Lecture 6 GEOTECHNICAL INPUT FOR THE DESIGN OF RETENTION STRUCTURES J.J. DavidDavid Rogers,Rogers, Ph.D.,Ph.D., P.E.,P.E., P.G.P.G. Karl F. Hasselmann Chair in Geological Engineering Missouri University of Science & Technology for the course GEGE 441441 GeotechnicalGeotechnical ConstructionConstruction PracticePractice StackedStacked cyclopeancyclopean masonrymasonry wallswalls havehave providedprovided excellentexcellent serviceservice forfor manymany centuriescenturies ThisThis isis aa cyclopeancyclopean wallwall usingusing nativenative rockrock andand cementcement mortar,mortar, onon TiogaTioga PassPass highway,highway, justjust outsideoutside YosemiteYosemite NationalNational Park,Park, whichwhich waswas completedcompleted inin 19641964 Masonry walls can be very resilient Masonry gravity retaining wall systems have been constructed since 6600 BC in Jericho. We can learn much from observing what has survived and what has not. The basic objective of early walls was to provide military bastions and protection from attack. Most of these structures were built using rock facing with random fill, as sketched above. StackedStacked masonrymasonry wallwall atat MacchuMacchu Pichu,Pichu, thethe lostlost IncaInca citycity sittingsitting atat anan altitudealtitude ofof 74007400 feetfeet inin thethe PeruvianPeruvian AndesAndes WithoutWithout mortarmortar oror reinforcement,reinforcement, thesethese wallswalls havehave withstoodwithstood centuriescenturies ofof earthquakes,earthquakes, precipitationprecipitation andand neglectneglect Above left: Stacked cyclopean masonry wall in Guatemala Above right: Undulating masonry rock retaining wall about 100 years old in Glenwood Springs, CO. Gravity walls can withstand minor foundation movements and differential settlement Masonry retaining wall in Cincinnati accommodates the grade by employing vertical offsets, or “steps.” Steps are usually employed because there is some sacrifice in load capacity if the wall is built on-grade. The difference in cost can be substantial, while the increased active pressure is usually between 1 and 11 %, for 2% to 20% grades. PartPart 11 GRAVITYGRAVITY WALLSWALLS GravityGravity Walls:Walls: Oldest and Most Common Wall System GravityGravity wallswalls are designed to use their effective weight to resist the lateral pressures engendered by soil backfill, considering seepage, surcharge, and earthquake loads CommonCommon formsforms ofof gravitygravity retainingretaining wallwall systemssystems inin thethe modernmodern eraera ConventionalConventional gravitygravity wallswalls likelike thosethose shownshown atat leftleft areare graduallygradually beingbeing replacedreplaced byby combinationcombination systems,systems, suchsuch asas thethe MechanicallyMechanically StabilizedStabilized EmbankmentEmbankment structure,structure, shownshown atat rightright FourFour basicbasic failurefailure modesmodes areare consideredconsidered inin thethe designdesign ofof gravitygravity retainingretaining systems:systems: basalbasal sliding;sliding; overturning,overturning, tilting/bearingtilting/bearing failurefailure andand globalglobal slopeslope stability.stability. You’dYou’d bebe surprisedsurprised howhow oftenoften #4#4 isis forgottenforgotten oror ignoredignored TheThe mostmost basicbasic requirementrequirement ofof aa gravitygravity PA = active load wallwall isis thatthat thethe resultantresultant against wall thrust, R, must project W = dead weight thrust, R, must project The Resultant Thrust, R, of the wall must fall within middle third withinwithin thethe middlemiddle thirdthird of the base R = resultant vector thrust ofof thethe footingfooting BatteredBattered MasonryMasonry WallsWalls Above left & right: Ancient masonry walls were usually constructed with a healthy batter, and drained through natural gaps. Lower left: modern equivalents are employ weepholes and/or geomembrane drains Roosevelt Dam on the Salt River in Arizona was constructed as a cyclopean masonry gravity dam by the US Reclamation Service in 1903-11 TaperedTapered oror flaredflared toetoe Ancient fortifications, like Hemeji Castle in Japan, which dates to 1346, employed flared walls designed to spread load over a broad area, while discouraging ascent of the increasingly steep face GravityGravity RetainingRetaining WallWall StabilityStability CriteriaCriteria –– ContinuedContinued 2.2. ResistanceResistance toto SlidingSliding T = effective thrust of wall, after subtracting the pore pressure The Bayless Dam near Austin, Pennsylvania failed by basal sliding along a shale foundation in 1911, despite the reservoir being lowered by notching the dam (left). MethodsMethods commonlycommonly employedemployed toto increaseincrease basebase frictionfriction ThoughThough notnot common,common, basalbasal slipslip cancan alsoalso occuroccur ifif thethe footingfooting embedmentembedment isis compromisedcompromised andand thethe basebase ofof thethe footingfooting isis lyinglying withinwithin saturatedsaturated groundground AA moremore complicatedcomplicated checkcheck isis thatthat againstagainst bearingbearing capacitycapacity andand differentialdifferential settlement,settlement, whichwhich cancan resultresult becausebecause ofof eccentriceccentric forcesforces andand inadequateinadequate footingfooting designdesign PassivePassive reactionreaction wedgeswedges formform underunder differingdiffering conditionsconditions ofof soilsoil friction.friction. NoteNote thatthat highhigh frictionfriction soilssoils influenceinfluence aa muchmuch greatergreater areaarea thanthan lowerlower frictionfriction soilssoils ChartChart showingshowing decreasedecrease inin designdesign bearingbearing capacitycapacity versusversus slopeslope setbacksetback versusversus phiphi angleangle ofof foundationfoundation materialsmaterials .