jetty foundations on sediments zeevaert 1957

8/12/2019 Jetty Foundations on Sediments Zeevaert 1957

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CHAPTER49 JETTYOUNDATIONSON INEEDIMENTS

LeonardoZeevaertProfessorofSoilMechanicsandFoundations

University ofMexicoMexicoCity,Mexico

SYNOPSISThe SecretariadeMarinaNacional ofMexico,contemplatesthe construc-tionoftwo longjettiesat themouthoftheGrijalva River topermit safenavigationintotheportofProntera inthestateof Tabasco, FigloheportofFrontera is located in the estuary ofthe GrijalvaRiver 9Km.from

itsmouth*he proposed jettiesshouldreachintotheseato adepthof wa-ter of 6 mts.hisrequires alengthfromthemouthof theriver ofabout2000mts, Fig2.ockfilljettiesconstructed inthepast inthisareaonthe fine sediments havefailedby spreadingandpenetration into thefine cohesionless sediments encounteredat theseabottom.eavystructurescannotbeconstructed onaccountofthelowshearingstrength ofthe submarine deltclaydeposits thatmay be encountered at themouthofthe QrijalvaRiver*

Subsoil investigationswereperformed bytheauthorto learn the mech-anicalpropertiesofthematerialsatthemouthofthe GrijalvaRiver,andmade possiblethe designoflight-weightandstrong structuraljettiestoresist the seaand riverforcestowhichthese structureswillbe subjected*The problem involvedandstabilityconsiderationsofthejettiesareexplaixedbythe authorinthis paper*

GEOTECHNICAL STUDIESSubsoilinvestigations atthemouthof theGrijalvaRiverindicateda

surface depositofabout 5 mts thickof fine sandwith frequentdiameter r a igingbetween 0*15and0*20mm.hismaterial is theproductofthepresentsedimentationof theriverandcoversuniformlyallthemouthand extends i athesea at least3 Km.fromthemouth of theriver*hetypical subsoil p r


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JETTYOUN TIONSON INEE IMENTS

IOOCANA tnoroTOFig. PictureofmouthofGrijalvaRiver.

Fig.. Proposedlocationofjetties819


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COASTALENGINEERING

TCQ

is 9z% 3> O 'ocfffi

cr 3

co aa

JZ-C5

yuoa.

r

s i:o >W a8Se c

820


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JETTYOUNDATIONSONINEEDIMENTSother*ollowing this stratumauniformdepositofsiltysand withlittleclay maybe foundwithathickness of3*1 mts correlatingvery wellin alltheboreholes.he averagewater contentis 23$ andthe unconfinedeom-pressivestrengthas lowas0,2 Kg/cm.

From10 to 29.5mtsdepthitmay be encountered athickgreenish-grayclaydepositwith microscopicshells,andunderlainby graysiltysandwithsmallshells*hisdepositistheproductofthesedimentation of thefineralluvial sediments deposited inthe sea.his fines cominginto the seaflo-cculatedandsettledforming thesubmarinedelta oftheriver.owever*theveryfinesubmarinesedimentsfound in the present at themouth ofthe riverwereundoubtedly formedwhenthemouthwasseveral kilometersbehind itsac-tualposition*he recentadvanceofthemouthoftheGrijalvaRiver towardthe seahas produced theupper coarser sedimentsunderlainbytheclaydepo-sit.

Theaveragewatercontent of the claydeposit isabout 55 andshowspracticallyconstant withdepth Fig3*he clay isnormallyconsolidated*Thisfactmay be demonstrated bythegood agreementshownbetween thebreakin the compressibilitycurvesandthecomputedoverburdenpressures*

The shearingstrengthofthe claydepositbetween 10and29*5tsdepthobtained fromunconfinedcompressiontestsshowsaveryerraticvalue* pro-bably because of thestrongdifferenceinthesalinityof thewaterduringtheprocessofsedimentationandflocculationofthefine grains*his factmay have hadanimportantinfluenceinthe shearing strength of theclay$However,the minimum value ofthe shearingstrength isof0*15Kg/cm 2at10mtsdepthand increasesonlyto0*2Kg/cmt 29.5 depth*

From numerousconsolidation tests onlaterally confined specimensit wasfound anaveragevalueofm 0.033to 0,038 cm 2/Kgbased ona period of100years,whenthe secondarysettlement istakenintoaccount(1).rimarycon-solidationtakesplace in only20yearsafter loadapplication*Usingthesettlement analysis(1)(2)equations

S-m*,F(T)ApH ;US [mF(T ) m toqf^Ap-H ,t

Thecorrespondingvaluesoftheparametersare:

1 )EcuacionCompletadeConsolidaoion para DepositosdeArcillaqueExhiben Fuerte CompresionSecundarfa,by Leonardo Zeevaert,Juno 1957Published inBevista Ingenieria*

2 )Consolidation ofMexicoCity Volcanic ClaybyDr.LeonardoZeevaert,ProceedingsJointMeetingofASTM andSMMS,MexicoCity Dec.9-13-57

821


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COASTALENGINEERING mw0 027 to 0035 cm z/kgmt - 0.01 I to 0 .012 cm 2/kgT l.oC v 00 0 / 59 to 0 .00/46 c / 7 ? 2/sec

dzi / - L I ?0years.Thetotalthickness of the deposit maybe takenas 19*5mts drained ttopandbottom*

SUBSIDENCE PROBLEM

Whena heavy rockfillis constructed onthesefine sand,silt and csediments toforma jettyor anywaterwave protection,it isnecessarytconsider inthe design thefollowingphenomenala.- penetration of therockinto thefine cohesionless sedimentsbe-cause of thepossibilityof spontaneousliquefaction*b.- spreading ofthe slopes of the rockfill because ofmaterialeresioninthefoundation*c subsidence of thefill because excessive shearing stresses in t hunderlying clay deposit.d*-largesettlement of thefillbecause of excessive compression*Items ( a )and (b)are produced by wave actionand (c)and( d )becautof exceeding, respectively,the allowablemechanicalproperties of shear;strengthandcompressibility of the soft claydeposit*( a )Penetration oftherockintothe fine cohesionlessmaterialisused bythereductionof shearing strength in thesandat the passage ofwaterwaves*hatis tosay,asa waterwavepasses over certainpointthe slope of the jettythe hydrostatic pressures inthesoil mass cannotjust themselves totheinstantaneous hydrostatic excesswater pressures*erefore,an importanthydrodynamic lagmaybe created inthesand supportherock materialof the jetty,thus the hydrostatic excess pressure oremomentarilyan spontaneous liquefaction condition of the sandpermitting

822


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JETTYOUNDATIONSON INEEDIMENTSr o c k t o p e n e t r a t e i n t o t h e f i n e s a n d . h i s e f f e c t i s a f u n c t i o n o f t h e w a v eh e i g h t . o d e m o n s t r a t e t h i s p h e n o m e n o n l e t u s c o n s i d e r F i g 4 a v a v e c o m i n ga l o n g t h e j e t t y . h e c r e s t r i s e s f r o m t h e n o r m a l w a t e r l e v e l i n a p p r o x i m a t e -ly h M a n d a s t h e v a l l e y o f t h e w a v e p a s s e s a l o n g , t h e w a t e r l e v e l d r o p s f r o mt h e a v e r a g e l e v e l i n a p p r o x i m a t e l y h . n e p i e z o m e t e r A i n s t a l l e d a t d e p t hz i n t o t h e v e r y f i n e s a n d d e p o s i t w i l l p r e s e r v e t h e n o r m a l o r a v e r a g e w a t -e r l e v e l a s t h e w a t e r w a v e p a s s e s r a p i d e l y . h e r e f o r e * a n i m p o r t a n t a t e ru p l i f t p o r e p r e s s u r e i n t h e s o i l w i l l t a k e p l a c e a s t h e v a l l e y o f t h e w a v ep a s s e s o v e r t h e p o i n t c o n s i d e r e d * a k i n g i n c o n s i d e r a t i o n , F i g U t h e f o r -c e s a c t i n g f o r e q u i l i b r i u m i n t h e u p p e r p a r t o f t h e d e p o s i t o f t h i c k n e s s z may b e o b t a i n e d t h e f o l l o w i n g e q u i l i b r i u m e q u a t i o n s

t h u s :

&= z-yJ_ T h e e f f e c t i v e i n t e r g r a n u l a r p r e s s u r e a t d e p t h z i s e q u a l t o t h e w e i g h tf o f t h e s u b m e r g e d s a n d , m i n u s t h e u p l i f t p r e s s u r e y ^ * . ^p r o d u c e d b y t h ew a v e h e i g h t * h e e q u i l i b r i u m i s u n s t a b l e u p t o a d e p t h w h e r e :/D=oh e n c e ;Z --z~- Als i n c e :p = ~ / t h e n X. T h i s i m p l i e s t h a t t h e w a v e s p r o d u c e a n u n s t a b l e c o n d i t i o n i n t h e e o -h e s i o n l e s s f i n e s a n d a n d s i l t y s e d i m e n t s o f t h e b o t t o m o f t h e s e a t o a d e p t ha p p r o x i m a t e l y e q u a l t o t h e s e m i - h e i g h t o f t h e w a v e s *

A h e a v y r o c k f i l l w i l l p e n e t r a t e i n t o t h e f i n e s e d i m e n t s t o a d e p t h w h e r ey t ot a r t s to b e l a r g e r t h a n z e r o *T h e r e f o r e * i f t h e s u b m e r g e d w e i g h t o f t h e r o c k f i l l i s a n d i t s t h i -c k n e s s a b o v e t h e s a n d i s D t h e n : r

h-t^+fromwhich

z r D~h>1 823


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COASTALENGINEERINGThe value ofMzMrepresents thetheoretical depthat whichtherockfill willpenetrateinto the sand*racticallyno penetrationwilltakeplace fordepthsoffills

D 5 )However,atthe footofthe slope there isalways thetendencyto haveapenetrationequal tothatgiven byformula. (2)

WavcS QmhTTrrrrn

Erosion of th


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JETTYOUNDATIONSON INEEDIMENTSc )reakingof the rockfillinto theground may be controlledby notexceedingtheshearingstressesinthesoft claydeposit.

(d)xcessive settlement may be avoidedby notoverloading theclay de-posit.ince theclay deposit isnot of highcompressibilityitappearsfromsettlement analysis that thecomputed settlementswithsafebearing loadingcapacity will producereasonablesettlements that can betakensafelybythestructure

STABILITY OF PROPOSEDSECTION

Thedecision of theproposedsectionwasobtained after oarefull studyofthesubsoilmechanicalpropertiesin conjunction withthetheoreticalstu-diesconcerning thepossibility ofspontaneous liquefaction ofthefinesandsediments,erosion andthephenomenon ofpossible spreadingofthepartiallysubmerged rockfill.urthermore,sincetheshearingstrength ofthe clay isonly 0.15Kg/cm 2itwas decidedtoabandon,for safety,the idea of apar-tially submergedrockfilljetty.fterstudyingother possible sectionstheauthordecidedtorecommend astructureofreinforcedconcreteboxesbuiltwith asystemofsheetpiles driven previously andforminga cofferdam-likestructure,Fig 6*hestructure,thusformed is confinedlaterallyagainststrong erosion ofthefinesedimentsby meansof aminimumrock fillplacedatthebottomoftheseaagainsttheproposedstructure.iththepurposeof resistingthelargedynamic lateralforcesbecause ofthe impact ofthebreaking wavesagainst thestructure,it wasnecessary to anchorproperlythesheet-piles into the clay deposit.nthisfashion itwas possible tomake averyfavorable use of the shearingstrength of theclay.

Fig.6.Proposedsection.825


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COASTALENGINEERINGHowever,the coffer-dam formedofhollow reinforced concreteboxesFig 6,is alonenot enoughtoholdthe large thrust ofthebreaking wavesonthe structure*herefore,to increase thesafebearing capacity itwasnecessaryto confine thebottomof the seaclosetothereinforced concretestructurebymeans of a rockfill,thus increasingalso thestabilityof t lstructure asa whole and reducing the dimensions and cost of thecentral p

jetty foundations on sediments zeevaert 1957

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