The site team has undergone underwater excavation in rock using rockwheel aka cold milling. The main scope for the project comprises of repairs to the existing masonry walls, using a combination of concrete, structural grouts and mortars, and replacement stone blocks from a local quarry.
Scour and erosion to the toe wall was recognised as a potential source of weakness in the future, so concrete protection was proposed as a ‘toe beam’, along the main pier walls. The method proposed by the client/designers for excavation from the toe into the sea bed (the bed material is mudstone, similar to slate) was Hydro-demolition.
- Hydro-demolition is slow and costly – requires divers ‘hands on’ – so it is a high-risk operation.
- It requires a supply of fresh water (potable/near potable quality) – this would increase vehicle movements as it needs to be brought in by tanker – up to 50 m3 of water would be required per day or around 1500 m3 over the course of the project.
- The water jet is of high temperature and disrupts the local marine environment as it reduces salinity.
- Hydro-demolition can cause ‘arisings’ in the form of silt which are bad for the marine environment.
- It is a noisy operation above and below water.
Excavator mounted rockwheel or ‘cold mill’
Elimination of a high-risk operation, improvements to the marine environment, reduced noise, reduced usage of fresh water, reduced vehicle movements, improved programme and reduced cost. No visible silt during excavation.
Monitor Report. Balfour Beatty. North Yorkshire. August 2019.