以有限元素法分析桩基础压密行为的探讨

摘要根據研究顯示台灣西南部部分區域的土壤具有較高的壓縮性，致使工程興建完成後仍持續進行壓密沉陷，如嘉義布袋港、麥寮台塑六輕廠等，因此如何於設計階段就考量壓密沉陷對結構物的影響就成了一個重要的課題。本研究針對國道三號竹田系統交流道橋墩沉陷為案例，配合現場監測資料，利用有限元素法「Plaxis8.2」進行施工順序、地下水位變化、地盤改良等對樁-筏基礎的壓密沉陷行為作一探討，並進行後期沉陷預測。經由分析結果顯示：（1）利用土壤鑽探資料所估得之土壤材料參數，並以此作為建立土壤壓密沉陷的模式，其沉陷量的計算值與監測值大致吻合；（2）利用所建立的土壤壓密模式模擬基樁與路堤不同的工序，先完成路堤後再施作橋墩及先做橋墩後完成路堤兩者工序後期的沉陷相差9.5公分，有此可知路堤的預壓密將可減少橋墩的沉陷；(3)模擬連續豪雨造成地下水位上升1m時橋墩會有微升0.1cm的現象，但當地下水位回降至原來平常水位後將導致橋墩下陷1.2cm；（4）地盤改良對橋墩沉陷僅在初期發揮效果，但隨著時間的增加成效則相對的遞減，其安全性增益不大；（5）經由橋墩沉陷歷時曲線可發現該橋墩經過1,785天之後才壓密完成，顯示該地區於工程設計時應特別考慮壓密沉陷問題，本研究預測8年後（2117年4月）的累積沉陷量共為2.1cm，經判斷該區域應已完成壓密。關鍵詞：有限元素法、壓密沉陷、二次壓縮沉陷、參數敏感度、樁-筏基礎AbstractAccordingtothestudying,thesoilinsomeareasofthesouthwestTaiwangetshighercompression.Thefinishednew-builtconstructionsstillcontinueintheconsolidationsettlementcondition,suchasBudaiinChiayiandFormosaPlasticssixthnaphthacrackerplantinMailiao.Thatconsiderationsofconsolidationsettlementaffectthebuildingstructuresinthedesignstagebecomesanimportantissue.ThestudyisaimedaboutthesinkingpiercaseatC386intersectionsystemofFreewayNO.3.Basedonmonitoringdata,usingfiniteelementmethod「Plaxis8.2」todotheresearchaboutconstructionprocedure,watertablechangeandsoilimprovement.Theseconditionsaffecttheconsolidationsettlementofpiled-raftfoundationandsettlementpredictioninthefuture.Analyticresultsareshownasfollows：（1）Thesoilconsolidationsettlementmodelisbuiltbytheestimatedsoilparametersusingthedataofsoilexploration.Thecalculationvalueofsettlementandthemeasuredvaluealmostmatch.（2）Utilizingthesoilconsolidationsettlementmodeltosimulatetheconstructingprocedureofthepileandtheembankment,thedifferenceinthesettlementbetweentheembankmentandpierfinishedfirstis9.5cm.Thereforetheconsolidationsettlementincaseoftheembankmentfinishedfirstcanreducesinkingvalueatthepier.（3）Simulationofcontinuousheavyraincausesthe1mriseofwatertable.Thenthepierwillheaveslightlyabout0.1cm.Thepierwillsinkabout1.2cmafterthenormalwaterlevel.（4）Soilimprovementonlyshowseffectiveininitialstagebuttheinfluenceisprogressivelydecreasedbythetime.However,increasingsafetyisnotenough.（5）Basedonthecurveofsettlementatpier,itisfoundthattheconsolidationhasbeencompletedthrough1785days.Therefore,inthedesignstageconsideringtheconsolidationsettlementinthisareaisanimportantissue.Thepredictionvalueofcumulativesettlementwillreach2.1cmafter8years(April,2117).Accordingtotheevaluation,theconsolidationsettlementshouldhavebeenalmostcompletedinthearea.Keyword：FiniteElementMethod、CompactionSubsidence、SecondaryCompressionSettlement、ParameterSensitivity、Pile-RaftFoundation。