Pioneering Hong Kong’s first horizontal bridge rotation over the East Rail Line under Fanling Bypass Eastern Section Project

By the Civil Engineering and Development Department

The Fanling Bypass Eastern Section (FLBP(E)) is one of the major road infrastructures connecting Fanling North New Development Area (FLN NDA). This dual two-lane carriageway, approximately four kilometres long, provides a direct link with the existing highway network to cope with the anticipated traffic demand arising from FLN NDA.

Figure 1: Artistic impression of the Fanling Bypass Eastern Section

[Source: Public Works Subcommittee (PWSC) Paper PWSC(2018-19)41 Annex 1]

The Civil Engineering and Development Department (CEDD) plays a pivotal role in the development of FLN NDA. AECOM Asia Co. Ltd. was employed as the project manager for the first phase development, including the site formation and engineering infrastructure works commenced in 2019. The FLBP(E) is part of the major works under the first phase development of FLN NDA (Figure 2).

Figure 2: Artistic impression of the FLN NDA

(Source: https://www.ktnfln-ndas.gov.hk/tc/2-1-4-about_project.php)

Innovative construction methods to overcome difficult site constraints

The entire FLBP(E) comprises 3.3-km long viaduct and 0.7- km long underpass. CEDD Contract ND/2019/05 involves the construction of 2-km long viaduct between Shung Him Tong and Kau Lung Hang. The alignment of the viaduct meanders across Ma Wat River and runs through the industrial zones and low-density village residential areas. It spans over the MTR East Rail Line (EAL) and connects to the existing Fanling Highway. The bridgeworks were carried out within a limited working space congested with major underground utilities, including high voltage power cables, large diameter above-ground Dongjiang watermains. These site constraints imposed huge challenges on the bridge construction, and the project team had to carefully plan the overall construction sequence and methodology to overcome them.

The project team had to carefully plan the overall construction sequence and methodology to overcome the huge challenges imposed by the site constraints on the bridge construction.

Challenging site constraints across the East Rail Line

Multiple site constraints and construction challenges

The construction of viaducts spanning over the East Rail Line (EAL) was the most challenging part of the project due to complex site constraints, attributed to the presence of underground 132 kV high voltage cables, large diameter Dongjiang watermains (ranging from 1.4 m to 2.3 m in diameter), adjacent footbridge structure, existing Ma Wat River and the operational railway corridor.

To avoid conflict with the existing utilities and infrastructures, the bridge spans across EAL at Piers D2-01 and E2-01, which were designed as long-span prestressed concrete structures with span length of 100 m and 128 m respectively. Unlike the other sections of precast segmental bridges with a typical span length of 60 m and uniform structural depth, these spans were supported by single-cell prestressed concrete box girders with a haunched profile to provide the required structural capacity. The depth of T-span D2-01 (66 m long) varied from 6 m to 3 m, while T-span E2-01 (136 m long) ranged from 7.5 m to 3 m (Figure 3). In addition, the T-span at Pier E2-01 was designed in a curved alignment with a minimum radius of 340 m.

With the bridge configurations above, the conventional segment erection method by lifter within such congested works area would be rendered impractical by the following:

(i) Transportation limitations of over-height segments fabricated in Mainland China

(ii) Disruption to nearby residents during assembly of segments at ground which involved extensive works near residential areas

(iii) Restricted working hours during non-traffic hours of railway services between 1:30 am to 4:30 am, imposing high risks for heavy lifting operations above the railway tracks

Figure 3: Overview of Bridge D2 and E2

Innovative horizontal bridge rotation method

These challenges were addressed by the adoption of an innovative horizontal bridge rotation method (HBRM) with the bridge deck constructed in-situ using a pair of travelling formwork systems (“form travellers”) in a balanced cantilever mode. The HBRM has been widely adopted in Mainland China with many successful cases of highway projects. This method allowed the bridge deck to be constructed in parallel to the railway track under normal working conditions during daytime. Upon completion of the bridge deck in its initial position, the whole T-span was rotated horizontally in one night, and thereby significantly reducing the risk of disturbance to railway operations.

The form travellers facilitated the sequential construction of large-sized segments at the pier head, and overcame the constraints on site access and storage area. A pair of bridge segments could be constructed within a 14-day cycle during daytime. After achieving the required concrete strength, prestressing was applied to the segment and the form traveller was then launched to the next segment position.

The HBRM was introduced to the project through a collaborative approach by the project team, which took into consideration the following advantages of the method:

• Eliminating the risk of carrying out heavy lifting over the existing railway line

• Reducing the amount of night works adjacent to MTR facilities, minimising potential delay to the project due to uncertain availability of working time slots during non-traffic hours of railway services

• Causing less environmental nuisance to nearby resident

After obtaining MTR’s agreement, in principle, on the use of HBRM, the project team commenced the development of the construction sequence and site logistics for bridge rotation works to address the site constraints.

Collaborative safety design and construction planning

To ensure the successful implementation of the bridge rotation works, the project team engaged the Mainland specialist (CSSC Sunrui (Luoyang) Special Equipment Co., Ltd) to design and fabricate the bridge rotation systems. The bridge designer (YWL Engineering Ltd.) also worked collaboratively with the Mainland specialist to determine the loading that would be experienced by the T-span structure during construction stage, as well as under rotation, and reviewed the structural stability of the rotating structure based on the requirement of Structures Design Manual for Highways and Railways (SDMHR) published by the Highways Department. The integration of advanced engineering technology of the Mainland specialist with local design standards ensured the stability of the bridge structures throughout the whole construction process—under local typhoon conditions especially.

Precision in pre-rotation bridge configuration

The T-spans at Piers D2-01 and E2-01 were constructed in a temporary position parallel to the EAL outside the railway’s operational zone (Figure 4). The balanced cantilever method with form travellers was employed to incrementally build each 4-metre segment of the box girder bridge deck.

To achieve the required clearance from railway facilities while maintaining the original pier positions, the bridge structures were meticulously designed to optimise pre-rotation positioning. The curved T-span E2-01, with a total length of 136 m, was engineered to rotate 33 degrees, allowing it to span across the railway after rotation. However, the final configuration of E2-01 imposed a spatial constraint on the pre-rotating T-span D2-01, requiring it to be cast with a shorter cantilever of 66 m with a rotation angle of 62 degrees. To address the shorter deck length, the form travellers were mounted on T-span D2-01 before rotation, enabling the continuation of its construction immediately after the bridge was positioned.

Before rotation, the bridge deck of T-span D2-01 was positioned just 1 m from the railway boundary fence, requiring an extremely precise execution. To further optimise space, the segment wing of T-span E2-01 and the upper portion of lift tower on an adjacent footbridge were trimmed, thereby creating additional clearance for safe rotation operation of T-span D2-01.

The longest rotating T-span at Pier E2-01 was 136 m in length and weighed over 7,000 tonnes. Due to limited space, the other rotating T-span at Pier D2-01 could only be partially completed before rotation and four pairs of segments were constructed after the planned 62-degree rotation. The configuration of the two rotating T-spans are summarised in Table 1.

Table 1: Technical information of T-spans and bridge rotation system

Figure 4: Layout plan for T-spans (before rotation)

Bridge Rotation Mechanism

The bridge rotation system (Figure 5) comprised the following components, which were installed between the upper and lower turntables:

• A rotational bearing at the centre – This high-strength steel bearing was equipped with low-friction polytetrafluoroethylene (PTFE) sliding plates, sandwiched between the upper and lower parts of the spherical bearing, designed to withstand loads of up to 14,000 tonnes.

• Eight pairs of stanchions installed at the perimeter of the turntable – These stanchions provided additional support and enhance the stability of the T-span during rotation. They rested on a stainless-steel sliding track lined with low-friction PTFE plates with friction coefficient below 0.03.

• Strand jacks and reaction blocks – Two sets of strand wires, casting into the upper turntable, were tensioned by hydraulic strand jacks acting on reinforced reaction blocks to generate the necessary traction for rotation.

During rotation, the majority of the structural load was borne by the spherical bearing after aligning the centre of gravity of the rotating T-span with the centre of the bearing through weight balancing. A pair of stanchions served as secondary supports to resist the out-of-balance loads and prevent overturning of the structure.

A series of temporary stabilisation measures were implemented to ensure that the bridge structure remained secure before, during, and immediately after the rotation:

• Sandboxes for load transfer control – Seven pairs of sandboxes, preloaded with sand in a controlled factory environment, were placed between the two turntables. When the upper turntable was cast, the sandboxes made full contact between the two turntables, resisting the eccentric loading on the system and pre-activating the stanchions. Before rotation, the sand was released, allowing load transfer to the stanchions and spherical bearing.

• Shear steel supports for stability – Ten pairs of massive vertical steel temporary supports (“shear steel supports”) were embedded into the upper turntable and bolted to the lower turntable during casting. These supports were designed in accordance with the requirements of SDMHR to counteract potential overturning forces.

Figure 5: components of the bridge rotation system; special design for bridge rotation structure during construction and rotation of the T-span

During the construction of the T-span in its initial position, the design considerations closely mirrored those of conventional cantilever bridge construction, including verifications for its Ultimate Limit State (ULS) and Serviceability Limit State (SLS) performance. The key distinction lies in the fact that the T-span was supported by a turntable instead of a permanent pile cap. To ensure structural stability and performance, the turntable was restrained using temporary works that transferred loads to the lower turntable.

A comprehensive computer model was crucial for simulating the global and local behaviour of the entire rotating T-span. This model was designed to capture the load effects across various parts of the structure.

During the initial construction of the T-span, several critical load scenarios were considered to ensure structural stability. Wind loads were calculated according to the BS EN 1991-1-4, a Eurocode Standard, and the specifications outlined in Section 3.4 of SDMHR. A critical load case was the out-of-balance cantilever moment, which resulted from differences in loads on the two cantilever arms caused by potential discrepancies in design segment weight, asymmetric live loads or failures in erection equipment. To balance these forces and ensure the stability of the structure, temporary shear supports were installed to connect the upper turntable and the pile cap with the spherical bearing idealised as a pinned joint, sandwiched between these two structures.

During the rotation phase, the system’s configuration changed continuously as the T-span was moved into its final position. This required the removal of the temporary works used during the casting stage to allow for rotation. While the load effects on the deck and pier remained consistent with those in the initial position, the process was carefully monitored to ensure stability. Wind speed was a key factor, with rotation planned for the non-typhoon season to minimise risks to the structure. Real-time monitoring of wind conditions was conducted to check whether the design wind speed of 26 m/s had been exceeded, factoring in any local variations in topography or sheltering.

Sophisticated work sequence of HBRM

Upon completion of the bridge rotation system, the bridge pier and deck were constructed atop the upper turntable in a pre-set alignment parallel to the railway line. Once the T-span structure was completed, pre-rotation preparations commenced. This included removal of the temporary supports installed during construction and weight-balancing of the superstructure using hydraulic jacks placed between the turntables. Counterweights were deployed and positioned on the bridge deck based on the analysis of the structure’s centre of gravity. A trial rotation was then conducted to verify the functionality of the equipment and establish the correlation between the applied jacking force and the rotational movement.

With all technical parameters confirmed, the bridge structure was rotated clockwise using a pair of strand jacks with continuous monitoring by a high-precision survey system. The rotation was executed within the time frame agreed upon with MTR based on a minute-by-minute programme as illustrated in Figure 6. The T-span was stabilised after rotation using temporary supports before the railway service was resumed.

Figure 6: Minute-by-minute programme for bridge rotation operation

Engineering techniques in controlling bridge rotation

As horizontal bridge rotation at this scale was implemented for the first time in Hong Kong, the project team engaged another Mainland specialist (namely, Wuhan Tebridge Engineering Consulting Co. Ltd.) to monitor the rotation operation with smart devices.

Key points for ensuring smooth execution

Wind speed and weather forecasting: Wind speed was a critical factor in ensuing the structure’s stability. The pre-rotation schedule had to be agreed upon with MTR, with particular emphasis on not removing temporary supports prematurely. To determine the optimal timing for removing temporary stabilisation measures, the project team relied on close liaison with the Hong Kong Observatory, which provided detailed predictions on rainfall and wind speed near the rotation time. In addition, real-time monitoring of wind speed was conducted on site to avoid sudden gusts exceeding operational limits. Special “Alert, Alarm, and Action (AAA)” alert levels were established to warn the team if wind conditions became unsafe.

Weight balancing: Weight balancing is a critical step in preparing the rotation operation. It is carried out after the removal of the temporary stabilisation works, including the shear supports and sandboxes, which were installed to ensure the structural stability during construction stage. At this stage, two 500-ton hydraulic jacks were installed at the perimeter of the turntable, on the railway side, to initiate the weight balancing procedure. The objective was to verify the centre of gravity (CoG) of the bridge structure and its alignment with the centre of rotation. To adjust the CoG, counterweights were added to the bridge deck, based on the design CoG, to strategically shift the CoG towards the target operation range. In the next stage, where the T-span was able to be rotated by the hydraulic jacks, the frictional force and the CoG were evaluated by monitoring the movement curve versus jacking force chart. An additional 17.6 tons of counterweight were placed to the bridge deck, ensuring the CoG remained within 100 mm of the target position and away from the railway line. After the weight balancing operation, the bridge deck was slightly tilted away from the railway. This tilt was maintained throughout the entire rotation process, ensuring a smooth and controlled movement while managing the load distribution at all times.

Trial rotation: A 1-degree trial rotation was conducted to test the “inching movement” of the bridge under jacking by the strand jacks. During the pre-rotation stage, the structure was not permitted to encroach into the railway bar fence, restricting the trial bridge rotation operation to just 1-degree rotation. This limited rotation was considered sufficient to gather critical technical data on the traction force required to overcome static and kinetic friction during rotation, as well as the kinetic behaviour of the system throughout the operation. The success of the trial rotation provided the team with crucial data allowing for precise adjustments in the final stage of rotation, specifically in the last 1.5 degree rotation angle before achieving the final alignment.

Rotation speed control: The primary concern was to maintain a uniform rotation speed to prevent sudden jerks that could damage the structure or disrupt the alignment. To mitigate this risk, the rotation was executed in several small increments with each step carefully monitored by the operation team.

Control of rotation speed was of paramount importance. Given the tight timeframe during railway non-traffic hours, the rotation of each T-span (E2-01 and D2-01) had to be completed within the 2-hour non-traffic hour (NTH) window. To ensure synchronised movement, the speed was carefully regulated across a pair of hydraulic jacks, with real-time monitoring of rotation speed, bearing stress, and wind speed (Figures 7 and 8). Alarm levels were set to reduce traction force by 80% if necessary, and a complete suspension would trigger if further investigation was required. Both T-spans were successfully rotated within the prescribed limits.

Figure 7: Monitoring dashboard before rotation

Figure 8: Monitoring dashboard after rotation

Bridge Adjustment and Final Stabilisation

Upon completion of the rotation, the bridge geometry was fine-tuned to meet the designed alignment within a 1-hour allowable timeframe. Displacement calculations were pre-determined to assist in decision-making during the final adjustments. Additional hydraulic jacks were deployed on standby to address any deviations in the centre of gravity. Survey teams monitored the displacement in real time, ensuring the structure met the allowable longitudinal and transverse tolerances.

After all the adjustments were made, the geometry of the bridge was fixed, and the bridge rotation was considered complete.

The bridge rotation operation, along with its temporary stabilisation works, needed to be conducted during NTH, following the isolation of traction power supply, typically starting at 02:00. The line-clear certificate needed to be signed no later than 04:30 to ensure that railway traffic could be resumed in the morning.

Temporary stabilisation works in Hong Kong

To cope with the frequent typhoon visits in Hong Kong and the significant out-of-balance forces generated by the curve alignment of T-span bridge section, twenty massive shear steel supports were provided to maintain the stability of the structure during casting stage. During structural monitoring, it was observed that the shear steel supports played a critical role in counteracting the unbalanced moment when only one side of the T-span was constructed. While essential for ensuring structural stability during the pre-rotation stage, their removal required careful planning and execution due to the constrained layout of the turntable. A meticulous dismantling process was undertaken to ensure the supports were safely removed in a timely manner before proceeding with the bridge rotation operation.

Reverse rotation mechanism for the busy EAL

A reverse rotation mechanism was integrated into the bridge rotation system, necessitating the construction of an additional pair of reaction blocks and traction strands. This mechanism served as a final contingency measure in the event that the bridge rotation could not proceed as planned. The project team, collaborating with the MTR and other stakeholders, formulated a detailed contingency plan to outline the necessary actions for various scenarios. This comprises temporary stabilisation measures and a step-by-step response strategy for each possible situation, including the extreme case of structure toppling.

Figure 9: Pre-rotation rehearsal

To accommodate the reverse rotation mechanism, the project team had to overhang the existing high voltage cable on the side of the enlarged Excavation and Lateral Support (ELS) shaft to provide sufficient working space for the structure construction and bridge rotation operation.

Pre-rotation rehearsal

The bridge rotation operation had to be completed within the 2.5-hour NTH window allowed by MTR. A detailed minute-by-minute programme was developed to control and monitor the time spent in each step. To ensure the whole operation team was familiar with the workflow and line of communication, an on-site rehearsal followed by a trial rotation were conducted (Figure 9). The exercise involved over 50 contractor workers and 20 supervisory staff prior to the full rotation operation. Moreover, a pre-work trial for the installation of temporary supports after rotation was carried out under the simulated working environment to ensure the operation could be completed within the allocated timeframe. With effective planning and coordination, the project team successfully (and without any disruption to railway services) completed both bridge rotation operations on 28 September and 3 November 2024 within the allowed timeframe, with rotation speed up to 1.20 º/min and 1.80 º/ min at Piers E2-01 and D2-01 respectively.

Promotion of HBRM for Future Application

The successful application of the bridge rotation method in FLBP(E) has set a new benchmark for railway-overbridge construction in Hong Kong. This innovative approach mitigated inherent railway safety risks and streamlined the consultation process with MTR. The significant reduction in night works minimised the safety risks for site personnel, environmental nuisance to nearby residents, and the potential impact on the major railway system serving North District in the event of a lifting operation failure.

The successful application of the bridge rotation method in FLBP(E) has set a new benchmark for railway-overbridge construction in Hong Kong.

Before rotation

After rotation

The introduction of new construction technology, with collaborative efforts of professional engineers from Hong Kong, Mainland and other countries, fostered technology transfer. This initiative has also inspired the new generation of engineers to explore transformative solutions, addressing complex challenges in infrastructure development.

To promote wider adoption of this method in future projects, the project team is preparing a Practice Note that consolidates the key design and construction considerations based on the successful experience of the Fanling Bypass project. The Practice Note will be distributed to relevant works departments by the Development Bureau, serving as a technical guideline for implementing HBRM in future infrastructure projects.

Acknowledgement

CEDD would like to express its heartfelt gratitude to MTR for their invaluable insights, unwavering support, and active collaboration throughout the project. In particular, their efforts have been instrumental in pioneering the adoption of the innovative horizontal bridge rotation method above the existing railway.

CEDD would also like to thank the project manager, AECOM Asia Co. Ltd., the contractor, CRCC-Paul Y. Joint Venture, along with their subcontractors, including bridge alternative design and construction engineering designer YWL Engineering Ltd., subcontractor of bridge structure construction China Railway 15th Bureau Group Road and Bridge Construction Co. Ltd. and the bridge rotation specialists CSSC Sunrui (Luoyang) Special Equipment Co Ltd. and Wuhan Tebridge Engineering Consulting Co. Ltd. The collaborative efforts in utilising innovative bridge construction methods and digital technologies were instrumental in overcoming the challenges encountered throughout the project. CEDD also wishes to acknowledge the invaluable contributions of all project stakeholders in support of this initiative.

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