高等土力学高等土力学 (2).pdf

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1、Consolidation Analysis of Nuozhadu High Earth-Rockfill DamBased on the Coupling of Seepage and Stress-DeformationPhysical StateAbstract:A consolidation analysis method for an earth-rockfill dam based on the coupling of physical state,stress deformation,and seepagewas established.In this method,a per

2、meability coefficient model considering physical state and shear stress level was introduced,and it wasintegratedwith Biots consolidation theory to simulate thecoupling relationship.Taking the Nuozhadu high earth-rockfill dam as an example,a parameterback-analysismethodusinga neural networkandevolut

3、ionaryalgorithmwasappliedtoback-analyzethepermeability coefficientmodel parameters of the core material based on observation data of pore water pressure in the dam.Then,consolidation numerical analyseswere performed by using the back-analyzed parameters and the constant permeability measured in situ

4、,respectively.The spatial and temporaldistribution of pore water pressure calculated by the back-analyzed parameters coincided quite well with the monitoring data during construc-tion and operation.However,the numerical results with constant permeability are different from the observed values,which

5、indicates that thepermeabilitycoefficientmodelandtheconsolidationanalysismethodcanbeappliedeffectively,reasonably,andnecessarilytonumericalsim-ulationsforhighearth-rockfilldams.DOI:10.1061/(ASCE)GM.1943-5622.0000555.2015AmericanSocietyofCivilEngineers.Author keywords:Highearth-rockfilldam;Consolidat

6、ion;Coupling;Seepage;Stressdeformation;Physicalstate;Back-analysis.IntroductionEarth-core rockfill dams account for a large proportion of highdams with a large reservoir in western China.Because of growingsocial demand and the development of construction technology,earth-rockfill dams built in China

7、 are being built higher andhigher.Many of the dams under construction or planning weredesigned with heights of 200300 m,or even higher(e.g.,theLianghekou dam,with a height of 292 m,located on the YalongRiver,and the Shuangjiangkou dam,with a height of 314 m,located on the Dadu River).The core wall i

8、s a key part of thiskind of dam;it bears extremely complex changes in seepage andthe stress-deformation state in the processes of construction,im-pounding,and operation.Also,pore water pressure in the corewall becomes an important indicator to assess the safety of highearth-rockfill dams.Earth-core

9、rockfill dams that were built withheights of more than 250 m are rare,so there has been little expe-rience designing and constructing high ones.In high earth dams,the internal stress is high,and large shear deformation exists,which result in significantly different physical states(the void ra-tio,th

10、e water content,the density,etc.)of the materials and seep-age behaviors from those of low dams.Therefore,it is necessaryto further study such coupling effects on high earth-core damprojects.During construction and operation of several high earth-rockfilldams in China,the observed values of pore wat

11、er pressure,defor-mation,and their trends are quite different from the numericalresults obtained by using the traditional consolidation analysismethod.For instance,the Xiaolangdi earth-rockfill dam(Chen etal.2005),with a height of 160 m,was completed 12 years ago,but the pore pressure in the core wa

12、ll stays at a high level andchangesperiodicallywiththereservoirwaterlevel.TheNuozhadu dam was completed at the end of 2012,and variationsin pore water pressure in the core wall have similar features.These phenomena are difficult to explain reasonably with previ-ous theories and methods,which may be

13、because the distributionsof physical state,stress deformation,and seepage have a complexcoupling relationship during construction,impounding,and opera-tion.Thus,more reasonable models and methods should be devel-oped for analyzing and simulating the coupling relationship inhigh earth-rockfill dams.I

14、npreviousstudiesofconsolidationandstress-deformationanal-yses,the influence of changes in seepage(e.g.,seepage force andpore pressure)on the stress deformation has been taken intoaccount.In consolidation analyses,the permeability characteristic,as oneof themost importantsoil properties,wasmostlyhigh

15、lysim-plified(Luo et al.2008;Huang et al.2010;Elia et al.2011;Yao etal.2012),which will cause big errors in the numerical results ofstress deformation and seepage.The distributions of physical state,seepage,and stress deformation actually influence each other.Thechanges in seepage will induce the ch

16、anges of seepage body forceandseepagepressure,whichmakethestressstateinthedamchangeaccordingly,but the changes in the stress state will make the strain,the void ratio,the permeability coefficient,and then the seepage in1Ph.D.Candidate,State Key Laboratory of Hydroscience andEngineering,Dept.of Hydra

17、ulic Engineering,Tsinghua Univ.,Beijing100084,China.E-mail:2Professor,State Key Laboratory of Hydroscience and Engineering,Dept.of Hydraulic Engineering,Tsinghua Univ.,Beijing 100084,China.E-mail:3Professor,State Key Laboratory of Hydroscience and Engineering,Dept.of Hydraulic Engineering,Tsinghua U

18、niv.,Beijing 100084,China(corresponding author).E-mail:4ProfessorofEngineering,HydroChinaKunmingEngineeringCorporation,115 Renmin East Rd.,Kunming 650051,China.E-mail:zhang_Note.This manuscript was submitted on October 28,2014;approvedon June 10,2015;published online on December 30,2015.Discussion p

19、e-riod open until May 30,2016;separate discussions must be submitted forindividual papers.This paper is part of the International Journal ofGeomechanics,ASCE,ISSN 1532-3641.ASCE04015085-1Int.J.Geomech.Int.J.Geomech.,2016,16(3):04015085 Downloaded from ascelibrary.org by TSINGHUA UNIVERSITY on 09/29/

20、16.Copyright ASCE.For personal use only;all rights reserved.the dam change.Therefore,it is of great theoretical and practicalsignificance to study the coupling effects of seepage,physical state,andstressdeformation.To obtain a more reasonable coupling relationship between per-meability and other fac

21、tors,there have been many studies on thepermeability of cohesive soil,and a series of permeability modelshave been proposed with parameters e.g.,void ratio,watercontent,density,effective stress,and confining pressure(Mesri 1971;Samarasinghe et al.1982;Kodikara and Rahman 2002;Xie et al.2005).These r

22、esults reflect the permeability characteristics of co-hesive soil from different aspects.However,most of the studies ontheinfluenceofphysicalstateonpermeabilitywereconductedwith-out considering the loading and deformation;that is,they cannotreveal the permeability characteristics under loading.Most

23、of theexperimentalstudiesontheinfluenceofstressstateonpermeability,generally performed in isotropic consolidation conditions,cannotreflect the influence of shear deformation caused by the practicalanisotropic stress in consolidation(Carpenter and Stephenson1986;Nagaraj et al.1993;Chu et al.2002).Thu

24、s,relevant simula-tions based on the theories can hardly reflect the permeability char-acteristicsinfluencedbyshearstressanddeformation.Moreover,the traditional experimental test methods for deter-mining the parameters of a constitutive model of soil involve manylimitations,such as the specimen repr

25、esentation,the size effect oflaboratory tests,and so on.With the recent development of optimi-zation algorithms,intelligent back-analysis methods were intro-duced to geotechnical engineering and have become an accurateand effective way to verify the soil parameters(Sakurai andTakeuchi 1983;Swoboda e

26、t al.1999;Feng et al.2004;Zhang et al.2006).Among them,artificial neural networks and generic algo-rithms were used and showed great potential applications,whereasthe studies mainly focused on the back-analysis of parameters of aconstitutivemodel basedondisplacementinundergroundengineer-ing.There ar

27、e few research results concerning the back-analysis ofpermeabilitycoefficientbasedonporepressureindamprojects.This study was performed on the basis of the observation datafrom the Nuozhadu high earth-rockfill dam.A large number ofmonitoring instruments were installed in the dam during construc-tion,

28、andagreatdealoffieldobservationdatahavebeenobtainedsofar.In this study,a permeability coefficient model consideringphysicalstateandshearstresslevelwasintroducedintheconsolida-tion analysis,which was discussed in detail in another paper(Leiet al.2015).Here,this model was integrated into a finite-elem

29、entprogram based on Boits consolidation theory(Biot 1941,1955)tosimulate the coupling effects in the consolidation analysis.In thisway,a consolidation calculation method based on the coupling ofthe physical state,the stress deformation,and the seepage was setup.Then,aback-analysismethodusinganeuraln

30、etworkandevolu-tionary algorithm was applied to obtain the parameters of this per-meability coefficient model on the basis of the observation data ofpore pressure.Last,the consolidation numerical analyses of theNuozhadu dam were performed by using the back-analyzed param-etersandtheconstantpermeabil

31、itymeasuredinsitu,respectively.Project DescriptionThe Nuozhadu hydropower station is located on the main stream ofthe lower Lancang River,near Puer City in the Yunnan Province,China.The total installed capacity is 5,850 MW,and the designedannual average power output is 2.39?1010kWh.This project isco

32、mposed of the earth-core rockfill dam,an open spillway on theleft bank,flooddischarge tunnels,andundergroundwater-diversionstructures.The earth-core rockfill dam has a maximum heightof 261.5 m,and the total storage capacity of the reservoir is2.37?1010m3.The maximum cross section of the dam,with mat

33、erial zoning,isshowninFig.1.Therearesixtypesofdammaterialsintotal,amongwhich,rockfill I,rockfill II,and gravelly clay are the three mainmaterials.To reduce the differential deformation between the corewallandtherockfill,acertainamountofgravelismixedintoclay toincreasethemodulusofthecorematerial.Moni

34、toring instrumentsforpore water pressure were installed inthedam,and thelayoutofthe15piezometersinthemaximumcrosssection is shown in Fig.1.They were installed in the gravelly claycorewallatfiveelevations:626,660,701,738,and780m.The construction process of the core wall and the impoundingprocess of t

35、he reservoir are shown in Fig.2.Dam construction wasstarted in January 2008 and completed at the end of December2012.The upstream water level was almost unchanged at approxi-mately 605 m before December 2011,and then the water level rosewith theimpoundingofthereservoir.AttheendofDecember2012,theupst

36、reamwaterlevelreached774m.Consolidation Theory,Material Models,and Back-Analysis MethodA modified consolidation analysis program was used to calculatethestress,deformation,and pore pressureoftheNuozhadu dam andtogeneratethetrainingsamplesfortheneuralnetwork.Theanalysisprogramwasdevelopedbytheauthors

37、onthebasisofBiotsconsol-idation theory(Biot 1941,1955).The element types of the finite-element program contain eight-node hexahedron elements,six-Fig.1.MaximumcrosssectionoftheNuozhaduearth-rockfilldam ASCE04015085-2Int.J.Geomech.Int.J.Geomech.,2016,16(3):04015085 Downloaded from ascelibrary.org by

38、TSINGHUA UNIVERSITY on 09/29/16.Copyright ASCE.For personal use only;all rights reserved.node triangular prism elements,five-node pyramid elements,andfour-nodetetrahedronelements.Inthisprogram,apermeabilitycoefficientmodelwasintroducedto describe the coupling effects,and Duncan and Changs E-Bmodel(D

39、uncan and Chang 1970)was used to describe the stress-strain relationship of dam materials in the construction andimpounding process.A back-analysis method based on an artificialneural network and evolutionary algorithm,which was successfullyapplied to several earth-rockfill dams in China with reason

40、ableresults(Zhang et al.2005;Wu et al.2014),was used to obtain thepermeabilitymodelparameters.Biots Consolidation Theory DescriptionBiots theory applies only to the consolidation problems of satu-rated soil.In the numerical simulations of this work,the initial stateof the core material was assumed t

41、o be fully saturated(i.e.,the ini-tialdegreeof saturation was100%).In reality,theinitialstateof thecore material was not fully saturated,just approximatively satu-rated.During theconstruction process of thedam,the degree of sat-uration after roller compaction will exceed 90%.Along with damconstructi

42、on,the stress in the dam increases,and the void ratio ofcore material decreases due to compression;as a result,the degreeof saturation increases.Moreover,the dam project,located in a plu-vial region of western China,is significantly influenced by precipi-tation;thus,the degree of saturation will inc

43、rease further.In thiswork,thetheory ofsaturated soilsisused tostudy thecouplingcon-solidationproblemsofhighearth-rockfilldams.InBiots consolidationtheory,soil isregarded asa porousskele-ton filled with water,and the interaction between soil skeleton andpore water is determined by the principle of ef

44、fective stress and thecontinuity relation.In conjunction with the principle of effectivestress,theequilibriumequationsofaninfinitesimal soil elementcanbeexpressedass0 xxtxyytxzzpwx 0(1a)tyxxs0yytyzzpwy 0(1b)tzxxtzyys0zzpwz Fg 0(1c)where Fgis the body force of the soil skeleton;and pwis the porewater

45、pressure.Another relation between deformation and pore water pressureisgivenbythecontinuityequation.Physically,itmeansthatthevol-ume of fluid flowing in or out is equal to the volume change of thesoilmass?kx2hx2 ky2hy2 kz2hz2!vt 0(2)where h is the total water head;vis the volumetric strain;and kx,ky

46、,and kzare the permeability coefficients in three directions,respectively.Thetotalwaterheadisdefinedbyh pwgw z(3)wheregwistheunitweightofwater;andzistheelevationhead.In general,the compacted core materials exhibit an anisotropicseepagebehavior,andthehorizontalpermeabilityisgreaterthanthevertical per

47、meability.However,theanisotropic seepage behaviorofthecorematerialofgravellyclayoftheNuozhadudamisnotobviousandisnegligiblecomparedwiththeeffectsofstressanddeformation(Lei 2010).The anisotropic seepage behavior was not taken intoaccount,and the permeability of the compacted core material wasapproxim

48、ativelyassumedtobeisotropic.The weighted residual Galerkin method is adopted to perform fi-nite-elementanalysisofBiotsconsolidationproblem.Discretization of the consolidation equations in the space and timedomains is required.The global coupled-element equation of incre-mentalformisasfollows:K?Kc?Kc

49、?T?uDtKs?Dd?D?h?DRF?DtRq?Ks?h?n?1?(4)whereK?isthesoilstiffness matrix;Ks?istheseepage matrix;Kc?is the coupling matrix;Dd?is the vector of increment of elementnodal displacements;Dpw?is the vector of increment of elementnodal pore pressures;uis a time-integrating parameter;DRF?istheincrement of equi

50、valent nodal load;and DRq?is the increment ofequivalentnodalflux.Constitutive ModelIn Duncan and Changs E-B model,the nonlinear stress-straincurves are represented by a hyperbola,the instantaneous slope ofwhich is the tangent modulus,Et,which can be expressed asfollows:Et 1?Rf1?sinf2c?cosf 2s3?sinfs