Water and Effluent Treatment Magazine
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In addition to the Mega Brace frame, the excavation was supported by five MP125 hydraulic props Holy Molesey! • Replacing the last gates at Molesey Weir, has been no mean as the site's irregularity presented a challenge. • Groundforce deployed its Mega Brace frame system and its MP125 and MP250 hydraulic props for the project • Mackley Construction replaced one set of gates with new electronically-controlled radial gates, and returned to replace the second set at a later date • 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 1 All the work has to be done in the river itself 2 Molesey Weir, built in 1815, is one of the longest locks on the Thames 3 The first excavation was a fairly straightforward rectangle, the second is very irregular and more difficult to support G roundforce has completed its second contract for joint venture contractor Team Van Oord (TVO) on the Molesey Weir replacement project on the River Thames at Hampton Court. Molesey Weir, built in 1815, is one of the longest locks on the Thames. Time having taken its toll, the En- vironment Agency (EA) decided to replace the weir, but due to its size and the need to maintain the river's navigability the gates were replaced in two phases. Last year TVO partner compa- ny Mackley Construction replaced one set of gates with new elec- tronically controlled radial gates. This year, it has returned to r eplace 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 ac- cess to the river bed and weir structures. Groundforce supplied struc- tural 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. Hydraulic rams Following the successful comple- tion of the first phase in Decem- ber last year, Groundforce re- turned in the spring to help construct the cofferdam for the second phase. "This was quite different from the first cofferdam," explains Ste- "Whereas the first excavation was a fairly straightforward rectangle, the second is very irregular and more difficult to support" Stephen Brown "By all accounts we're ahead of the game here. We're approximately four to five weeks ahead of where we were this time last year" Stephen Brown, site manager, Mackley ASSeT RenovATIon phen Brown, site manager with Mackley. "Whereas the first exca- vation was a fairly straightfor- ward rectangle, the second is very irregular and more difficult to support." The irregularity of the coffer- dam was further complicated by the inclusion of a permanent sheet-pile section housing a fish pass to allow the passage of mi- grating salmon and sea-trout. "The design was more difficult than last year's," says Ground- force sales engineer Matthew Hughes, "and because of its shape it required more linear me- tres 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 com- plex structure. Weak soil and the pressure exerted by up to 3m of water resulted in high lateral forc- es on the steel sheet piles of the cofferdam. Therefore, in addition to the Mega Brace frame, the ex- cavation was supported by five MP125 hydraulic props – each with a load capacity of 125 tonnes – and two 250-tonne capacity MP250 prop. Ažer Mackley had driven the sheet piles to their full depth, the barge-mounted crane ližed the Groundforce components into place – a process complet- ed in just one day – before exca- vation began within the coffer- dam. To maximise the space within the excavation, and reduce the total number of props re- quired, Groundforce positioned the MP125 and MP250 props across the corners of the excava- tion. This followed the pattern set last year when the same design concept was used. Impossible As with the previous cofferdam, special end-bearings were sup- plied 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 im- possible 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. This involved providing two sluice gates, upstream and down- stream, within the cofferdam wall. "Basically, we'd have to leave the site under instruction and let the river flood the cofferdam" says Brown. "Luckily that hasn't hap- pened." The last piece of Groundforce equipment was removed during the first week of September last year 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 Brown. "We're approximately four to five weeks ahead of where we were this time last year." • Replace the last set of gates at the Molesey Weir on the River Thames • Construct sheet-piled cofferdams to give provide access to the river bed and weir structures • Construct a permanent sheet-pile section housing a fish pass 12 WET NEWS MARCh 2015