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June 05, 2009

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Sandcastle secrets could help revive ancient building technique

June 5, 2009
Courtesy Durham University
and World Science staff

The se­cret of a suc­cess­ful sand­cas­tle could aid the re­viv­al of an an­cient, eco-friendly build­ing tech­nique, ac­cord­ing to en­gi­neers at Dur­ham Un­ivers­ity in the U.K. They have car­ried out a study in­to the strength of rammed earth, which is grow­ing in pop­u­lar­ity as a sus­tain­a­ble build­ing ma­ter­ial.

Rammed earth was de­vel­oped in an­cient Chi­na around 2,000 B.C., when peo­ple used the tech­nique to build walls around their set­tle­ments, ac­cord­ing to ar­chae­o­lo­gists at Dur­ham. The tech­nique then spread through­out the world.

Part of The Al­ham­bra pa­lace com­p­lex in Gra­na­da, Spain, built in 1238 and now a world her­it­age site. (Im­age © Paul Jaquin/Dur­ham Uni­ver­si­ty )

Parts of the Great Wall of Chi­na and the Al­ham­bra pa­lace at Gra­na­da in Spain were built us­ing rammed earth. In the U.K. the tech­nique was used to build ex­pe­ri­men­tal low cost hous­ing, in Ames­bury, Wilt­shire, af­ter the end of the First World War, and it is a rec­og­nised build­ing meth­od in parts of Aus­tral­ia and the U.S.

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

Just as a sand­cas­tle needs a lit­tle wa­ter to stand up, Dur­ham en­gi­neers found that the strength of rammed earth de­pends much 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.

They found 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.

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. The team hopes their find­ings may al­so aid the con­serva­t­ion of an­cient rammed earth build­ings by cre­at­ing safe­guards against too much wa­ter en­ter­ing a struc­ture, which would weak­en it.

“We know that rammed earth can stand the test of time but the source of its strength has not been un­der­stood prop­er­ly,” said the re­search proj­ect lead­er, Charles Au­garde of Dur­ham. “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.”

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The secret of a successful sandcastle could aid the revival of an ancient, eco-friendly building technique, according to engineers at Durham University in the U.K. They have carried out a study into the strength of rammed earth, which is growing in popularity as a sustainable building method. Rammed earth was developed in ancient China around 2,000 B.C., when people used the technique to build walls around their settlements and the technique spread throughout the world, according to archaeologists at Durham. Parts of the Great Wall of China and the Alhambra at Granada in Spain were built using rammed earth. In the U.K. the technique was used to build experimental low cost housing, in Amesbury, Wiltshire, after the end of the First World War, and it is a recognised building method in parts of Australia and the U.S. 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. Just as a sandcastle needs a little water to stand up, Durham engineers found that the strength of rammed earth depends much 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. They found 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. 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. The team hopes their findings may also aid the conservation of ancient rammed earth buildings by creating safeguards against too much water entering a structure, which would weaken it. “We know that rammed earth can stand the test of time but the source of its strength has not been understood properly,” said the research project leader, Charles Augarde of Durham. “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.”