Dorset residents had almost forgotten what Sturminster bridge looked like – but finally the bank stabilisation work is complete. Rachael Rowe reports
In August 2019 they were put up as a temporary safety measure – now, almost five years later, the plastic railings on the approach to Sturminster Newton have finally been removed and the view of the bridge looks remarkably different, with a new footpath and the striking absence of barriers and temporray traffic controls.
Work to stabilise the bank of the River Stour is complete at last – but there was much more to this complex project than most people probably realised while they were stuck in a the traffic queue.
The bank between the ancient Town Mill and the early 16th century town bridge has been shifting for many years – it’s a man-made structure which was formed when the road was relocated more than 100 years ago and was therefore never intended to support the weight of modern vehicles. Over time, vibrations, over-loading and water ingress have weakened the material and caused it to start failing, while repeated storm events and flooding removed fine material from the base of the slope, causing further movement. The footway has been repeatedly patched and repaired, but by 2019 the movement had become too great and it was decided that a more permanent solution was required to protect the highway. A temporary solution using gabions to make the footway useable had already failed: it started to move even before those initial works were completed.
The problem and the solution
The A357 runs along the south bank of the Stour. To stop any further soil movement and protect the busy road, the bank has now been strengthened by Dorset Council, in a major project costing £1.3 million – when The BV first reported on the issue in 2021, the anticipated budget for the completed project was £400,000.
Almost 500 soil nails were drilled into the embankment, with grout securing the soil nail to the stable material beneath. The erosion matting was then placed over the bank, and stainless steel mesh on top of that – the nut on each soil nail was then installed and tightened down to tension the mesh and matting, which holds the bank in place. The nail heads remain exposed so that they can be maintained periodically – ensuring the mesh remains tensioned so that it can retain the slope effectively.
The final part of the work – sowing the bank with wildflower seeds, reinstating the footway bordering the road and installing the rail fence, is now complete.
Project team manager Zoran Maric explained why the unsightly barriers were in place for so long: ‘Over the course of two years, we monitored the embankment for movement and also recorded vibrations in order to establish a baseline. The monitoring told us that we had a significant problem with slippage from the embankment. This is why we were forced to complete the project through the winter months. It could not wait until next year.
‘We had to compete with seven big storms before Christmas, which caused around ten days delay to the project timeline.
‘We had looked at the various options for completing the task. One was taking the work back to include the entire road, but that would have been a huge issue, as it would have completely closed the road. Another option was to use sheet piles, but that was also a huge engineering job. The most effective option was to use the nails and a mesh.
‘The nails will be there forever and a day. The nails themselves are 90mm in diameter and are between eight and 12 metres long, depending on how they interface with the geology. The mesh is critical – it holds everything in place and prevents loose material from falling.
The protective matting holds the wildflower seeds in place. A specific wildflower mix, that is low in maintenance, has been chosen, so that the banks don’t need to be continually trimmed. Once it starts to grow and seed, that iconic picture postcard view will be back.
‘We are continuing to monitor the work for any problems.
We monitored vibrations during the work and because we had a baseline measurement we were able to demonstrate there was no risk to property in the immediate area.
During the works we also removed the large willows that were found to be damaging the gabions at the river edge – which provide scour protection – but other trees are being planted in the area to mitigate the environmental impact.’
Dorset Council spokesperson said: ‘Our team, and our contractors Heidelberg, worked in some challenging conditions, including heavy rainfall that caused higher than normal river levels. They have done a fantastic job and have created a safer place for all.’
A succesful project
During the works, the eastbound lane of the A357 was closed with traffic signals in place to maintain traffic flow both ways. Zoran explained that an extra sensitive system called Urban 64, which has a more intelligent way of operating than the standard highways traffic lights, was used during the project: ‘The system was more effective at monitoring queue lengths and also allowed us to use a fourth arm so the residents who directly fronted the works had an opportunity to join the traffic flow.’
‘The workforce has been fantastic,’ said a Dorset Council spokesperson. ‘It really is so wonderful not to see the awful plastic railings any more. I would like to thank all of the residents for their patience. We got there!’
Zoran highlighted a lot of the unseen work of civil engineering that was invisible to most people in traffic queues, waiting to cross Sturminster bridge.
‘I am proud of this whole project. We have protected the highway, and also saved the bank from collapsing into the river. We also kept communications working – we had a lot of stakeholders to
coordinate. And of course we have helped restore the iconic entrance to Sturminster Newton.’
