• Offshore extension

Spectacular vibrocompaction

18 August 2018 - 14h18

Another new stage. Now that the fall pipe vessel Simon Stevin has left the Principality, the summer period will be noteworthy for two major events: the vibrocompaction operations, using 50 metre long needles, and the arrival of the first caisson at the end of July.

NP460. This “codename” conceals an 80 metre long and nearly 30 metre wide barge, with two cranes weighing 400 and 500 tons. The NP460, which arrived on 9th May, will be on the water for several months. Its role is to compact the million and a half tons of backfill that will hold the 18 caissons that will ring the future expansion. The backfill varies in thickness between 30 metres near Port Hercules and 15 near Larvotto. Buried in the backfill, the needles vibrate and rise in stages of 70cm in 30 to 40 seconds, both needles working simultaneously. “Our objective is to move forward between 500 and 600 linear metres a day so that we can stabilize the entire area by the deadline”, says Christophe Hirsinger, the director of Bouygues TP Monaco. “This is an important activity that began in May, with the first phase due to be completed at the end of August: that is, for the caissons located near the Fairmont”.

Another smaller barge will also take part in the vibrocompaction operation from mid-July on, alternating among various sites: vibrocompaction and creating ballasted columns. As a reminder, these ballasted columns, which are required in the eastern section of the site to avoid any risk of liquefaction because of the nature of the ground, consist in simple terms of inserting columns of a very significant depth, which will then be filled with a granular material. This barge was being prepared in Toulon in mid-June so that it will be fully operational by the planned deadline.

“A full summer”

Like the Simon Stevin, the Fabio Duo left Monaco’s waters in the past few days after completing its work of installing 5/50 materials and removing blocks of rock (after working at the foot of the existing rock). The same applies to the Francesca, which was responsible for rock-breaking, and whose task has also been completed. Conversely, a new barge, owned by Trasomar and called Edmond, has reached the Anse du Portier, where it will begin laying anchorages for various vessels (including barges) and for the first caisson, which is scheduled to arrive at the end of July. This first caisson will then be put into position towards the end of August, signalling the first of a series of eighteen, which will make up the ring of the future expansion. We must therefore expect “a full summer” as Christophe Hirsinger puts it, especially because between the vibrocompaction and the positioning of the caissons, the clipping and levelling procedures must also be proceeded with. In order to ensure that the caissons and therefore the entire future land area are perfectly stable, a foundation must be provided that is not only stable, and therefore compacted, but also as uniform as possible.

Clipping consists in “rubbing out” the small wave-shaped pieces of material which have inevitably appeared on the surface of the backfill following the vibrations caused by the NP460’s needles. Levelling will then enable a top coat to be created that provides the most effective surface for holding the caissons, which, we recall, will be brought from Marseille by sea, positioned as precisely as possible and sunk into their final location before being ballasted (filled with material) so that they will be stable. To this end, preparation of the so-called technical ballast materials for the caissons and the backfill – that is, material added after the caisson has been placed around its perimeter, again with the aim of stabilizing, has begun in a quarry near Toulon. Two kinds of granules will be used: 350,000 tons of 0/20 (between 0 and 20mm in size) for the ballast for the caissons, and 520,000 tons of 20/180 for the technical backfill. Delivery of these materials to the Principality will begin in September, and will last for a year

Georges-Olivier Kalifa

Photo : © DR

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