Saturday , 31 October 2020

Lalbaug Flyover Flying over nine signals in four minutes flat

Type of Superstructure: Precast Segmented Technology was used in the construction instead of box girders, slabs and segmented technology. Precast segments can be cast even when the construction of the substructure is on at the site (i.e. flyover). Thus, these 25m segments could be cast simultaneously at the yard at Wadala making it possible to exercise high control over the quality. The segments can be cast precisely as per geometry thus allowing greater dimensional accuracy. “This technology of segmented construction is like putting bread slices. The segments are cast at the yard, transported to the site, placed atop the superstructure, grouped together, threaded and locked. So, it becomes one single piece. It is like a wire running through the holes of the segments, holding them together and integrating them,” explains Naik. Segmented technology has been used for the Bandra Worli Sea Link too.

Vertical Clearance: MCGM and MMRDA decided to provide vertical clearance as 7.1m.

Span Length: Since the average length of the spans used in the flyover is 26m and the maximum, 36.1m, it was decided to design the superstructure as “simply supported with full length cable” and thereby avoid blisters. Such an approach makes post tension easier, faster and allows greater control.

Overhead launching of segments: The method of overhead launching girder was opted for instead of the under slung method to launch the segments. The under slung method occupies a nine metre wide road for launching while overhead launching with girder leaves the entire road free for traffic.

Width during alignment: Since two curves of +145m and -142m radii with two straight spans exist, while aligning not just the curves but also the spans were widened in order to provide more safety and driving comfort. IRC does not make it mandatory to do so though.

Pedestrian Subway: For people visiting Jijamata Gardens, a pedestrian subway is provided in the ramp.

No provision for public utility below the flyover: No public utility like parking has been proposed below the flyover as it leads to accidents, unhygienic conditions, sanitation problem and security risks. This also ensures that illegal elements, scrap dealers and encroachers are kept at bay.

The construction of the flyover was carried out through contractors – there is no concessionaire in the picture as it is a budgeted project.

Obstacles and Challenges

Many challenges and constraints were faced by the teams planning and executing the project. And quite a few were surprises.

The biggest challenges for the team lay under the ground. “Though the initial activity for construction is laying down the foundation, it cannot be done immediately after taking position of the site as it might lead to rupturing of some or the other utility,” says Naik. More than 15 utility agencies including storm water drains, water supply lines, BEST lines, old calendar lines, optical fibre cables, Tata lines, two-three kinds of lines of the BEST and Mahanagar gas lines are normally involved in such an exercise.

As there is no proper geological information system (GIS) with the BMC for providing this data, it is mostly done at random. No mapping of the utilities was available. Most of the lines are from the time of the British. Hence, the engineers had to establish the alignment of all these lines and then feed it into the data base. Getting the alignment of underground utilities in itself is a very big project. This made the task difficult and complex, and slowed down the pace of work as fast work could damage the utilities during excavation. Even with modern technology and non destructive methods, it is impossible to get an accurate diagram of the underwater lines as there are high chances of “judging some inches here and there”.

As a result, some utilities got ruptured a few times. One 1200mm diameter water mains supplying water to entire south Mumbai was accidentally ruptured a couple of times. This resulted in emergencies and BMC had to rush with its equipment to handle them. Though the civic body could handle them efficiently, man-hours were often lost as a result.

The challenges didn’t end here. Quite often, the pile cap and pile cap foundations had to be redesigned all over again to accommodate major utilities like water mains, claims a MMRDA official. Each of the 79 foundations was tailor-made to suit the conditions. That makes each foundation unique. Also, it was clear that the water mains would not be accessible for repairs and maintenance once the flyover was made. Hence, they were provided permanent protection by concrete encasing in approaches and near pile caps.

There was another challenge the team faced. It found during the construction of the South end ramp that a municipal arch drain existed below the alignment of the South bound RE wall. It was not possible to divert it. So, another arch shaped slab was cast “well above the drain to transfer the load from RE panels to the base on to the sides of the drain.”

The construction of the superstructure at sharp curves and two reverse curves posed another major problem. Experts opine that it is easy to construct a flyover in a straight portion at an interline but constructing sharp curves with huge segments weighing 80 tonnes and having a width of 17.2m in a heavily congested area is a big task. The team had to pull all its expertise in handling this difficult situation.

The 74m long Launching Girder (LG) rests on three supports. The spans are 26m long. Auto launching in a curved alignment is complex as it involves shifting of supports in transverse direction. The rule is that supports should never be allowed to position beyond line of webs of the span as it may endanger the entire machinery and put the span at risk. Great care had to be taken during auto launching that involved many cycles of incremental longitudinal movement of the girder and transverse relocation of the supports.

But despite “great care”, a mishap did take place. A portion of the flyover collapsed at Lalbaug during the time of construction (November 2010), injuring three persons and damaging four vehicles. This happened when the driver of a crane which was being used to auto launch a LG (weighing 320 tonnes) at span P72-P73 on the sharp curve of radius 218m on the flyover lost control and hit two pillars. The rear trolley slipped on to the cantilever portion of the span and damaged three segments. A concrete beam fell on a taxi, a motorcycle and two cars. Such accidents too were big challenges as the project involved lifting heavy segments.

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