Project Sheet: Foryd Harbour
- Denbighshire County Council
- CMP Group Ltd, Dawnus Construction, LM Engineering Service, Martin Wright Associates
- Site Investigation
- Desk Study
- Designing an alternative slip way
- Designing a causeway onto Horton’s Nose
- Design of a reinforced earth retaining system using CPM blocks
- Producing a sheet pile design
- Designing the access road and platform for the cranes to install the bridge.
Description of Project
As part of the regeneration scheme on Rhyl seafront, Denbighshire County Council proposed a scheme including the £2 million redevelopment of the quayside for a boat storage area, car park, a floating pontoon and boat hoist. The scheme included the construction of a 188m extended quay wall, a new public square and a deepened harbour channel to improve navigation.
Rhyl is in a low-lying area at risk from the sea and the River Clwyd estuary. Denbighshire County Council commissioned Martin Writes Associates to deliver a coastal defence scheme in Rhyl, comprising upgrading the existing training and harbour walls and creating a new revetment along the promenade. GroundSolve Ltd was appointed to carry out a desk study and a site investigation for the proposed area.
The site investigation comprised boreholes drilled in tidal areas, with several also drilled off large scaffold in the River Clwyd.
On completion of the site investigation, GroundSolve Ltd carried out the following:
- Designed a sheet pile supported causeway onto Horton’s Nose
- Designed a replacement slip way into the channel
- Design of reinforced earth, with concrete block facing using CPM blocks
- Provided geotechnical recommendations for the development
- Designed a crane platform for the construction of a new bridge connecting the public square with the West Parade.
The causeway and seawalls were constructed from reinforced redi-rock blocks (precast concrete blocks which incorporate a geo-grid). The geo-grid took the form of polyester multifilament yarns which interact with the soil to generate friction, providing strength and stability. The wall was designed by GroundSolve Ltd and constructed by CPM Group Ltd. This was the first use of this system in the UK using current British Standards.
Also, because the width of the causeway is 6m, the reinforcing polyester yarns had the purpose of holding the upper walls on either side together at the end of the structure.
The wall was supported by 3.5-4m long sheet piles at its base, necessary to support the weight of the walls as well as providing protection from the undercutting processes of the waves and tide. The walls were going to be under high levels of strain due to the weight of vehicles and force of waves crashing into them. Therefore, dowel bars and tie rods were installed for further stability.
The construction of the redi-rock wall required the existing ground level to be lowered to sea level. The excavated material was then used as backfill, which provided a sustainable solution because no aggregate needed to be imported to site, hence reducing the number of construction vehicles traveling through residential areas of town.
The public square was built at 7.5mAOD and the adjoining slipway lowered to -1.5mBOD. The construction of the public square required the infilling of an existing inlet with materials excavated from the dredging of the channel.
Earthworks on this project also included the re-grading of sand dunes to the north of the square.
The construction of the bridge connecting the public square with the West Parade required the construction of an access road and a piling platform for the crane to install the sheet piles and caissons. GroundSolve Ltd was approached to design the piling platform.
As the channel is tidal, water needed to be retained so that construction work could continue at all times. This required the installation of caissons which created a dry working environment.
The new reinforced earth retaining wall required a temporary excavation to the north of the river. Dawnus appointed GroundSolve Ltd to determine the stability of the excavation.