Est. Completion Date:
May 31, 2017
This project replaces the main traffic carrying elements of the Forth Road Suspension Bridge carrying the A-90 over the Firth of Forth near Edinburgh, Scotland, constructed in 1960. The project deviates from the existing A-90 right of way south of the Firth and re-joins at the north end, for a total right of way replacement of about 6km. Approximately 2.6km of this replacement represents a major bridge crossing of two ocean shipping and military navigation channels. Construction began in 2012 and will be completed in 2017. The design/build contractor is Forth Crossing Bridge Constructors, a joint venture of Hochtief Construction AG (28% ownership), American Bridge International (28%), Dragados SA (28%) and Morrison Construction (16%).
The bridge structure has an overall length of 2,638m, including a cable supported structure of 2,090m. The two main navigation spans are 650m each. The bridge has 14 spans, three concrete towers in the center of the transverse cross section up to 210m in height, two planes of stay cables that anchor in the center of the structure and a composite steel tub/concrete deck superstructure. The cable stay bridge superstructure was erected mainly by deck mounted gantries.
Overall there are 13 main bridge foundations, including three excavated caissons, one precast cofferdam, five marine sheetpile cofferdams, one land sheetpile cofferdam and three conventional pile or spread footings.
The foundations for the main span are founded on soft clay materials over glacial till and rock at depths of 30m to 40 m for the South and North Towers.
The foundations required a variety of methods. The three major circular excavated caissons were designed by FCBC and fabricated in Poland. They were shipped to the site, floated into place, moored on specially constructed dolphins and sunk in their precise position. They were excavated and loaded with additional casing segments and concrete around the perimeter to enable sinking, and steered to assure proper positioning and verticality. After the caissons reached bedrock at +/- 40m depth, the perimeter of the steel caisson was sealed with jet grouting (to address sloping bedrock conditions) and the core rock surface was cleaned with airlifts and inspected with drones and divers prior to placing the large interior concrete seal plug. These seal plugs contained large tremie placed concrete pours, one reaching almost 17,000M3 – the largest known underwater concrete placement.
During sinking operations for the caissons, stability and vertical control was a key consideration. Joint venture technical staff were constantly monitoring progress and controlling locations for adding concrete weight to control the vertical process.
There were six marine cofferdams for bearing foundations, one of which (at CT) was prefabricated. These were each driven, excavated, rock surface cleaned, tremie seal poured, and dewatered to enable foundation construction. There were also four land based conventional spread footings within excavated pits, and two abutments.
Steel and other bridge components were sourced from all over the world. For the foreign fabrication, the joint venture utilized fabricators prequalified by audits performed by the Quality Department and Owner. During fabrication ABC utilized expatriate and non-expatriate staff to setup project management teams located at the various fabrication yards in several locations in China, Poland, Spain, and the UK. These teams were responsible for quality oversite/assurance, perform geometry control and survey, schedule and sequence the work, oversee shipping methods and sea fastening design, and manage shipping. The size of staff depended on the fabricator’s experience with European standards and the type of work, but reached a peak of about 50 persons. The team provided key cross cultural communication functions and interfaced with the fabricators to ensure that shipping and schedule dates were compatible.
26 large, high load capacity, spherical bearings were fabricated in China for supporting the approach girders. Two large, approximately 1.5 m movement, expansion joints were also fabricated in Europe and China and shipped to site.
The component manufacturers were accustomed to working within a variety of global standards, and utilized programs acceptable to the joint venture for quality control and assurance. However, AB provided audits of the facilities and ongoing checking and quality assurance overchecks during fabrication of the components to ensure the products met specifications prior to shipping to site.
The six-span, twin box, 543m South approach viaduct was incrementally launched in twelve stages. This structure includes concrete ‘V’ shaped piers, twin steel trapezoidal box girder superstructures, cast in situ concrete deck, guardrails, windshields and wearing surface overlay. The concrete V piers are constructed with conventional jump forming techniques. Access to the piers is provided by dredged channel (for marine craft and material delivery) and walkway (for personnel). The superstructure was constructed by incremental launching from the south landside. The alignment is tangent. The steel box girders were assembled in the launch area due to shipping restrictions. The in-situ concrete deck was placed using collaborating formwork (precast forms that become composite with the deck) that was erected prior to launch except at the leading edge.
The twin box 75m north viaduct was preassembled on land along with 158m of single box cable stay bridge units, and launched as a single 233m long unit.
Cable Stay Bridge
The cable stay bridge includes three concrete towers designated as ST (south), CT (central) and NT (north) that range in height from 202-210m. They are positioned in the center of the bridge cross section and inclined in two slopes, one through deck level and one from the deck to the top. The tower shape is flat on the longitudinal sides and radius on the transverse sides. They are hollow, with steel box cable anchorages in the top 60m of the tower. The central tower is fixed to the steel deck superstructure by post tensioning and the bridge deck floats around the north and south towers, protected by lateral bearings.
The towers were formed utilizing an electric self-climbing system (designed by Cantilever Engineering) and 20m tall “birdcage” to maximize work time in the high wind conditions prevalent in Scotland. Most concrete was pumped. Decks and tie-ins were designed and built to accommodate the material and manhoist elevators. A fixed platform was constructed on top of the caissons and with an additional pile support for the tower cranes and personnel hoists. A floating platform was also provided to accommodate the additional space needs for materials, equipment, personnel mess facilities, etc. Concrete was batched by the joint venture, delivered in hoppers fixed to barges, and pumped to position. The concrete was placed in 5m lifts. Rebar and steel core segments were delivered to the workpoint by barge and lifted by tower crane. The structural steel core segments weighed a total of 1,200mt, and were fabricated in China. The tower cranes were located outside of the deck footprint to enable continuous climb, unrestricted by superstructure erection activities. This required unusually long support connections (+/- 18m) to the tower.
The main bridge superstructure consists of composite steel tub girders fabricated into 128 segments ranging in length from 6 to 20m. The 35,000 tonnes of structural steel was fabricated in China delivered in 5 shipments to the site. The fabrication for each shipment was carefully sequenced by our team, and coordinated with storage capacity at the site and the schedule for casting the decks.
After delivery to the site, the segments were unloaded, cast with the composite deck, fitted with MEP and underdeck gantry mountings, re-loaded and floated to the erection point. There are four unique conditions: 1) Segment erection at towers. This was undertaken by lifting the segments onto special falsework supports, using a shearleg crane. 2) Typical segment erection. This was undertaken with specially designed traveling gantries lifting from a barge. These custom gantries were designed by FCBC and weighed about 700mt each. Due to the width of the deck, the gantries have a built-in mechanism to control transverse deflection. 3) Segment erection at Piers S1-S2. In order to avoid re-mobilizing a shearleg crane, a special procedure was developed for these pier segments. Erection proceeds in a north-south direction. A falsework bent with a launchable section was constructed and attached to the north of the completed pier S1. The S1 segment was erected by gantry crane north of the pier, with the launchable section retracted. After reaching final elevation, the falsework section was launched underneath the just-erected pier segment, and that segment was then lowered to the falsework and skidded over pier S1 to its permanent position. The falsework was removed, allowing the segment just north of the pier to be erected. The segments were closed and the gantry was then launched forward to erect the segment just south of pier S1. The same procedure was completed at pier S2, enabling the cable stay bridge to be closed to the Approach Viaduct South. 4) Segment erection at N1-NA. These two spans also required special procedures due to access restrictions caused by a bluff at the shoreline. In this area the superstructure was erected in its entirety on land, and launched over the abutment and Piers N2 and N1. This was complicated by the fact that this 233m segment contains both twin box viaducts and a single box, and also by the fact that the single box portion is ultimately cable supported.
The cable stay bridge erection cycle included segment lifting, rib bolt-up, welding, concrete stitch pour installation, cable stay installation and tensioning, and gantry launching. A special narrow gantry was conceived and developed by the team that took full advantage of the center planes of cable stays and their attendant stay webs.
The road networks feature both greenfield divided highway approaches and complex sequenced tie-ins to the existing networks. On the South side the existing Route B800 was widened and reconstructed including a new bridge over the M90 mainline, and the existing Route A904 was reconstructed to include a new South Queensferry elevated roundabout junction. The existing route B924 was partially relocated, and Society Road was reconstructed. Extensive protection and redundancy work was carried out on a 1m diameter BP Oil pipeline, which had 3 crossings of the new ROW aggregating 331m. This required rough excavation by the DBJV, close excavation, cleaning, and wrapping by BP, construction of 500mm thick concrete containment footers and sidewalls by the DBJV, close backfill by BP, then placement of the 500mm top and final backfill by the DBJV. The DBJV also constructed a 900mm spare pipeline encased in concrete for possible future use.
On the North side the new M90 tie-in required the sequenced reconstruction of two roundabouts underneath the new and existing alignments, the construction of a new mainline elevated viaduct over Ferrytoll Road (B981), relocation of the existing B981 on a new embankment for a total of 700m, reconstruction of Castlandhill Road, and reconstruction of numerous local streets. Utility works included the construction of 500m of 1.5m and 300m of 1.2m dia combined sewer diversion and 1.1km of 355mm HPPE water pipe diversion. Total earthwork quantities for upland work was XX M3.
New transit links and stations were provided on both sides of the Forth.
The north road networks contained variable depth and soil types. A patchwork quilt of soil improvement methods was developed during design to address the variability and the viability of suggested methods.
There were five site installations for the FCBC JV – four on the north and one on the south: 1) Start up installation was in an existing office building within the Babcock marine complex on the north side. It was rented for seven months until the permanent facilities were completed. 2) South compound for FCBC and Transport Scotland, 1,500m2 at Echline Fields on the south side. This housed the south networks and South approach viaduct teams for both the JV and Transport Scotland. The facility was constructed from modular buildings on land provided to the JV by the owner free of charge. 3) Main FCBC and Transport Scotland Site installation on King Malcolm Drive on the north side, 5,500m2. This modular facility was purchased from the London Olympics 2012, and housed the job headquarters, canteen facilities, training center, and some yard storage area. This site was leased. 4) Port of Rosyth facilities and yards, North side, also leased. This was the location of the main project stores and storage area, medical center, carpentry and rebar workshops; the concrete batching plant occupying 5,000M2, laboratory, precast fab plant of 9,000M2 including workshops and cable/PT storage; 46,000M2 of improved segment storage area; waterfront marine facilities (for crewboats, barge fleeting, multi-cat docking, tug storage, etc): 685LM; Deep water quay wall: 325LM, and a 1,000M2 office building housing the marine operations center, towers team, and cable stay bridge team. 5) Launch area offices, North side. This was a modular complex that served the North Approach Viaduct assembly and launch operations as well as North road networks teams.
Equipment and temporary works
Major equipment utilized included a Concrete batch plant with capacity of 240m3/h, which produced some 200,000m3 including about 110,000m3 that was marine delivered, with 99.8% compliance; numerous concrete pumps and barge mounted concrete rotators, three tower cranes on towers w/35t capacity @ 18m; three tower cranes on piers w/12t capacity@ 20m; three sets of self-climbing tower forms, V-pier forms, Doka collaborating concrete deck forms for the approach viaduct deck placement, a floating shearleg crane with 800t capacity; six bespoke erection travelers fitted with strand jack lifting systems; numerous telehandlers, track cranes, barges and crew boats; six Alimak material and manhoists; three tugs; two multi-cat propelled barges with integral cranes; extensive welding equipment; and earth moving and grading equipment. A 1,600t shearleg was also rented for erection of the tower platforms and segments and erection travelers.
As the project has a wide range of design conditions, extensive temporary condition engineering and works were required. Temporary trestles were utilized to enable access to the foundations that are in the tidal flats on both sides of the bridge. An extensive network of temporary roads were also constructed to allow access to the various works.
The six erection travelers were conceived by FCBC to accommodate the superstructure geometry and reinforcement, and were utilized to erect the 700mt cable stay bridge segments. Kingposts were designed, fabricated, and erected to control deflection in the launches. Many types of temporary support were conceived and designed by the team, including triangular tower segment falsework bents, pier falsework bents with skid frames, and a wide variety of smaller temporary support systems.
The Forth Replacement Project has an environmental plan approved by Scottish National Heritage and a wide variety of other groups. The scheme approval was undertaken by the owner, Transport Scotland, during the planning for the project. FCBC has been responsible for securing final approvals based on our specific designs. Environmental features at the site include ancient monuments, listed buildings, wetlands of international importance (Ramsar sites), Great Crested Newt sites, historic garden and designed landscape sites, special protection areas, and European Union sites of special scientific interest and ancient woodlands. Protections are designed for badgers, otters, bats and a wide variety of fauna. A badger sett (house) was designed and submitted, and over 100 of these protected animals were caught and relocated to new homes.
FCBC has reduced environmental impacts from what was allowed in the approved environmental plan by the use of trestles instead of dredging and embankments, launching instead of crane lifting and avoidance of the nearshore areas.
Structural Health Monitoring
A robust structural health monitoring system was specified by the client, and final design was performed by the joint venture. The health monitoring system is a key component in helping the OMR team, subcontracted separately, to execute and update the operations, maintenance, and replacement plan developed during design. The comprehensive system ties into the OMR plan, and includes vibration monitoring, corrosion sensors, anemometers, strain gages, and displacement/load monitoring of the bearings and expansion joints. The system provides the OMR operator extensive data for updating inspection plans, risk registers, and rehabilitation/repair planning.
FCBC’s innovations have included a wide variety of technical and commercial measures that enabled the venture to be the low bidder for the project by a significant margin. This included the re-design of the foundations, the raising of the central tower to reduce rock excavation at Beamer Rock, the complete re-design of the structural steel superstructure, the substitution of embankment for structures in the approach road networks and the global sourcing and procurement of materials for the project.