Innovative Precast Retaining Walls Built Underwater for Estuary Waterfront Development
Tuesday, 3rd October 2023
Precast Retaining Wall – Key challenges
Working in a tidal zone always presents issues. The fundamental one is often the programme constraints when work can be conducted on either side of high tides. Instead of considering a standard dewatering method such as a cofferdam using steel sheet piles for support, which has a limited lifespan in water. By providing an off-site precast retaining wall the client immediately had a durable fully finished structure with an aesthetic surface to match existing estuary walls and an installation that offered extended whole life cycle costs.
The scheme therefore designed an approach using selective areas of stone column foundations, which were installed below the area to be built upon. In total around 60km of columns were installed. Granular fills were then deposited onto these and an area of levelled fills was later deposited where the thin-walled precast counterfort panels would be placed. The cranage of the precast units had to also ensure lifts of these panels could be made up to 115m from the side in some instances.
Innovation
Called TechWall, the low carbon precast panels are capable of being cast in factory conditions to heights of over 16m, widths of up to 2.5m and have only very thin sections of 140mm. The panel is reinforced and in hydraulic situations, GRP or Basalt can replace standard steel reinforcement bars. The panel is further strengthened by a counterfort system on the back face of the panel, allowing a variety of selective backfills to be utilised. TechWall has significant benefits not only for use in hydraulic situations but also where a cut situation is small, where the base can be suited accordingly to accommodate the situation.
Benefits
The significant speed of build even underwater. In Bilbao up to 10 units a day 160m2 were installed.
Cost benefits over conventional coffer dam solutions and whole life cycle costs were significant.
Surface aesthetics on the front face of the panels were possible and indeed could easily support biomimicry for coastal habitats in the intertidal zones.
Utilising GRP or basalt reinforcement further enhances the design life of the units and ensures no corrosion.
