Mumbai: the financial capital of India. A tiny island bursting at the seams with its teeming millions. With real estate prices among the highest anywhere in the world, land is an extremely precious commodity in the city. Expanding the existing infrastructure especially transport has been a nightmare for the city planners.

But, one thing that Mumbai does have plenty of is the sea – surrounding the island on three sides. Finally when push came to shove, with the only connection between the western suburbs and ‘town’ - the Mahim causeway choked to its limit, the planners looked out to the sea.

The idea about a bridge in the sea seemed far fetched at first, but where there is a will there is a way. Many years in the planning and more years in the making, but finally Mumbaikers can now speed across the Mahim bay on a swanky new 8 lane 4.7 km bridge, cutting the earlier travel time of up to 60 minutes to just 15 minutes.

The marvel that has brought this sea change to the life and landscape of Mumbai is the Bandra-Worli Sea Link – the first ever sea link to be built in India.

The Bandra-Worli Sea Link (BWSL) is a part of the Western Freeway Sea Project, which, in turn, is a part of a larger proposal to upgrade the road transportation network of greater Mumbai. The project has been commissioned by the Maharashtra State Road Development Corporation Ltd (MSRDC) and the Maharashtra Government and has been built by Hindustan Construction Company (HCC).

The entire project was originally conceived as one large project comprising, different components, but in order to accelerate the overall construction schedule, the project was divided into five construction packages. Four of these packages involved work on the island, while one – Package IV involved construction in the sea.

“Package IV”

The largest and main phase of Bandra- Worli Sea Link Project was Package IV which included the construction of Cable- Stayed Bridges together with viaduct approaches extending from Worli up to the Toll Plaza at Bandra end and an Intelligent Bridge System (IBS). The work under this package was awarded to HCC.

The Bridge

The bridge consists of twin continuous concrete box girder bridge sections for traffic in each direction. Each bridge section, except at the cable-stayed portion, is supported on piers typically spaced at 50 meters. The bridge layout is categorized into three different parts:

• Part 1 - The north-end approach structure with Pre-Cast (PC) segmental construction.

• Part 2 - The Cable-Stayed Bridge at Bandra channel is with 50m -250m- 250m-50m span arrangement and the Cable-Stayed Bridge at Worli channel is with 50m-50m-150m- 50m-50m span arrangement.

• Part 3 - The south end approach structure with Pre-Cast segmental construction.

Toll Plaza

A modern toll plaza with 16 lanes is provided at the Bandra end. The toll plaza is equipped with a stateof- the-art toll collection system. Intelligent Bridge System

The intelligent bridge system provides additional traffic information, surveillance, monitoring and control systems. It comprises CCTVs, traffic counting and vehicle classification system, variable message signs, remote weather information system and emergency telephones.

The marvel in the making

When one sets out to achieve a first, the challenges are bound to be many. Some problems on the project were unique and needed special solutions, while other problems were those faced on any other such project around the world. The project team of over 3000 workers, HCC engineers, foreign engineers and technicians met the challenges head on and overcame them to give Mumbai its latest landmark.

Foundation and Substructure

The project’s site geology consists of basalts, volcanic tuffs and breccias with some intertrappean deposits. This highly variable geotechnical condition of the foundation bed in the intertidal zone needed suitable solutions solutions to lay the foundation and the substructure.

The foundations for the BWSL project consist of 2000-mm diameter piles numbering 120 for the cable-stayed bridges and 1500-mm diameter piles numbering 484 for the approach bridges. The key to success was a program of pier by pier in-situ testing. An extensive subsurface exploration and drilling program (total 191 bores inside sea) was undertaken to define the subsurface stratigraphy, determine the rock types and obtain material properties for optimizing the foundation design. The working load on the approach piles ranges from 700 tons to 1500 tons whereas for the piles below the cablestayed bridge working load is 2500 tons.

For conducting the load test on the piles, the load to be applied varied from 4500tons to 9600tons. This was accomplished by a careful planning of load test using the Osterberg load cell method.

The award winning Osterberg Cell, or “O-Cell”, gets its name from the inventor, Dr. Jorj O. Osterberg. The O-cell is a hydraulically driven, high capacity, sacrificial loading device installed within the foundation unit. Working in two directions, upward against side-shear and downward against end-bearing, the O-cell automatically separates the resistance parameters. By virtue of its installation within the foundation member, the Osterberg Cell load test derives all reaction from the soil and/or rock system. Load testing with the O-Cell continues until one of three things occurs: ultimate skin friction capacity is reached, ultimate end bearing capacity is reached, or the maximum O-cell capacity is reached.

At BWSL, four test locations were selected. Reverse Circulation Drilling method was adopted for foundation construction. The highly uneven foundation beds and the presence of intertidal zone brought in lots of difficulty in terms of Liner pitching. This problem was solved by constructing a gabion boundary at the bed level around the casing, pouring concrete between the casings to make an artificial penetration of the casing. After setting of the concrete under the water, drilling was commenced using RCD.

For several locations, cofferdam construction using steel liner and sheet piles was not possible due to very hard and uneven strata. Here the problem was solved using circular steel caissons. These caissons were fabricated outside and towed to location using A-frame barge. The caissons were sunk at the location using counterweights. The unevenness at the bottom was sealed using the gabion method. The benefit of this method was that it completely eliminated deployment of resources like Jack up Platform, Crane, Vibrohammer, Compressor, etc for liner pitching. It also eliminated substantial amount of field works and is pre-fabricated in principle.

Superstructure

The BWSL Project has (9+2) approach bridge modules. These modules range from 3 continuous span units to 8 continuous span units. The deck of the carriageways consists of triple cell precast box girders supported on piers founded on independent substructure. The Concrete Grade for the superstructure is M60. The average weight of the span is 1800 tons, whereas the heaviest span in the bridge weighed 2000 tons. In addition, the trusses were designed to receive the segment from the already erected deck as well as from barges parked directly under the truss.

The erection gantry was a 1260MT truss designed to erect spans for the superstructure configuration. The unique feature of the truss was the maximum span weight it could handle and that it could launch the pier and EJ segment

itself. The truss also had the capacity to align the total span in hanging condition after the gluing was completed. The truss was fully mechanized for self launching and aligning. An individual segment could be aligned on the truss using a set of four hydraulic jacks mounted on each suspension frame. In order to eliminate the casting or erection errors within a span, two wet joints were provided on either end of the span. The wet joints were cast after finalization of the span alignment. A Typical 50m span of the approach bridges comprises 15 field segments, a Pier segment and 200mm (nominal) in-situ wet joints. During the span construction, all field segments were suspended from the Gantry, glued and temporarily stressed together. Once the gluing operation was

completed, span alignment to the Piers was followed. After alignment, the wet joints were cast including grouting of bearings top plinth. Once the wet joints achieved the required strength, stressing of longitudinal PT was commenced followed by load transfer of Span to Piers.

The Cable Stay Bridges

It is for the first time that a cable stay bridge has been attempted on open seas in India. The aesthetically designed pylons have an extremely complex geometry and one of the longest spans for concrete deck – presenting a formidable engineering challenge.
The complex pylon geometry was another challenge for surveyors. HCC’s Principal Surveyor devised a sophisticated technology to measure coordinates through a combination of total station and prisms mounted on pylon legs. The pylon legs were constructed within an accuracy of ±5mm, which speaks volumes about the technique employed.

Erection of Segments of Cable Stay Bridge by Derrick

The balance cantilever method was used for erection of segments at Cable-Stayed Bridge. During construction, the length of free cantilever for Bandra Cable-Stayed Bridge was 215m and for Worli Cable Stayed Bridge it was 73m. The segments were lifted by a Derrick which was fixed on both ends of the pier table segment and then forwarded. Lifting operation was done simultaneously on both ends. At a time, Derrick could lift one segment. Deck is constructed of alternate stay and non stay segments joined to pier table segments.

Dry Matching, Epoxy and Temporary Stressing for Gluing

When the segment was positioned, it was to be joined with the existing segment. Therefore, the segment was first drymatched with the already erected segment. On completion of dry-matching, the segment was moved back by sliding the lifting beam for a distance of 400mm of the derrick and epoxy was applied on the face of both segments. After application of the glue, the segments were joined together and were stressed by Temporary PT bars. Post this step, the segment lifting beam on derrick was moved forward to lift the next segment i.e. stay segment.

Erection of Stay Segment

These segments were also erected similarly as the non-stay segment and were also joined in the similar way. After this, guide pipes were installed over the ducts left behind during segment casting.

Stay Cable

Stay Cables used are ‘Parallel Wire Stay Cables’. Each cable consists of a group of different number of steel wires. Each wire is made up of high tensile steel. Diameter of single wire was 7mm with a breaking limit of 6.28 Tones. Six different sizes of cables were used in the cablestayed portion. The difference between them was only on the basis of number of steel wires in each cable. Six different types used were of 61, 73, 85, 91, 109 and 121 steel wires. Group of these wires was packed in two layers of HDPE (High Density Poly Ethylene) material to protect them from atmospheric effects.

Closure Pour

In Bandra Cable-Stayed Bridge, closure pour is provided between main