🕑 Reading time: 1 minute
The Bandra-Worli Bridge, now known as the Rajiv Gandhi sea link, is a civil engineering marvel traversing an arc along the coastline of the city of Mumbai. The towers of the cable-stayed bridge rise gracefully towards the sky, manifesting the growing repertoire of modern infrastructure the city possesses. The iconic bridge was constructed with an intent to reduce the traffic congestion in Mumbai, India's commercial capital.
The 5.6 km long bridge has two parallel independent four-lane carriageways for northbound and southbound traffic. In India, the bridge has demonstrated many firsts: the first sea link bridge constructed using only precast segments, the first cable-stayed bridge across a sea, the first cable-stayed bridge with a single central pylon. The project has two cable-stayed bridges on each carriageway along its alignment making it a masterpiece in bridge aesthetics.
The bridge has a 16-lane toll plaza with 20 m wide walkway together with traffic monitoring, surveillance, information, and control systems. A total of 2342 precast segments, 40,000 tons of reinforcement, 2,30,000 cum of concrete, and 5400 tons of post-tensioning strands and bars were used to construct the bridge.
Osterberg cell technology was used for the first time in India to check the pile strength of the Bandra-Worli Bridge. The Asian Hercules, one of the largest floating shear leg crane in the world, was used to launch the truss girder from Bandra end to Worli end.
1. Geology of the Bandra-Worli Bridge Site
The Bandra-Worli bridge project is located in the Deccan trap province within India's central-west coastal region. The following points describe the major geological features of the Bandra-Worli Bridge site:
- Mumbai and surrounding areas had a series of volcanic eruptions that resulted in lava formation around the project area. These lava flows were primarily deposited under the subaqueous region.
- The rocks of the project site are the weathered remnants of these lava flows. Successive sheet of lava flow accumulations resulted in the Deccan trap of basalts. The basalt rocks were found in the thickness range of 5 m to 30 m.
- The project site geology falls within the Malabar Hill and Nana Chowk lava flow unit of the Bombay island formation. Rock types consist of basalts, volcanic tuffs and breccias, and some intertrappean deposits.
- The fresh basalts, volcanic tuffs, breccias, and intertrappean deposits are overlain by completely weathered rock and residual soils.
- The strength of these rocks ranged from extremely weak to extremely strong at conditions ranging from highly weathered and fractured to fresh, massive, and intact.
- These weathered rock beds are overlain by transported soils, calcareous sandstone, and thin beds of the coarse-grained conglomerate.
- The tops of these strata are overlain by a marine soil layer ranging from 1 m to 9 m in thickness. It consists of dark brown clayey silt with some fine sand overlying weathered dark brown basaltic boulders embedded in clayey silt.
- Water depth ranges from 3 m to 6 m below mean sea level.
2. Foundations of Bandra-Worli Bridge
Foundations of the Bandra-Worli Bridge project were constructed using drilled shaft method. Drilled shafts with diameters of 2 m and 1.5 m were selected as the foundation elements for the entire project because of their high load-carrying capacity. Drilled shafts of 2 m diameter were selected for the approaches and the main tower pylon of the cable-stayed bridge.
The approach structures to the north and south of the cable-stayed bridge were founded on 1.5 m diameter drilled shafts. Overall, the project consists of 676 drilled shafts, 72 with a diameter of 2.0 m and 604 with a diameter of 1.5 m. Working loads on the shafts range from 3.3 Mega Newton (MN) to 25 MN.
The approach piers were supported by four 1.5 m diameter drilled shafts placed within a pile cap and spaced with a center-to-center spacing of 4.5 m. Figure-3 shows a typical pile cap with dimensions and shaft layout. A similar configuration and spacing were used for the approach piers to the cable-stayed bridge, where 2 m shafts were placed within a pile cap. The working loads for the approach piers range from 3.3 MN to 14.9 MN.
Foundations for the towers comprised of 52 piles of 2 m diameter arranged in an H-shape to support the legs of the pylon. Piles were provided up to 34 m in length. The structural loads acting on the bridge were varying considerably from 2 MN to 25 MN. Therefore, the shafts for the foundation were designed with different diameter based upon the structural loads.
2.1 Drilled Shaft Construction
The drilled shaft construction method includes the casing installation, drilling for shaft, concreting for shaft, and testing. These operations are explained below.
2.1.1 Casing Installation
Before drilling each shaft, a permanent casing was lowered into place using the 60 tons Maxi Traction vibratory hammer (see Figure-4). The average installation time was approximately 1 hour. Typical penetrations in the rocks were generally in the order of 1 m to 3 m, with the occasional refusal of rock fragments in the soil layer.
2.2.2 Drilling Equipment
Drilling of the shafts was accomplished with a Bu-Ma BM R200 pile top drilling unit using the reverse circulation drilling procedure. The BM R200 is designed to drill both 1.5 m and 2.0 m diameter shafts.
Setup procedures for the BM R200 consisted of placing the base of the drilling unit on top of the casing using the crane positioned on the jack-up platform and hydraulically clamping the unit to the outer casing wall. The top of the drilling unit is tiltable above the working platform. The BM R200 is pivoted back to allow the bottom-hole assembly to be placed inside the casing. The bottom-hole assembly consisted of the drill bit, stabilizer, and drill string.
The crane on the jack-up platform was used to lower the bottom-hole assembly into the casing and drill pipe was added until the drill bit reached the seabed floor. Once the bottom-hole assembly was placed, the working platform was returned to the vertical position and locked in place.
Final preparations for drilling included connecting air and water lines from the external hydraulic power pack and compressor to the BM R200. Air pressure was applied around the perimeter of the drill bit assembly.
The design unconfined compressive strength for the concrete mix used for the drilled shafts was 50 MPa with a slump of 200 mm. Concreting was batched and mixed onshore and barged to each pier location. Transit times ranged up to 2 hours. Neglecting transit time from shore, the average time for placing approximately 42 m3 of concrete was 2 hours and 30 minutes.
The concrete was tremied into the shaft through a 250 mm tremie pipe. The tremie pipe was gradually withdrawn as the level of concrete rose while maintaining a head of concrete above the bottom of the pipe.
2.2.4 Integrity Test
Integrity test of the concrete was performed using the cross-hole sonic logging method. During the fabrication of the reinforcing cage, steel tubes were tied to the inside perimeter. Four tubes were used for 1.5 m shafts, and 6 tubes were used for the 2 m shafts.
3. Structural System of Bandra-Worli Bridge
The elements of superstructure of the bridge consist of pylon tower, cables, and the deck. The construction details of each elements are discussed below.
Pylons are the most important components of a cable-stayed bridge. The main span bridge has two pylons, each with four legs. Each tower is inclined towards the other tower by 10° and eventually merging at 98 m above the deck to become a single tower. Transverse and longitudinal post-tensioning was provided to the tower head to resist local cable forces. The single tower was tapered towards the top. Beneath the superstructure of the bridge, the four legs merge into two points, which were carried into the ground through the pile caps.
The cables were arranged in four planes of a semi-fan configuration. The total length of cables used in the construction of the Bandra-Worli Bridge project was 2250 km and the cables were made up of high strength galvanized steel wires. These cables can support 20,000 tons of cable-stayed bridge.
Each deck section has two planes of inclined cables attached to the top of the tower in one plane. This layout of cables was suitable for the large spans as the inclined arrangement provides the required lateral stiffness. The advantage of this layout was that the deck could be slender. It does not have to account for the torsional inadequacies of a single plane of cables whilst taking advantage of the preferred aesthetics of a single plane attachment to the pylon. Cable spacing was kept at 6.0 m along the bridge deck.
A hollow concrete box section made up of three cores was used for the construction of the deck. Precast concrete segments of 1.5 m to 3.1 m length were used throughout the length of bridges for the construction of the box section. After the installation of each box section, post-tensioning was carried out. The idea behind using the precast segments was to develop a slender and lightweight deck so that the overall longitudinal stiffness of the deck can be reduced. Thus, a flexible deck helps in reducing the bending and torsional stresses of the deck under lateral suspension.
The transverse moments acting on the deck due to various load combinations and the point loads at the anchorage block were used to determine the size of the deck. The top slab of the box section used for the construction of deck was continuous over the webs and props. The use of webs and props established a continuous multi-box section. Thus, the large width for the deck was formed and it allowed the wider lanes for traffic in each direction.
4. Construction Methodology
A truss system (known as the balanced cantilever method) was used to launch the precast concrete segments of the deck between pillars. The girder was supported by temporary means on the outer pier. Due to this, it was possible to move carriage between two pillars to install the segments for deck formation by winching them into the appropriate position.
After that, to hold the precast segments in position, they were epoxied together and prearranged degree of pre-stress was also applied. Once each span of the girder was completed and final adjustments as per the geometry were completed, the main tendons of precast segments were stressed to the prerequisite level.
Further, the installation for cable work started only after the deck of the bridge had reached its final position. All the cables were transported to the site and unwounded using a winch. Subsequently, the cables were secured into an anchor hook at the pylon tower and the other end of the cables were secured to the foundation anchorage block through deck connection. Once cables reached the desired location, they were stressed to a prerequisite level by hydraulic jacks.
The time period for construction was limited due to deadlines and the major activity for transporting the truss girder without dismantling it from Bandra to Worli could have consumed more time. Therefore, to save time, a floating shear leg crane was used to lift the truss girder from Bandra to Worli end of the main cable-stayed bridge.
Once the main span of the pylon was constructed, construction for approach section of the bridge started using span by span method. As per this method, the formwork was setup for each span up to the contra-flexural point to the pier. Further, the cycle was repeated by shifting the formwork to the next span.
For constructing the pier, pylon, and diaphragm wall, all the rebar and formwork were constructed off-site. Temporary transverse and longitudinal compression struts were used to support the construction of the pylon tower because of the inclination and height of the tower.
4.1 Construction Sequence
The following points describe the construction sequence of the Bandra-Worli Bridge:
- Construction of pier foundation
- Pylon tower construction below the deck profile
- Pylon tower construction above the deck profile
- Pier table and diaphragm construction
- Construction of deck using precast concrete segments
- Launching and stressing of cables
- Construction of approach span
- Final setup of the deck by wet joint construction
- Grouting the cables for lock-in position
- Fine-tuning and adjustments
The Bandra-Worli bridge is 5.6 km long.
The Bandra-Worli Bridge was constructed at a cost of 16.5 billion INR.
The construction of the Bandra-Worli Bridge started in 2000 and it was opened for public in 2010.
It was constructed to reduce the travel time (from 90 minutes to 10 minutes) between Bandra and Worli.
The Bandra-Worli Bridge is India's first bridge constructed using precast segments only.
The Bandra-Worli Bridge is India's first cable-stayed bridge across a sea.
The height and width of Bandra-Worli Bridge is 413 ft and 66 ft respectively.