10 October 2018 - 16h09
Tuesday 11th September, 6:17 p.m. The first in a series of eighteen caissons takes its final position. Inaugurated on 25th July in the presence of the Sovereign, among others, the 10,000 ton colossus will only need a single day to be put into position. Subject to unusual constraints.
The margin of tolerance was less than 20 centimetres. Clearly, the theoretical positioning described in the plans needed to be complied with along the two horizontal dimensions within less than one-fifth of a metre. This is quite an achievement if one takes into account the fact that it is being carried out in a liquid environment, which is unstable by nature, and that a 27-metre-per-side cube placed on the water is being “played with”. In any event, two attempts were needed. After being towed towards its installation location, the caisson was attached to land by two winches and kept in place by two tugs. It was then ballasted with liquid: that is, it was partially and gradually filled with water. The caisson then sank little by little. When it was a few dozen centimetres from the seafloor, final adjustments and checks were carried out. This was the crucial moment. The pumps spewed out the thousands of cubic metres of water needed to conclude the operation. A few drops of sweat appeared on the teams’ foreheads. Everything was in play now. The caisson touched the sea floor. It was in position. More checks. The verdict was announced: the colossus had exceeded the margin of tolerance by just a few centimetres. A drop in the ocean. But it was impossible to begin the installation of a series of 18 caissons because of the non-compliance with this margin of tolerance, however small it may have been. This was obviously a scenario that had been envisaged long ago.
The caissons will be definitively ballasted using 0/20 solids – that is, a material that is almost sand, but has about a 1.6 times greater mass – but the “trick” is first to carry out this operation with water. In this way, if necessary, it is possible to use the pumps to empty the water out and so enable the caisson to be lifted from the ground by a few centimetres and then put back. At the second attempt, hopefully. Once perfectly in position, this caisson validated the protocol that had been developed by the teams from Bouygues TP Monaco, and gave the starting signal for an installation schedule for all the caissons that will last until next June, at an average rate of one caisson every 15 days. This is an even greater relief, because the Port of Marseille has become saturated due to the storage of the 15 caissons that have already been produced. This why the second caisson, which arrived in the morning of Wednesday 19th September, was put into position on the 21st. “The levelling layer designed to flatten out the seafloor and allow the caissons to be properly installed was impeccable”, Christophe Hirsinger, Director of Bouygues TP Monaco, explained happily. “We had a slight contretemps later: one of the jacks on the underwater leveller broke down. We didn’t lose any time from our schedule in the end because operations were suspended during the Monaco Yacht Show, and so we took advantage of this break to make repairs and start levelling again in the meantime”.
It was therefore possible to begin the ballasting of the first two caissons with solid material at the end of September. Christophe Hirsinger: “We are extremely satisfied with these first two installations of the caissons”. And for good reason. Draconian margins of tolerance, challenges associated with the primary importance of getting the positioning of all the 18 caissons off to a good start and particular uncertainties arising from the initial implementation of the protocol. The installation of the caissons between 11th and 21st September was also made more complicated due to the unusually high occupancy of the water as the time for the Monaco Yacht Show approached.
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