Polder Enclosure Dike in France | 2021

The lot M02 project consists of constructing an 860 m long sea dike starting from the existing polder's embankment and connecting to the quay of lot M01. The initial solution provided for the construction of an earth dike consisting of a 105 m wide gravel base course (10 to 80 mm), a dike core made of alluvial and gravelly materials ranging from 30 to 500 mm in size, a filter layer of rocks weighing 60 to 300 kg, and a carapace made of hard rocks weighing 1,000/3,000 kg, to reach a crest level 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 course 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 lead contractor of a group composed of Pigeon Bretagne Sud, Liziard, STPA, and Sodraco, proposed a variant by replacing the traditional core with a gabion structure made of metal sheet piles. This solution saves 500,000 m3 of imported fill and increases the debris storage volume inside the polder by 120,000 m3. The execution studies were conducted by SETEC International with the assistance of TERRASOL. The gabion structure is made up of 26 circular gabions with a diameter of 21.14 m and 50 intermediate arches with a radius of 6.30 m. The gabions and arches are composed of straight sheet piles of the AS 500 type with a core thickness of 12.50 mm. The S 355 GP sheet piles have a pull-out resistance of 5,500 kN/m. The sheet pile lengths vary between 20.6 and 33.50 m to take into account the variability of their foundation level along the entire dike.

Sheet pile driving works down to the altered shale layer began in the summer of 2017 following the installation of vertical drains. The construction phasing of a gabion structure made of sheet piles is quite complex, and its implementation requires the use of special driving guides. These guides, designed by the Technical Department of BOUYGUES TP Régions France and custom-built according to the specific needs of the site, including the diameter of the gabions, serve for the proper installation of the straight sheet piles and also for the internal support of the gabion structure until it is filled with imported material. 

A floating pontoon was used as a service pontoon and working guide for the crane that was installing the foundation piles for the jetty built inside the gabion structure. The foundation piles of the jetty also supported the driving guide of the gabion structure. The jetty subsequently served as a working platform for the straight sheet pile driving workshop and for trucks carrying imported fill and excavators that unloaded them. 

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