To establish the true condition of the cable system, and make the case for full replacement, Soundprint acoustic monitoring devices were installed on the bridge to report any breakages. These inspections led to several separate contracts to replace four cables.
However, after further monitoring of the overall system and analysis of the removed cables, it was finally decided in 2007 to replace the entire cable system.
To carry out this world-first operation while keeping the bridge open, Freyssinet engineers had to develop procedures for installing a temporary support system and transferring load to it from the existing cables which then had to be safely cut and removed.
The old anchors both at deck level and in the towers had to be cored out, and a new cable system erected and tensioned.
To form the space for the new system, custom-designed equipment was necessary to safeguard the tight tolerances that were required in coring out the anchors, to support the self-weight of the loosened existing cables, and to achieve perfect alignment for the performance of the replacement cables.
The existing cables were held in cradles looped around the temporary stays, before they were cut into pieces and removed in a controlled fashion. Checks on the structural performance of the full reinforced concrete sections at deck and tower had to be carried out to ensure they were not compromised by the cores.
Freyssinet’s technical department developed customized specifications and appointed a specialist company to design a machine capable of drilling holes up to 520 mm diameter at 130 rpm for this purpose.
As a crucial transport connection, the bridge had to remain open to traffic throughout, and operations were restricted to a single traffic lane’s width on each side. Work started with the longest cables and followed a strictly specified order of replacement, working simultaneously on corresponding cables to safeguard the stability of the structure and remain within admissible stress in both the original and the new stay cables.
Access equipment was particularly critical with the bridge open to traffic. Freyssinet designed a C-shaped platform that was installed below the deck to allow staff to easily move from one side of the bridge to the other and to access the deck anchors. A combination of fixed tower-top platform and mobile platform was designed for each of the four towers to smoothly bring the coring machine to the required elevation for the cable anchor.
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