Molesey Weir, built in 1815, is one of the longest locks on the Thames. Time having taken its toll, the Environment Agency decided to replace the weir – but due to its size and the need to maintain the river’s navigability the gates are being replaced in two phases.
Last year TVO partner company Mackley Construction replaced one set of gates with new hand-wound radial gates. This year, it has returned to replace the second set.
All the work has to be done in the river itself, which has required the use of a 95-tonne barge-mounted crane and the construction of sheet-piled cofferdams to give access to the river bed and weir structures.
Groundforce supplied structural support for the sheet-piled cofferdam on both phases of the project, deploying its Mega Brace frame system and its MP125 and MP250 hydraulic props.
Following the successful completion of the first phase in December last year, Groundforce returned in the spring to help construct the cofferdam for the second phase.
“This was quite different from the first cofferdam” explains Stephen Brown, site manager with Mackley. “Whereas the first excavation was a fairly straightforward rectangle, the second is very irregular and more difficult to support”.
The irregularity of the cofferdam was further complicated by the inclusion of a permanent sheet-pile section housing a fish ladder to allow the passage of migrating salmon and sea-trout.
“The design was more difficult than last year’s” says Groundforce sales engineer Matthew Hughes, “and because of its shape it required more linear metres of Mega Brace frame around the perimeter”.
Whereas only four hydraulic rams were required to pre-load last year’s Mega Brace frame, five were required for this more complex structure.
Weak soil and the pressure exerted by up to 3m of water resulted in high lateral forces on the steel sheet piles of the cofferdam. Therefore, in addition to the Mega Brace frame, the excavation was supported by five MP125 hydraulic props (each with a load capacity of 125 tonnes) and two 250 tonne capacity MP250 props.
After Mackley had driven the sheet piles to their full depth, the barge-mounted crane lifted the Groundforce components into place – a process completed in just one day – before excavation began within the cofferdam.
To maximise the space within the excavation, and reduce the total number of props required, Groundforce positioned the MP125 and MP250 props across the corners of the excavation. This followed the pattern set last year when the same design concept was used.
As with the previous cofferdam, special end-bearings were supplied to fix the hydraulic props securely to the Mega Brace frame at an angle of 45o.
TVO began work on this phase in March, the weather and river conditions making it virtually impossible to attempt the task in the winter months. Even so, Mackleys had to ensure plans were in place to restore full capacity of the weir in the event of summer flooding.
“Basically, we’d have to leave the site and let the river flood the cofferdam” says Stephen. “Luckily that hasn’t happened”.
The last piece of Groundforce equipment was removed during the first week of September following concreting of the new weir structure. And despite the greater complexity of this phase, the lessons learned in 2013 have ensured the work all went smoothly.
“By all accounts we’re ahead of the game here” comments Stephen Brown. “We’re roughly three or four weeks ahead of where we were this time last year”.
The project is due for completion before Christmas 2014.