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Site investigation: Barking up the right tree

 v9 a6453 print crown crescent from north

A 170ha site in Barking is set to become one of the largest regeneration sites in the UK, but with it comes its own set of problems with a disused power stations and historic landfill.  

Barking Riverside in east London is set to become a new town when it is completed in 2035 but there is a lot of work to be undertaken before the first new residents move in. The 170ha site hugs the Thames for 2km and is being developed with 10,800 new homes, a new London Overground station and riverboat service connecting it to central London, 65,000m² of retail, commercial and leisure space and seven new schools.

The ground investigation of the former industrial site, which includes three disused power stations and a historical landfill, has been challenging according to CGL, which was awarded the contract for the site investigation by Bellway in 2012.

The site is divided into lots for developers and four construction stages, which has seen CGL working continually on the site for the past six years.

Drone footage of CGL's ground investigations at Barking Riverside

“Our role has been to represent and advise Barking Riverside in developing the remediation strategies and dealing with the geotechnical constraints for their infrastructure, for the masterplan,” explains CGL project manager Adam Branson.

“The geology of the site spans the full London sequence including London clay, to Lambeth to Thanet and through to Chalk. We have literally undertaken every type of site investigation on this site.”

Quite apart from the geotechnical consultancy, the firm has undertaken desk studies, intrusive site investigation using boreholes, pits and CBTs, as well as road, bridge, flood bund (see box), and embankment design and analysis, land surcharge and consolidation, gas, groundwater and settlement monitoring, landfill and contamination remediation and asbestos mitigation.

A large part of our work has been calculating what the future settlement of the land parcels will be and how long it will take to reach a residual settlement for the development to proceed

Prior to CGL working on the site, there were around 750 historic exploratory holes including a significant number of shallow trial pits. Since 2012, the firm has completed a further 1,200 exploratory holes including boreholes, cable percussive and rotary, cone penetration tests, deep and shallow trial pits, as well as the in-situ testing and monitoring.

For the fourth and final stage of the development, covers the site of three Victorian coal powered power stations – one has been demolished, while the remnants of the other two remain.

“Across the whole site there are large deposits of fuel ash from the power stations, placed loosely everywhere,” says Branson. “There has been a lot of ground engineering and enabling works to make the land developable. The site also has a number of contamination hotspots including the discovery of Blue Billy, which has been treated, remediated and encapsulated.

CGL project director Dan Matthews adds: “The reason we did this was raise the ground levels to accommodate the flood level. There was a lot of material brought on to site during the 1990s to raise levels and surcharge the pulverised fuel ash and the underlying alluvium. But that was left during the recession.”

capture from no9

capture from no9

Since 2012, the firm has completed a further 1,200 exploratory holes including boreholes, cable percussive and rotary, cone penetration tests, deep and shallow trial pits

The ground across the site needs raising to a minimum of 8.2m, with some areas needing 11m. This is between 4m and 6m from existing levels on the site. 

“When CGL started on the project, our first involvement was to look at what was left to be done,” explains Matthews. “There was a new masterplan which changed the ground levels so it was really what needed to be done to finish off that ground improvement work.”

Much of the material that came to the site was from HS1, the Jubilee Line Extension and more recently London Clay from the Crossrail tunnels.

For the first two years that CGL worked on the site, its work mainly involved redesigning the remedial strategy, competing risk assessment and gaining approval from local authority and the Environment Agency, especially remediation of the historic commercial landfill site which closed sometime in the 1980s.

Branson explains: “Our strategy to separate the contamination from the future site users, included capping for the future soft landscape areas, gas protection measures and the permeable reaction barrier which runs along the east and southern boundary of the disused landfill and deals with a lot of the water control issues.”

Settlement of the landfill has been a concern to Branson. “With the soft ash deposits on alluvial ground and then with the need to control the settlement for future development we have had to surcharge in parcels by moving earth around and surcharging different land plot for different times and for different periods.

“A large part of our work has been calculating what the future settlement of the land parcels will be and how long it will take to reach a residual settlement for the development to proceed.”

The site will have an automated waste collection system, where waste inlets installed near people’s homes across the development will vacuum waste at 64km/h through a 15km underground pipe network. The main collection building is nearing completion towards the rear of the site and will remove the need for door to door waste collection.

The geology of the site spans the full London sequence including London clay, to Lambeth to Thanet and through to Chalk. We have literally undertaken every type of site investigation on this site

“It helps explain the sensitivity and important of the settlement analysis and ground levels,” explains Matthews. “If the system works by vacuum and, if you get differential settlement along that route, then that is where you are going to problems. This is the same for all the utilities.”

Branson adds: “This is one of the biggest geotechnical challenges on the site. We could be analysing one plot and the one next to it, but critically it is not the settlement of the buildings within the plot, but the interaction or differential between settlement of the plot and the settlement of the road route which might not have been surcharged to the same level. And with all the utilities crossed that boundary, it becomes a bigger geotechnical challenge.”

Matthews agrees and adds that they are only the first part of the site investigation work. “When Barking Riverside hand over the land, we produce an interim verification report, so it quite clear what is left for the developer to do.”

CGL has also been involved in the design of an embankment over a water course, that will eventually link two roads on the scheme.

“We have Buzzard’s Mouth Creek, a large water course that flows into the Thames, and other smaller water courses,” explains Branson. “Originally the design was for two bridges, but that was not economically viable with pretty serious foundations in the thick and soft deposits.

CGL suggested an alternative solution, although was more financially viable, was more technically challenging.

Branson continues: “We designed an 6m high embankment that would sit on a reinforced earth raft beneath the whole embankment to ensure that differential settlement doesn’t rotate it.

“We undertook investigation before the construction, and then we have installed vibrating wire piezos into the alluvium to provide readings of the pore water pressures to ensure the alluvium wasn’t overloaded.”

The construction of the embankment was staged, with work pausing to allow the pore water pressure to reduce again, and then construction would continue.

“It worked extremely well,” says Branson. “The actual settlement was monitored for pore water pressures and regular physical surveying of the construction and it worked as we predicted.”

cpt on soft alluvium underlying proposed embankment stage 1

cpt on soft alluvium underlying proposed embankment stage 1

CPT on the soft alluvium underlying the embankment

Perhaps the most exciting area of the site investigation on the project is the fourth and final stage - the plot of land where the three power stations were sited.

“This is where we know little in comparison for the rest of the site. We haven’t got a legacy to work from, we need to start from the beginning,” says Matthews.

Branson adds: “There is a lot of unknown there. As well as remaining constraints, large basements which are unknown structurally, probably contamination, and various active electricity infrastructure such live sub stations that need decommissioned and re-engineered, as well buried cables and tunnels. The structural drawings don’t really exist.”

In addition, the power stations were targets during the Second World War and unexploded ordinance have been found and cleared from the site.

“It’s been a great experience for our engineers,” explains Branson. “They have gained a wealth of experience just working on the one project, not only the site investigation, but also the geology.”

The firm has clocked up 14,750 hours of work so far on the project and this looks unlikely to taper off anytime soon.

Flood bund

CGL’s work at Barking Riverside includes design of a 1.4km long flood bund for the site.

The longer part – at Foreshore Park – is around 900m long and has involved earthworks to increase the ground level to meet planning flood defence levels. CGL estimates that the total volume of earthworks will be 80,000m³ to raise the ground to at least +8.2mAOD.

The second bund at the District Centre is approximately 500m long and will comprise of a mix of earthworks and new retaining wall structures.

Branson said: “Site investigation for this element of the work was undertaken over two phases and included a mix of deep boreholes, machine trial pits, including long-reach pits close to the low-tide line, window samples, and hand pitting within the tidal mudflats. We also undertook structural investigations of the existing intertidal revetments.”

The design is currently underway and will include intertidal ecological habitats, and leisure facilities including cycle routes and foot paths.

Construction is expected to start in April next year.

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