In June last year, Midlands-based specialist steel sheet piling contractor Ivor King commenced piling work on one of their largest flood prevention scheme contracts yet at Rhyl on the North Wales coast.
The work was carried out for main contractor Galliford Try as part of Denbighshire County Council’s £10.4m West Rhyl Coastal Defence Scheme.
The work involves extensive sheet piling to form a replacement harbour wall and to permit placing of rock armour by others and has proved to be challenging yet rewarding.
“The existing coastal flood defences, comprising the promenade wall at Foryd Harbour and eastwards along West Parade, were between 60 and 110 years old,” explains Ivor King project supervisor Andy King.
“Due to deterioration and anticipated rising sea levels, the defences needed to be improved and raised to protect over 2,100 homes and 550 commercial properties in the town of Rhyl situated below high tide level.
“The scheme involves the stabilisation and refurbishment of the existing harbour wall and placing sheet piling to create a toe for the 70,000 tonnes of rock armour revetment.”
Over 1,000 sheet piles
Ivor King is on site for a five-month contract period. The firm is using a combination of an ABI TM13/16 leader rig, an IHI DC700 70 tonne crawler crane and a Movax excavator-mounted rig, to install approximately 1,070 PU12 steel sheet piles supplied by Galliford Try, varying in length from 3.5m to 12m. Ivor King provided the tracked excavators, tractors and trailers to load and distribute the piles from the central stockyard to site.
“The ground conditions generally comprised storm gravel beach deposits, including sandy gravel with cobbles and occasional boulders, with localised deposits of looser silt and sand,” says King. “We envisaged problems in vibrating piles to the specified depths but our rigs were powerful enough to install the piles without difficulty.”
The steel sheet piling, designed by Martin Wright Associates, comprises two main sections, with a total length of about 630m:
- 200m of 12m long steel sheet piles and associated anchors to form a new harbour wall; and
- 430m of 8.5m or 3m long steel sheet piles as a toe to rock revetment.
The western section forms a new inner harbour wall in front of the original wall, extending eastwards from Foryd Harbour Bridge at its western end to the location of the proposed Cycle Bridge due to be built across the River Clwyd opposite the end of Quay Street.
From this point eastwards (and seawards), the outer harbour works comprise sheet piles forming the toe for the outer harbour rock armour protection and the transition zone to the western end of the rock armour river training wall.
Use of crawler crane
“The 350 12m-long inner harbour piles were installed working from the existing promenade using the crawler crane, which was able to reach out to install the piles up to 5m in front of the existing harbour wall, and a rope-suspended vibratory hammer,” explains King. “The steel sheet pile wall will finally be topped with a reinforced concrete curved harbour wall.”
For the outer harbour section, 714 piles, which were at greater distance from the promenade, were installed from beach level during the approximately six-hour long, twice-daily low tide periods by the Movax excavator-mounted rig working in tandem with the leader rig.
These outer harbour piles comprise alternating pairs of piles 8m and 3.5m long, driven to the same toe depth thus creating a crenellated wall designed to dissipate the wave and tidal energies.
“The Movax unit was used to 'pitch' the piles which were then vibrated in to the required depth by the leader rig,” says King. “The versatility of the Movax unit also proved invaluable as it was able to track out at low tide along the rock armour forming the existing river training wall to install navigation marks.”
In addition to installing steel sheet piles, Ivor King has supplied and installed seven navigation marks, over 1,500m of steel angle walers, fitted tie backs and tied in the new sheet piles to the existing sheet piles at several locations, and constructed drainage pipe outfalls.
The firm's stock of pontoons and “navvy mats” were also employed to safely execute the work and safeguard Rhyl’s town centre from future flooding.