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Construction : Cover Story | March 2016 | Source : Equipment India

BWSL: A Triumph of Precision Engineering

Given the gigantic size of the project, mega equipment were used in construction; bringing them to the project site and operating them was a feat in itself.
Bandra-Worli Sea Link (BWSL) is the first-of-its-kind civil engineering marvel on Mumbai´s coastline. With its cable-stayed towers soaring gracefully skywards, the sea link is a reflection of the modern infrastructure that Mumbai added in its progress towards becoming a world-class city.

BWSL was part of the Western Freeway Sea Project, part of a larger proposal to upgrade the road transportation network of Greater Mumbai. As on date, the first phase Bandra and Worli have been connected, rest will follow.

The project, built by HCC under the auspices of Maharashtra State Road Development Corporation Ltd (MSRDC) and the Maharashtra Government, has been commissioned to offer a quicker alternative to the north-south traffic.

As a builder of landmark infrastructure projects around the country, HCC handled numerous challenges both in terms of location and technology. The BWSL project offered HCC an opportunity to accomplish one more feat: to construct this eight-lane freeway over the open sea for the first time in India.

Project overview
The entire project was originally conceived as one large project comprising different components, but in order to accelerate the overall construction schedule, the project has been divided into five construction packages:

  • Package I: Construction of flyover over Love Grove junction at Worli
  • Package II: Construction of cloverleaf interchange at Mahim intersection
  • Package III: Construction of solid approach road from the Mahim intersection up to the start of the Toll Plaza on the Bandra side and a public promenade
  • Package IV: Construction of cable-stayed bridges together with viaduct approaches extending from Worli up to the Toll Plaza at Bandra end, Intelligent Bridge System (IBS).
  • Package V: Improvement to Khan Abdul Gaffar Khan Road

Package IV, the largest of the lot, was the most challenging: Cable-stayed bridge including viaduct approaches extending from Worli up to Toll Plaza at Bandra end.

Challenges encountered
Engineering challenges:
BWSL project is a unique and pleasing structure, but before undertaking the construction, the following were the major challenges to be addressed:

  • The foundations of the bridge included 604 large diameter shafts drilled to lengths of 6 m to 34 m in geotechnical conditions that varied from highly weathered volcanic material to massive high strength rocks.
  • The superstructure of the approach bridges were the heaviest spans in the country to be built with span-by-span method using overhead gantry through a series of vertical and horizontal curves.
  • A one-of-its-kind, diamond shaped 128 m high concrete tower with flaring lower legs, converging upper legs, unified tower head housing the stays and a throughout varying cross section along the height of tower.
  • Erection of 20,000 MT Bandra cable-stayed deck supported on stay cables within a very close tolerance of deviations in plan and elevation.

The challenges were varied and started right from the pre-cast yard.

Ground stabilisation for pre-cast yard
The pre-cast yard was located on reclaimed land. The yard caters to casting, storing and handling of pre-cast segments for the project totalling 2,342 in numbers. The storage capacity requirement of yard was to be about 470 units. As the area available was limited, the segments were to be stored in stacks of three layers.

The bearing capacity of the ground was of paramount importance to enable three-tier storage of segments. As the pre-cast area was on reclaimed land, the bearing capacity of existing ground was very poor and found to be less than 2 T/sq m.

Hence, detailed ground stabilisation was carried out, which involved the following:

  • Excavation of the ground to a depth of ~ 2.5 m
  • Strengthening the ground using rubble soling and filling the voids with sand.
  • The soling thus done was compacted layer by layer using vibratory rollers.
  • Total area of the pre-cast yard was covered with a layer of PCC.
  • RCC footing done to facilitate storing of segments.

These measures offered the required strength to the casting yard.

Foundation and substructure
The foundations for the BWSL project consist of 2,000-mm diameter piles numbering 120 for the cable-stayed bridges and 1,500-mm diameter piles numbering 484 for the approach bridges. The project´s site geology consists of basalts, volcanic tuffs and breccias with some intertrappean deposits. These was overlain by completely weathered rocks and residual soil. The strength of these rocks range from extremely weak to extremely strong and their conditions range from highly weathered and fractured, to fresh, massive and intact. The weathered rock beds was further overlain by transported soil, calcareous sandstone and thin bed of coarse grained conglomerate. The top of these strata was overlain by marine soil layer up to 9 m thick consisting of dark brown clay silt with some fine sand overlying weathered, dark brown basaltic boulders embedded in the silt.

The major engineering problems that needed suitable solutions before proceeding with the work were as follows:

  • Highly variable geotechnical conditions of the foundation bed as explained above.
  • Highly uneven foundation bed even for plan was for each pile. Presence of Intertidal Zone (Foundation Bed exposed in low tide and submerged in high tide).

The key to success was a programme of pier by pier in-situ testing. An extensive subsurface exploration and drilling programme (total 191 bores inside sea) was undertaken to define the subsurface stratagraphy, determine the rock types and obtain material properties for optimising the foundation design. Owing to a highly variable geology, the design calculations were performed on a pier-by-pier and the unit side shear values were checked that they did not exceed the load test results under similar rock conditions. The working load on the approach piles ranged from 700 tonne to 1,500 tonne whereas for the piles below the cable-stayed bridge working load was 2,500 tonne.

For conducting the load test on the piles, the load to be applied varied from 4,500 tonne to 9,600 tonne. Arranging reactions for such loads either by normal kentledge method or by soil anchor required massive scale arrangements in the sea waters. This was completely avoided by careful planning of load test using the Stenberg load cell method.

The award winning Osterberg Cell, or ´O-Cell´, was 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 was not restricted by overhead structural beams and tie-down piles. Instead, the O-Cell derives all reaction from the soil and/or rock system. End bearing provides reaction for the skin friction portion of the O-Cell load test, and skin friction provides reaction for the end bearing portion of the test. Load testing with the O-Cell continues until one of three things occurs:
ultimate skin friction capacity was reached, ultimate end bearing capacity was reached, or the maximum O-Cell capacity was reached.

Each Osterberg Cell was specially instrumented to allow for direct measurement of the end bearing and skin friction. O-Cells range in capacities from 0.7 MN to 27 MN. By using multiple O-Cells on a single horizontal plane, the available test capacity can be increased to more than 200 MN. At BWSL, four test locations were selected for the following criterion.

Reverse Circulation Drilling method was adopted for foundation construction. The highly uneven foundation beds and the presence of intertidal zone brought in a lot of difficulties 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.

Interestingly, loss of water head during continuous drilling operation was a major problem while working in the intertidal zone. This water head loss led to very slow production rate and very high consumption of drill bits. To overcome this problem, pits were made in the low tide at each foundation location using an excavator and the casing was placed at the bottom of the pits. Then the casing was placed in the pits and was concreted to make an artificial penetration, maintaining the proper water head for continuous drilling.

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 casings. Pre-fabricated and pretested casings were towed to location using A-frame barge and the caissons sunk at the location using counterweights. The unevenness at the bottom was sealed using the gabion method. As a result, it completely eliminated deployment of resources like jack up platform, crane, vibrohammer, compressor, etc for liner pitching and substantial field works.

Superstructure
The BWSL project had (9+2) approach bridge modules. These modules ranged from 3 to 8 continuous span units. The deck of the carriageways consisted of triple cell precast box girders supported on piers founded on independent substructure. The concrete grade for the superstructure was M60. The average weight of the span was 1,800 tonne, whereas the heaviest span in the bridge (erected with the launching gantry) weighed 2,000 tonne. In addition, the trusses were to be designed to receive the segment from the already erected deck as well as from barges parked directly under the truss.

The erection gantry was 1,260 MT truss designed to erect spans for the above configuration. The unique feature of the truss indeed was the maximum span weight it could handle and launch both the pier and EJ segment. The truss also had the capacity to align the total span in hanging condition after the gluing was completed; was fully mechanised 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 final alignment of the spans.

A Typical 50 m span of the approach bridges comprised 15 field segments, a Pier segment and 200 mm (nominal) in-situ wet joints. During the span construction, all field segments was 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.

Most formidable Challenge! Relocation of the mammoth launching truss

After the successful erection of the deck on Bandra side, the trusses had to be shifted across the Bandra cable stay bridge by 600 m to Worli side for erecting the spans beyond the bridge.

Conventional process: Option 1

  • Dismantling
  • Lowering on components on floating barges
  • Move to new locations
  • Lift all components to deck level
  • Reassemble entire truss
  • Re commissioning & testing

Disadvantage: Process time nearly 12 months???
Method deployed: Innovative Challenger

  • Enter Asian Hercules: One of the world´s biggest floating shear leg cranes.
  • Mounted on a floating barge 240 long x 130 ft wide weighing 5,900 tonne
  • Lifting capacity 1,600 tonne, equivalent to 2,000 small cars!
  • Sourced from Singapore
  • Equipment selected considering various challenges, like the draft and space available at working locations, tide limitations, and other weather constraints.
  • Operations commenced on November 6, 2006

Innovative Process:

  • Hurdles
  • Draft required for safe operation was not good enough
  • VSNL communication cables lying underneath cold not be disturbed
  • Innovative Solutions
  • Use of sophisticated Global Positioning Systems
  • Carrying out the entire process in series of small steps
  • Using favourable high tide conditions for operations
  • Positioning Asian Hercules at lifting position
  • Balancing the crane vessel with pre-determined stage ballasting
  • Using a customised pre-fabricated lifting spreader.
  • Truss weight transferred to crane in computer controlled stages till the entire weight of the assembled truss was hung on the crane.
  • Computer system also prevented overloading at each loading point.
  • Asian Hercules loaded with the heavy truss withdrawn to safe location.
  • Loaded crane moved to location during the next available high tide.
  • Truss lowered at new location

Phenomenal achievement: Actual time taken was, just a few days!!!

Cable stay bridges
It was for the first time that cable stay bridges have been attempted on open seas in India. Coupled with the fact that the aesthetically designed pylons have an extremely complex geometry and one of the longest spans for concrete deck, the challenges encountered were indeed formidable.

Pylon tower legs construction
The salient characteristics of the pylon tower that made it complex and challenging from the point of view of constructability was as follows:
a)The section decreased gradually with height;
b)There were horizontal grooves at every 3 m height and vertical grooves for circular portion that requires special form liners as well as it required attention for de-shuttering;
c)The tower legs were inclined in two directions, which created complexities in alignment and climbing of formworks.
d)Construction joints permitted only at 3 m level. Inserts were permitted only in horizontal grooves provided at 3 m height.

Since immediate solution from reputed worldwide formwork manufacturers, the project design team designed an automatic climbing shutter formwork system, which was fabricated on site and employed to execute all tower leg lifts below deck level. To affect further reduction in time cycles, HCC approached Doka, Austria. Doka then devised a customised solution based on their SKE-100 automatic climbing shutter system.

Survey of tower legs
The complex pylon geometry was another challenge for surveyors. Coupled with geometry, the construction stage analysis indicated leaning and progressively increasing inward inclination of pylon legs during construction. HCC´s Principal Surveyor devised a sophisticated technology to measure coordinates through a combination of total station and prisms mounted on pylon legs. The temperature and construction stage analysis factors were applied to derive the corrected coordinates. The pylon legs were constructed within an accuracy of ¦5 mm, which speaks volumes about the technique employed.

Anchorage box for Worli cable stay bridge
Anchorage box was used as inner shuttering for tower head. Bearing plates with guide pipes was fixed to the anchorage box. Guide Pipe and Bearing Plates actually transfer the deck loads to tower concrete which was generated due to stressing of stay cables. The anchorage box was fabricated with 12 mm thick high grade steel plates. It was fabricated in pieces and then bolted at tower head portion. The bearing plates and guide pipes of anchorage box was galvanised and the remaining portion was painted with anti-corrosive polyurethane based paint. Anchorage boxes was fixed with the help of co-ordinate system for accurately fixing the anchorage point and angle of stay cable.

Compression struts
Compression struts were provided at various levels of tower legs. These were basically provided to keep the alignment of all tower legs in their required position. During construction, due to geometry it was possible that the tower legs might lean inwards due to weight and stresses involved in the base. In order to avoid that, compression struts were provided and jacking done to desired load to maintain the alignment.

Erection of pier table segments
The pier table segments numbering 42 for both the carriageways were another hurdle encountered. The reasons being:

  • Launching truss could not be deployed;
  • Being generally over the pylon pile cap, lifting segments from the sea was not possible. To overcome this hurdle, HCC´s Expat devised a brilliant and ingenious solution in the form of:
  • Pier Table Trusses (PTT): One each was erected for each carriageway. It had rails on top to move segments with the help of hydraulic jacks from one end to another.
  • Lifting Frame: This was an ingenious little devise mounted at either end of PTT.

Strand Lifting Units (SLUs) were mounted on top to lift the segment from barges anchored in the sea. After lifting the segment, the front frame closed down, where the segment was lowered on the rails. The rear frame lifted up to enable the segment to slide across the PTT hydraulically.

Erection of segments of cable stay bridge by Derrick

The method used for erection of segments at cable-stayed bridge was balance cantilever construction method. During construction, the length of free cantilever for Bandra cable-stayed bridge was 215 m and for Worli cable-stayed bridge it was 73 m. The segments were lifted by the instrument named 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 can lift one segment. Deck was 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 dry-matched with the already erected segment. On completion of dry-matching, the segment was moved back by sliding the lifting beam for a distance of 400 mm 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 was ´Parallel Wire Stay Cables´. They were manufactured by Shanghai Pujiang Cable Co Ltd China. Each cable consisted of a group of different number of steel wires. Each wire was made up of high tensile steel. Diameter of single wire was 7 mm with a breaking limit of 6.28 tonne. Six different sizes of cables were used in the cable-stayed 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 High Density Poly Ethylene (HDPE) material to protect them from atmospheric effects.

Closure pour
In Bandra cable-stayed bridge, closure pour was provided between main cable-stayed cantilevers and back span. In Worli cable-stayed bridge, closure pour was provided between two cable-stayed cantilever decks.

Longitudinal stressing and grouting
When all the segments and cables were erected, the segments were post tensioned longitudinally. This post tensioning was done by stressing the steel tendons placed in the ducts provided inside the body of segments. This helps the members to stay together and to increase their load carrying capacity as a large number of segments were joined together to make single unit. Once the stressing was done as per requirement, these holes or ducts were filled with cement grout and were plugged at both ends.

Fine tuning
After completion of closure pour and post-tensioning of the deck, fine tuning of stay cables was done. Fine tuning was fine force adjustments of the stay cables to achieve the required stresses in the deck and profile of the deck. During fine tuning, forces in the stay cables was adjusted to suit further addition of super-imposed dead loads such as wearing coat, crash barriers, handrails and also vehicle loads. During fine tuning operation, longitudinal and transverse deck profiles was also monitored to provide smooth curve.

Wearing coat over south bound bridge deck
Bridge deck surface of south bound carriageway was provided with 40 mm thick polymer modified bituminous pavement in conjunction with water-proofing system to seal the bridge deck.

Working during monsoon The Maritime Board does not allow marine traffic in monsoon season. Thus, work was halted mid-May only to re-commence in October, effectively reducing the work schedule to only seven months in a year. To overcome this hurdle and to use this time to speed up the construction activities at Bandra Pylon, HCC put forth the solution in the form of an innovatively designed temporary bridge. This bridge had a total length of 325 m. It had the facility of a walkway, a concrete pipe line, an electrically-operated trolley mounted on rail, water line and a pipe line. It paved the way for successful continuation of work during the monsoon season when the sea was rough and the winds were strong.

Logistics
Another challenge was ensuring effective supply chain at all working locations spread across the alignment in the sea and formulating measures to ensure the same. A diligently worked out logistics plan was put into action to ensure that commodities were handled at dedicated location and dispatches monitored meticulously. State-of-the-art electronic devices were placed on the barges to cut down on idle timings.

During peak construction activities, innovative procedures and specialised equipment were required to enable high accuracy. Expert crews had to also exercise good judgment in assessing sea behaviour and priorities during foundation/substructure constructions and final placement of concrete in situ. Navigation and transporting 19 precast segments in 24 hours at different open sea locations was a challenge. Secondly, concrete consumption at the peak had been at the rate of 50 cum/hr. Under marine conditions, the consumption rate has been in the order of 700 cum per day. To add to this, maintaining adequate food supply for around 2,500 people (in a shift) working in the sea at over 30 locations was a big challenge. These complete requirements were met with an effective utilisation of a fleet of 30 marine vessels including 13 barges for concrete, segments and material transport, eight steel boats for material and workers transport, three tug boats and six smaller passenger boats. Around four passenger boats were used for carrying food to approximately 30 locations in the sea. Each employee, while starting his day, entered the log indicating the location at which he would be working. Thereafter began the clockwork of gathering tiffin boxes, washing and cleaning, allocation and dispatch as per the log entries along with the drinking water supply including tea supply at two time intervals per shift. During rough sea conditions in the normal working season, extra tiffins were carried to take care of possible spillage while transferring the tiffins from boats to working locations. Thus workers were also suitably cared for, while meeting the engineering challenges posed during construction of the Bandra-Worli Sea Link project.

Psychological conditioning
With a long track record and experienced in building large infrastructure projects, HCC follows strict guidelines for occupational health and safety and environment protection. Safety is extremely important to HCC and the company officials worked towards sensitising labour and creating greater awareness of safety standards with gentle persuasion, consistent motivation and tool box meetings. The kind of structured processes that were implemented by HCC for ensuring safety was nothing short of phenomenal.

Lack of awareness was the biggest hazard for safety. Since the primary safety hazard was related to engineering control, equipment, job methodology, material handling, structural fabrication and emergency preparedness, HCC made sure that every worker was taken through the HSE programme. The orientation programme made them aware of the various safety hazards associated with a project and necessary precautions to be taken to prevent them. They were also taught how to evacuate during any emergency. For its meticulous planning and implementation of safety practices for the BWSL project, HCC has won the prestigious ´Golden Peacock Award´ for safety, health and environment in June 2007.

Key people
Over 3,000 workers were employed to work on the project. Several teams of HCC engineers and foreign engineers and technicians have been involved in specialised tasks on the structure of the Sea Link. These include professionals from China, Egypt, Canada, Switzerland, Britain, Serbia, Australia, Singapore, Thailand, Hong Kong, Indonesia and the Philippines. In terms of language, cultural differences and methods of work these key people were different, yet the engineering challenges kept the group creatively involved, and they worked enthusiastically as a team. The foundations for the BWSL project consist of 2,000-mm diameter piles numbering 120 for the cable-stayed bridges and 1,500-mm diameter piles numbering 484 for the approach bridges.

Erection Gantry configuration

  • Main truss
  • Front/rear pylons
  • Front/centre/rear legs
  • Front /rear trolley
  • Cross beams
  • Stressing gondola
  • Suspension frames
  • Connection beams-Type A/B
  • Spreader beams- Type A/B
  • Pier bracket
  • Chain Support

Partners involved

  • VSL Singapore Pvt Ltd: Technical Consultants
  • Ultra Tech: Cement
  • Metco group: Bearings
  • Tata Steel, RIN Ltd & SAIL: Steel
  • ELKEM International Ltd, Norway: Norway: Micro Silica
  • SPCC: China-Stay Cable
  • DOKA Australia: Pylons formwork

Equipment categories used

  • Jack up platform
  • Launching truss
  • Reverse circulation drilling machine
  • Floating barrages
  • Boats
  • Crawler crane
  • Tower crane
  • Gantry crane
  • Derrick crane
  • Placer boom
  • Concrete pump
  • Transit Mixer
  • diesel generators,
  • ´A´ frame barrage.
  • Segment precast yard facilities

The equipment was sourced from various countries.

Specialised Lifting Equipment
Note: Huge and high capacity lift equipment was used to transport, handle and raise the heavy and oversized structures from on shore and off shore locations. Some examples:

  • Segment launching truss: 112 m long weighing 1250 tonne, lifting 130 tonne
  • Floating jack up platform: 18.3 m wide x 30 m long x 2.1 m deep with 4 nos. 30 m depth legs, for marine works.
  • Flat barge: 30x12x2 m for in sea transportation.
  • Self propelled barges for concrete transport.
  • 75-150 tonne capacity crawler cranes for heavy lifts.
  • Tower cranes

Workers were also suitably cared for, while meeting the engineering challenges posed during construction of the Bandra-Worli Sea Link project.

Article Courtesy: HCC

 
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