90

16 piersV-shaped design

Pile foundationL=25-50 m,

8 m into the limestone

Fjord bed

Pile Cap

Water surface

1.360 m long

83 m span

492 segments

19,7 m

22 mnavigational clearence

Crown Princess Mary’s Bridge

DesignIt was crucial for the architects to unite landscape and bridge. The bridge is designed as a light and open structure, where the lightness is achieved by a very slim deck and 16 V-shaped piers. Moving the abutments away from the coastline emphasizes the lightness even more when looking at the bridge in elevation.

NatureThe bridge is built in a Nature2000 area meaning stricter requirements for the Contractor with regards to construction methods. To mitigate any damage to the area temporary embankments were built to protect the fjordbed in shallow waters, all excavated materials from the fjord were removed immediately on barges, and curtains were erected under the bridge deck to prevent building material from falling into the fjord.

The high bridge across Roskilde Fjord is 1.360 m long and a new landmark for the area. The dual carriageway offers the commuters an alternative to the exist-ing old Crown Prince Frederiks Bridge in Frederikssund.

Crown Princess Mary’s Bridge Bridge FoundationsThe bridge foundations are designed to minimise the impact on the Roskilde Fjord, which is a designated Natura2000 area, and provide a cost-effective solu-tion for the complicated geotechnical conditions in the fjord.

The bridge spans have been optimised to reduce the number of pier foundations in the fjord, giving 13 piers in the water. Each pier is founded on four cast-in-place bored piles rock-socketed in the limestone bedrock. The properties in the limestone is very varia-ble; the piles range from 1.35m to 1.83m in diameter, from 25.2m to 50.6m in length and the rock socket lengths vary from 8 to 13m. Given the conditions, the design philosophy adopted assume the pile ca-pacity to rely on shaft resistance under SLS and as end-bearing in the ULS. To validate the calculation method two pile load tests were undertaken with Os-terberg-cell©.

To ensure the piles’ ULS performance, pile tip clean-ing and flushing were performed prior to installa-tion of the reinforcement cage. All piles were base grouted to ensure good contact between pile tip and the limestone thereby reducing the impact of any soft toe. Crosshole Sonic Logging was performed to ver-ify the integrity of the bored piles.

The offshore pile caps were constructed well below the mean fjord level to reduce the impact on the fjord, the bridge aesthetics and the navigation. To build the pile caps, pre-cast concrete shells were fitted with

temporary steel cofferdams and lifted in place and dewatered prior to the assembly of the reinforcement cage and concrete pour. With the aim of reducing offshore operations, the concrete mix design was ad-justed after a construction trial to allow casting of off-shore piles and pile caps from a pumping line, which reached up to 250 m, from shore instead of direct tremie placement.

Integral piers To minimise the operation and maintenance (O&M) cost, the design incorporates a semi-integral connec-tion between the deck and the four central piers, pier 8 to pier 11. A total of 8 pot bearings have consequently been removed from the bridge. This solution results in the deck being restrained for 249 m. Integral connec-tions (concrete hinges) consist of 20 mm thick con-crete layer reinforced using stainless steel arranged into the cage with confinement reinforcement. Con-crete hinges are almost maintenance-free and very durable. In addition to an obvious O&M benefits, the use of semi-integral connections allowed for a better transfer of horizontal forces, which resulted in a more optimised foundation layout as the proposed solution allows the transfer of longitudinal forces through four piers rather than one.

Crown Princess Mary’s Bridge Match cast segments and balanced cantilever erection with overhead launching gantry

The erection of the bridge deck used the balanced cantilever method with an overhead launching gan-try, with deck segments built as pre-cast match cast. This method of construction was selected as it is par-ticularly advantageous for long spans in marine oper-ations where access beneath the deck is difficult.

Match-cast precast segmental construction allowed offsite prefabrication of the entire bridge deck as a parallel activity to the construction of the bridge sub-structure. Key achievements were reductions to the overall construction programme and a more gentle impact on the local fjord environment compared to the in situ fabrication of the deck. Segments were manu-factured at a precast yard in the port of Szczecin, Po-land, between December 2017 and February 2019, using three sets of short line match cast forms for the typical segments and one additional set for the pier and abutment segments.

For each cantilever, casting the pier segment was the first step in the construction process. All succeeding cantilever segments were then match cast against the previously cast segment to ensure complete bearing and proper alignment on mating surfaces. Aspects such as pre-camber was included in order to ensure the final bridge geometry. This was accounted for by carefully adjusting the orientation of previously cast segments and the soffit formwork of the new seg-ment to reproduce the relative position of each mem-ber in the structure.

Complex geometry control was carried out in the pre-cast yard by use of 6 survey points on each segment and a computer algorithm. After each segment was cast the actual position of the survey points were recorded and compared with the theoretical posi-tion to identify required geometry corrections in the fabrication of the next match cast segment. Further detailed geometry control was applied during erec-tion by monitoring the cantilever tip deflection and making adjustments, as required, as construction progressed. This was essential to ensure the com-pleted cantilevers aligned at mid-span between the piers where they were joined by a narrow in-situ stitch of reinforced concrete cast.

Transversal and external post- tensioning The geometry of the deck cross section is a sin-gle-cell box girder, 19.7m wide, with relatively long cantilevers of approximately 4.5m. Given the heavy traffic loading, a transverse post tensioning of the top slab has been used. This technique allows for the reduction in the thickness of cantilevers and deck slab while achieving all code requirements in terms of strength and serviceability (section is much slen-der in comparison to a more traditional reinforced concrete solution). It also allowed for the reduction in weight of the deck segments and increased the overall transverse stiffness of the box section, reduc-ing the reinforcement usage per segment.

In addition to internal bonded cantilever and conti-nuity post-tensioning cables, the bridge design also

Crown Princess Mary’s Bridge incorporates external post-tensioning. It allows to take full advantage of the deck cross section and maximize/optimize the length of the spans. The cross sections of box girders with an external post-ten-sioning are slenderer and thus lighter as the thick-ness of the webs is kept to a minimum. As the cables are external and unbonded to the concrete section, they can be removed and, if required, replaced at any time during the lifetime of the structure: Each of the cable can be replaced without traffic closure.

CooperationThe cooperation between RBAI and the Danish Road Directorate (DRD) has been exceptionally good and as a result the pro-ject was handed over 3 months before contract completion date. Both parties agreed to a mutual goal; building a great bridge in unique and demanding surround-ings, keeping up a positive and helpful at-mosphere and working environment.

Client: The Danish Road Directorate (DRD)

Architect: DRD in cooperation with Claus Bjarrum Arkitekter

Consultant: ARUP

Contractor: RBAI JVRazzani de Eccher s.p.a. N.V. Besix S.A Acciona Infraestucturas S.A.

H&SThe design is carefully elaborated to ensure that the highest H&S standards are met for those who in the future will work on the bridge. Access provisions at the abutment are provided with lifting points to lift material in and out of the bridge. The box girder floor is continuous and openings in the diaphragms are step-free and 2m tall by 0,8m wide, allowing per-sonnel to push materials and equipment on trollies along the entire deck without requiring heavy lifting. In addition, lighting and electrical supply is provided regularly throughout the bridge, providing a safe en-vironment where it is easy to work in.