关于地基承载力如何取得合理值

HowtogetthereasonablevalueofbearingcapacityoffoundationIntroductionWiththerapiddevelopmentofcityconstruction,citylandincreasinglytense,whetheronthegroundorundergroundspacearerequiredtomaximizetheuseoflarge,complexshape,theupperloadsizedisparityofbuildingshaveconstantlyappeared,easilyleadtotheunevenfoundationdeformation.Theresultsoftheinvestigationshallbeapplieddirectlytotheconstructionproject,inordertofacilitatetheunderstandingandtherationaluseofspecification,weonengineeringsurveyinformationindeterminingthebearingcapacityandthevalueswerediscussedandputforwardthetreatmentprinciple.2．CharacteristicvalueofsubgradebearingcapacitydeterminationThebearingcapacityofthefoundationforthefoundationdesigniscrucial,"codefordesignofbuildingfoundation(GB50007-2002)"cancelled"codefordesignofbuildingfoundation."[GBJ7-89]aboutbearingcapacitytablearticlesandappendix.Butinthepracticalworksomeindustriesandlocalcodes,regulationsalsoretainsthebearingcapacityofthefoundationofexperienceandexperienceformulatable.Withthelargenumberofcomplexarchitecture,thebuilding'supperloadoftendifferenceisbigger,easytocausetheunevenfoundationdeformation,designisneededwhenusingavarietyofgroundandfoundationtypeorconsidertheuseofsubgradeandfoundationandsuperstructureofdeformationcalculation,solvetheunevensettlementoffoundationandupperstructureeffectsoftheproblem,notsimpleaccordingtodatastatisticvalue,look-uptabletodeterminethebearingcapacityofthefoundation.Therefore,determinationofbearingcharacteristicvalue,thereasonableselectionofengineeringcharacteristicsofrepresentativevalueatthesametime,shouldbefullytakenintoaccounttheeffectofgeologicalenvironmentontheground.Forexample,avenueofthetwoengineeringdistanceofabout20meters,belongtothesameengineeringgeologicalunit,thewatercontentofsoilindex,voidratio,compressionmodulusdifference,project1,project2maincompressionlayersptthemeasuredaveragevalueNwere22hits,butthecharacteristicvalueofsubgradebearingcapacityforFA220kPa,160kPa.Fromtheaboveresultscanbeobtainedindeterminingthebearingcapacitycharacteristicvalue,selecttheengineeringcharacteristicstestingforthevalue,theselectionprincipleisdifferent,project1selectthestandardvaluesfortherepresentativevalueengineering2,slightlyhigherthantheminimumvalueofselectedasrepresentativeofvalue.Evidentinthesameengineeringgeologicalunitselectionofrepresentativevalueofdifferentengineeringproperties,todeterminethebearingcapacitycharacteristicvalueisrelativelylarge,sothescientificandreasonableselectionofengineeringcharacteristicsofrepresentativevalueistodeterminethecharacteristicvalueofsubgradebearingcapacityinthekey.Fromthegeologicalenvironmentonthefoundation'sinfluence,isalsooneofthekeyfactors.Forexamplefromtheundergroundwaterlevelimpactanalysis,theengineeringofgeomorphicunitsoftheYellowRiveralluvialIterrace,5metershighriverterrace.Bearinglayersofsand,groundwaterdepthis2meters,thegroundwaterofQuaternaryporewater,groundwaterfluctuationandatmosphericprecipitationandtheYellowRiverhaveclosehydraulicconnection.Determinationofbearinglayercharacteristicvalueofsubgradebearingcapacitytofullyconsidertheburieddepthofgroundwatertableandwaterlevelvariationfactors.Throughthewaterlevelandwaterlevelabovetwogroupsofplateloadingtestresultstodeterminethecharacteristicvalueofsubgradebearingcapacitywere280kPa,460kPa.Engineeringpracticeshowsthattheengineeringpropertiesofdiversity,engineeringgeologyandhydrologicgeologycondition,engineeringandtechnicalpersonnelinthedeterminationofcharacteristicvalueofsubgradebearingcapacityshouldbeobjective,scientific,reasonableselectionofengineeringpropertyindexvalue,takingthegeologicalenvironmentonthefoundationofeffectfactors,sotodeterminethecharacteristicvalueofSubgradebearingcapacityisthesafetyinreal.3．ThefoundationbearingcapacitycalculationNationalstandard"codefordesignofbuildingfoundation."theundersideofthebasepressurecalculatedbythefollowingequation:Pk(Fk+Gk)/APkfa(1)OnbuildingfoundationwidthanddepthofthemodifiedcharacteristicvalueofsubgradebearingcapacityofFApresstypecalculationamendments:FA=FAK+BB(B-3)+gammaETAgammaMD(d-0.5)(2)Atpresent,manyhigh-risebuildingsexistinwholestructureofbuildingwithpodiumandundergroundgarageislocatedadjacenttothetwocases.Onthecharacteristicvalueofsubgradebearingcapacitycorrectionisoftenoverlooked,butsimplybybuildingthebasewidthanddepthiscorrectedandestimation,ledtothesurveyresultsconclusionmistakesorunreasonable.地基承载力如何取得合理值一、前言

随着城市建设的高速发展，城市用地愈来愈紧张，无论地上还是地下空间均要求最大限度地利用，众多体型复杂、上部荷载大小相差悬殊的建筑不断出现，很易导致地基的不均匀变形。勘察成果须直接应用于建设工程，为便于理解和合理使用规范，我们对工程勘察资料中承载力确定和取值进行了分析并提出处理原则。

二、地基承载力特征值确定

地基承载力对于基础设计至关重要，《建筑地基基础设计规范》(GB50007-2002)取消了《建筑地基基础设计规范》[GBJ7-89]有关承载力表的条文和附录。而实际工作中一些行业和地方规范、规程还保留地基承载力经验表以及经验公式。随着众多复杂大型建筑的出现，这些建筑的上部荷载往往差别较大，易导致地基的不均匀变形，设计时需要采用多种地基和基础类型或考虑采用地基与基础和上部结构共同作用的变形计算，解决不均匀沉降对基础和上部结构的影响问题，不能简单地按数据统计取值，查表来确定地基承载力了。因此，确定地基承载了特征值时，在合理的选取工程特性的代表值同时，须充分考虑地质环境对地基的影响。例如某场地的两个工程相距约20米，隶属同一工程地质单元，其土工试验指标含水量、孔隙比、压缩模量差异不大，工程1、工程2主要压缩层标准贯入试验实测平均值N均为22击，但是地基承载力特征值fa分别为220kPa、160kPa。从上面的结果可得出在确定承载力特征值、选取工程特性测试代表值时，两者选取的原则不同，工程1选取的标准值为代表值，工程2选取略高于最小值作为代表值。明显看出在同一工程地质单元选取的工程特性代表值不同，对确定承载力特征值影响比较大，所以科学合理的选取工程特性代表值是确定地基承载力特征值的关键。从地质环境对地基的影响来看，也是关键因素之一。例如从某工程地下水位的影响分析，该工程地貌单元属黄河冲洪积Ｉ级阶地，阶地高出河床5米。持力层为中砂，地下水位埋深为2米，地下水为第四系孔隙潜水，地下水的升降与大气降水及黄河有密切的水力联系。确定持力层地基承载力特征值要充分考虑地下水位埋深和水位变幅因素的影响。通过水位以下和水位以上两组平板载荷试验结果确定地基承载力特征值分别为280kPa、460kPa。工程实录表明因工程性质多样，工程地质与水文地质条件不同，工程技术人员在确定地基承载力特征值时要客观、科学、合理地选取工程特性指标代表值，充分考虑地质环境对地基的影响因素，这样确定的地基承载力特征值才是安全真实的。

三、地基承载力修正计算

国标《建筑地基基础设计规范》基础底面的压力按下式计算：

Pk(Fk+Gk)/APk≤fa(1)

对建筑物基础宽度和埋置深度修正后的地基承载力特征值fa按下式计算修正：

fa=fak+ηbbγ(b-3)+ηdγm(d-0.5)(2)

目前众多高层建筑存在着主裙楼一体结构和地下车库紧邻的两种情况。在对地基承载力特征值修正时往往被忽视，而只是简单地按建筑物的基础宽度和埋置深度进行了修正和估算，导致勘察成果结论的错误或不合理。对第一种情况主裙楼一体结构地基承载力的深度修正时，将基础底面范围内的荷载，按基础两侧的超载考虑，当超载宽度大干基础宽度两倍时，可将超载换算成土层厚度作为基础深度；当基础两侧超载不等时，按小值考虑。

对第二种情况主楼与地下车库紧邻的地基承载力的深度修正时，宜按不利组合的受力状况考虑。往往还存在上部荷载大小相差悬殊，结构刚度和构造变化复杂，很易出现地基的不均匀变形情况，设计时需要采用多种地基和基础类型。例如某高层建筑工程，其主楼为15-28层，其层数差异较大，拟采用筏板基础，基础埋深7.2米，基底压力为500kPa；主楼两侧为地下车库，基础埋深6米。主楼地基持力层为④层粉细砂，地基承载力特征值为160kPa。如不考虑周边地下车库受力状况条件的影响，地基承载力特征值按(2)式进行修正计算结果为691kPa。

基底标高位于④层粉细砂上，经基础深度和宽度修正后的地基承载力特征值，在不考虑周边地下车库受力状况条件时能满足天然地基设计要求。按实际情况两侧地下车库基础埋置深度为6米，已改变了主楼地基和基础的受力状态，按不利组合的受力状况考虑，其主楼的基础埋深D应