Welsh Water’s Bryn Cowlyd treatment works is on the east side of the Carneddau mountain range on the outskirts of Snowdonia National Park. It delivers clean water to residents in the Conwy Valley and parts of the North Wales coast.
The existing facility was commissioned in 1998 but is now getting a £31m upgrade toprovide additional levels of treatment, including hydrogenation dissolved air filtration (DAF), rapid gravity filter (RGF) units, sludge handling, a new chlorine drum store, chlorinators, generators and a high lift pumping station.
The work is being carried out by Skanska to a design by Arcadis as part of Welsh Water’s capital delivery alliance. It includes a large cofferdam for the RGF structure. One of the challenges for the design and construction team is the location of the works: it sits on the southern bank of the Afon Ddu, approximately 600 metres from its confluence with the Afon Conwy, and the entire site is on the tidally influenced flood plains of both rivers.
The original treatment works was built on a rock shelf below the Carneddau mountains, but the new facilities are being constructed on soft, silty ground, mixed with fluvial and boulder material. These poor ground conditions were a challenge for the Skanska temporary works team responsible for designing the RGF cofferdam, which consists of a 55.6- by 29-metre excavation, 5.5 metres deep, constructed within a steel sheet piled wall driven to a depth of 8 metres.
Skanska senior temporary works engineer Matt Parker devised a support solution that would enable the excavation to be carried out within the sheet piled cofferdam. The solution involved constructing an external concrete retaining beam and then installing temporary props suitable for the precast concrete structure to be erected inside.
Groundforce Shorco was brought in to supply the props, with the solution using a variety of hydraulic struts over two levels.
For the upper level Groundforce supplied four of its 150-tonne capacity, 620mm diameter MP150 props as knee braces to span 17.5 metres across the corners; and two 250-tonne MP250 cross struts formed of 1,220mm diameter props to be installed over the width of the excavation. These cross struts were fitted with load monitoring, and all the props had mechanical lock off.
“Because of the poor ground conditions, there was the possibility that the temporary structure may go into tension at the top of the sheet piles during the excavation,” Matt Parker explained. “The props are designed for compression, so the props in the upper level had to be capable of being locked off if the structure went into tension.”
The upper level props were attached directly to the concrete ring beam cast on the outside of the sheet piles at ground level. To enable this to happen, the top of the sheet pile had a notch cut out to allow the prop end bearing plate to be bolted directly to the concrete face.
The support of the lower level consists of eight 610mm diameter MP250 knee braces across the corners, and five 1,200mm diameter MP250 cross struts spanning the width of the cofferdam. Three of the cross struts were fitted with load monitoring, and all the props in the lower level were bolted directly to a modular steel waling beam that was designed and installed by Skanska.
Groundforce supplied the props in October 2016, and the lower frame was removed in January 2017 once the base slab had been cast. The top frame will stay in place for another few months, until the cofferdam has been excavated and permanent support is in place. Construction of the RGF unit is now under way, and the entire project is due to be completed at the end of this year.