Redevelopment work at Battersea Power Station is gathering pace and basement excavation work is set to get underway soon.
London’s Nine Elms district is a real hub of construction activity and the Northern Line Extension and Thames Tideway are also set to join the action soon. While there are numerous developments now coming out of the ground, one of the most iconic in the area must be the redevelopment of Battersea Power Station.
Repairs to the structure, including demolition and reconstruction of the chimneys, are now underway and upcoming work will involve excavation around the power station itself. This work will not only expose the infamous scour feature below the site but is also expected to be interesting in terms of ground behaviour.
Further repairs will be undertaken after the excavation work has been completed as ground movement affecting the structure is considered inevitable given the scale of the work.
Overall the work will involve 750,000m2 of development over seven phases and take up to 12 years to complete.
“Three phases are already underway,” says Buro Happold director of environment and infrastructure Justin Phillips, who has been involved in the development for 15 years through changes of owner and development plans.
Many developers have tried to get the redevelopment off the ground but failed. The current project is being led by Battersea Power Station Development Company which bought the site in 2012.
Carillion is working on phase one – known as Circus West – which lies between the power station and the railway lines into Victoria Station, and is due to be completed at the end of next year. Phase two was secured by Skanska and is focused on regeneration of the power station itself and this work is scheduled for completion in 2019.
The contract for phase three of the development, which forms the over site structures and those around the new Northern Line extension, has just been awarded to Bouygues.
Also underway is construction of the primary infrastructure that will serve the rest of the development with an energy centre planned as a below ground facility between the power station and the Thames. Once complete this will form a new public park that will open up the river frontage at the site for the first time and connect through to Battersea Park.
While building the new structures alongside all the other development underway in the Nine Elms area and up against the power station is a challenge, the main geotechnical obstacles lie within the power station itself. Many of the existing foundations will be reused but with the presence of – what Phillips believes is London’s biggest – scour feature below parts of the structure, which is not a straightforward task.
Many people wonder why the structure was built across the scour feature but there was no other land available at the time. Historically the site was used as a water works, which left a legacy of water tanks, underground pipes and areas of compacted ground.
The site was also used as a railway sidings that wrapped around the power station building. Today the site is still constrained by rail and road routes and crossed by a Thames Water ring main, electricity tunnels, disused cooling tunnels and underground pipework from both the water works and the power station.
The power station was built in two phases – each with two of the iconic chimneys separated by a central boiler house – and are known as A and B. Part A was built in the late 1920s, while part B was completed in 1953. Both were decommissioned in 1983.
The new development will have a connected basement that extends to 10 to 15m below ground level, making it large not just in depth but also in area.
Phillips says that lots of ground investigation has been undertaken at the site and, outside of the scour feature, the ground conditions are typical of London with up to 4m of made ground over alluvial clay in some areas, which is around 1m thick. This overlies up to 4m of Terrace Gravels and the London Clay is generally found at 10m below ground level and is around 35m thick. Below that is the Lambeth Group and Thanet Sands with the Upper Chalk located at 70m below ground level.
The scour feature cuts through the London Clay by 35m at its deepest and almost connects with the Lambeth deposits.
“Initial thoughts about the feature was that it might have been a dissolution feature but we haven’t found evidence to support this,” says Phillips. He believes that the feature could be the result of Pleistocene pingos.
“It is not one feature, but four or five interconnected hollows,” he says. A lot of work has been undertaken to understand the geometry and also the geotechnical parameters of the infill material, which Phillips describes as a mix of clay, peat, sand and silt with water-filled voids.
“There is evidence of ground relaxation around the edge of the feature so the properties of the London Clay in this area are also different as a result,” explains Phillips.
The scour feature is located under the boiler house and was well documented at the time of the construction of the first phase of the power station. “There is an excellent paper published by the ICE in 1932 on the feature,” says Phillips.
The foundations for A and B stations are different too. Not because of the ground conditions but more due to the time difference between the construction and the availability of materials. There is also a marked difference within the internal architecture too with the 1920s A station appearing considerably more ornate compared to the utilitarian postwar look of B station.
Part A was generally supported by piles, which at the time were described as vibro piles but are now known as driven cast insitu piles. These 430mm diameter piles were driven through the gravel and into the London Clay and are generally 10 to 12m in length and have a capacity of 500kN.
Part B is mostly built on mass concrete “pad” foundations measuring 7m by 7m in plan and are 6m deep. The differential settlement risks posed by the redevelopment both within and around the power station are likely to be affected by the scour feature and the different foundation types.
Phillips says that lots of work has been undertaken to gain knowledge about the existing foundations to allow reuse wherever possible. This testing has included parallel seismic testing for length and integrity testing and a phase of load testing has just been completed by Walsh to check the findings of previous analysis. “The tests took the foundations to three times the working load and the results were fairly consistent with our predictions,” says Phillips.
While excavation of the new basements at Battersea is a significant step forward for the scheme, it is clear that, with a further nine phases of work planned, there is plenty to keep the geotechnical community busy for years to come.