Making of Bandra-Worli Sea Link

A triumph of precision engineering

Introduction The Bandra-Worli Sea Link (BWSL) is a civil engineering marvel spanning an arc of the Mumbai coastline. With its cable-stayed towers soaring gracefully skywards, the sea link is a reflection of the modern infrastructure that Mumbai is adding in its progress towards becoming a world-class city.
The BWSL project 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. In the first phase it will connect Bandra to Worli whereas in the subsequent phases the plans are to take it further to Haji Ali and then to Nariman Point. It is a connecting bridge linking the city of Mumbai with its western suburbs and has the potential to bring about permanent and far reaching changes in the travel patterns of the area. The Bandra-Worli Sea Link is primarily meant to provide an alternative to the Mahim Causeway route that is presently the only connection between South Mumbai and the Western and Central suburbs. The project starts from the interchange at Mahim intersection, i.e. intersection of Western Express Highway and Swami Vivekanand Road at the Bandra end, and connects it to Khan Abdul Gaffar Khan Road at the Worli end. The project has been commissioned to offer a quicker alternative to the north-south traffic that presently amounts to approximately 125,000 cars a day. The project has been commissioned by the Maharashtra State Road Development Corporation Ltd (MSRDC) and the Maharashtra Government and is being built by HCC (Hindustan Construction Company). As a builder of landmark infrastructure projects around the country, HCC has handled numerous challenges both in terms of location and technology. The BWSL project offered HCC an opportunity to accomplish one more feat: to construct an eight-lane freeway over the open sea for the first time in India. Highlights in brief • India’s first bridge to be constructed in open-sea conditions
• 4.7 km, twin, 4-lane independent carriageway bridge across the open sea
• 16-lane toll plaza with 20-m wide promenade together with state-of-the-art traffic monitoring, surveillance, information and control systems
• 2342 pre-cast segments for total bridge with varied width
• 40,000 MT of reinforcement, 23,0000 cum of concrete, 5,400 MT of Post tensioning strands and bars used
• Osterberg cell technology used for the first time in India to check pile strength (for up to 9600 MT).
• Engagement of Asian Hercules, one of the largest floating shear leg crane in the world for shifting 1,260 MT launching truss from Bandra end to Worli end of the main cable stay bridge
• Largest span for cable-stayed bridge in India
• Up to 25-m high pier in open sea, giving ample headroom to marine traffic
• Use of Polytron Disc in bearings on piers 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 is the largest and main phase of Bandra-Worli Sea Link Project.

Main features of this technically challenging package are:
• Cable-Stayed Bridge including viaduct approaches extending from Worli up to Toll Plaza at Bandra end
• Modern Toll Plaza The work under this package was awarded to HCC. Details of Package – IV Main Bridge structure 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. Each section is meant for four lanes of traffic, complete with concrete barriers and service side-walks on one side. The bridge alignment is defined with vertical and horizontal curves. 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.
Part – I North End approach structure The bridge is arranged in units of typically six continuous spans of 50 meters each.
Expansion joints are provided at each end of the units. The superstructure and substructure are designed in accordance with IRC codes. Specifications conform to the IRC standard with supplementary specifications covering special items. The foundation consists of 1.5 meters diameter drilled piles (4 nos. for each pier) with pile caps. Bridge bearings are of Disc Type.
The bridge has been built utilising the concept of Pre-Cast, post-tensioned, segmental concrete box girder sections. An overhead gantry crane with self-launching capability is custom built by the company to lay the superstructure of the precast segments. The Pre-Cast segments are joined together using high strength epoxy glue with nominal prestressing initially. The end segments adjacent to the pier are short segments “cast-in-situ joints”. Geometrical adjustments of the span are made before primary continuous tendons are stressed.

Segment types are further defined by the changes in the web thickness and type of diaphragms cast in cell. The segment weights vary from 110 tons to 140 tons per segment. The segment length varies from 3000 mm to 3200 mm. Deck post tensioning is performed at the completion of the erection of each 50m bridge span. Part- II Cable-Stayed Bridge The cable-stayed portion of the Bandra channel is 600 meters in overall length between expansion joints and consists of two 250-meter cable supported main spans flanked by 50 meters conventional approach spans. A centre tower, with an overall height of 128 meters
above pile cap level, supports the superstructure by means of four planes of cable stay in a semi-harp arrangement. Cable spacing is 6.0 meters along the bridge deck.
The cable-stayed portion of the Worli channel is 350 meters in overall length between expansion joints and consists of one 150 meters cable supported main span flanked by two 50 meters conventional approach spans. A centre tower, with an overall height of 55 meters, supports the superstructure above the pile cap level by means of four planes of cable stay in a semi-harp arrangement. Cable spacing here is also 6.0 meters along the bridge deck.
The superstructure comprises twin precast concrete box girders with a fish belly cross sectional shape, identical to the approaches. A typical Pre-Cast segment length is 3.0 meters with the heaviest superstructure segment approaching 140 tons. Balanced cantilever construction is used for erecting the cable supported superstructure as compared to span-byspan construction for the approaches. For every second segment, cable anchorages are provided.
A total of 264 cable stays are used at Bandra channel with cable lengths varying from approximately 85 meters minimum to nearly 250 meters maximum. The tower is cast in-situ reinforced concrete using the climbing form method of construction. The overall tower configuration is an inverted “Y” shape with the inclined legs oriented along the axis of the bridge. Tower cable anchorage recesses are achieved by use of formed pockets and transverse and longitudinal bar post-tensioning is provided in the tower head to resist local cable forces.
A total of 160 cable stays are used at Worli channel with cable lengths varying from approximately 30 meters minimum to nearly 80 meters maximum. Like the Bandra channel, the tower here is also cast in-situ reinforced concrete using the climbing form method of construction but the overall tower configuration is “I” shape with the inclined legs. Similarly, tower cable anchorage recesses are achieved by use of formed pockets.
The foundations for the main tower comprise 2 meter-drilled shafts of 25 meters length each.
Cofferdam and tremie seal construction have been used to construct the six-meter deep foundation in the dry. Part – III South End approach structure This portion of the bridge is similar to the North end approach structure in construction methodology with span by span match cast concrete box girder sections. Toll Plaza A modern toll plaza with 16 lanes is provided at the Bandra end. The toll plaza is equipped with a state-of-the-art toll collection system. A structure is provided at this location to house the control system for the ITS. Intelligent Bridge System The toll station (TP) and collection system will provide for three different types of toll collection, as follows:
– Fully automatic system: Electronic payment through On board Units mounted on the vehicles which allow passage without stopping.
– Semi-automatic system: Electronic payment through a smart card, which allows payment without having to pay cash.
– Manual toll collection: Payment of toll by cash, requiring vehicle drivers to make cash payment to a toll attendant, and stopping for cash exchange.
The intelligent bridge system will provide 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 control centre located near the toll plaza is housed with the
electronic tolling controls. The transmission system comprises fiber-optic cable housed in PVC conduits running parallel to the Bandra-Worli corridor. In addition, facilities to assist enforcement are provided in the form of pullout locations, which will allow drivers and enforcement officers to safely pullout of traffic. Power Supply Distribution and Road Lighting System A reliable and dependable power supply has been arranged for the entire project. It will also house diesel generator sets and auto mains failure panels to cater to critical load, e.g., monitoring, surveillance and communication equipment emergency services like aviation obstruction lights. Adequate levels of lighting levels have been maintained and energy saving luminaries have been installed. Special emphasis has been given to incorporate lighting protection at bridge tower and control room building to protect those building/ structures and the sophisticated monitoring and communication equipment installed therein.
Challenges encountered during execution of the project Engineering challenges BWSL Project is a unique and pleasing structure, but before undertaking the construction, following were the major challenges to be addressed:-
• The foundations of the bridge included 604 large diameter shafts drilled to lengths of 6m to 34m 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 128m 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 20000 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 is located on reclaimed land. The yard caters to casting, storing and handling of pre-cast segments for the project totaling 2342 in numbers. The storage capacity requirement of yard is to be about 470nos. As the area available is limited, the segments are to be stored in stacks of three layers. The bearing capacity of the ground is of paramount importance to enable three-tier storage of segments. As the pre-cast area is on reclaimed land, the bearing capacity of existing ground was very poor and found to be less than 2 T/Sqm.
Hence detailed ground stabilization was carried out, which involved following:
• Excavation of the ground to a depth of ~ 2.5Mtrs.
• 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. Marine works Foundation and 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 project’s site geology consists of basalts, volcanic tuffs and breccias with some
intertrappean deposits. These are 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 are further overlain by transported soil, calcareous sandstone and thin bed of coarse grained conglomerate. The top of these strata are overlain by marine soil layer up to 9m thick consisting of dark brown clayey 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:
1. Highly variable geotechnical conditions of the foundation bed as explained above.
2. Highly uneven foundation bed even for plan area of one pile.
3. Presence of Intertidal Zone (Foundation Bed exposed in low tide and submerged in high
tide).
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. Owing to a highly variable geology, the design calculations were performed on a pier-by-pier basis 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 ranges from 700 tons to 1500 tons whereas for the piles below the cable-stayed bridge working load is 2500 tons. For conducting the load test on the piles, the load to be applied varied from 4500tons to 9600tons.
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 a careful planning of load
test using the Osterberg load cell method (Refer sketch 1).

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 is 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 is reached, ultimate end bearing capacity is reached, or the maximum O-cell capacity is reached.
Each Osterberg Cell is 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 is 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.
It is interesting also to mention that loss of water head during continuous drilling operation was a major problem while working in the intertidal zone. This water head loss leads 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 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.

Construction of Pylon Tower Legs
The salient characteristics of the pylon tower that make it complex and challenging from the point of view of constructability are as follows:
(a) The section decreases gradually with height;
(b) There are horizontal grooves at every 3m height and vertical grooves for circular portion that requires special form liners as well as it requires attention for deshuttering;
PIER TABLE
CABLE ANCHOR ZONE 10
LIFTS
UPPER TOWER LEGS 24
LIFTS
LOWER TOWER LEGS 8
LIFTS
SIDE VIEW OF TOWER P19 FOR BANDRA CABLE-STAYED
BRIDGE
(c) The tower legs are inclined in two directions, which creates complexities in alignment and climbing of soldiers;
(d) Construction joints permitted only at 3m level. Inserts were permitted only in horizontal grooves provided at 3m height.
On not being able to get 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 customized solution based on their SKE-100 automatic climbing shutter system. Construction of tower legs below deck level DOKA SKE-100 Automatic Climbing Scaffolding System erected on tower legs
a. 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 ±5mm, which speaks volumes about the technique employed.]
b. Anchorage Box
Anchorage Box for Bandra Cable Stay Bridge placed on Tower Head Junction Anchorage Box for Worli Cable Stay Bridge Anchorage Box is used as inner shuttering for tower head. Bearing plates with guide pipes are fixed to the anchorage box. Guide Pipe and Bearing Plates actually transfer the deck loads to tower concrete which are generated due to stressing of stay cables. The anchorage box is fabricated with 12mm thick high grade steel plates. It is fabricated in pieces and then bolted at tower head portion. The bearing plates and guide pipes of anchorage box are galvanized and the remaining portion was painted with anticorrosive polyurethane based paint.
Anchorage boxes are fixed with the help of co-ordinate system for accurately fixing the anchorage point and angle of stay cable. c. Compression struts
Compression struts are 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.
d. Erection of Pier Table segments The pier table segments numbering 42 for both the carriageways were another hurdle encountered. The reasons being –
(a) Launching Truss could not be deployed;
(b) 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.
SLU (Strand Lifting Units) 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.
STAGE: 1
Open the Support Bracket and Lift the Segment. Close the Support Bracket, Slide in the Trolley and Lower the Segment on Sliding Trolley
STAGE: 2
Opening of the Lifting Boom & Strut, Slide out the Segment and Close the Lifting Boom and & Strut. Repeat operations for the other segment lifting.
e. 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 215m and for Worli Cable-Stayed bridge it was 73m. 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 is constructed of alternate stay and
non stay segments joined to pier table segments.
Lifting of Segment with Derrick
f. Dry Matching, Epoxy and temporary stressing for gluing When the segment is positioned, it is to be joined with the existing segment. Therefore, the segment was first dry-matched with the already erected segment. On completion of drymatching, 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 is moved forward to lift the next segment i.e. stay segment.
g. 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.
h. Stay cable
Stay Cables used are ‘Parallel Wire Stay Cables’. They were manufactured by “Shanghai Pujiang Cable Co. Ltd” China. 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 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 HDPE (High Density Poly Ethylene) material to protect them from atmospheric effects. Typical Cross Section of Stay Cable i. Closure pour
In Bandra Cable-Stayed Bridge, closure pour is provided between main cable-stayed cantilevers and back span. In Worli Cable-Stayed bridge, closure pour is provided between two cable-stayed cantilever decks
j. 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.
k. Fine tuning
After completion of closure pour and post-tensioning of the deck, fine tuning of stay cables is done. Fine tuning is 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 are adjusted to suit further addition of superimposed dead loads such as wearing coat, crash barriers, handrails and also vehicle loads.
During fine tuning operation, longitudinal and transverse deck profiles are also monitored to provide smooth curve.
l. Wearing Coat over south bound bridge deck Bridge deck surface of south bound carriageway is provided with 40mm 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 metres. 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 logistic 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 specialized equipments were required to enable high accuracy. Expert crews had to also exercise good judgement in assessing sea behavior 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 50cum/hr. Under marine conditions, the consumption rate has been in the order of 700cum per day. To add to this, maintaining adequate food supply for around 2500 people (in a shift) working in the sea at over 30 locations was a big challenge. These complete requirements were met with an effective utilization 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 they 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. 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 is nothing short of phenomenal.
Lack of awareness is the biggest hazard for safety. Since the primary safety hazard are related to engineering control, equipment, job methodology, material handling, structural fabrication and emergency preparedness, HCC made sure that every worker is taken through the HSE program. The orientation program made them aware of the various safety hazards associated with a project and necessary precautions to be taken to prevent them. They are 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 3000 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.

“This project gave us an opportunity to showcase our equipment”

		Rakesh Kaul says Rakesh Kaul, General Manager, Elcome Technologies Pvt. Ltd., with reference to the survey equipment that they supplied for the Bandra-Worli Sea Link.

When did Elcome Technologies first get associated with HCC on the Bandra- Worli Sea Link Project? Leica equipment has been used on most of the Sea Link projects around the world and based on this experience we approached Hindustan Construction Company (HCC) sometime at the end of 2000 with our range of specialised equipments for the Bandra-Worli Sea Link (BWSL). The first Leica Total Station was supplied by us to HCC in early 2001. What were the equipment supplied for this project? To meet the demand for high accuracy coordinate measurements on the BWSL project we supplied high performance Leica Total Stations including the TCA 2003, the TCA 1800, the TCA 1201, the TCRM 1201 R 300 and the TC 1800. We also supplied the SR 510 GPS equipment. What kind of support did you provide HCC vis-à-vis the equipment that you supplied to them? We gave comprehensive application trainings at their site towards effective and optimal usage of the equipment. Moreover, these equipments in keeping the desired accuracies that are required for such a project, needed timely calibration checks and corrections – for this, besides providing them complete service support during the warranty, we also got into annual maintenance contracts for these equipment post their warranty period. We were thus able to provide timely service and calibration of the equipment at our service centre. Do you think being associated with the project gives Elcome Technologies any leverage for other similar projects? It has been a privilege to be associated with BWSL and the HCC team we worked with. Moreover the challenges in geometric control on the project were highly demanding and exacting. This gave us an opportunity to showcase our equipment and our expertise. Our experience with BWSL project will be a basis for us to promote our technology on other such projects too.

Partners involved 1. VSL Singapore Pvt Ltd : Technical Consultants
2. Ultra Tech : Supplier of cement
3. Metco group of companies: Supplier of bearings
4. Tata Steel, RIN Ltd & SAIL: Supplier of steel
5. ELKEM International Ltd.: Norway-based company supplier of micro silica
6. SPCC: China-based company supplier of stay cable
7. DOKA Australia: supplier of Plyon fromwork
Equipment used
The major equipments deployed for this project are:
• Jack up platform, launching truss, reverse circulation drilling machine floating barrages, boats, crawler crane, tower crane, gantry crane, derrick crane, placer boom, diesel generators, concrete pump, transit mixers & ‘A’ frame barrage.
• The equipment was brought together from various countries.
The construction of the mammoth bridge structure required huge cranes and other structures to lift material for off shore and on shore structures. Some of these included:
1. Launching Truss: Weighing 1250 tonnes and measuring 112 m in length, it was used for lifting segments each weighing 130 tonnes. This has been fabricated in India.
2. Jack up platform: Size 18.3x30x2.1m (Width x Length x Depth) having four legs of 30m. It is a floating equipment used for marine work.
3. Flat barge: Size 30x12x2m. Like motor boats, they are driven inside the sea for material transportation.
4. Self-propelled barge: It is a barge with a machine component and is used for concrete transportation.
5. Crawler crane: Capacity ranges from 75-150 tons. It is used for material and heavy lifting activities.
6. RCD drill bit: Dimension 1.5m x 2m diameter. Imported from Korea, the RCD drill bit is used for pile drilling work.
7. Vibro hammer (PTC): Imported from France and used for driving of steel liners.
8. Fushun crawler crane: Imported from China, Capacity 80 tons.
9. NCK Eiger crane: Imported from England, capacity 65 tons.
10. Kobelco crawler crane: Imported from Hong Kong, capacity 150 tons. Fascinating facts • The project has already been acclaimed by the viewers as an engineering marvel of modern India.
• First Cable-Stay Bridge in India in open sea.
• The length of the bridge is 63 times the height of the Qutub Minar in Delhi.
• Its weight is equivalent to 50,000 African elephants.
• The length of the steel wires used is equivalent to the circumference of the earth.
• The height of the cable-stayed tower is 128 m, which is equal to a 43-storey building.
• A total of 424 cables were used for both Bandra cable stay as well as Worli cable stay bridges.
• The cables have been sourced from Shanghai Pujyang Cable Company, China. The cables were subject to a series of quality and engineering tests to meet the special requirements including fatigue tests of two million cycles.
• The cables are made of high tensile steel and are designed to take the maximum load of 900 tons.
• 92,000 tons of cement was utilized to make BWSL.
• Environment friendliness was top priority during the construction – fly ash, a waste product extracted from thermal power plants, was mixed with concrete, to make the construction durable as well as eco-friendly, thus making good use of waste material.
• The construction team is like a mini United Nations: several teams of foreign engineers and technicians have worked on specialized tasks on the structure; these include professionals from China, Egypt, Canada, Switzerland, Britain, Serbia, Singapore, Thailand, Hong Kong, Indonesia and the Philippines, Australia.
• The Launching Trusses, each 112 meters long, were custom built to precision by HCC for this project. The pre-cast concrete segments of this four-lane road are fabricated at the Bandra site location. These segments are then carried on a barge to the construction location and are lifted by the Launching Truss to the designated height and assembled between two piers, each 50 meters apart. Fifteen such segments are fitted between two piers and the Launching Truss can lift all fifteen segments together, weighing 130 tons each, between two piers. Once these segments are fixed between two piers, the Launching
Truss crawls to the next piers on its mechanical legs.
• Given the gigantic size of the project, mega equipments were used in construction; bringing them to the project site and operating them was a feat in itself. Asian Hercules, one of the biggest floating shear leg cranes in the world, was hired from Singapore to lift the massive 1250 tonnes, custom-built Launching Trusses with its mechanical arm and relocate them on the Worli side of the bridge.
Hindustan Construction Company (HCC)
1. VSL Singapore Pvt Ltd : Technical Consultants
2. Ultra Tech : Supplier of cement
3. Metco group of companies: Supplier of bearings
4. Tata Steel, RIN Ltd & SAIL: Supplier of steel
5. ELKEM International Ltd.: Norway-based company supplier of micro silica
6. SPCC: China-based company supplier of stay cable
7. DOKA Australia: supplier of Plyon fromwork Equipment used The major equipments deployed for this project are:
• Jack up platform, launching truss, reverse circulation drilling machine floating barrages, boats, crawler crane, tower crane, gantry crane, derrick crane, placer boom, diesel generators, concrete pump, transit mixers & ‘A’ frame barrage.
• The equipment was brought together from various countries.
The construction of the mammoth bridge structure required huge cranes and other structures to lift material for off shore and on shore structures. Some of these included:
1. Launching Truss: Weighing 1250 tonnes and measuring 112 m in length, it was used for lifting segments each weighing 130 tonnes. This has been fabricated in India.
2. Jack up platform: Size 18.3x30x2.1m (Width x Length x Depth) having four legs of 30m. It is a floating equipment used for marine work.
3. Flat barge: Size 30x12x2m. Like motor boats, they are driven inside the sea for material transportation.
4. Self-propelled barge: It is a barge with a machine component and is used for concrete transportation.
5. Crawler crane: Capacity ranges from 75-150 tons. It is used for material and heavy lifting activities.
6. RCD drill bit: Dimension 1.5m x 2m diameter. Imported from Korea, the RCD drill bit is used for pile drilling work.
7. Vibro hammer (PTC): Imported from France and used for driving of steel liners.
8. Fushun crawler crane: Imported from China, Capacity 80 tons.
9. NCK Eiger crane: Imported from England, capacity 65 tons.
10. Kobelco crawler crane: Imported from Hong Kong, capacity 150 tons. Fascinating facts • The project has already been acclaimed by the viewers as an engineering marvel of modern India.
• First Cable-Stay Bridge in India in open sea.
• The length of the bridge is 63 times the height of the Qutub Minar in Delhi.
• Its weight is equivalent to 50,000 African elephants.
• The length of the steel wires used is equivalent to the circumference of the earth.
• The height of the cable-stayed tower is 128 m, which is equal to a 43-storey building.
• A total of 424 cables were used for both Bandra cable stay as well as Worli cable stay bridges.
• The cables have been sourced from Shanghai Pujyang Cable Company, China. The cables
were subject to a series of quality and engineering tests to meet the special requirements including fatigue tests of two million cycles.
• The cables are made of high tensile steel and are designed to take the maximum load of 900 tons.
• 92,000 tons of cement was utilized to make BWSL.
• Environment friendliness was top priority during the construction – fly ash, a waste product extracted from thermal power plants, was mixed with concrete, to make the construction durable as well as eco-friendly, thus making good use of waste material.
• The construction team is like a mini United Nations: several teams of foreign engineers and technicians have worked on specialized tasks on the structure; these include professionals from China, Egypt, Canada, Switzerland, Britain, Serbia, Singapore, Thailand, Hong Kong, Indonesia and the Philippines, Australia.
• The Launching Trusses, each 112 meters long, were custom built to precision by HCC for this project. The pre-cast concrete segments of this four-lane road are fabricated at the Bandra site location. These segments are then carried on a barge to the construction location and are lifted by the Launching Truss to the designated height and assembled between two piers, each 50 meters apart. Fifteen such segments are fitted between two piers and the Launching Truss can lift all fifteen segments together, weighing 130 tons each, between two piers. Once these segments are fixed between two piers, the Launching
Truss crawls to the next piers on its mechanical legs.
• Given the gigantic size of the project, mega equipments were used in construction;
bringing them to the project site and operating them was a feat in itself. Asian Hercules, one of the biggest floating shear leg cranes in the world, was hired from Singapore to lift the massive 1250 tonnes, custom-built Launching Trusses with its mechanical arm and relocate them on the Worli side of the bridge. Hindustan Construction Company (HCC).

Meeting challenges with innovation!

		Col S Diwanji Project Manager, Bandra-Worli Sea Link Project, Hindustan Construction Company satish.diwanji@hccindia.com

When did the work on the Bandra- Worli Sea Link project start? Hindustan Construction Company (HCC) was awarded Package IV of the Project and work started in September 2000, but was held up due to several reasons including environmental issues and protests by fishermen. In right earnest the work started in December 2004. On what basis was the distance between the piers and the height of the bridge decided? The span between the piers is 50m. This distance was arrived at after considering various factors, which included optimization between the foundation cost vs. the superstructure cost. If the piers are wide apart then the foundation cost comes down, but the superstructure becomes heavier and its cost goes up.

Also the navigational requirements as in an emergency the smaller trawlers and boats should be able to pass between the piers. Moreover, more number of piers provide better wind resistance to the bridge. Why was the cable stayed bridge design chosen for the Bandra-Worli Sea Link? A cable stayed design has some inherent advantages compared to other conventional designs, the main being that it allows for larger spans. There were three main reasons why a large span was needed in case of the Bandra-Worli Sea Link:

• A navigational channel for the fishermen and other sea faring vessels had to be maintained.
• There are plans to expand the present jetty.
• There are overseas communication cables on the seafloor which keep shifting and this had to be taken into consideration. At what stage of the bridge construction was the need for precision survey instruments felt ? We knew from the start of the project that high precision equipment would be needed and one of the first things we did was to mobilise the Total Stations – the first of which were procured in early 2001.

Survey Challenges! Bandra Worli Sea Link Project

	Len Gower, Principal Surveyor on the Bandra-Worli Sea link Project for Hindustan Construction Company, shares his experiences on the project in an exclusive interview with Coordinates.
		Len Gower len.gower@gmail.com

1. For those of us who use them, a bridge is a bridge; but for those who build them each bridge has its individuality. Please tell us about some of the bridges that you have worked on in the last few years. In the past 10 years, I have worked on 7 long span bridges. Each one posed unique technical challenges that were overcome by my teams.

The Rion Antirrion Bridge, between the Peloponnese peninsula and the Greek mainland, is a four pylon, 2200 metre span across a seismic fault in the Strait of Corinth. The pylon bases are not fixed to the seabed, but rather rest on an engineered bed of gravel and inclusion pipe piles. Thus they are free to move during a seismic event, without sustaining damage. The deck is mounted to the pylons with telescopic ‘shock absorber’ energy dampers, so minor pylon movement can be accommodated safely.

The Puente de las Americas cable stayed bridge spans the Panama Canal, about 15 km east of Panama City. The size of the ships that pass through the canal necessitated a very high navigation envelope beneath the deck, and the commercial ramifications of interrupting canal traffic meant that the entire bridge had to be constructed without the use of marine barges or floating cranes. The deck was cast in situ with movable shutters and one of the largest formwork travellers ever utilized in bridge construction. The other major technical challenge was that the deck erection started before the pylon construction was complete. This meant that if the deck was out of balance for some period of time, the pylon lifts had to be cast ‘out of plumb’ with the embedded cable anchors still set to tight angular tolerances.

The Cooper River Bridge in Charleston, South Carolina was (at the time) the longest cable stayed bridge in North America. The slenderness of the pylon legs and the lack of a cross beam below the cable anchorages made pylon leg construction a challenge. The selection of PERI self climbing formwork made it impossible to place an instrument bracket on the formwork support frame, so we employed a Trimble DGPS solution. Early in the morning, immediately after a section of the pylon leg was cast, with the concrete in place for 4-6 hours, we installed a special instrument pedestal onto a ‘cast in’ pipe flange. The tower crane picked up a 15 ton test block (to make the mast load neutral) and then swung to a position that permitted the GPS receiver to receive the maximum number of ‘clean’ satellite signals. We used a set of three ‘one minute’ static position averages, capturing one position every second, to determine our XY position and an approximate Z. We corrected the weaker Z position by taping a ∆elevation up from the previous lift. We then replaced the GPS antenna with a Leica total station and obtained an angular orientation from a back sight, set about 2 km away. We set two auxiliary instrument brackets on the inside face of the exposed cable anchor box, and then surveyed their positions for use in a 3Dimensional ‘Best Fit’ solution.

The actual as-built of the most recently cast lift took place 3-4 hours later, long after sunup and with the tower crane in full operation, but we could use a least squares solution to correct for temperature gradients and eccentric load induced tower deflections by turning off the internal compensators, observing the auxiliary bracket positions again, then taking measurements to our concrete lift as-built positions last. During post processing, we applied a 7 parameter similarity transformation to the observed data to perform the scale factor, rotations and transformations necessary to get the instrument position and the auxiliary bracket positions to match the 06:00 AM positions, before sunup and with the tower crane neutral. These transformation parameters were of course applied to the as-built measurements, to convert them to 06:00 AM readings.

The Shaikh Zayed Bridge was not a cable stayed bridge, but rather a cast in place concrete deck supported by cable stays from a series of asymmetrical steel arches. This project was designed by Zaha Hadid, engineered by HPR and proved to be almost un-buildable, and absolutely unprofitable for the main contractor. It is currently 2 years behind schedule, with about 18 months to go until completion. I stayed on this project only until I found an exit strategy – the Sutong Bridge, in Nantong PRC.

The Sutong Bridge is currently the world’s longest cable stayed bridge, with a clear span length of 1088 metres! By comparison, the Bandra Worli’s main span is 500 metres. Ultra long span bridges bring a myriad of technical challenges, which directly impact the project costs and construction schedule. Long decks require tall towers (to be able to accommodate the increased number of stay anchors, and still maintain the necessary angular cable geometry). Sutong’s pylons were 306 Metres in height, as compared to Bandra Worli’s tallest pylon at P19, at 124 Metres. The crossing of the Yangtze River, between Nantong and Suzhou has strong winds during most of the year so deck flutter and tower deflection were much more of a problem than thermal gradient induced deflections. The pylons were so tall, that after 200 metres, two auxiliary instrument positions had to be transferred onto the pylon, to perform set out and as-built surveys, similar to the Cooper River Bridge, however the Chinese chose a more traditional approach – a reciprocal observation procedure with a Leica 2003 total station. The rebar for the lifts above were much too tall to permit a GPS antenna to receive satellite data free from multi-path errors.

The anchor boxes for the pylons were fabricated close to Beijing, and then barged down to Nantong. These boxes were quite similar to the Bandra Worli boxes, and the survey control methodology chosen was mine, a combination of metrology and steel fabrication ‘dimensional’ quality control. The only serious challenge in anchor fabrication is to achieve angular accuracy in the three planes (α, β and g), AFTER the welding is completed. Anticipating angular errors due to weld shrinkage, and mitigating unexpected results is almost an art – not a science. If the acceptance criteria for angular errors is +- 0.5 degrees (fabrication and installation errors combined) that means that the post weld fabrication error must be between +- 0.25 degrees and the pre-weld fit-up errors between +- 0.125 degrees. In an anchor plate of 400x400mm, that means making repeatable survey measurements of sub-millimetre accuracy. This level of accuracy demands the best 1st order instrumentation, on-board software, customized targeting and methodology available. Both the Nantong and the Bandra Worli anchor boxes were manufactured and installed within the designer’s tolerances.

I also consulted to a bridge project in Canada, a long floating bridge in the province of British Columbia. The challenge they faced that prompted my involvement was completing the pylon leg construction once the pontoons were afloat. Again, I designed a system based on a 7 parameter similarity transformation, utilizing a total station that could operate with the internal compensators deactivated, and a series of control points originally established when the pontoon was still in the dry dock. 2.Could you please tell us briefly how a cable stayed bridge is different from other bridges? Cable stay supported bridges are a type of suspension bridge. A typical suspension bridge, like the Golden Gate Bridge in San Francisco, consists of two large diameter incrementally spun cables, hanging between the two main towers, on a catenary, with much smaller diameter vertical hangers spaced evenly along the deck, connected to the suspension cables.

Cable stayed bridges have many smaller diameter cables, connecting the pylon legs to the deck at evenly spaced intervals. The pattern of pylon connection can vary. The parallel stay system is called a ‘harp’, the system that bunches the pylon anchors close to the top of the pylon is called a ‘fan’, but the most common style is to space the anchors from the pylon top downwards towards the deck. This system is called a modified fan.

The longest spans still require a suspension bridge, but for the medium spans (100 – 1000 metres), cable stayed bridges may offer cost benefits and shorter construction schedules to the client.

The suspension bridge needs large, expensive abutments to ‘anchor’ the suspension cables, and the time it takes to spin the suspension cables is lengthy. If the span distance permits, a cable stayed bridge alternative offers about 12 months gain in the schedule, as no spinning delays are required. A possible compromise is a hybrid design, where a portion of the bridge deck is partially suspended by cable stays, while the suspension cable is being spun, and then upon cable spinning completion, the mid span portion of the deck is suspended by hangers from the catenary cables. The cost benefit is slight with this alternative, so is rarely chosen. Multiple span cable stayed bridges are also a cheaper alternative for long span bridge designers, such as the Rion Antirrion Bridge in Greece. The minor drawback to this solution is that the navigational channel is cut into smaller portions by the extra pylons. 3.Please tell us about some of the survey related challenges you faced on the Bandra Worli Sea Link Bridge? The survey related challenges for the Bandra Worli were similar to most cable stayed bridges. The accuracy requirements are always demanding, especially in the fabrication of the cable anchor assemblies. The angular misalignment permitted is+- 0.5 degrees in the completed structure, so the fabrication and assembly tolerances are much tighter. We fabricated the bearing plate / guide pipe assemblies to +- 0.06 degrees from perpendicular. We placed them in the deck slab formwork (prior to concrete placement) within +- 0.125 degrees, to ensure that they would still be within +- 0.25 degrees after the concrete had been placed and the concrete curing shrinkage was complete. This procedure required custom design / manufacture of very accurate bearing plate/pipe sleeve assembly jigs, as well as special dual turnbuckle pipe sleeve support yokes and customized targeting and tooling for surveying the anchor assemblies in the deck sections.

The fabrication of the pylon head anchor boxes was even more complex, as weld shrinkage had to be anticipated, and unforeseen results dealt with during erection. The size of the base plate that the bearing plate rests on is only 700×400 mm in dimension, so that means the fit-up survey measurements had to be accurate to sub-millimetre, to ensure the 0.125 degree angular misalignment specification was met.

The pylon legs (below the tower head) were very slender, so were susceptible to thermal gradient deflections. Care had to be taken to ensure that all important surveys were performed in a thermally neutral state. The legs also deflected towards the pylon centre after concrete placement, so the ‘as-set’ positions were always different from the ‘as-built’ positions.

The reference geometry supplied by the designer is based on Time ∞, whereas we were constructing every element at Time 0, so allowances had to be made for future creep, shrinkage and elastic shortening. These allowances are referred to as pre-cambers and over heights/lengths. For example, the over height for the P19 pylon’s tower head was +35 mm. As we completed the south carriageway first, the deck load was transferred into the pylon legs, causing the shared centre legs to shorten less than the single outer leg. This caused the pylon to temporarily incline away from bridge centreline by nearly 30 mm at the top of the tower head. This meant that we had to construct the north pylon’s tower head on a similar inclination, with the expectation that the pylon would come back to plumb when the load of the north deck was in place, bringing the pylon sub structure and common foundation back into equilibrium. 4.Could you please elaborate on the role of the pylons in a cable stayed bridge and the survey methodology that was used to put them up in the Bandra Worli Sea Link Bridge? The pylons of a cable stayed bridge are used primarily to anchor the upper cable stay sockets. Many times the deck is firmly attached to the pylons (as in the case of the Bandra and Worli spans) but other bridges only have sliding pot bearings at the pylons (Ting Kau Bridge, in Hong Kong), or elastomeric bearings between the tower’s cross beam and the underside of the deck (Alex Fraser Bridge, Cooper River Bridge).

The pylon must be tall enough to provide sufficient space for all the cable anchors, and still yield a decent vertical angle at the uppermost cables. Obviously, as the alpha angle decreases with the height of the anchor above the deck, the amount of cable force in the vertical direction decreases as the force in the horizontal direction increases. This is why the diameter of the longest cables is greater than those nearest the pylon (with the steepest alpha angles). They have to be strong enough to resist the extra cable force applied, to yield sufficient upward lift to support the dead load and the live load (traffic) in the worst case scenario.

The survey method utilized to construct the Bandra Worli pylons was based on the fact that the pylon legs were inclined. Inclined pylon legs pose a significant challenge to the contractor, as the rebar cage will have a natural tendency to sag down hill during construction. If a rebar cage sags, it will be out of tolerance when completed and clash with the formwork adjustment procedure during the final as-set survey. Our solution was to implement a sacrificial rebar template assembly, to guide the construction of the rebar cages and to ensure that there would be no clashes of the steel embedments, like crane tie-ins and DOKA climbing cones.

Our surveyors set the rebar templates in the early morning hours, after the self climbing formwork was fixed for the next lift. These templates had 3 or 4 key points stamped onto them that the surveyors could shoot, and once the support framework was completely interconnected, formed a local survey network that moved with the pylon’s thermal deflections, yet was still based on a thermally neutral pylon. At any time, and with any amount of pylon leg deflections present, our surveyors could set up their instrument on the special brackets attached to the DOKA framework, disable the internal compensators and then perform a ‘resection’ or ‘free station’ operation to determine instrument co-ordinates and orientation, for set-out work. Once the concrete was placed, the pylon would deflect downwards towards the bridge centreline, so new co-ordinates of the rebar template were measured (again in the early hours of the morning). The instrument was again transferred up to the top of the pylon, installed on the same bracket, where the as-built survey could be completed quickly and accurately.

The main challenge to these surveys was in the formwork construction. On most bridge pylons, there are 2 fixed panels and two adjustable panels, so fine adjustment at each corner is possible. For the Bandra Worli pylons, the formwork had no adjustability. Each panel butted up to the adjacent panels, so the entire shutter assembly acted as a solid body. To move the top into position, the entire shutter had to be tipped, similar to the survey alignment procedure of an elevator core shutter. If an error in a panel length cutting operation occurred, there was no way to eliminate this. Small errors could be mitigated by setting the shutters so that half the error was on one corner and the other have was on the opposite corner. Our survey alignment criteria was therefore based on the centroid of the entire shutter (6 point average in the pylon leg sections & 12 point average in the tower head sections), and not on individual corner positions. 5.In the Bandra Worli Sea Link project what were the phases in the construction where survey played a critical role. The phases of construction most dependent on survey were the following:

1. Offshore pile driving, cofferdam placement and marine foundation construction
2. Deck segment match casting
3. Deck segment installation
4. Pylon leg casting
5. Pylon leg junction below the tower head
6. Cable anchorage fabrication
7. Tower head anchor box fabrication
8. Tower head installation
9. Deck profile surveys
10. Wet joint alignment between 16 deck segment ‘blocks’
11. Deck closure surveys, pre cable length fine tuning
12. Post fine tuning deck profile surveys
13. Kerb and asphalt grades

6. What kinds of survey instruments are best suited for the different survey works in a typical cable stayed bridge construction. The best instruments for cable stayed bridge surveying are state of the art, high accuracy, vibration tolerant electronic total stations, with ATR (automatic target recognition) and on-board software for Free Station and Resection.

GPS receivers can also play a role, when the pylons are extremely tall (as in the Millau Bridge in France) or far out to sea. Their lack of accuracy in Z measurements is their only weakness, in my opinion. With improvements in multi-path error mitigation and the implementation of the Glonass satellites, the availability issue and PDOP are much improved.

The exception to this high tech equipment is utilizing a pair of old fashioned tilting levels to perform accurate deck profiles. The vibrations present in cable stay supported decks makes internally compensated survey equipment susceptible to ‘compensator excitation’, producing a blurred image of the crosshairs in an auto level or randomly inaccurate vertical differences in total stations. A split bubble tilting level exhibits the deck vibrations in the movement of the tilting bubble – while the cross hair image remains completely stable. By adjusting the level so as to balance the bubble movement evenly, a level observation is possible. The purpose of having two instruments observing a single staff is that long circuits can be run ‘one way’. Each set-up produces two back sights and two foresights, so constitutes a closed level loop. The next set-up again produces 2 back sights and two foresights, which is again a closed loop. It is like building a chain, link by link. It was quite common to be able to level from P17 to P21, a distance of 600 Metres, with a misclosure of only 1-2 mm. 7.Could you tell us about the kind of accuracies that are needed for various aspects of a cable stayed bridge? There are many different accuracy requirements, in the steel fabrication/concrete casting and their related survey control measurements, as some types of errors can propagate or systematically multiply and others are essentially ‘one off’ – with no knock-on effects.

Deck segment lengths are a typical dimensional component that has potential for systematic error propagation. A +1mm error on every 3M long deck segment of a 600 metre span will produce +100 mm errors at each expansion joint at the end spans, or roughly 10% of the thermal gradient expansion range. This is still an acceptable range of error, but 200 or 300 mm wouldn’t be, so deck lengths have to be measured accurate to the millimetre, and significant errors must be tracked during deck segment installation, and compensated for in the last in situ stitch joints cast.

Installation of the first deck segment of a 16 segment block is another example of a potential systematic error situation. For every 1mm rotational error (in either the horizontal or vertical directions) there will be a 16mm error at the next wet joint. When setting these segments during wet joint construction, we measure the horizontal positions to the millimetre and the vertical differences to better than 0.5 mm.

Cable anchorage placement errors in either the deck or pylon are minor, as there is usually a fairly generous range of cable length adjustment at the live end socket – either by split shims or by threaded sockets and lock nuts. A shift of 1 or 2 centimetres in longitudinal or transverse directions is insignificant, so normal survey procedures are quite capable of controlling installation and identifying absolute errors. The exception to this is angular misalignments. The principal of multi-strand stays, or parallel wire pre-formed stays is that each wire or strand carries an equal proportion of the cable force. If the bearing plate isn’t perpendicular to the cable force vector, then some wires will carry much more of their respective share of the force, and other wires will carry much less of the force. The over-stressed wires are therefore susceptible to premature fatigue failures. Most manufactures will provide a warranty period for their stays, providing the final angular alignments are within +-0.01 Radians (+- 0.57 degrees). Even with this ‘less than generous’ installation tolerance, longer guide pipes with a misalignment close to the limit will pose problems during damper installation. This is the one phase of works that requires the best survey equipment and methodology available, to produce repeatable measurements at sub millimetre accuracy.

The actual positions of the anchors need to be measured accurately, only IF the cables are to be installed to length, instead of force. As absolute cable lengths at the installation forces are very difficult to determine, engineers rarely use length as an installation criteria, and instead choose force (as measured at the hydraulic jack pump). As long as the cable anchor socket has sufficient capacity for minor length errors, the cable length, deck anchor and pylon anchor locations need only to be within 2 cm of design.

Pile driving and coffer dam positioning can be performed to +- 5mm without any detrimental effects, so is a perfect application for DGPS. 8.How important is the use of GPS for survey purposes in a cable stayed bridge project? How was it used in the Bandra Worli Sea Link Bridge project. The application of GPS in cable stayed bridge construction is quickly gaining acceptance, for specific tasks. While it can’t replace all traditional survey equipment – it does have cost benefits in certain applications.

Bridges far from shore, very tall pylons, marine plant positioning, bathymetric vessel positioning, and construction site control networks are all perfect applications for DGPS. You can even use static GPS receivers for as-builts, provided the Z co-ordinates are not critical.

In dynamic structures that require periodic monitoring, a DGPS system that logs reading once per second, over 24 hours is a much more cost effective solution than a two man crew with a total station and prism pole.
As the Bandra Worli Bridge is fairly close to shore, GPS played a limited role in construction control.