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地基附加应力是指什么?

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正确答案:

建筑物建成后作用于基底上的平均压力减去基底原先存在于土中的自重应力才是新增加的压力,此压力称为基底附加压力。

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StandardmuseumpracticecanwashawayDNA. 1.Washing,brushingandvarnishingfossils—allstandardconservationtreatmentsusedbymanyfossilhuntersandmuseumcuratorsalike—vastlyreducesthechancesofrecoveringancientDNA. 2.Instead,excavatorsshouldbehandlingatleastsomeoftheirbountywithgloves,andfreezingsamplesastheyarefound,dirtandall,concludesapaperintheProceedingsoftheNationalAcademyofSciencestoday. 3.AlthoughmanypalaeontologistsknowanecdotallythatthisisthebestwaytouptheoddsofextractinggoodDNA,Eva-MariaGeigloftheJacquesMonodInstituteinParis,France,andhercolleagueshavenowshownjusthowimportantconservationpracticescanbe.Thisinformation,theysay,needstobehammeredhomeamongthepeoplewhoareactuallyoutinthefielddiggingupbones. 4.Geiglandhercolleagueslookedat3,200-year-oldfossilbonesbelongingtoasingleindividualofanextinctcattlespecies,calledanaurochs.ThefossilsweredugupatasiteinFranceattwodifferenttimes—eitherin1947,andstoredinamuseumcollection,orin2004,andconservedinsterileconditionsat-20oC. 5.Theteam'sattemptstoextractDNAfromthe1947bonesallfailed.Thenewlyexcavatedfossils,however,allyieldedDNA. 6.Becausetheboneshadbeenburiedforthesameamountoftime,andinthesameconditions,theconservationmethodhadtobetoblamesaysGeigl."AsmuchDNAwasdegradedinthese57yearsasinthe3,200yearsbefore,"shesays. Washin,washout 7.Becausemanypalaeontologistsbasetheirworkontheshapeoffossilsalone,theirmethodsofconservationarenotdesignedtopreserveDNA,Geiglexplains. 8.Thebiggestproblemishowtheyarecleaned.Fossilsareoftenwashedtogetheron-siteinalargebath,whichcanallowwater—andcontaminantsintheformofcontemporaryDNA—topermeateintotheporousbones."NotonlyistheauthenticDNAgettingwashedout,butcontaminationisgettingwashedin,"saysGeigl. 9.MostancientDNAspecialistsknowthisalready,saysHendrikPoinar,anevolutionarygeneticistatMcMasterUniversityinOntario,Canada.Butthatdoesn'tmeanthatbestpracticehasbecomewidespreadamongthosewhoactuallyfindthefossils. 10.GettingholdoffossilsthathavebeenpreservedwiththeirDNAinmindreliesoncloserelationshipsbetweenlab-basedgeneticistsandtheexcavators,sayspalaeogeneticistSvantePbooftheMaxPlanckInstituteforEvolutionaryAnthropologyinLeipzig,Germany.Andthatonlyoccursinexceptionalcases,hesays. 11.Pbo'steam,whichhasbeensequencingNeanderthalDNA,continuallyfacestheseproblems."WhenyouwanttostudyancienthumanandNeanderthalremains,there'sabigissueofcontaminationwithcontemporaryhumanDNA,"hesays. 12.Thisdoesn'tmeanthatallmuseumspecimensarefatallyflawed,notesPbo.TheNeanderthalfossilsthatwererecentlysequencedinhisownlab,forexample,hadbeenpartofamuseumcollectiontreatedinthetraditionalway.ButPboiskeentoseesamplesoffossilsfromeverymajorfindpreservedinlinewithGeigl'srecommendations—justincase. Warmandwet 13.Geiglherselfbelievesthat,withcooperationbetweenbenchandfieldresearchers,preservingfossilsproperlycouldopenupavenuesofdiscoverythathavelongbeenassumedclosed. 14.Muchhumanculturaldevelopmenttookplaceintemperateregions.DNAdoesnotsurvivewellinwarmenvironmentsinthefirstplace,andcanvanishwhenfossilsarewashedandtreated.Forthisreason,Geiglsays,mostancientDNAstudieshavebeendoneonpermafrostsamples,suchasthewoollymammoth,oronremainsshelteredfromtheelementsincoldcaves—includingcavebearandNeanderthalfossils. 15.Betterconservationmethods,andafocusonfreshfossils,couldboostDNAextractionfrommoredelicatespecimens,saysGeigl.Andthatcouldshedmorelightonthestoryofhumanevolution. (640wordsnature) Glossary Palaeontologists古生物学家 Aurochs欧洲野牛 Neanderthal(人类学)尼安德特人,旧石器时代的古人类。 Permafrost(地理)永冻层 Questions1-6 AnswerthefollowingquestionsbyusingNOMORETHANTHREEWORDSforeachanswer. 1.Howdidpeopletraditionallytreatfossils? 2.WhatsuggestionsdoGeiglandhercolleaguesgiveonwhatshouldbedonewhenfossilsarefound? 3.Whatproblemsmaybeposediffossilbonesarewashedon-site?NameONE. 4.WhatcharacteristicdofossilboneshavetomakethemsusceptibletobecontaminatedwithcontemporaryDNAwhentheyarewashed? 5.Whatcouldbebetterunderstoodwhenconservationtreatmentsareimproved? 6.Thepassagementionedseveralanimalspeciesstudiedbyresearchers.Howmanyofthemarementioned? Questions7-11 Dothefollowingstatementsagreewiththeinformationgiveninthepassage?PleasewriteTRUEifthestatementagreeswiththewriterFALSEifthestatementdoesnotagreewiththewriterNOTGIVENifthereisnoinformationaboutthisinthepassage. 7.IntheirpaperpublishedintheProceedingsoftheNationalAcademyofSciences,GeiglandhercolleagueshaveshownwhatconservationpracticesshouldbefollowedtopreserveancientDNA. 8.ThefossilbonesthatGeiglandhercolleaguesstudiedareallfromthesameaurochs. 9.Geneticistsdon'thavetoworkonsite. 10.OnlynewlyexcavatedfossilbonesusingnewconservationmethodssuggestedbyGeiglandhercolleaguescontainancientDNA. 11.Paaboisstillworriedaboutthepotentialproblemscausedbytreatmentsoffossilsintraditionalway. Questions12-13 CompletethefollowingthestatementsbychoosingletterA-Dforeachanswer. 12.“Thisinformation”inparagraph3indicates: [A]ItiscriticaltofollowproperpracticesinpreservingancientDNA. [B]ThebestwayofgettinggoodDNAistohandlefossilswithgloves. [C]Fossilhuntersshouldwearhome-madehammerswhilediggingupbones. [D]Manypalaeontologistsknowhowoneshoulddointreatingfossils. 13.ThestudyconductedbyGeiglandhercolleaguessuggests: [A]thefactthatancientDNAcannotberecoveredfromfossilbonesexcavatedinthepast. [B]thecorrelationbetweentheamountofburyingtimeandthatoftherecoveredDNA. [C]thepaceatwhichDNAdegrades. [D]thecorrelationbetweenconservationpracticesanddegradationofDNA.
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