USE OF GEOGRIDS IN FLEXIBLE PAVEMENT GUIDED BY, PRESENTED BY, Prof. K.A.MOHANDAS CHINNU MOL.M ROLL NO 24 S8CA

INTRODUCTION Difficulty in the development and maintenance of pavement with limited financial resources Unavailability of high quality materials Geosynthetics- commercial construction aid Separation Reinforcement Filtration Drainage Containment Geogrid gained increasing acceptance in road construction

GEOGRIDS Made from polymers Openings between adjacent longitudinal and transverse ribs called apertures Formed by regular network of integrally connected elements with apertures > ¼” Longitudinal and transverse ribs meet at junctions (nodes) Both rib strength and junction strength are important

TYPES EXTRUDED GEOGRIDS WOVEN WELDED GEOGRID COMPOSITES

EXTRUDED GEOGRIDS UNIAXIAL BIAXIAL PRE-TENSIONED IN ONE DIRECTION PRE-TENSIONED IN TWO DIRECTIONS

FUNCTIONS Reinforcement Improves engineering properties of pavement Separation Reinforcement Secondary function Primary function Improves engineering properties of pavement system

USES Improve service life Construction aid over soft subgrades Improve service life Reduce structural cross-section for given service life

APPLICATIONS Mechanical subgrade stabilization Aggregate base reinforcement Asphalt Concrete (AC) overlay reinforcement

REINFORCEMENT MECHANISMS LATERAL RESTRAINT IMPROVED BEARING CAPACITY TENSIONED MEMBRANE EFFECT

LATERAL RESTRAINT

IMPROVED BEARING CAPACITY

TENSIONED MEMBRANE EFFECT

MATERIAL PROPERTIES Commonly Reported Engineering Properties of Geogrids Engineering Property Test Method Units 2% or 5% Secant Moduli ASTM D 6637 kN/m Coefficient of Pullout Interaction GRI GG5 - Coefficient of Direct Shear ASTM D 5321 Degrees Aperture Size Direct Measure Mm Percent Open Area COE CW-02215 % Ultimate Tensile Strength Junction Strength GRI GG2 Ultraviolet Stability ASTM D 4355

Minimum Geotextile Specification Requirements Geotextile Property ASTM Test Method Min. Requirement Grab Strength (lb) D 4632 200 Puncture Strength (lb) D 4833 80 Burst Strength (psi) D 3786 250 Trapezoid Tear (lb) D 4533 Apparent Opening Size (mm) D 4751 < 0.43 Permittivity (secˉ¹) D 4491 0.05 Ultraviolet Degradation (% Retained Strength @ 500 hr) D 4355 50 Polymer Type - PET or PP

Minimum Biaxial Geogrid Specification Requirements Geogrid Property ASTM Test Method Min. Requirement Mass per Unit Area (oz/yd²) D 5261 9.0 Aperture size-MD (in.) Direct Measure 1.0 Aperture Size-CMD (in.) 1.3 Wide Width Strip Tensile Strength (lb/ft) Strength @ 5% Strain-MD Strength @ 5% Strain-CMD Ultimate Strength-MD Ultimate Strength-CMD D 6637 700 1200 2096 Manufacturing Process _ Punched $ Drawn

AGGREGATE-SURFACED REINFORCED PAVEMENT DESIGN First step is to determine the properties of subgrade Grain-size distribution Atterberg limits In-situ shear strength Vane shear test CBR test DCP test

GEOSYNTHETIC APPLICABILITY FOR SUBGRADES OF DIFFERENT CBR RATIO Nonwoven Geotextile Biaxial Geogrid Reduction in aggregate thickness Geotextile is required for fine- grained subgrades Geogrid reinforcement generally considered cost-prohibitive RELATIONSHIP BETWEEN CONE INDEX, CBR AND SHEAR STRENGTH (C)

Design Procedure Determination of the Subgrade Shear Strength (C) Determination of Design Traffic Directly measured using vane shear devices Converting from CBR to Shear Strength Based upon the gear configuration of heaviest vehicle expected in the traffic mix The combined weight on the selected gear is used as the design vehicle weight Single-wheel Load Dual-wheel Load Tandem-wheel Load

Determination of the Reinforced Bearing Capacity Factor (Nc) CBR VALUE The Unreinforced Bearing Capacity Factor = 2.8 The Reinforced Bearing Capacity Factor Recommendations:- CBR VALUE GEOTEXTILE GEOGRID BOTH ≤ 0.5 0.5 < CBR ≤ 2.0 0.5 6.7 2.0 < CBR ≤ 4.0 >4.0

4 . Determination of the Required Aggregate Thickness The minimum aggregate thickness is 6 inches Thickness is obtained from Design Curve Subgrade Bearing Capacity (CNc) on X-axis Required aggregate thickness on Y-axis Subgrade Bearing Capacity = Shear Strength x Bearing Capacity Factor

REINFORCED FLEXIBLE PAVEMENT DESIGN DESIGN PROCEDURE:- 1. Determination of the Pavement Layer Properties CBR ≤ 0.5 0.5 < CBR ≤ 4.0 4.0 < CBR ≤ 8.0 CBR > 8.0 No aggregate thickness reduction is recommended Geotextile is recommended for fine-grained subgrades generally not The design subgrade strength exceeds the existing database Consider a base course reduction factor

2. Determine Design Traffic The Design Index (DI) combines the effect of average vehicle axle loadings and expected traffic volume

3. Determination of Required Layer Thickness

MINIMUM THICKNESS FOR SURFACE AC AND BASE COURSE Design Aggregate Thickness = Total Required Pavement Thickness – The Minimum AC Thickness

CONSTRUCTION OF GEOSYNTHETIC-REINFORCED PAVEMENTS RECOMMENDED PROCEDURE:- Geotextile Installation Geogrid Installation Aggregate Placement

GEOTEXTILE INSTALLATION If CBR < 2.0 vegetation and surface mats should not be removed If CBR ≥ 2.0 If CBR > 0.5 geotextile is laid in the direction in which the aggregate will be laid If CBR ≤ 5 geotextile is laid transverse to the direction of the traffic lane subgrade should be compacted stumps, roots, etc. should be removed Geosynthetic Overlap Requirements For Use Pavements

GEOGRID INSTALLATION Site should be cleaned Geogrid should be rolled out by hand directly on subgrade/geotextile for design aggregate thickness < 14 in. For thickness > 14 in. it should be placed in the middle of aggregate layer It can be held in places using staples, pins or sand bags for windy conditions

AGGREGATE PLACEMENT CBR < 1.0 Aggregate is placed in the centre at a depth greater than the final design grade Spread excess aggregate laterally to the shoulders of the roadway It is compacted to the design thickness Full design aggregate thickness is placed in one lift CBR between 1.0 and 3.0

CONCLUSION The positive effects of geogrid reinforced base courses can economically and ecologically be utilized to reduce reinforced aggregate thickness The design engineer should be well aware of possible problems and should use this relative new tool for solving them For this he should understand properties and capabilities of the geogrid materials at hand

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