Morpeth Flood Alleviation Scheme

A coarse tree screen had to be installed on the river Wansbeck at Morpeth – a significant white clawed crayfish habitat in the North-East. This involved installing 7 large tubular steel piles into the environmentally-sensitive river. Each tube weighed around 7.5 tonnes and was installed in a pre-drilled shaft through gravel, glacial till and boulders.

Substantial plant (cranes, drill rigs, and so on) was needed to work in the watercourse, at restricted times to ensure that they were disturbing the river bed as little as possible.

The original scheme proposal was to have a piling rig travelling across the river moving from one pile to the next. This was significantly more expensive than working from a rig at ground level and had addition risk. At tender stage we proposed to construct a traditional stone temporary piling platform across the river, using concrete pipes to channel the flow or use specialist expensive piling kit that uses an installed pile to drive the next pile. This meant importing around 3,000 tons of clean aggregate to make a 1m deep platform, and would have taken around 4 weeks to install and remove or require expensive kit which had a long lead in time.

The JBA Bentley project team devised an alternative solution using precast concrete Legato blocks placed directly onto the river bed, which minimised the load on the pipes. In addition, because these could simply be placed and lifted, we disturbed the river bed as little as possible. We did not place the Legato blocks by the tree poles, simply filling these voids and topping the surface with an imported aggregate.

‘Legato’ blocks, supplied by Elite Precast Concrete, are high-strength interlocking blocks designed for a huge variety of projects. We have also used them at the Foss Barrier to construct a temporary platform to lift M&E equipment above the potential flood zone. Benefits included:

  • Reduced piling matt thickness
  • Reduced need for clean, imported stone
  • Speed and ease of construction
  • Fewer material deliveries to site
  • Legato blocks can easily be re-used, minimising waste
  • Minimal disturbance to the river bed

We estimate this approach reduced the programme by 2 weeks and saved around £34,000.