Ancient buildings yield modern, eco-friendly secrets

The secret to an ancient eco-friendly building technique could lie in the structure of your typical successful sandcastle, according to researchers at Durham University.

The building technique, known as rammed earth, is growing in popularity today as a sustainable building method. Experts at Durham’s School of Engineering have been studying the technique and have found the strength of rammed earth is heavily dependent on its water content.

Rammed earth is a manufactured material made up of sand, gravel and clay which is moistened and then compacted between forms to build walls. Sometimes stabilisers such as cement are added but the Durham research focussed on unstabilised materials.

The researchers tested small cylindrical samples of rammed earth using “triaxial testing,” in which external pressures are applied to model behaviour of the material in a wall. They found the suction created between soil particles at very low water contents was a source of strength in unstabilised rammed earth.

Theie research showed that rammed earth walls left to dry after construction, in a suitable climate, could be expected to dry but not lose all their water. The small amount of water remaining provided considerable strength over time.

As the link between strength and water content becomes clearer, that discovery could have implications for the future design of buildings using rammed earth.

The growing interest in the rammed earth technique lies in the fact it might help reduce reliance on cement in building materials. Cement production is considered a significant contributor to climate change, as its production is responsible for about five per cent of manmade carbon dioxide emissions. Rammed earth materials can also usually be sourced locally, which reduces transport needs.

“We know that rammed earth can stand the test of time but the source of its strength has not been understood properly to date,” said research project leader Charles Augarde. “By understanding more about this we can begin to look at the implications for using rammed earth as a green material in the design of new buildings and in the conservation of ancient buildings that were constructed using the technique.”

The rammed earth technique was developed in ancient China some 4,000 years ago, when people used the method to build walls around their settlements. Parts of both the Great Wall of China and the Alhambra at Granada in Spain were built using rammed earth.

In the UK, the technique was used to build experimental low-cost housing, in Amesbury, Wiltshire, following the end of the First World War. It is also a recognised building method in parts of Australia and the USA.

Augarde is a co-director of Earth Building UK (EBUK), a new association established this year to foster the conservation, understanding and development of building with earth in Britain. EBUK brings together builders, academics, researchers, architects, engineers, manufacturers and many more to work in areas of common interest at a national and local level.

“This kind of research is very valuable as the construction industry analyses environmentally sound, traditional ways of building and adapts them for sustainable construction in the 21st century,” said Tom Morton, secretary of Earth Building UK.  “Such low-carbon technologies are most likely to succeed by marrying the expertise of our research universities, such as Durham, with the commercial understanding of the wider industry and we are seeing a number of very exciting developments in this area.”