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Link found between sand castles and rammed earth structures

The secret to buiding a successful sandcastle could aid the revival of an ancient eco-friendly building technique, according to research published by Durham University.

Ramboll engineer Paul Jaquin and researchers from Durham University’s School of Engineering, have carried out a study into the strength of rammed earth, which is growing in popularity as a sustainable building method.

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

The research, funded by the Engineering and Physical Sciences Research Council (EPSRC) and published in the journal Geotechnique, showed that a major component of the strength of rammed earth is due to the small amount of water present.

Just as a sandcastle needs a little water to stand up, the engineers found that the strength of rammed earth is heavily dependent on its water content.

Small cylindrical samples of rammed earth underwent “triaxial testing” – where external pressures are applied to model behaviour of the material in a wall. The researchers found that the suction created between soil particles at very low water contents was a source of strength in unstabilised rammed earth.

The researchers say their work could have implications for the future design of buildings using rammed earth as the link between strength and water content becomes clearer.

There is increasing interest in using the technique as it may help us to reduce reliance on cement in building materials (cement production being responsible for five per cent of man’s CO2 output (1)). Rammed earth materials can usually also be sourced locally hence reducing transport needs.

The researchers 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 well as informing new build designs the team hopes their findings could also aid the conservation of ancient mud brick buildings by putting methods in place to protect against too much water entering a structure, which would reduce its strength.

“It’s remarkable that one third of the world population live in mud brick buildings, and we’re only just discovering how they work - that the science of sandcastles is the same as most peoples houses is amazing,” said Jaquin

“The results of this research are a major step towards finding another sustainable method of building, and what’s great is that it is not a new technique, it is one we are all familiar with from our childhood days playing on the beach. Everyone knows a sandcastle can’t be too wet or too dry, it’s the same with mud buildings, and we’ve explained why.”

Research project leader, Dr Charles Augarde, of Durham University’s School of Engineering and co-director of Earth Building UK (EBUK), said: “We know that rammed earth can stand the test of time but the source of its strength has not been understood properly to date.”

“Our initial tests point to its main source of strength being linked to its water content. 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.”

Rammed earth was developed in ancient China around 2,000 years before Christ, when people used the technique to build walls around their settlements and the technique spread throughout the world. Parts of the Great Wall of China and the Alhambra at Granada in Spain were built using rammed earth.

The popularity of eco-friendly homes showcased on television programmes such as Grand Designs has further brought the technique to people’s attention.

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