Celtest team's exemplary adherence to strict protocols results in successfully overcoming tidal river challenges.
Project/Client
Detailed non-destructive investigation of the two reinforced concrete pier heads supporting the A494 Queensferry River Bridge for client North & Mid Wales Trunk Road Agent (NMWTRA). NMWTRA is responsible for managing, maintaining and improving the strategic road network in North and Mid Wales on behalf of the Welsh Government.
Challenge
Constraints included working within the tide range and river flows of the River Dee, avoiding the high Spring tides with flow rates of up to 4-6 knots; preventing debris falling and polluting the river, which is a Site of Special Scientific Interest (S.S.S.I.) and a SAC/RAMSAR site; and using a floating pontoon (supplied by National Marine) while being aware of constraints due to Air Bus transits.
Results/Conclusion
Successful testing was completed within the ten specified 'Test Areas', while ensuring there was no contamination of the river. Celtest satisfied all the client’s needs.
Project/Client
NMWTRA operates on the basis of a partnership between the eight North and Mid Wales local authorities. The agency is responsible for maintaining the condition and value of the Welsh Government’s trunk road assets, including the highway, bridges and structures. Activities through its service providers includes structures' routine maintenance.
The River Dee Bridge was built in 1960, carries 60,000+ cars daily and is a strategic link between North Wales and North West of England. It was identified in the Welsh Government’s ‘Moving Wales Forward’ to address existing issues in the area and ensure it is fit for the future. The upgrade is a key part of the Welsh Government’s plans to improve transport infrastructure and resilience in North Wales. The Preferred Option (announced in September 2019) will include a new river crossing for westbound traffic and the partial reuse of the existing River Dee Bridge for eastbound traffic.
Following a competitive tender process in October 2019, Celtest was selected as the preferred contractor for the detailed structural investigation of two reinforced concrete pier heads supporting the A494 Queensferry River Bridge, accessed via River Dee. The work involved drilling and taking concrete cores, testing the concrete for carbonation and chloride content, and mapping the location and condition of the steel reinforcement. The contract also included testing concrete samples at an UKAS-accredited laboratory and providing a factual report.
Because the base of the pier heads are fully exposed at low water, while high-water levels reach up to half the depth of the pier cross heads, a floating pontoon was provided by Bromborough-based National Marine, the nominated sub-contractor. The family-run business offers services in safety boats and work boats, pontoon hire and water safety training. As well as the company's core team, it has fully-qualified skippers and crew based all over the UK. The company believes in supporting the local people and economy, and uses crew from the project area wherever possible.
Challenges
The main challenge for the Celtest team was working within the tidal range and river flows of the River Dee – at low water, there is only a limited channel width. There aren't many jetties on that stretch of the river, so National Marine assembled a floating pontoon to access the boat.
Both pier heads are at the same level, are 34.5m long and approximately 4.3m high. There can be a four meter variation between low and high tides (low water drops to approximately 1.4m below the soffit of the pier head and high water comes up approximately 2.9m above the soffit), so the working area could either be exposed above the crew's heads, or be almost a metre underwater. The river's tidal current upstream varies between 4-6 knots, and it can be flooded after heavy rains – both elements creating challenging flow conditions. The work was planned to take advantage of the Neap tides between 11-22nd November.
Another challenge for the Celtest team was preventing any pollution entering the river, because it is a Site of Special Scientific Interest (S.S.S.I.) and a SAC/RAMSAR site. Marine biosecurity measures also had to be enacted.
There was also the planned movement of Air Bus wings between 14-21 November 2019. Air Bus transfer wings down the River Dee on the Afon Dyfrdwy barge to Mostyn Dock on a regular basis. Therefore, the pontoon could not remain in the main channel, nor be moored on the main channel side of either pier head during the Air Bus transits.
A fourth challenge was the issue of vandalism. The site investigation area, and the proposed outfall headwall and jetty on the eastern bank of the River Dee is a renowned vandal-prone area. Celtest had to provide and maintain 24-7 on-site security, and a small welfare unit.
The Celtest team, led by Operations Manager Jason Chinery, were Daley Thomas, Keiren Williams, Geraint Wyn Jones, Will Gareth Pritchard and John Parry.
Results
Kyle Allen, Business Development Manager, National Marine, explains how the project access was tackled: "We visited the site at various tide levels, including low and high tides. Based on that information, we decided to use a regular modular pontoon system to help with debris, and another pontoon down the river to get on and off, using a small RIB [rigid inflatable boat] boat to go back and forth. The pontoon had a tapered plan layout about 16m x 6m wide, and a lightweight scaffold on board helped the team access the upper regions of the pier heads as the water levels changed."
The pontoon was tied to two central piers, and at different heights to accommodate the tide, with a scaffolding tower built on one side. Equipment was placed on the pontoon and the crew and the equipment were then ferried out to the bridge. Kyle comments that the brown mudbanks are above water at low tide: "The banks have steep sides and a sticky consistency. Men and heavy equipment would sink into it they tried to cross it, so we had to use a separate pontoon tied to the wall, with constantly adjusting ropes to manage it in the tide."
The RIB was moored up, so it had to be started and ropes tied up before picking up the Celtest crew and ferrying them back and forth. (True to the warnings about vandalism, someone tried to steal the RIB. It was found abandoned on rocks downstream, but National Marine ensured it was back in service just three hours later.)
The modular pontoon couldn't be left in the main channel, so it was moved to the outside pillar every night, with the help of the RIB to swing the pontoon around using the tide.
"It's a heavy pontoon, and small-engined RIB," comments Kyle. "One person on the pontoon used ropes to help swing the pontoon around the pillars, while the RIB driver used the boat to nudge the heavy pontoon in the right direction.
The river current was used to help manoeuvre the pontoon – although at one point the pontoon was perpendicular to the bridge, pointing downstream, and slowly being pulled and nudged into place on the other side of the pillar. This was a daily operation. "Kyle adds that the use of Celtest’s LPG generator reduced the risk of any pollution through oil or diesel spillage.
Onsite, the Celtest crew performed the following testing within the ten specified areas (each 2m x 2m) on each pier:
- Concrete tests: cover-meter survey.
- Half-cell potential survey;
- Chloride Ion testing,
- Depth of carbonation testing.
- Rebound hammer surface hardness.
- Drilling and extraction of seven concrete cores for determination of compressive strength.
- delamination survey of all areas (vertical/pier head soffit).
- Sig sulphide tests.
- Steel reinforcement investigation by exposing bars and verifying section loss.
- All samples were tested in Celtest’s UKAS-accredited labs.
As Jason Chinery, Celtest Operations Manager, comments: "We were very aware of the need to contain any possible polluting elements, so we used LPG instead of petrol or diesel to power the drills, and took steps to ensure no concrete debris or drilling slurry entered the water course. We installed water catchers onto the wall to contain the drilling slurry, along with tarpaulin attached to the concrete pier draped on to the pontoon to ensure no debris could enter the water course."
Celtest performed laboratory testing for chloride ion content and compressive strength testing of concrete cores, with electronic data supplied for all tests, along with a factual report.
Conclusion
All testing was completed within the schedule, and without any contaminant in the river.
The team followed strict procedures to ensure compliance with anti-pollution regulations. All jetties, pontoons and the scaffold tower were dismantled.
Kyle Allen adds: "The Celtest crew, led by Jason Chinery, was really helpful. They adapted quickly to the varying tide conditions, and also understood the rigorous anti-pollution strictures. And they helped with the pontoon navigation where possible, which was much appreciated."
In this case study:
Airbus - https://www.airbus.com/
Welsh Government - https://gov.wales/