Dyke for a new polder, FR | 2021

Development of port of brest through construction of enclosure dyke and gabion structure in brest

The Brittany Region has decided to develop the Port of Commerce in Brest by increasing commercial activity, improving maritime accessibility to eventually accommodate larger ships, and developing the surfaces of the current polder. 

This development project includes both land and maritime work lots. The maritime work includes the construction of a 400-meter long dock with a draft of 12 meters and platforms for the lifting of heavy loads (10 t/m2), an enclosure dike to close a polder of approximately 14 hectares, dredging operations in the port, storage of excavated materials in the polder, and subsequent consolidation of the polder for exploitation loads suitable for the needs of future industries. The EMR quay works (lot M01) and the enclosure dike works (lot M02) were carried out in a first phase.

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en English AMCRPS_CS040_Dyke_polder_Brest_EN_HD_web.pdf fr French AMCRPS_CS040_Digue-polder-Brest_FR_HD_web.pdf
Coupe type Digue d’enclôture du polder
Digue d’enclôture du polder
Digue d’enclôture du polder
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The lot M02 project consists of constructing an 860-meter dike in the sea, starting from the existing polder and connecting to the quay of lot M01. 

The initial solution provided for the construction of an earthen dike consisting of a 105-meter-wide gravel base layer (10 to 80 mm), a dike core of alluvial and gravelly materials ranging in size from 30 to 500 mm, a filter layer of 60 to 300 kg rock, and a hard rock carapace of 1,000/3,000 kg to reach a crest elevation of +13.50 CM.

Due to geotechnical stability reasons, the dike construction had to be carried out in two phases with an intermediate consolidation phase and vertical drains to be installed under the base layer up to the shale roof located between -23.00 CM and -12.00 CM to accelerate this consolidation phase.

The company BOUYGUES TP Régions France, as the leader of the consortium composed of the companies Pigeon Bretagne Sud, Liziard, STPA, and Sodraco, proposed a variant by replacing the traditional core with a structure made of gabions of metal sheet piles. 

This solution saves 500,000 m3 of fill and increases the storage volume of excavated materials inside the polder by 120,000 m3. The execution studies were carried out by SETEC International with the assistance of TERRASOL. The gabions consist of 26 circular gabions with a diameter of 21.14 m and 50 intermediate arcs with a radius of 6.30 m. The gabions and arcs are made of AS 500 straight sheet piles with a core thickness of 12.50 mm. 

The sheet piles of grade S 355 GP have a resistance to pull-out of 5,500 kN/m. The lengths of the sheet piles vary between 20.6 and 33.50 m to take into account the variability of their foundation level along the dike.

The straight sheet piles were implemented by a PTC 24HFV pendulum vibro hammer with a high frequency and variable moment attached to a crawler lattice crane Hitachi Sumitomo SCX-2800. With a centrifugal force of 1,419 kN, the vibro hammer was well suited for driving the straight sheet piles, even those of 33.5 m up to the roof of the shale at -23.00 CM.