Through a process of Early Contractor Involvement, Van Oord UK Ltd, in collaboration with the Environment Agency and project designers Jacobs Babtie, instigated a redesign of the revetment structure in 2003, with design testing of scale model defences at Technical University Delft in the Netherlands leading to a finalised plan by September 2004. Workability sessions and other planning took place over the winter. When the EA budget came through in April of this year, all that was required was to mobilise equipment for a start of works in May. Team Van Oord was three weeks ahead of schedule when visited by Maritime Journal last month, with Phase One works expected to finish by late November.

Phase One involves the construction of 1,200m of concrete seawall spaced some 2m seawards from the promenade atop the existing defences. The gap is backfilled with debris from the excavation of footings and topped with concrete to give a new promenade some 7m wide. The existing concrete slope up to the old defenses has deteriorated under the relentless pounding of wind driven seas off the Strait of Dover and did little to slow or disperse the energy of the onrushing waves. There was risk of breaching, and overtopping was a recurring problem for traffic on the busy A259 immediately behind and below the old defences.

To eliminate these threats, 150,000 tons of rock armour is being placed over a length of 1,500m to protect the toe and slope of the existing structures.

Phase Two will start in the spring of 2006, involving the construction of a further 900m on concrete wall to the immediate south of Phase One as well as an additional 200,000 tons of rock revetment. Team Van Oord is also constructing seven new sets of steps and three new access ramps for maintenance and emergency vehicles along the entire project.

The new rock revetment over the existing concrete slope will be placed to an average depth of 2.4m and consists of irregular granite blocks produced by Stema Shipping at their dedicated rock armour quarry at Larvik in Norway.

The granite is loaded onto a 20,000 ton capacity flat barge and towed to an agreed anchorage point off the site. Approximately 2,000 tons at a time is loaded onto a smaller Stema transhipment barge and brought as close to shore as possible just before the high tide, when an onboard loading shovel tips the rocks, each one weighing between three to ten tons, onto the beach.

During a five hour low tide window twice a day, Team Van Oord workers take to the beach and defences. An excavation team prepares 20m of footings per day, which is the same rate at which the concreting team is able to place forms and pour the seawall. Meanwhile, excavators equipped with grab attachments pull rock armour from the tide and place them two at a time in the sand filled bed of a large dump truck which delivers them to a powerful 100 ton capacity final placement excavator, which will eventually bring rock back as far as the toe of the new seawall.

Unlike the old concrete slope it sits atop, this irregular, multifaced rock armour effectively dissipates the force of the waves before they even reach the new seawall and will significantly reduce overtopping.

For reasons of plant removal access, Phase Two must finish next year before the final rock armour is placed for Phase One, at which time the people and places behind these renewed defences can expect another 50 years of the vital protection they need.

MJInformation No: 21123