5/8/2015 Whydoesstructuralbehaviorchangeindifferenttypesofsoil?|MITSchoolofEngineering

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- POSTED: November 13, 2012

WHY DOES STRUCTURAL BEHAVIOR CHANGE IN DIFFERENT TYPES OFSOIL?

Theres a recipe for getting it right, depending on what you need

Asany5yearold can tell you, the secret to theperfectmudpie is the correctamountofwaterand the rightkindofdirt. Theyknow fromexperience that sandand claybehavedifferentlyand thatusingoneor theotherwill spell the successor failureof aplayhouse recipe.

Thebehaviorand strengthof soil is largelydeterminedby the sizeof itsparticlesand themigrationofwater through the soil skeleton, saysAndrewWhittle(http://cee.mit.edu/whittle) , theEdmundK.TurnerProfessorwhospecializes ingeotechnical engineeringandgeomaterials and isheadofMITsdepartmentof civil andenvironmental engineering.

Soil types range fromsiltwithparticles in the submillimeter range, to clay, sand,gravel, andboulders, andmost containwater,heexplains.Thelarger thegranules, thegreateramountofwater that can flowbetween them,determining the soils stability. Inessence,a change inwatercontentdrives change in soil strength saysWhittle.

Water flows freely fromsand, for example, inwhich thegrainsare roundedandpore spacesare largeandnumerous.Asbeachgoers canattest,their every stepupon the shore squeezeswaterup through the sand.Water flowsout, leaving the sandsporous,unstable skeleton.

In clay,on theotherhand,watermovesmuchmore slowly through its flatnanoparticles. A lot ofwater isheldon the surfacesof the tiny clayparticlesand isnt free tomovewithin its skeleton, saysWhittle. Thewaterholds theparticles together throughcapillary force,providingadditional strength.

Understanding thebehaviorof various soils is ofutmost concern to the civil engineer. Soil strength is extremely important in thedesignof allsupport systems, fromabuildings foundation toahighwayoverpass, saysWhittle,who servedon theMassachusettsgovernors safety reviewpanel ofBostonsBigDig tunnels. Whenwebuildhighwayembankmentson clay,we relyonaprincipleknownas consolidationof clay.Webuildone layerandallow thewater to flowout so the claygains strengthbeforeweaddanother layer.

Farmers, too,mustknowhowvarious soilsbehave if theyexpect to successfully reapwhat they sow.Natural soils areablendof sandand clay,and theirbehaviordependsonwhats in themix. In fieldsmadeupof toomuch sand, rainwaterwashesnutrients through theporous soil skeletonand far from the spinachand sweet cornand soybeans.Toomuch clayand thewatermoves too slowly to transportnecessaryminerals theplantsneed togrow.Whileagricultural soils arenothis specialty,Whittle says theprinciples remainare the same. Whileparticles in somesoils arebonded togetherbyminerals that reprecipitate in the soil,most relyon the interlockingand frictionbetweenparticles, and capillary forcesexertedby theporewater. Its all about cohesion the relativeabilityofwater tomove.

Its aprinciplemudpiemakersknow intuitively.Mix just the rightproportionsofwaterand clayandallow it todryabit, and thepiewill standupverywell on theplate.But stir in toomuch sand,and it crumbles likea sandcastleon the sunny seashore.Sarah Jensen

Thanks toMahabubarRahman fromDhaka,Bangladesh, for submitting thisquestion.

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