A complex site, multiple landowners and restricted access made sheet piling a challenge for a Welsh flood defence scheme
The Crindau Pill area of Newport in South Wales has a long history of tidal flooding from the River Usk, which has the second highest tidal range in the world. This risk was combined with poor condition defences that offered only a one-in-10-year flood event protection for around 660 homes and businesses.
The JBA-designed solution, led by Natural Resources Wales (NRW), is being constructed between the M4 motorway, along the west bank of the River Usk and into the lower end of Crindau, and will future-proof the area against flood risk.
“The scheme we are putting in offers one-in-200-year protection with 50 years adaptation for climate change,” explains NRW civil engineering project manager Melissa Mahavar-Snow. “So, in 50 years’ time if we need to upgrade the defences all the foundations will already be in place.”
In 2014, NRW submitted a business case to secure funding from the Welsh government for the £14M scheme, that will not only provide flood defence, but will also improve the local area, which is classified as a deprived community.
The area is mainly made up of terrace housing, mixed industrial units, with high levels of drug use and crime, and it is hoped the new flood defences will encourage investment and regeneration into the area.
The nature of the surroundings for the 2.6km scheme can be divided into north and south. Towards the north of the project – nearest to the M4 - there is a mixture of rented homes, industrial units and commercial premises with many landowners, which has proved challenging for arranging access for the construction. Whereas towards the south, the surroundings are more residential and include a park, and the community has been much more engaged and aware of the flood risk.
The scheme is being delivered by main contractor Galliford Try with Keltbray Sheet Piling undertaking the £420,000 installation of the sheet piling for the scheme.
“It’s a complex project, more than just a flood defence scheme,” explains Mahavar-Snow. “There was a lot of contaminated land, around the industrial part of the scheme. Hydrocarbons and made ground where material had just been dumped over the years from the factories that were once there.”
“There wasn’t really a great deal of soil,” says Galliford Try senior project manager on the scheme Lawson Etheridge. “It was pretty much all the nasty stuff, mostly made ground. We separated all the large potential obstructions, crushed it and then compacted it to make the piling mat.”
The sheet piles were installed using two different methods. Vibratory methods using an ABI TM14/17 telescopic leader rig with 20VV vibratory hammer installed the majority of the sheet piles. And in the work areas behind the existing buildings where there were access restrictions, a Giken Double Z pile press was used to install the sheet piles to the top of the river bank.
The northern part of the scheme - the smaller part of the works accounting for £115,000 of the total piling cost of £420,000. The work took around four weeks to complete, and consisted of the installation of 190 pressed pairs of ZZ38-700 sheet piles, in lengths of 12m and 16m adjacent to river bank and topped with a 30m long steel reinforced concrete capping beam, replacing the existing earth mound bank.
“This included a HGV curve, with a concrete infill with a de-bonding agent. There is also a fillable expansion joint every 9m and an induced joint every 3m,” explains Etheridge.
“In addition, we have installed all the new drainage with 13 new outfalls and left space either side so the asset can be inspected.”
”It’s a complex project, more than just a flood defence scheme. There was a lot of contaminated land, around the industrial part of the scheme. Hydrocarbons and made ground where material had just been dumped over the years from the factories that were once there.”
In areas where access for the telescopic leader rig was not possible, the sheet piles were installed to this area using a Giken Double Z pile press. The pile press and sheet piles were handled using a 150t mobile crane which was positioned away from the installation area, with some of the sheet piling operations involved lifting sheet piles over properties.
Kelbray sheet piling managing director Andy Appleton says: “Due to the challenging tight access restrictions on the site, the logistics and the transportation of the sheet piles from the docks to the site had to be carefully managed. We achieved this by ensuring just in time deliveries to the work areas.”
Extra security mesh fencing has been fixed along the 1.5m tall barrier to not only protect businesses from theft, but also to prevent fly tippers throwing rubbish into the river. In addition, oil interceptors have been installed along the defences too to ensure that the river isn’t polluted by the industrial units.
To ease the construction NRW purchased areas of land, as well as strips of land running alongside the defences, where the landowners received compensation. Several derelict buildings have also been demolished and any imported 2C material used on the scheme came from works at junction 28 of the M4.
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Further south on the scheme, a Victorian bridge over the Crindau Pill was originally planned to be closed for around eight weeks for ground investigation works, but when one of the bridge parapet walls was found to be unsafe, it had to be removed and replaced with a new, slightly higher 1.3m flood defence wall, clad with a locally sourced Pennant stone.
In addition, services need to be diverted and in some cases replaced. Some services were not included in the original plans, or were not known about.
“Part of the process was that we had to divert low voltage cables, but once they were exposed Western Power said they weren’t fit for use and they needed to be upgraded.
“This made life very difficult as we needed to get a cantilever beam and the services were essentially in the way,” adds Etheridge.
The sheet piling for the southern part of the scheme took around eight weeks made up the majority of the sheet piling installation budget at £305,000, and included the installation of 17,000m² of ZZ17-700 and ZZ38-700 sheet piles 4m to 16m long to the sheet pile retaining wall.
The majority of the sheet piles were installed using vibratory methods, with an ABI Telescopic Leader Rig used to install the longer sheet piles and a Movax piling rig for the shorter lengths.
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In the park area, the flood defence bund is mainly embankment, and has been piled through. Improvements to the public park area, include a new seated terrace area, landscaping, new footpath and cycle path on the top of the embankment, refurbished changing rooms and the river has been cleared of fly tipped rubbish.
“It’s going to be a massive enhancement to the community,” says Mahavar-Snow. “It’s the largest scheme that NRW are working on, so it is very important to us.
“We work under is the Wellbeing of Future Generations Act, with seven main goals centred on making Wales a better place to live. But with this project is we have achieved a lot more.”