3FAILURE CRITERIA FUNCTIONAL STRUCTURAL SERVICEABILITY RUTTING FATIGUE DRAINAGE/ MOISTURERUTTINGFATIGUEREFLECTIVE AND LONGITUDINAL CRACKINGCONTAMINATIONSERVICEABILITYLOW TEMPERATURE CRACKING
4Increase in traffic volume Contamination of road material WHAT CAUSES ROAD PAVEMENTS TO FAIL?Poor drainageIncrease in traffic volumeContamination of road materialShort term designIncrease in traffic loadsInsufficient strength of road materialPoor existing soil propertiesDefects in construction method and quality control
5Pavement – Typical components Typical Asphalt PavementsWEARING COURSEBINDERBASEFOUNDATIONSUBGRADEMain structural element (durable)BOUND MATERIALPavement foundationGRANULAR MATERIAL OVER SOILExisting soilBOUND MATERIAL: high stiffness, crack and deformation resistantGRANULAR MATERIAL OVER SOIL: adequate platform to place layer above2
6Geosynthetics in Pavements - Today More common and well known, well established solutionUsed as an alternative to conventional systemConsidered as cost-effective solutionBasic Application includes:1. Subgrade Separation and Stabilization2. Base reinforcement3. Overlay stress absorption and reinforcement4. Drainage arrangement
8Subgrade SeparationThe placements of a flexible porous textile between dissimilar materials so that the integrity and functioning of both materials can be remain intact or are improved.Nonwoven Geotextile is used between subgrade and base course to prevent the intermixing of subgrade and base course aggregateAdopted when subgrade is strong enough (CBR > 3.0)If CBR is weak then subgrade stabilization is also required by providing high strength geotextile or geogrid
9Concept of geotextile separation in roadways (after Rankilor, 1981) Subgrade SeparationConcept of geotextile separation in roadways (after Rankilor, 1981)
10Without Separation – Base Stone Fouling Occurs Fouling of the base aggregate gradually reduces the quality and strength of the base aggregate.Rigid PavementFlexible Pavement
11Intermixing problem of aggregate and subsoil “If you combine 10 kg of stone and 10 kg of mud, you have 20 kg of mud” and the associated loss of support!
12SUBGRADE STABILIZATION For larger rut depths, more strain is induced in the geosynthetic. In this case, considerable reduction in aggregate thickness is possible by the use of a stronger geosynthetic.If subgrade CBR is very weak (CBR<1.5) proper stabilization is required by providing high strength geotextile or geogrid.It provides lateral restraint to the subgrade which increases the allowable bearing capacity of the subgrade
13SUBGRADE STABILIZATION A stabilization geotextile/geogrid facilitates ease of construction over weak subgradeA geogrid is effective in reducing the required fill over a weak subgrade
14Summary for Separation and Stabilization The following general conclusions can be drawn relating to a typicalroad base:A geosynthetic that functions primarily as a separator (typically when the subgrade CBR 3) will increase the allowable bearing capacity of the subgrade by 40-50% (separation geotextiles)A geosynthetic that functions primarily to provide confinement of the aggregate and lateral restraint to the subgrade (typically when the subgrade CBR < 3) will both increase the allowable bearing capacity of the subgrade and provide an improved load distribution ratio in the aggregate. The combined benefits can enhance load carrying capacity of the road by well over 50% (stabilization geogrids or geotextiles)With very weak subgrade (CBR<1.5), it is often beneficial to combine the benefits of both separation and stabilization
16BASE REINFORCEMENTPermanent roads carry larger traffic volumes and typically have asphalt or portland cement concrete surfacing over a base layer of aggregate.With the addition of an appropriate geosynthetic, the Soil-Geosynthetic-Aggregate (SGA)system gains stiffness via“confinement” of theaggregate.
17Geogrid base reinforcement, confines and stiffens the aggregate base layer providing long-term support for the paved surface by: A. Preventing lateral spreading of the base B. Increasing confinement and thus stiffness of the base C. Improving vertical stress distribution on the subgrade
18The unique structure allows the grid to get a good “grip” on the aggregate particles and results in effective mechanical interlock.The unique shape of the geogrid ribs confines aggregate particles due to its high stiffness and the strength at the corners (junctions), just like a rack confines billiard balls.
20Reflective Cracking in Pavements Many pavements, which are considered to be structurally sound after the construction of an overlay, prematurely exhibit a cracking pattern similar to that which existed in the underlying pavement – Reflection CrackingReflective cracksA. destroy surface continuityB. decrease structural strengthC. allow water to enter sub-layers.Thus, the problems that weakenedthe old pavement are extended upinto the new overlay.
21Solution for Reflecting Cracking Overlay reinforcement by:Glass GridGeogrid made of glass fibers
27GS against Reflective Cracking 0 Not reinforced1A Nonwoven GT without emulsion1B Nonwoven GT with emulsion1C Nonwoven GT impregnated with elastomeric bitumen2 Polyester geogrids3 Fiberglass geogrid4 SAMI5 Woven GT510152025303540453214781A1B1CCrack propagation (cm)GS against Reflective CrackingLoads number (x1000)
30Importance of proper & effective drainage Undesired water can lead to damage to pavementsThe pore water pressure built-up in subgrade may lead to pavement permanent failureLoss of subgrade supportReduction of granular layer stiffness
31Conventional method of drainage Side trenches are constructed in pavements along the length to drain away the subgrade waterTrenched are filled with gravel
32Drawbacks of Conventional method Costlier solutionDifficult to installAs the time pass, the efficiency of drainage reduces due to the clogging of gravel layerConsumption of natural resources
33Solution with Geosynthetics Edge Drain: by Drainage Composite
34Quantifying the Geosynthetic Benefit Traffic Benefit Ratio (TBR) (also known as Traffic Improvement Factor or TIF) is a ratio comparing the performance of a pavement cross-section with a geogrid-reinforced base course to a similar cross-section without geogrid reinforcement, based on the number of cycles to failure, with failure defined as a selected depth of rut.In general, geosynthetics have been found to provide a TBR in the range of 1.5 to 20, depending on the type of geosynthetic, its location in the road, and the testing scenario.
35Advantages of Geosynthetics in Pavements Improved service life, lower maintenanceBetter drainage arrangementImproved load carrying capacityBetter load distribution by pavement sectionCost effective if compared to conventional solutionPossible to construct over very weak soilReduction on rut depth and crackingConsiderable saving in design thicknessChemically inactive, non biodegradable material hence high durability
37Installation of Geosynthetics For Separation, Stabilization & Base Reinforcement 1. Subgrade Preparation- Roadway subgrade preparation typically involves removal of all vegetation, roots, and topsoil.- Localized soft soil or otherwise unsuitable subgrade areas may be require to be excavated and backfilled with selected materials.
382. Geotextile Placement- The geotextile is usually laid in the direction of construction traffic- On very soft subgrade (CBR < 1.0) the fabric layout and aggregate placement should begin on firm soil on the site perimeter.- The geotextile should not be dragged across the subgrade.- Wrinkles and folds in the fabric shall be removed by stretching and staking as required.
393. Geotextile OverlapRolls of geotextiles must be overlapped, sewn, or jointed as requiredOverlap as per FHWA
404. Aggregate Placement- Construction vehicles should not drive on fabric- First lift should be placed at minimum 300 mm thick- Rut depths should be less than 75 mm- Initial lift should be compacted by tracking-additional lifts with smooth drum vibratory