Such a building material is usually associated with pleasant rural settings like Cornwall's Eden Centre, not a main artery connecting London to Kent where traffic relentlessly thunders past day and night. Indeed, this is the first time rammed earth has been used for an inner-city, multistorey structure in the UK for more than 100 years.
The project team hopes this nursery, which has three storeys of flats above it, will be a blueprint for future multistorey rammed-earth residential buildings. It is part of a research project being carried out by Rowland Keable, head of specialist contractor the In Situ Rammed Earth Company, which is building the walls, and the department of architecture and civil engineering at Bath University. The main contractor, YJL London, reckons it can also benefit from a new wave of these buildings. "We would like to sell this and do more of it - it's a specialisation that other people can't offer," says managing director Nigel Wilson. "There will be more of these types of projects because they are environment-friendly, and this is being driven by the government." The team has had to go through a long, hard learning process to ensure that rammed earth can be used safely and economically on other commercial projects.
The tiny Peckham site is in the middle of a terrace of shops facing the road, and the nursery and three storeys of flats above will form part of that terrace. The nursery extends behind the main terraced building in a single storey that will contain play areas for children, storage and a staff room. All the ground-floor walls are made from rammed earth, including two oval-shaped interior areas for babies. The walls are nearly finished, and those on the team are keen to talk about their experiences.
Martin Waters, director at engineer HRW, outlines the difficulty of working with rammed earth. "The main issue is shrinkage – one of the biggest challenges is making that work," he explains. "Overall, the walls shrink 20-25 mm – 10-15 mm of that is in the first week or two."
Site manager Carl Peploe has found this has kept him busy. "From our point of view, it's been a steep learning curve and very time consuming," he says. "The tender documents said the shrinkage should take place within four weeks, but it actually takes up to eight weeks for the walls to stop shrinking as it depends on the weather. This suggests that many people do have a good knowledge of the product, but not a 100% understanding."
Getting right the moisture content of the earth, which came from a quarry 30 miles away in Kent, was critical. Ideally it would be sourced on site to save on emissions from transporting it, but the local earth was unsuitable. The first walls to be built had to be demolished because the earth was too wet, and the Bath University researchers worked out that the optimum moisture content would be 12%. Luckily, an existing building on site was being used for storage and could be heated. Heating and turning the earth reduced its moisture content to 14%, rendering it usable.
Bath University then carried out strength tests on the earth. A rammed-earth section with a footprint of 350 × 350 mm, 2.8 m high, was compression tested. "I was taking bets; I thought it would take maybe 4.5 tonnes," says Keable. "It took 10 tonnes – the weight of a London bus." The walls supporting the three-storey timber-framed flats are 350 mm thick. The engineer HRW has built in a safety factor of 10 but reckons that, because the walls won't be perfectly vertical, the actual safety factor will be five – the same as standard brickwork.
The walls are built onto a concrete raft foundation with small, cast-in upstands to help locate the formwork at ground level. Each section of formwork is positioned and 150 mm of earth shovelled in and mechanically rammed. When the wall is up to the top of the shuttering, another layer is bolted on and the process repeated until the wall reaches its design height of 3 m. Each wall is built in a series of short, castellated sections, 2-3 m long. Once the sections have dried out, the remaining gaps are filled in. According to Keable, a section of wall can be built in a day and the shuttering removed without it falling over.
YJL found the differential drying rates of each wall caused the most problems. The sequencing of the wall construction means each section is in different stages of drying out, so the wall has shrunk more in some places than others. This caused problems when building the roof in the single-storey part of the building.
A timber beam, known a wall plate, rests on top of the walls to which the roof joists are attached. To make the top of the wall perfectly level, the contractor have inserted wedges between the wall plate and the wall. These are progressively tapped in as the wall dries out so the wall plate is evenly supported. Once the wall stops shrinking, the gap is packed with a mix of cement and sand.
YJL has also had to rethink conventional techniques when building the multistorey part of the building. The earth walls are capped with a reinforced-concrete beam running around the top of the walls, which in turn supports steelwork that engages with the timber-framed upper part of the building. "You have to wait until the last section of wall has stopped shrinking before you can cast the beam, as it needs to be cast in one piece," explains Peploe. Managing director Wilson agrees, adding: "It has an impact on us as a construction company because it can prevent us moving forward. Here we are affected by all sorts of factors that we are not used to. Next time we would have a bit of float in the programme."
Despite the problems, YJL is looking forward to its next rammed-earth project. "It has taken perhaps 25% longer than conventional construction," says Peploe. "But if there was a textbook, I think it would be quicker than using concrete." Wilson is already convinced it is a practical proposition. "We would like to team up with Rowland [Keable] and move this forward," he explains. "The chairman and I have had discussions about partnering with Rowland and using our structural expertise and his expertise on other projects." Wilson then corners Keable and earnestly begins a discussion on partnering.
Perhaps there will soon be a time when rammed-earth buildings on the Old Kent Road hardly raise an eyebrow at all.
Won’t it fall down when it rains?
Earth buildings can last for hundreds of years. The roof needs a decent overhang to prevent the wall from getting wet. However, according to Keable, rammed-earth walls don’t dissolve when that happens. “Once the walls are dry and the moisture content has dropped to 4%, there is an enormous buffer,” he says. The In Situ Rammed Earth Company had to build a test wall when working on the visitor centre at the Eden Project that was left unprotected from the elements. Two years later, Keable says it has lost just 50 mm in height. “The surveyor was quite upset about it and asked me why the wall hadn’t slumped,” he adds, laughing.The earth walls can be insulated either internally or externally. At the Bird in the Bush Centre, both permutations are being used. In some areas, the walls are left bare internally so children and babies can enjoy touching them. The walls will be insulated externally and rendered. In other areas, the insulation will be internal and the walls left bare outside for aesthetic reasons. Keable says the ideal combination is a hard render on the inside of the wall, and external insulation protected by a soft render. This means the earth can be used to act as a heat store to help regulate the temperature of a building. Differential pressure will force moisture through the hard, inner layer of render where it can easily pass to the outside. Keable says this approach isn’t often adopted because architects like to leave the earth visible in the finished building.
Ramming it home: Lessons learned on the Old Kent Road
Rowland Keable has been waiting a long time for this project. For years he has been trying to promote rammed-earth construction because of its very low embodied-energy content, but has been hampered because there is no standard for using it in the UK. This scheme is a practical part of a DTI Partners in Innovation-funded research project called Developing Rammed Earth Walling for UK Housing Construction.“This project is my mission statement – it’s loadbearing, it’s inner city, it’s domestic and it’s publicly funded,” says Keable.
Rammed earth has to be commercially viable if it is to be widely adopted. “I am trying to make this match the price of blockwork,” Keable says. “We have power tools, a front-end loader and a standard formwork system. In other areas, it can be a job creation thing, but on this project we’re trying to keep the team as small as possible.” Keable supervises a team of four. He points out that the job is not particularly skilled, and could be one answer to the shortage of bricklayers.
The project has been a learning experience. For example, other trades don’t always appreciate the precautions they have to take to help the walls to dry out. “Things that have come up are keeping the walls covered up in wet weather,” says Keable. “The other trades don’t know about this. They take the covers off and leave them off and you don’t know they have done it.” Keable hopes that once the research project is finished and the guidelines are published, the UK will lead Europe in rammed-earth construction. “What we are doing is so basic and simple, it’s almost pathetic,” he says. “This country could be so far ahead of the rest of Europe – you only have to put in pennies and you could be miles ahead.”
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