SANDCASTLES could hold the key to reviving a forgotten eco-friendly building technique, according to North-East scientists.

As every child at the seaside knows, a little water is needed to keep a sandcastle standing up.

In just the same way, the strength of ''rammed earth'' - a ''green'' construction material made from sand, gravel and clay - heavily depends on its water content, scientists have found.

The use of rammed earth is growing in popularity as a sustainable building method.

Typically it is moistened and compacted within a supporting framework.

The technique was developed in ancient China around 4,000 years ago and then spread around the world.

Parts of the Great Wall of China and the Alhambra palace in Granada, Spain, were built from rammed earth.

It has also been incorporated into the Eden Project, a visitor attraction in Cornwall that recreates different natural environments.

There is increasing interest in rammed earth because it is environmentally friendly and reduces reliance on cement, the manufacture of which generates five per cent of man-made carbon dioxide emissions.

Scientists from the University of Durham tested small cylindrical samples of rammed earth, applying external pressures to model the behaviour of the material compressed in a wall.

They found that the suction created between slightly damp soil particles was an important source of strength.

Rammed earth walls behaved as if they were part of a sandcastle. Left to dry after construction, they retained a small amount of water and remained strong.

Study leader Dr Charles Augarde, from the university's school of engineering, 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.

''Without this understanding we cannot effectively conserve old rammed earth or make economic designs for new buildings.

''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.''

The research is published today in the journal Geotechnique.

Tom Morton, secretary of Earth Building UK, an organisation set up this year to promote earth building techniques, said: ''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.

''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.''