As part of a process of ongoing improvements and maintenance to the site, the original constructors of the basin, Kier Construction, was approached by Sovereign Harbour to undertake the placement of additional rock armour protection onto the perimeter revetments. However, as silt had built up onto these revetments, this was not as simple as it seemed.

In order to guarantee 100% coverage from the rock armour onto the revetments, the surface would need to be in a clean state. This would also need to be done without disturbance or impact to the silt within an environmentally sensitive area.

Ongoing consultation with DEFRA, the MCEU, the EA, the local council and other key stakeholders revealed that large scale dredging would not be supported on environmental grounds. Kier Construction therefore had a highly challenging project on its hands and approached a number of specialist contractors to provide an answer to this unusual problem.

Land & Water, a specialist marine civil engineering firm with over 25 years’ experience of working in complex marine environments, provided a unique design specifically tailored to overcome this exact problem. It was an innovative method that had never been attempted before and which resulted in minimal disturbance for the stakeholders with a vested interest in the project including residents living adjacent to the existing harbour walls – an important factor in the concept, development and ultimately confirmation of the scheme to be utilised.

The answer lay in actually dredging the silt before placing the rock layer to the revetment, but without generating silts for disposal. This was undertaken using a unique, submersible steel caisson (or box), specifically designed and fabricated by Land & Water for the project. The caisson was in fact a three sided structure, with sides engineered to mimic the side slope gradients of the harbour. The rear wall of the caisson was 4m deep, and suitable to retain the mass of silt to allow temporary excavation within the box.

By excavating in a controlled manner within the caisson, Land & Water was able to expose the former base and side slopes of the harbour, check the status of the profile using computerised dip survey techniques, and then place the rock in a controlled and clean manner, allowing a post placement survey within the caisson for quality standards purposes.

The advantage of the use of the box was not only to ensure quality and minimise silt disturbance during excavation (as it acted like a “moon-pool”), but also to increase the efficiency of the dredging operation. Sediment which came from one footprint of the caisson could be placed directly into the adjacent void created by the previous day’s operations.

Land & Water also had to relocate the 28 ton cassion (held in location on the harbour floor by high suction stresses) on a daily basis and then make allowances for the tidal cycles of the outer harbour, which only exposed the silts for a maximum of two hours a day.

Ultimately the tidal restrictions were converted into tidal advantages. This was achieved by converting one of their modular pontoons into a double duty dredge platform and a tidal lifting frame.

Large floatation fingers on the side the pontoon were used to take advantage of the tidal cycle to pluck each completed caisson from the sea bed. The works therefore started at the top of the tidal cycle. Here the ‘U’ shaped pontoon with the caisson suspended between the buoyancy fingers was positioned over the location for the next rock panel to be placed. Then, as the tide receded, a long reach excavator (sited on the floating pontoon) was used to excavate and side cast up to 400m³ of silt from within the caisson, allowing it to progressively sink through the silt until it landed on the harbour floor, at approximately one hour before low tide. At this time the spudded pontoon, which was also used to help guide and steer the caisson through its descent, was also beached on the exposed silts outside the caisson.

Having cleared the silts from the footprint of the basin floor, a check survey was then conducted before the 1,000mm thick rock blanket was placed. Here a second long reach excavator mounted on the harbour wall passed the rocks from a nearby stockpile down to the beached pontoon, where the pontoon excavator placed and trimmed it to profile.

Following a survey, and at precisely low tide, the caisson was reconnected to the ‘U’ shaped pontoon using a series of deck mounted winches and once secured within the buoyancy fingers, the tide was then allowed to lift the pontoon and thus the caisson off the bed of the harbour (The displacement effect of the pontoon giving suitable buoyancy to lift the pontoon and the submerged caisson simultaneously).

Once high tide had been achieved, the pontoon and suspended caisson were moved along the revetment wall so that one side of it rested on the newly placed rocks and the other side into the next section of silt as the next cycle approached.

The timing of the entire process was dictated by the tidal cycle and the constraints laid down by the adjacent occupants of the development. As a result, the Land & Water team had to work uneven shifts to make the most of the available working times and favourable tidal conditions.

The system developed takes only six people to operate the machinery and manage the works to ensure compliance with the project requirements. Land & Water are making minor adjustments and improvements to the original caisson design, such as replacing the winches and cables and utilising a hydraulic ram system.

With the improved design, Land & Water are tendering for projects of a similar nature.

MJ Information No: 22503