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Chasing the tides

Marine specialist Kaymac has helped devise plans to lay a new treated water sea outfall on the Welsh coast for Welsh Water. NCE reports.

Marine works contractor Kaymac has recently completed the installation of a treated water sea outfall at Newport, Pembrokeshire, for Welsh Water, working as a subcontractor for Morgan Sindall. Kaymac worked with the main contractor on the design and construction work under an early contractor involvement contract.

The 643m long, 630mm diameter outfall runs across a beach with a high tidal range and then into the sea. It was placed in a trench and weighed down by more than 200 precast concrete collars, each weighing 320kg, which were attached to the pipe by Kaymac operatives.

Kaymac decision

The decision to weigh the pipe down with concrete collars and mattresses was influenced by Kaymac.

It reviewed the design which originally involved using thousands of tonnes of backfill to be dumped on top of the pipe on the seabed to weigh it down.

“Over 530 sheet piles were installed to support trench excavation work, using two piling rigs. The sheet piles were propped to withstand ground pressures”

The concrete collars solution reduced the need to transport materials to site, although it did mean that the excavation for the pipe had to be deeper.

Getting to the worksite for the beach works part of the project was constrained by the high tidal range. Plant took an hour to reach the beach excavation where the pipe was to be laid.

Sheet piles

Over 530 sheet piles were installed in the beach to support trench excavation work, using two piling rigs. The sheet piles were propped to withstand external ground pressures.

Once the sheet piles were in place, the trench was excavated using a Beach-20T Zero swing excavator, and two Volvo A30 dumpers were used to remove spoil.

For the seaworks, excavation took place from a Sea-Doreen Dorward dredging barge, supported by a storage barge, dive support vessels and a Sabrina work boat.

Seaward dredging was carried out using an on-board GPS and dig software, which was used to check the location and depth of the excavation.

The pipe was manufactured in Norway and towed to site by tug boat. It was stored in Fishguard Harbour until it was needed.

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