Key to the construction is the form-traveller,the custom-built steel equipment that holds the concrete forms.
As there are no pylons or pillars in the middle of the highway,the bridge is a balanced cantilever construction,a structure that projects horizontally with support only on one end. The 50-tonne form traveller remains anchored to the previously cast segment,where it provides a rigid frame for the concrete that is poured at that height using a boom. Unlike the cable-stayed portion of the Bandra-Worli Sea Link and unlike the rest of the elevated Versova-Andheri-Ghatkopar route itself,this cable-stayed bridge does not use pre-cast concrete segments.
Concrete pouring is done only between midnight and 6 am; we have strict instructions regarding that, says senior engineer Sree Kumar,one of the supervising staff-members from SEW,civil works contractor for MMOPL. Traffic on the Jog flyover,busy even at 11 pm on most days,continues to run with minor barricading just below the section where the teams from SEW and cable-stay consultants VSL India Ltd are hard at work.
The form traveller system is provided by VSL,and is designed to not deflect under load. After all,the heaviest concrete segment will weigh 62 tonnes. Once the segment is cast and cured,the steel cables stressed,the form traveller inches forward on rails to allow the next segment to be cast.
Four form travellers are in use to speed up work from either end,their total worth Rs 5 crore.
With work on the stay cables also underway,each of the remaining segments will take about 17 days to be readied,cast,stressed and bolted into place.
Every alternate segment also has a vent for the stay-cables,made of Korean Kiswire,among the worlds most expensive. And to prepare the Kiswire for Mumbais yearlong mugginess,the already galvanised steel wires,not for the feeble-wristed with its high tensile strength,is coated with a viscous rustproof grease and then enclosed in a High Density Polyethylene (HDPE) duct,the brilliant blue casing thats set to become another Mumbai icon.
By 6 am,one set of exhausted SEW and VSL engineers get ready to leave,to be replaced by another shift. No more concrete will be arriving until midnight. Even transporting the concrete is done only late at night to ensure it stays at its prime temperature,with retardants to make sure it does not set in transit. The concrete is a self-compacted variety,including a cocktail of micro-silica and flyash to prevent its temperature from rising and causing the concrete to crack.
As day breaks,a fresh batch of MMOPL guards arrive,to help man traffic and prevent curious onlookers from attempting to climb the steep metal stairs to the mid-air construction site. Traffic on the flyover begins to pick up again. At a later stage,when work progresses to the centre of the Jog flyover,the flyover will be closed for traffic at night. Vehicles taller than 4.2 metres will be prevented from using the flyover during the day too. For now,though,its another segment built,the first of the stay cables installed,a small step in the construction of the corridor,still at least six months away from completion.
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