ArcelorMittal's straight web sheet piles optimise dyke for new polder at Port of Brest

Detailed information

The public authorities of the Bretagne Region in France have decided to develop commercial activity at the Port of Brest. The project aims to first improve marine accessibility to be able to receive larger ships, then to develop the area on the existing polder, and finally to reclaim land on the water for future expansion. The project has been separated into land work and marine work tenders. The marine works included the construction of a 400-metre-long quay wall with a draught of 12 m, platforms for the handling of heavy items (10 t/m2), and a dyke enclosing the 14-hectare-large polder. It also included dredging works in the commercial port, filling the polder with the dredged material, and its consolidation to allow service suitable for future industrial purposes. The construction of the quay wall (M01) and the dyke (M02) were part of the first stage of the project.

Circular cell sheet pile structure vs earth dyke

Lot M02 of the project consisted of the construction of an 860-metre-long dyke located between the existing polder shoreline and the M01 quay wall. The initial solution was an earth dyke built on top of a 105-metre-wide gravel substructure with a grain size from 10 to 80 mm. The core of the dyke was planned to be made of coarse fill material with a grain size of 30 to 500 mm. A filter layer of rock blocks of 60 to 300 kg and a cover made of hard rock blocks weighing between 1 and 3 tonnes were meant to complete the dyke design.

The top level of the dyke is +13.50 m. For geotechnical stability reasons, the weak soil below the substructure had to be consolidated. Vertical drain columns were therefore installed down to the bedrock at a level varying between -23.00 and -12.00 m to accelerate this consolidation phase.

Bouygues Travaux Publics Régions France (Bouygues TPRF) construction company was the leader of the tender-winning consortium consisting of Pigeon Bretagne Sud, Liziard, STPA, and Sodraco. They suggested an alternative solution to the earth dyke: replace the standard earth core with a circular cell structure made of straight web steel sheet piles. This solution saved 500 000 m3 of fill material for the dyke and increased the landfill volume inside the polder by 120 000 m3. Setec International, assisted by geotechnical experts Terrasol, was in charge of the design. The cell structure consists of 26 circular cells 21.14 m in diameter and of 50 intermediary arcs in a 6.30 m radius. The cells and arcs are made of AS 500 straight web sheet piles with a web thickness of 12.50 mm in S355 GP steel grade. The sheet piles have an interlock resistance of 5500 kN/m. The length of the sheet piles varied from 20.60 to 33.50 m to adapt to the variable level of the bedrock.

Sheet pile installation

Due to the complex nature of the construction process for a cell structure, the technical department at Bouygues TPRF designed and built bespoke installation guides. These frames met the specific requirements of the project - in particular, the diameter of the cells - and ensured the correct installation of the straight web sheet piles whilst also stabilising the cells during assembly and backfill operations.

A floating pontoon was used to stock the equipment and materials and to carry the crane that installed the foundation piles for the temporary pier built inside the cell structure. The foundation piles also supported the installation frame. The temporary pier then served as a working platform for the installation of the straight web sheet piles and for the trucks and excavators in charge of the filling works. The sheet piles were installed with a PTC 24HFV vibrodriver (with high frequency and variable moment) attached to the boom of a Hitachi Sumitomo SCX-2800 crawler crane. Acting with a centrifugal force of 1419 kN, this vibrodriver was well suited to drive the straight web sheet piles, even the 33.5-metre-long ones, down to the bedrock at -23.00 m.