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1、ban31881_ch04.qxd6/14/019:07AMPage63CHAPTER4GeometricDesignGeometricdesignfortransportationfacilitiesincludesthedesignofgeometriccrosssections,horizontalalignment,verticalalignment,intersections,andvariousdesigndetails.Thesebasicelementsarecommontoalllinearfacilities,su
2、chasroadways,railways,andairportrunwaysandtaxiways.Althoughthedetailsofdesignstandardsvarywiththemodeandtheclassoffacility,mostoftheissuesinvolvedingeometricdesignaresimilarforallmodes.Inallcases,thegoalsofgeometricdesignaretomax-imizethecomfort,safety,andeconomyoffacil
3、ities,whileminimizingtheirenviron-mentalimpacts.Thischapterfocusesonthefundamentalsofgeometricdesign,andpresentsstandardsandexamplesfromdifferentmodes.Theorderofpresentationofmaterialinthischapteristoconsidergeometriccrosssectionsfirst,thenverticalalignment,horizontalali
4、gnment,superelevation,intersec-tions,andvariousdesigndetails.Forpurposesofexposition,theorderofthetopicsisnotveryimportant.Inatypicaldesignproject,ontheotherhand,thereisadefiniteorderoftasks,inwhichtheestablishmentofatentativehorizontalcenterlineusuallyprecedesestablishm
5、entofverticalalignment.Thisisbecausetheelevationoftheex-istinggroundalongthecenterlineisanimportantconsiderationinestablishingthever-ticalalignment.Theprocessofdesigningtheverticalalignmentbeginswithplottingaprofileoftheexistingterrain,andatentativehorizontalcenterlinemu
6、stalreadybees-tablishedinordertodothis.4.1GEOMETRICCROSSSECTIONTheprimaryconsiderationinthedesignofgeometriccrosssectionsforhighways,run-ways,andtaxiwaysisdrainage.Detailsvarydependingonthetypeoffacilityand1,2agency.63ban31881_ch04.qxd6/14/019:07AMPage6464CHAPTER4:Geome
7、tricDesignHighwaycrosssectionsconsistoftraveledway,shoulders(orparkinglanes),anddrainagechannels.Shouldersareintendedprimarilyasasafetyfeature.Theyprovideforaccommodationofstoppedvehicles,emergencyuse,andlateralsupportofthepavement.Shouldersmaybeeitherpavedorunpaved.Dra
8、inagechannelsmayconsistofditches(usuallygrassedswales)orofpavedshoulderswithbermsorcurbsandgut-ters.Figures4.1
