The sea defences at Aberthaw Power Station site were constructed during the mid-1950’s. They comprise a perimeter sea wall, a series of groynes, and a revetment along the banks of the River Thaw. The sea wall surrounds the south and west sides of the site and comprises a site cast concrete gravity retaining wall. The front of the wall has a concave face to deflect waves and stands approximately 1.5m above the footpath and land behind.
The front of the wall drops down to the beach level on the north shoreline of the Bristol Channel. The beach has a relatively gentle slope and is mainly made up of a mixture of cobbles and pebbles atop a layer of sand with salt-marshes and tidal ponds visible at low tides. The gentle slope combined with the Bristol Channel’s large tidal range, 9-12m (mean approx. 9.5m), which produces a returning tide that advances quickly. The rear of the wall comprises an integral concrete slab at near site level.
There are 22 straight groynes projecting perpendicularly from the wall over the beach at 200 feet (61m) intervals along the southern wall; eight to the west of the River Thaw and thirteen to the east . These groynes are constructed from a parallel row of hardwood timber piles with cobbles set in concrete both between and outside the piles.
A coastal footpath runs the length of the southern boundary of the Power Station, from Limpert to main nature reserve, which is regularly walked by members of the General Public.
To the east of the River Thaw exists East Aberthaw Coast Special Site of Scientific Interest (SSSI), a 68.2ha site running from the end of the River Thaw embankment along the sea wall to Ffontygari Bay and includes the 14 of the 22 Groynes.
For safe access a stone access ramp was constructed to the beach level. Working at suitable states of the tide, the required plant accessed the works location from the access ramp and then travelled down the beach to each location. The plant and operatives had to remain within any restrictions set out in the Marine Licence
Before the works could being the beach level around the groins had to be surveyed so that once the project was complete, the beach could be re-instated to its original profile.
Repairs to the various missing timbers
At the required locations the damaged top sections of the piles were cut down to the point where good material was identified. Where the suitable material is below the existing beach level then an excavator was used to reduce the beach level. Using chainsaws and other hand cutting tools, the sections of timber were cut no greater than 500mm high to minimise the risk of the cut sections becoming unsteady, falling and striking operatives working at beach level. The void was then cleared of any loose beach material before steel dowels (10mm dia) were installed into the groyne core material. A modular formwork system was then installed with the anchors being drilled and fixed into the core of the groyne. Once the formwork was installed, the concrete was checked placed into the bucket of the dumper to be transported to each repair location and using the bucket of the 8t excavator, the concrete was placed into the formwork, with a poker being used to ensure that the concrete flows evenly throughout each void. Once cured, the formwork was removed and the next repair undertaken.
Breaking of various existing undercut concrete sections
The areas of damaged or undercut concrete needed to be broken up and crushed for the safety of other beach users. The sections were broken into smaller sections using the 21t excavator and hydraulic breaker and the broken sections (ranging between 250-750mm in size) were placed at the nose of the groyne as an added protection measure for the structure.
Sea Wall Repairs - Mass Concrete Repairs
At the required locations the seawall was repaired using modular formwork and concrete.
The existing voids were cleared of any loose material or debris before dowel bars (10mm dia) and steel reinforcement were drilled into the existing concrete surface. The formwork was positioned and then drilled and fixed to the existing seawall structure, and then the toe could be backfilled using beach material. Again, the concrete was transported down the beach using the dumper before being placed into the formwork. To ensure that the concrete flows evenly throughout each void a poker was used during pour. Once cured, the formwork was removed and the beach profile reinstated.
Upon completion of the works, the access ramp was removed and the stone spread out along the foot of the sea defence wall to act as further scour protection.
The project was delivered on time and to budget.