Replacement of sitra bridges: a mega- project for Bahrain · Sitra causeway The existing Sitra causeway is a dual, two-lane carriageway with two marine bridges – a northern bridge

C I V I L e n g I n e e r I n g

Keywordsbridges; geotechnical engineering;

project management

doi: 10.1680/cien.2009.162.6.34

proceedings of IceCivil engineering 162 November 2009

Pages 34–41 Paper 09-00024

The Sitra bridges are part of a busy 3.2 km causeway linking the main island of Bahrain to the island of Sitra, one of the most strategic road links in the kingdom. However, after just 30 years the structures have succumbed to the aggressive marine environment and are being replaced at cost of US$280 million, along with construction of a new causeway alongside the old one and a major new grade-separated intersection. This paper describes the design, construction and management challenges of delivering the country’s biggest ever road project in a sensitive marine environment and highly congested urban area.

Replacement of sitra bridges: a mega-project for Bahrain

Mostafa HassanainPhD, PEng, PMP

is head of bridge and flyover projects at the Ministry of Works, Manama,

Bahrain

The Sitra bridges project in Bahrain involves the replacement of a 3.2 km causeway linking the main island of Bahrain to the island of Sitra across the environmentally and politically sensi-tive Tubli Bay (Figure 1). The causeway is one of the most strategic road links in the kingdom’s highway network.

Since the causeway opened to traffic in

1976, the structural condition of its two marine bridges has deteriorated so significantly that it is no longer economically feasible to maintain or repair them. In addition, the causeway can-not accommodate the ever-increasing traffic volumes it must carry. This has led to long traffic queues that are frustrating to road users and have a negative impact on the movement

Figure 1. Sitra causeway in February 2007 (looking north)

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RePLACeMeNT oF SITRA BRIDGeS: A MeGA-PRoJeCT FoR BAHRAIN

of goods and services, and consequently a detri-mental effect on the local economy. The cause-way is currently being replaced.

The project, which commenced in November 2006, also involves the transformation of the northern approach to the causeway from an at-grade, signalised junction into a three-level, grade-separated interchange. This will be the first such interchange in Bahrain. Umm Al-Hassam junction is the main road junc-tion leading to the causeway from the capital, Manama, and is one of the most traffic-congest-ed junctions in Bahrain.

The approximately US$280 million project is the largest and most complex single road project ever undertaken in Bahrain. The king-dom is investing heavily in this road segment because most of the passenger and heavy traffic to and from Saudi Arabia passes through it. In addition, following completion of the pro-posed Qatar–Bahrain causeway, this segment of the road network will be the main access to Manama from Qatar.

Enhancement will bring significant benefits to the economy of the kingdom by easing the move-ment of people, goods and services not only on a local scale but also a regional one.

The client is the Ministry of Works of the Kingdom of Bahrain, the designer and construc-tion supervision engineer is Cowi of Denmark and the main contractor is Gamuda Berhad of Malaysia.

Sitra causeway

The existing Sitra causeway is a dual, two-lane carriageway with two marine bridges – a northern bridge 216 m in length and a southern bridge 576 m in length. The remainder is on embankments. The existing bridges are charac-terised by simply-supported spans of 12 m each (Figure 2). They carry several utility lines includ-ing high- and low-voltage cables, water pipe-lines, telecommunications cables and a natural gas pipeline. These are located beneath the deck and in the central median on an independent service deck. Some of the utility lines provide vital links in the overall services grid of the whole country. The existing bridges have low navigational headroom of 1.9 m above mean high-water level.

Due to the strategic importance of the causeway to the overall roads network, traffic movement and the utilities had to be maintained during its replacement. The new crossing is cur-rently being constructed approximately 50 m to the west of the existing one and mostly parallel to it. The new causeway will be a dual, three-lane carriageway. In addition, in each direction there will be a hard shoulder that could be converted into a fourth lane in the future when increased traffic volumes require it.

The two new bridges are 200 and 400 m

in length. The northern bridge has four spans (45.4, 54.6, 54.6 and 45.4 m), while the south-ern bridge has seven spans (50, 60, 60, 60, 60, 60 and 50 m). Each bridge consists of two separate and similar structures carrying one direction of traffic. The superstructures consist of cast-in-place, post-tensioned, voided concrete box girders with a maximum depth of 3 m (Figure 3). The substructures consist of cast-in-place, reinforced concrete piers (Figure 4) car-ried on reinforced concrete pile cap crosshead beams over bored, steel-encased reinforced concrete piles embedded in bedrock. The abut-ments comprise cast-in-place reinforced con-crete bank seats on similar piled foundations.

Figure 4. Piers of the northern bridge at various stages of construction – three temporary piled access platforms allow tidal flows to continue around the works

Figure 3. New marine bridges span 45–50m and provide navigational clearance of 5.2 m

Figure 2. Existing marine bridges have 12 m spans and only 1.9 m navigational headroom

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HASSANAIN

The new bridges have navigational headroom of 5.2 m above mean high-water level.

Aesthetics of the marine bridges was a major criterion set out by the client. This has resulted in a decision to eliminate the cluttered appearance of utilities on and beneath the bridge deck and to provide instead chambers for the numerous existing and future utilities inside the cross-section of the superstructures of the marine bridges.

The causeway embankment comprises a bund constructed out of quarry run protected by rock armour towards the sea. The bund serves as retainer of the dredged-sand filling forming the final embankment and also acts as temporary access road during construction.

Following the opening of the new causeway to traffic, the two old bridge structures will be demolished, while the existing causeway will be retained and converted into a landscaped public recreational area.

Umm al-Hassam junction

For years, the signalised Umm Al-Hassam road junction has held the undesirable distinc-tion as one of the most traffic gridlocked areas in Bahrain (Figure 5). It was determined that most of the traffic capacity benefits which would accrue from replacement of the causeway would not be realised if the junction was not completely overhauled.

The junction is located at the intersection of Shaikh Isa Bin Salman highway – the main road artery from Saudi Arabia to the new Shaikh Khalifa Bin Salman port in the east–west direc-tion – and Kuwait Avenue to the north and Shaikh Jaber Al Ahmed Al Sabah (which is carried by the Sitra causeway) to the south. It is physically constrained due to severe space limitations in the surrounding areas and the presence of a large number of significant under-ground utilities.

The solution adopted was to transform the junction from an at-grade, signalised junction into a three-level, grade-separated interchange (Figure 6). This involved the construction of a 560 m long underpass for east–west traffic, a 26 m long at-grade bridge for north–south traf-fic over the underpass, a 379 m long flyover for east–south traffic and a 183 m long ramp for east–north traffic.

The underpass consists of a watertight open-trough structure with a reinforced concrete bot-tom slab of thickness 0.5–1.2 m tied down with 942 vertical ground anchors to resist buoyancy. The side walls comprise permanent steel sheet piles tied back in the ground with inclined ground anchors to resist horizontal forces, and covered by cast-in-place, reinforced concrete cladding on the traffic side (Figure 7). The total width of the underpass is approximately 26 m and it carries three lanes plus a verge in each direction.

The at-grade north–south bridge comprises a two-span, continuous, reinforced concrete deck slab that is supported at each end by the top of the underpass walls and by a central reinforced concrete wall. The central wall, in turn, is sup-ported by bored, steel-encased reinforced con-crete piles. The bridge has an overall length of approximately 26 m and a total width of about 73 m. It accommodates eight traffic lanes in a variety of through and turning movements, in

Figure 5. Gridlocked Umm Al-Hassam junction in December 2005 Figure 6. Rendering of the proposed grade-separated interchange

Figure 7. Underpass trough during reinforcement placement for bottom slab – showing vertical anchor sleeves, under-slab waterproofing and permanent sheet piling



addition to a built-up reinforced concrete serv-ices corridor having a width of approximately 10 m to accommodate several utilities.

The east–south flyover has six spans (49, 68.61, 80, 66.39, 60 and 55 m), while the east–north ramp has five spans (31.823, 40, 40, 40 and 31 m). The superstructures’ consist of cast-in-place, post-tensioned, voided concrete box girders. The substructures structural system is similar to that of the marine bridges.

Durability design

The Middle East region is characterised by aggressive environmental and climatic conditions which make structural concrete vulnerable to chloride attack and rapid dete-rioration. The Sitra causeway is located in a severe marine environment exposed to high water salinity and temperatures, airborne chloride salt sprays and salt-laden dust, high temperatures and temperature gradients, and high humidity.

To ensure the structures would resist the adverse conditions and meet the client’s requirement for durability, a criterion was set by the client for the design life of the structures at 120 years in accordance with British Standard BS 5400-4, Steel, concrete and composite bridges.1 This was the bridge design standard adopted by Bahrain until March 2006, when it was replaced with the American Association of State Highway and Transportation Officials AASHTO LRFD bridge design specifications.2

A rational probabilistic, performance-based, service-life design methodology was used by the designer to model the transport of aggres-sive substances into concrete and the corre-sponding deterioration mechanisms. Life-cycle costing studies were carried out to compare the performance of different alternate materi-als with respect to the design life requirement.

The approach adopted was to use stainless steel reinforcement in those areas of the struc-tures most exposed to chlorides, combined with carbon steel reinforcement in other locations. The reinforcement is distributed as follows

n piles – carbon steel onlyn pile caps – carbon steel mostly with a few

stainless steel dowel bars for the piersn piers – stainless steel in the outermost

layers with carbon steel in inner layersn superstructures – stainless steel in the

outermost layers of exposed outer and inner surfaces with carbon steel in dia-phragms and as bursting reinforcement

n base slab in underpass – carbon steel onlyn cladding walls covering sheet piling –

stainless steel in the outermost layers of exposed surfaces with carbon steel in inner layers.

It is worth noting that, according to the literature, the Sitra causeway is the first project where such extensive use of stainless steel reinforcement has been adopted. Other examples for its use for bridge construction in North America, Europe and Asia have been recently reported, although the use of such reinforcement in each particular case was not this extensive and was constrained to certain structural elements.

Concrete is specified in six different classes for reinforced and prestressed concrete according to the structure exposure condi-tions and possible concrete deterioration mechanisms. Cement type Cem I (with minor changes and additions) in accordance with BS EN 197-13 and fly ash are used as binder. The fly ash content is 30% of the total binder content by weight. The most important ben-efit of using fly ash in concrete is reduced permeability to water and aggressive chemi-cals by creating a denser product, and thus improving the resistance of reinforcement to corrosion. The maximum water/binder ratio is 0.4. The gradation, type and source as well as conformity procedures of fine and course aggregates are diligently specified to avoid the problems exhibited by aggregates used in Bahrain.

Compressive strength of all reinforced and prestressed concrete is specified at C40/50 in accordance with BS EN 206-14 meaning the 28-day characteristic 150/300 mm cylinder strength of 40 MPa or 150 mm cube strength of 50 MPa.

The nominal concrete cover to reinforce-

ment steel is specified as 80 mm, except for piles where it is 100 mm. This may be reduced to 45 mm in locations where stain-less steel only is used. The tolerance on the cover layer measured at any point before and after concreting is set at ± 10 mm applied to the specified nominal cover.

Geotechnical conditions

Difficult ground conditions have been encountered in the project. In general, the bearing strata for the foundations consist of carbonate siltstone and mudstone deposits with a minimum thickness of about 15 m. These are underlaid by limestone deposits which are generally located at depths 25–35 m below seabed or ground level, and which are known to be very competent bearing strata. Compared to these limestone deposits, the strength of the carbonate deposits is more modest.

Ground investigations at Umm Al-Hassam junction and in Tubli Bay for the northern bridge showed that the carbonate siltstone and mudstone deposits were so weak that they could not be core drilled; they behaved more like slightly cemented soil than weak rock. For design purposes, these deposits were treated as slightly cemented very dense sand instead of very weak rock – an assumption considered to be on the safe side. A geological section along the causeway alignment is illustrated in Figure 8.

Assigning characteristic values for the relevant geotechnical parameters to be used in the design of the foundations was challenging due to the

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Figure 8. Geological section along the causeway alignment


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variability and relative incompetency of most of the underlying ground strata. This was accom-plished using the available ground investigation data in conjunction with experience with similar ground conditions in the Persian Gulf region. The presence of relatively competent bearing strata of carbonate siltstone and mudstone at variable depths below seabed or ground level called for pile foundations. Pile diameters varied between 1 m for the east–north ramp to 1.2 m for the piers of the southern bridge, and 1.5 m for the east–south flyover, the northern bridge

and the abutments of the southern bridge. Some of the piles were raked at 4:1 (Figure 9) to help resist the horizontal earth and surcharge pres-sures more efficiently.

Pile lengths in limestone (rock socket lengths) varied due to the competency variability of ground strata. The derivation of ultimate end bearing capacity and skin friction of the bored piles was based on the design principles outlined in AASHTO LRFD bridge design specifications.2 Rock socket lengths varied from 16.5 to 37 m depending on the characteristics of the bear-ing strata; the longest were for the piers of the northern bridge.

Pouring mass concrete

Several structural elements required pouring massive quantities of concrete. The casting of elements such as the base slab of the underpass, as well as the pile caps (some of which have dimensions of 19 × 7.5 × 3 m for the northern bridge) can generate significant heat from hydra-tion of the cement, leading to thermal cracking. This is even more challenging during the hot summer months.

The specifications limited the maximum concrete temperature to 65ºC. Additionally, the maximum temperature difference between the mean temperature of any structural element and the temperature at the surface of the element was limited to 15ºC. To reduce the temperature of mass concrete pours, the contractor used small-diameter steel pipes embedded in the concrete to circulate cool water (Figure 10). This decreased the core-to-surface temperature gradient. It also controlled the subsequent heat removal and accompanying concrete volume changes during the early stages of hardening in the first several days following placement.

Water with a temperature of 15ºC was cir-culated in pipes fitted with control valves that allowed small adjustments to the rate of water flow. The water then returned to the cooling machine at a temperature of about 18ºC, where its temperature was brought down to about 15ºC and recirculated in the pipes within the mass concrete.

Concrete temperatures were continuously monitored for 14 days after placement, after which the embedded cooling pipes were pres-sure grouted.

Silt treatment and removal

Deep silt deposits on the seabed in Tubli Bay proved problematic for the construction of the causeway as they are highly compressible and not suitable as a foundation base. North of the northern bridge, the silt layer thickness was about 1.5 m and had to be removed.

A simple method was utilised by the contrac-tor, involving building a temporary rock bund

Figure 9. Abutment piles for the marine bridges – some were raked at 4:1

Rock socket lengths varied from 16.5 to 37 m depending on the characteristics of the bearing strata

Figure 10. Cooling pipes within the reinforcement of the underpass bottom slab



3 m away from the toe of the proposed per-manent quarry run bund. The temporary bund fully enclosed the area to be cleared from silt (Figure 11). Water pumps were used to dewater the bunded area and the silty material was left to dry. Excavators then removed it for disposal in an area approved by the environment authori-ties. Over 15 200 m3 of silt was removed from this location. After removal of the silt, construc-tion of the permanent rock bund and embank-ment filling followed.

A similar method was used at the southern approach to the southern bridge, where silt deposits 3–3.2 m thick were present. However, this was a larger area and even after it had been dewatered, access for the removal of the silt was not as easy. Temporary rock access roads were therefore constructed within the bunded area; there was a central spinal access with ‘fingers’ leading off on both sides. These access roads overlaid the silt and were approximately 1.5 m thick. They were removed on completion of the work. About 40 000 m3 of silt was removed from this location.

The presence of much deeper deposits of silt than was expected in other areas of the project necessitated the adoption of a more complex and expensive approach. Some areas south of the causeway, where a permanent road was to be constructed, had deposits with depths exceeding 6 m. At the toe of the south abutment of the southern bridge, a ‘valley’ with an average thick-ness of silt of approximately 10 m was identified. At the north abutment of the southern bridge, silt layers approximately 2 m in depth were present. Dredging was utilised at these three locations to remove the silt.

A cutter-suction dredger pumped the silt to a separation and water purification area (Figure 12). The material was then allowed to settle. Clean water was later discharged into the sea and the remaining silt was removed by excava-tors to a stockpile area, where it was left to dry and then disposed of. A total of about 287 000 m3 of silt was removed using this method. Since construction of the southern bridge was a critical-path activity, removal of these unexpect-edly large quantities of silt caused a delay to the contractor’s schedule. This was the main reason for granting the contractor a 211 day extension of time.

working within Tubli Bay

The existing marine bridges allow coastal tidal flows into and out of Tubli Bay to the west and the passage of minor marine vessels mostly for small-scale fishing activities. Tubli Bay is an inshore coastal area of high environmental sensitivity and importance, and was once consid-ered to be one of the richest marine and coastal resources in Bahrain. However, over the past three decades, the bay has suffered significantly

from intensive reclamation, and the discharge of partially treated sewage and industrial effluents. Government and community bodies are now voicing the urgency of restoring Tubli Bay. The matter has also been a source of much political wrangling in the country’s parliament as well as in the local media.

The environmental and political sensitivity of Tubli Bay precluded any significant blocking with temporary backfilling of the northern and southern marine channels during construc-tion of the new bridges, in order to maintain

tidal levels and therefore flushing of the bay. The contractor thus erected three temporary platforms supported on driven steel piles across each marine channel to provide access for construction activities (see Figure 4). These platforms allow largely unrestricted water movement into and out of the bay.

Temporary cofferdams are installed to provide safe working platforms for the construction of the bored piling, pile caps and piers. Each coffer-dam has dimensions of 24.7 × 24.7 m, and con-sists of steel sheet piles, walers and cross-braces

Figure 11. Treatment of silt north of the northern bridge

Figure 12. Dredging of silt south of the causeway


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(Figure 13). Installation of the cofferdams is staged in such a way that minimises obstruction to tidal flows and thus the flushing of the bay. The cofferdams are dewatered through staged excavation with progressive installation of sump pits. Removal of silt and other unsuitable mate-rials is carried out until reaching the caprock level. Depending on the particular geotechnical conditions at each cofferdam, caprock is to be broken through to reach the formation level of the pile cap. The cofferdams are then filled with dredged sand for construction of the bored piles, following which the infill material is excavated to the formation level of the pile caps to allow the construction of the pile caps and piers.

Project management challenges

Mega-projects of such size and complexity require the cooperation of numerous stakehold-ers representing a variety of interests, priorities, opinions and agendas. There are over 40 primary stakeholders in addition to numerous other secondary stakeholders involved in the project. Achieving and maintaining stakeholder and public support and cooperation throughout the project has proved exceptionally difficult; in fact it has been the most challenging aspect of the project thus far.

Some of the numerous project management challenges encountered on the project are dis-cussed in more detail below. Other challenges included relocating 23 tenants of the northern strip of Mina Salman industrial area to make possible the widening of the southern side of Shaikh Isa Bin Salman highway, and compensat-ing the fishermen of Tubli Bay who were affected

by the construction works and accommodating them at another location within the project area.

Traffic management plansMega-projects attract intense pressure to mini-

mise, if not to eliminate completely, their adverse construction impacts on road users. In addition, some of the toughest project challenges tend to be concentrated in some of the oldest and most congested urban areas. The combined effect of these factors presented huge challenges to man-aging traffic in and around construction zones. These factors played a big role in determining the optimal traffic management (diversion) plans implemented in the project.

A key requirement by the client was to maintain traffic without interruption during all phases of construction. A key objective of the traffic management plans developed was to minimise construction-related traffic congestion by providing safe and comfortable detours to road users around the work sites. This was not only beneficial to the travelling public but also to the contractor by ensuring safe and efficient construction activities.

In addition to many minor and short-dura-tion traffic diversions, the contractor has thus far successfully implemented one major traffic management plan at Umm Al-Hassam junc-tion. This plan was put in place in May 2007 to allow the contractor to start excavating the western side of the underpass. The junction was converted from a signalised intersection into an elongated signalised roundabout, often referred to by the project team as the ‘tennis racket’ due to the obvious resemblance (Figure 14). Traffic was rerouted safely around the underpass work

area. Traffic counts and observations revealed that traffic conditions in the junction after the implementation of the ‘tennis racket’, in gener-al, did not get any worse than they were before the implementation.

Due to the anticipated effect of the traffic management plan on road users, an extensive media campaign was put in place to alert the public and to guide them to possible alternate routes away from the construction site. An extensive communications plan was implement-ed to secure the support and cooperation of the traffic planning authorities and the traffic police.

Underground services diversionsBahrain is a small country. As a consequence,

all road reserves are usually occupied with numerous underground utilities. At major road intersections, such as Umm Al-Hassam junction, myriad utility crossings exist. Some of these utilities were laid many years ago and accurate as-built drawings for their alignments and depths do not appear to exist. It is not always possible to confirm the locations of all utilities crossing major intersections during project planning as this would result in unac-ceptable interruptions to road traffic. This has resulted in situations where complete reliance on as-built drawings has caused problems dur-ing construction.

For example, early site investigation following the commencement of the project showed that a 900 mm diameter foul sewer pipeline running along the northern side of Shaikh Isa Bin Salman highway is located further south than anticipated at its western end, and that it encroaches into the excavation of the underpass. The contractor was instructed to construct a realignment of the encroaching segment of the pipeline away from the underpass excavation.

At the eastern end of the pipeline, the sewer was found to be very close to existing build-ings and the contractor decided to thrust-bore the sewer in the vicinity of these buildings. The sewer relocation was also affected by 11 kV and 66 kV electricity cables, and telecommunications fibre-optic cables, which either cross or run near the revised alignment. Thrust-boring was there-fore extended to overcome these obstructions.

The connection of the revised alignment to the existing alignment at the eastern end was affected by water mains, resulting in the instal-lation of a temporary segment of reduced sewer pipe cross-section until the water mains are decommissioned and removed in the future as part of the project. This situation delayed the contractor’s schedule by several months because construction of the underpass is a critical-path activity. The contractor was then granted a 100 day extension of time.

Numerous types of utilities other than those already mentioned exist around and across Umm Al-Hassam junction: 220kV and low-

Figure 13. A cofferdam with a completed pile cap and pier



voltage electricity cables, several sizes of water mains, several sizes of stormwater drains, treated sewage effluent lines and natural gas pipelines. Several of these utilities have to be diverted and/or protected during construction. Some will be abandoned, but several will be laid anew.

To complicate matters even more, the authori-ties in Bahrain ban all construction works on or near high-voltage electricity cables for about 6 months every year because there is not enough redundancy in the system to accommodate power cuts caused by damaged cables during hot summer months. Although this is gradually changing as the government upgrades the electri-cal power infrastructure, it remains an important constraint that must be taken into account in the contractor’s schedule. Any slips in carrying out any required cable diversions and/or protections could delay the schedule by several months.

Regular coordination with the various public and private utilities agencies was carried out diligently on this project, sometimes under the patronage and with the contribution of the high-est levels of senior management in the client’s organisation when needed. This helped over-come impediments to the timely completion of

utilities-related works and also instilled a sense of focus and accountability among the concerned stakeholders.

Conclusion

Mobility of people, goods and services is an important requirement for the economic growth of any country. Economic activities prosper where accessibility is good and mobility is fast. Thus, transportation infrastructure facilities are among the most important factors for the devel-opment of an economy.

Bahrain is investing heavily in the upgrading of its transportation infrastructure. Currently, the cornerstone of this investment is the replace-ment of Sitra bridges. This mega-project, when completed in mid-2010, will introduce a major enhancement to the roads network in the king-dom, and thereby will provide a significant boost to the country’s economy.

acknowledgement

The author thanks the Ministry of Works, Kingdom of Bahrain for permission to publish this paper.

Figure 14. The ‘tennis-racket’ signalised roundabout at Umm Al-Hassam junction

what do you think?If you would like to comment on this paper, please email up to 200 words to the editor at [email protected].

If you would like to write a paper of 2000 to 3500 words about your own experience in this or any related area of civil engineering, the editor will be happy to provide any help or advice you need.

references1. british standards institution. Steel, Concrete

and Composite Bridges. BSI, London 1990, BS 5400-4.

2. american association of State HiGhway and Transportation Officials. AASHTO LRFD Bridge Design Specifications, 4th edn. aaSHTO, washington D.C., USa, 2007.

3. british standards institution. Cement – Part 1: Composition, Specifications and Conformity Criteria for Common Cements. BSI, London, 2000, BS EN 197-1.

4. british standards institution. Concrete. Part 1: Specification, Performance, Production and Conformity. BSI, London, 2000, BS EN 206-1.

Documents

Replacement of sitra bridges: a mega- project for Bahrain · Sitra causeway The existing Sitra causeway is a dual, two-lane carriageway with two marine bridges – a northern bridge