Highway and Bridge Engineering 2014, International Symposium Iai, Romnia, December 12th, 2014 Geosynthetic reinforcement for base / subbase courses of road structures Oana Elena Col Department of Transportation Infrastructure and Foundations, Gheorghe Asachi Technical University, Iai, 700050, Romania Summary Geosyntheticsaredefinedasman-madeplasticplanarproducts,shapedinmany forms,thatareusedwithsoil,rock,orothergeotechnical-relatedmaterialsin order to solve different civil engineering problems. Thebenefitofusinggeosyntheticshavebeenseeninroadandhighway engineering. From this point of view, the geosynthetics may be used to increase the structuralorload-carryingcapacityofasystembytransfertheloadstothe geosyntheticmaterialortodecreasetheheightofthebaseandsurfacecourses above relatively soft subgrades. Geosyntheticsforbaseorsubbasecoursesofunpavedorpavedroadsand highwayshavebeenusedforseparation,filtration,lateraldrainage,and reinforcement purposes. The mechanisms by which geosynthetics provide reinforcement when placed at the subbase and subgrade interface include lateral restraint or confinement of subbase material, and increase in bearing capacity. Thispaperpresentsthemainaspectsofusinggeosyntheticsforbase/subbase courses of road structures. KEYWORDS: geosynthetics, base, subbase, highway, paved roads, unpaved roads. 1.INTRODUCTION Along the years, the advantages and efficiency of using geosynthetics were seen in different areas of application: reinforcedsoilsystems(embankmentsoversoftfoundations,reinforced soil walls, reinforced steepened slopes);subsurface drainage (road base drainage, structure drainage);erosion and sediment control; base reinforcement (stress reduction and reinforcement). 2O.E.Col Theadvantagesoftheuseofgeosyntheticsmaybedividedintwocategories: technical advantages and economic advantages. Between the technical advantages, we may talk about: a quickly installation with flexibility to construct during short period; the possibility of space savings; allowsabettermaterialqualitycontrolbecausethegeosyntheticsare homogeneous than soil and aggregates; the safety factor may be increased; better construction quality control at site; protection of the environment;the performance and service life are improved; the geosynthetics are compatible with field conditions. Theeconomicadvantagesandtheimportanceofgeosyntheticsaregivenbythe facts that: the natural resources are efficient use and the pollution is minimized; gives the possibility to increase the efficiency of structures, aredurable,longlastingandenvironmentallysafesolutionsforthegeotechnical engineering projects.minimizes the regular repair and maintenance costs directly; reduces the costs of the projects (up to 30%). Theseadvantageswereseenalsoforroadsandhighwaysconstructedwith geosynthetic-reinforcedsubbase,wereobservationsindicatebetterperformance (less rutting associated with increased numbers of load repetitions).2. THE USE OF GEOSYNTHETICS FOR BASE / SUBBASE COURSES OF ROAD STRUCTURES In order to increase the structural or load-carrying capacity of a road system by the transfertheloadtothegeosyntheticmaterialageosyntheticlayerisaddedatthe bottom or within a base course, Figure 1, resulting a base reinforcement. The main benefits of the base / subbase reinforcement are [3]: an improved service life of the road; an equivalent performance for a road using a reduced structural section. asafetyfactorincaseofweakersubgradefromdesignvaluesor inaccuracies in the design methodology.Highway and Bridge Engineering 2014 International Symposium3 Figure 1. Subbase reinforcement, [4] 2.1. Unpaved or paved roads Geosyntheticsapplicationsonroadsandhighwaycanbesplitintwoareas, dependingonthetypeofroadsthatareusedfor:unpavedandpavedroads.The separationbetweenthetwoisnecessary,sinceforeachofthemareutilized different theories, physical mechanisms, design methodologies and failure criteria, [1]. Anunpavedroadistypicallyconstructedbyplacingalayerofselectgranular material, upon the subgrade as a surface course. Initially a regrading or levelling of therutscanbeperformed.Thepurposeofthegranularlayeristotransferthe surfaceloadtothesubgradewhilespreadingouttheloadtothesubgrade,which effectively reduces the intensity of pressure on the subgrade [1]. For these types of roads,thegeosyntheticplacedbelowthegranularmateriallayerandonthe subgrade surface, assure the separation between them and so increases the services life of the road. Forpavedroadsthepurposeofgeosyntheticsplacedbelowthegranularbaseor subbase is to allow for increased numbers of load repetitions prior to failure due to rutting and/or fatigue. 2.2. Geosynthetics functions for roadways Thebenefitsofthegeosyntheticsusedforunpavedandpavedaregivenbythe different functions of these materials related to roadways applications:separation(theplacementofaflexiblegeosyntheticmaterial,bellowthe granularlayersothattheintegrityandfunctioningoftheselayercan remain intact or even be improved), Figure 2; 4O.E.Col Figure 2. Separation function (left separated, right mixed), [1] filtration(allowsforadequateliquidflowwithoutsoilloss,acrossthe plane of the geotextile over a service lifetime); lateraldrainage(thelateralmovementofwaterwithintheplaneofthe geosynthetic); reinforcement(theadditionofstructuralorload-carryingcapacitytoa pavement system by the transfer of load to the geosynthetic material), [3] The reinforcement functions of the geosynthetics is provided by, [1], [3]: lateral restraint of the upper granular course (occurs when a geosynthetic is placed at the subgrade/subbase or base interface to increase the support of construction equipment over a weak or soft subgrade), Figure 3; bearingcapacityincreasebyforcingthepotentialbearingcapacityfailure surface to develop along alternate, higher shear strength surfaces, Figure 4; tensionedmembraneeffectwhenstrainedextensively-apavement reinforcementmechanismmobilizedunderhighdeformationconditions, Figure 5. Figure 3. The lateral restraint function, [1] Highway and Bridge Engineering 2014 International Symposium5 Figure 4. Bearing capacity increase, [3] Figure 5. The tensioned membrane function, [1] Forunpavedroads,theallthreetypesofmechanism(lateralrestrain,bearing capacityincreaseandtensionedmembrane)mayactanddeterminethe reinforcement function. For the paved roads, the tensioned membrane effect is not appropriatebecausealargeruttingisrequiredtomobilizethereinforcement strength. 2.3. Geosynthetics for base / subbase courses Thetypesofgeosyntheticsusedforthebase/subbasecoursesofroadsare geotextiles, geogrids and geocelles. Theprimaryfunctionofthenon-woven(Figure6.a)andwoven(Figure6.b) geotextiles is to separate the layers between strata to prevent the upward migration offine-grainedparticlesfromthesubgradeintosubbaselayers.Theycanalso provide, lateral drainage (non-woven geotextiles) and reinforcement by the tension-membrane effect between the underlying weak soils and the road structure.6O.E.Col Thegeogrids(biaxialandtrixial)andgeocelles,Figure6.c-e,actprimarilyas reinforcement.Theyprovidelateralrestraintorconfinementofaggregatelayers abovesubgrade.Inordertohavethetensionedmembraneeffectisnecessarythat the geosynthetic to be extensively strained. a) non woven geotextileb) woven geotextile c) biaxial geogridsd) triaxial geogrids d) geocell Figure 6. Types of geosynthetics for base/ sub-base reinforcement [4] Highway and Bridge Engineering 2014 International Symposium7 3.CONCLUSIONS Themainbenefitsofusinggeosyntheticsinroadwaysaredependingofthefunctions of the material used and the type of road were is placed. From these point of view the principal benefits are: the reduction of the intensity of stress on the subgrade; the prevention of the aggregate to penetrate into the subgrade; the reduction of the differential settlement of the roadway that contribute to the integrity and uniformity of the pavement; thereductionofthefrostactioninfrost-susceptiblesoilsbyproviding capillary breaks; the reduction of the of seasonal water content of the swelling clays beneath the roadways on clays; the reduction of the thickness of granular aggregate required to stabilize the subgrade; the extension of life and reduction of maintenance of the pavement. Takinginaccountallthesebenefits,theuseofgeosyntheticsforthebaseand subbasecoursesofroadsshouldbeseenasaviableandpracticalsolutionforthe new projects in Romania. References 1.Maxwell S., Kim W.H., Edil T.B., and Benson C.H., Effectiveness of geosynthetics in stabilizing softsubgrades,FinalReport,GeotechnicalEngineeringProgramDepartmentofCivil Environmental Engineering University of Wisconsin-Madison, 2005. 2.WhiteD.J-SubgradestabilizationUsingGeosynthetics,IowaStateUniversity,Institutefor transportation, Tech Brief, April, 2014; 3.BergR.R.,ChristopherB.R.,PerkinsS.,GeosyntheticReinforcementoftheAggregate Base/Subbase Courses of Pavement Structures, GMA White Paper II, Ryan R. Berg & Associates, Inc. June 27, 2000. 4.www.lianyifiberglass.en.alibaba.com,accesat12ianuarie2014,LianYiBi-axialGeogrids, http://lianyifiberglass.en.alibaba.com/product/60063635418219068309/Biaxial_plastic_geogrid_for_Sub_base_reinforcement.html 5.IliescuM.,RatiuI,Geogridreinforcedroadsubgradestabilizationdesignmethodology,Acta Technica Napocensis: Civil Engineering & Architecture Vol. 5, No. 3 (2012).