The making of Dhola-Sadiya Bridge across the River Brahmaputra was a different task altogether, considering the location and an unpredictable flood situation. This mega structure was constructed by Navayuga Engineering Company Ltd. Dr Ir PV Chandramohan, President (Technical), shares his experience and challenges faced during the construction of this bridge.
Importance of the bridge
The Dhola-Sadiya bridge is located at the North-east corner of India. Geographically, the location is far away from the other parts of the country. Though this is supposed to be in Assam, it is far away from other parts of Assam, for e.g., Guwahati. The area is often referred to as upper Assam or Northern Assam. The nearest airport is Dibrugarh. From the airport, one has to travel 65 km by road to reach Dhola. Beyond the bridge site is Arunachal Pradesh and then China. This bridge will accomplish the much needed connection of the area beyond Dhola and Sadiya to the rest of the country.
Let us now focus on the site of the bridge across the river Lohit. This river originates from China. We have already built a crossing of 2.1 km in the upper part of this river in Arunachal Pradesh. There are two other river systems - Dibang and Siang - that also originate from China. A river system is the one that flows in a number of channels with high velocity flow but shallow depths.
A number of bridges are required to be put up to cross one river system. Lohit, Dibang and Siang join together downstream of the present Dhola-Sadiya crossing. Then the river gets its name Brahmaputra.
Navayuga Engineering Company Ltd is presently building five bridges across the Dibang river system. Foundations are finished and superstructure work is in progress. In arithmetical terms, the actual river crossing was only 4,000 m wide, but the bridge was 9,150 m because there are vast flood plains. The bridge has made an attempt to cover the whole flood plains but there are breeches during floods which cut the embankments.
This was a project based on deferred annuity basis. So design of the structure had to be done by the concessionaire. Without design, one will not be able to start on planning and construction operations. At the conceptual stage itself, we were aware of the constraints of the site and the environmental parameters. We were crossing a river in the process of installation of a bridge. And the river was prone to flash flooding. So, execution of the project should be independent of the water body. If we depend on the water levels and currents, we could be bogged down by the vagaries of nature. Design and construction took this factor into account.
This was a long bridge with a stiff time frame against flooding. It was decided that the foundation will be of piles that could be installed fast. Besides, the older method of doing the work of finishing the foundation piling and substructure, and thereafter continuing with the superstructure was ruled out. For faster completion, foundation work and superstructure should proceed simultaneously. So, it was decided to have segmental girders for superstructure. Casting of the segments will start simultaneously with piling for foundation. Once the substructure comes to the bearing level, segments already cast will be launched into position.
As far as regular construction is concerned, let us start with the foundation. First construction operation is piling. Of course, simultaneously, the casting yard will be busy with casting of segments. An important aspect of construction is design, without which, a structure cannot be constructed. So, designs are to be completed first. For the sake of piling, test piles are to be installed and load tested. Only then, regular piling could start. Hydraulic rotary piling rigs were mobilised for this bridge.
Batching plant was set up for weigh batching of concrete. In fact, the site will become a small self-contained township with men, material and equipment. As mentioned earlier, a substantial portion of the alignment is on dry land that would be submerged during floods. Piling in this reach could be straight away carried out in a routine manner. But due to the magnitude of the work, procurement and stock piling of material was to be attended to in that scale. Boring in the soil in land was routine. Requirement of drilling mud was also massive and this quantity had to be transported from far-away places. Steel and cement also had to be transported long distances.
Fabrication of reinforcing cage for piles had to be done on a factory set-up. Batching plant for the massive amount of concrete was set up in the beginning. This was set up near the casting yard for the segments to reduce lead. Since the piles would be along the alignment, the distances were high. Batching plant could not be moved. So, transit mixers were used to carry concrete to the pile boreholes.
But when the piling proceeds into waters, suitable arrangements had to be made. The waters were wide and shallow. Piling in water is a difficult proposition. The idea is not to depend on the water body for execution of piles. For a jetty, piling would have been carried out from a gantry by end-on method, without having to touch waters. But bridge foundations were 50 m apart and so end-on method was not feasible. Other option was to use either a jack-up platform so that piling could be done from above the water level. But possibility of flash floods would have interfered with the operation of a jack-up. Doing piling from a floating pontoon was ruled out due to unexpected floods. The methodology adopted was to form an embankment in the alignment. Piling rigs and cranes were transferred to this embankment. Boring, cage lowering and concreting were done on this filled up portion. Piling could be done up to the far end of this embankment. The embankment was lengthened by filling up and piling continued.
After completion of piling, the embankment is dismantled. There was always the impending danger of flash floods. Calculated risks were taken for the successful completion of the project. Once the piling rig moved out into filled up embankments, there was no turning back. Filling up of river portions were done by excavators.
Simultaneously with piling, casting of the segments would also progress. The casting yard is a big outdoor factory turning out segments round-the-clock. The yard is spanned by a number of electric overhead cranes. A number of moulds are required for the casting. Long line casting was done for segments. The moulds were moved by EOT cranes in the yard and the stacking area. The heavy segments were carried on to the point of launching by special hydraulic trailers.
Over water, the segments were launched using a launching girder. This equipment is specially fabricated for installation of segments. But in the flood plains where there is no water for long durations, launching could be done using a ground gantry. These are specially designed and fabricated supports on which segments are packed before post tensioning.
But this method is slow and requires removal of the ground gantry from one span and re-erection of the same in the next span. In our case, the launching gantry was not ready initially. So, work was commenced using the ground gantry. Once the launching gantry was ready, things moved faster.
Major equipment used
Main equipment for the launching of the segments over the waterway is the launching girder. This has a length of 120 m. This has to carry all the segments at a span of 50 m at a time and has to be very strong. The ground gantry consists of supports to the segments from the ground. The supports were specially designed and fabricated.
Batching plant was used for weigh batching of concrete. Segments to be placed over the ground gantry were moved by electric overhead cranes. Segments from the casting yard were moved by hydraulic trailers. Specially fabricated moulds were used for the long line casting of the segments at the casting yard. Electric overhead cranes were used for handling the moulds and segments in the casting yard. Piling rigs were used for making pile bore. These were ready-made and purchased. Mobile cranes were used for lifting and placing the reinforcement cages for piles. Excavators were used for formation and removal of the embankment in the river for piling. Specially fabricated moulds were used for casting of piers.
Major equipment in this project are the launching girder and the piling rig. Due to the initial delay in commissioning the launching girder, part of the installation of segments had to be carried out by ground gantry. This had upset the programme. This also required the ground gantry components to be mobilised additionally. Piling rig is another critical equipment for this work. But this is a bought-out item. We had to do two different diameters of piles. More than 750 piles were installed. Availability of piling rigs is an issue. We had been doing kilometres of piles for foundations in this country. But all these pilings are done using imported piling rigs. Till now not a single piling rig is made in India.
Launching girders are not being designed in our country. We have to get it designed outside. This involves time and money. Each launching girder is a specialised design and so each time when we build a bridge, we have to do this procedure all over again.
Any kind of breakdown of a machine would bring the work on a massive project to a standstill. This would have been unthinkable. We faced minor problems with the launching girder in the initial stages. This was a new design so, we had get used to the operation. Some modifications also became necessary.
The company has an equipment division with a well-equipped workshop. They would attend to emergencies. Besides, except for the launching girder, everything was routine as the operation was the same in other sides. This launching girder was designed by a different designer and the operating procedure was different. No training could be given for launching girder operation generally. Once a few spans are launched, things would fall in place.
The magnitude of this project was simply huge. Planning from the beginning had to be made to meet the massive demand. A sizeable period would be lost due to floods. The planning had to take into account the reduced time available. A total of 736 piles had to be installed which requires infrastructure of a massive nature. The batching plant and transit mixers had to keep pace with the piling. A number of hydraulic rotary piling rigs were mobilised. Cranes were required for lifting and placing reinforcement cages and liners. Since there were 182 piers, shuttering for the pile cap was also massive. The pier had a special shape. Moulds for these were a colossal requirement.
Casting yard was another massive entity. Total number of segments for the whole bridge was 2,745. Casting such a large number required a large number of moulds. These had to be fabricated at far away places. One salient feature was that practically everything had to come from far-away places.
Floods were damaging. On one occasion, we were able to just scramble out of the way a piling rig. But a couple of cranes were lost in the floods. We spent a lot of time and effort to salvage them, but couldn't.
Once during a destructive flood, the approach embankment was cut and the site was cut off from the rest of the world. The casting yard was totally flooded for days. Work could not be done during the flooded season. A lot of efforts had to be done before we were in a position to restart the work.
In a peculiar incident, after casting the first lift of the pier, when we came back to continue the remaining portion, we realised some thieves cut off the projecting steel rods and took them away. For the thieves it was the matter of a few rupees only. But we had to demolish some portion of the work and redo it.
Good times ahead
The river Lohit provides a difficult barrier between South and North Assam. Several projects have been planned on the northern side. Northern Assam is rich in hydro- electric potential and many hydro-electric projects are pending as the important crossing of the river Lohit was not completed. Now, opening of this bridge has thrown open immense possibilities. This will open a route from the northern Assam to Arunachal Pradesh. This is an all-important connection that would propel the area towards intense development. The whole area is geared up for good times.