CIVIL SEMINAR REPORT :USE OF GEOGRIDS IN FLEXIBLE PAVEMENT

1.INTRODUCTION
Geosynthetics provide significant improvement in pavement construction and performance. Figure 1 illustrates a number of potential geosynthetic applications in a layered pavement system to improve its performance. The reinforcement applications shown in Figure 1 can be provided by geogrids through friction or interlock developed between the aggregate and the geosynthetic. These applications include subgrade stabilization, base reinforcement and asphalt reinforcement. Subgrade stabilization refers to situations where geosynthetics are placed on weak subgrade prior to the placement of an aggregate layer. The reinforced unpaved road may be used as is or may serve as a construction platform for a permanent paved road. Base reinforcement is used for permanent paved roads and is typically applicable for low volume roads founded on weak subgrade. Reinforcement placed within asphalt layers are used to reduce fatigue, thermal and reflective cracking, control rutting and mitigate the effects of frost heave. The geogrid can also be combined with a geotextile separation layer to prevent fines from migrating into more open graded base layers and further enhance the roadway performance through improved drainage as well as reinforcement. The types of geogrid used in these roadway applications, the functions of the geogrid, design, specification, and construction requirements will be reviewed in detail in the following section

 

Figure 1. Potential applications of geosynthetics in a layered pavement system.

2. DEFINITIONS, MANUFACTURING PROCESSES AND IDENTIFICATION
A geosynthetic as a planar product manufactured from a polymeric material used with soil, rock, earth, or other geotechnical-related material as an integral part of a civil engineering project, structure, or system. A number of geosynthetics are available, including geotextiles, geogrids, geomembranes, geonets, geomeshes, geowebs, and geocomposites. Geogrids are formed by a regular network of tensile elements with apertures of sufficient size to interlock with surrounding fill material. Geogrids are primarily used for reinforcement. Geogrids may be combined with geotextiles to provide the best attributes of each material. These products are called geocomposites. Geogrids are made from synthetic polymers, and of these, polypropylene, polyester, and polyethylene are by far the most common. These polymers are normally highly resistant to biological and chemical degradation. Less-frequently-used polymers include fiber glass for the grid structure. Polyvinyl chloride (PVC) is also used for coating some geogrids. Natural fibers such as cotton, jute, etc., could also be used to make materials that are similar to geogrids. Because these products are biodegradable, they are only for temporary applications. Natural fiber geogrid type materials have not been widely utilized in the U.S. Geogrids can be manufactured with integral junctions are manufactured by extruding and orienting sheets of polyolefins (polyethylene or polypropylene). These types of geogrids are often called extruded or integral geogrids. Geogrids may also be manufactured of multifilament polyester yarns, joined at the crossover points by knitting or weaving process, and then encased with a polymer-based, plasticized coating. These types of geogrids are often called woven or flexible geogrids. A third type, a welded geogrid manufactured, as the name implies, by welding polymeric strips together at their cross over points. All these manufacturing techniques allow geogrids to be oriented such that the principal strength is in one direction, called uniaxial geogrids, or in both directions (but not necessarily the same), called biaxial geogrids.
Geogrids, as with all geosynthetics, are generically identified by: 1. Polymer (descriptive terms, e.g., high density, low density, etc. should be included); 2. Type of element (e.g., strand, rib, coated rib); 3. Distinctive manufacturing process (e.g., woven, extruded, knitted, welded, uniaxial, biaxial); 4. Primary type of geosynthetic (i.e., geogrid); 5. Mass per unit area; and 6. Any additional information or physical properties necessary to describe the material in relation to specific applications (e.g., opening size).
3. APPLICABILTY AND BENEFITS OF GEOSYNTHETICS IN ROADWAYS
Roads and highways are broadly classified into two categories: permanent and temporary, depending on their service life, traffic applications, or desired performance. Permanent roads include both paved and unpaved systems which usually remain in service 10 years or more. Permanent roads may be subjected to more than a million load applications during their design lives. On the other hand, temporary roads are, in most cases, unpaved. They remain in service for only short periods of time (often less than 1 year), and are usually subjected to fewer than 10,000 load applications during their services lives. Temporary roads include detours, haul and access roads, construction platforms, and stabilized working tables required for the construction of permanent roads, as well as embankments over soft foundations
3.1. Temporary Roads and Working Platforms
Geosynthetics are used in temporary roads to reduce rutting of the gravel surface and/or to decrease the amount of gravel required to support the anticipated traffic. Furthermore, the geosynthetic helps to maintain the aggregate thickness over the life of the temporary road. Where the soils are normally too weak to support the initial construction work, geosynthetics in combination with gravel provide a working platform to allow construction equipment access to sites. This is one of the more important uses of geosynthetics. Even if the finished roadway can be supported by the subgrade, it may be virtually impossible to begin construction of the embankment or roadway. Such sites require stabilization by dewatering, demucking, excavation and replacement with select granular materials, utilization of stabilization aggregate, chemical stabilization, etc. Geosynthetics can often be a cost effective alternate to these expensive foundation treatment procedures.
3.2 Permanent Paved and Unpaved Roads
For permanent road construction, a temporary working platform can be constructed to provide an improved roadbed using geogrid reinforcements with an aggregate layer to provide a form of mechanical stabilization. This mechanically stabilized aggregate layer enables contractors to meet minimum compaction specifications for the first two or three aggregate lifts. This is especially true on very soft, wet subgrades, where the use of ordinary compaction equipment is very difficult or even impossible. Long term, a geogrid or, in some cases, a geocomposite acts to maintain the roadway design section and the base course material integrity. Thus, the geosynthetic will ultimately increase the life of the roadway.
Another geogrid application in roadways is to place the geogrid or geocomposite at the bottom of or within the base course to provide reinforcement through lateral confinement of the aggregate layer. Lateral confinement arises from the development of interface shear stresses between the aggregate and the reinforcement and occurs during placement, compaction and traffic loading. A small residual restraint remains after each load application, thus increasing the lateral confinement of the aggregate with increasing load applications. Base reinforcement thus improves the long-term structural support for the base materials and reduces permanent deformation in the roadway section and has been found under certain conditions to provide significant improvement in pavement performance.
Table 1 Application and Associated Function of Geosynthetics in road way system

Application	Functionss	Subgrade Strength	Qualifier
Separator	SeparationSecondary;filtration	2000psf≤ca≤5000psf3≤CBR≤8	Soil containing highFines(SC,CL,CH,ML,MH,SC,GM,GC)
Stabilization	Separation,filtration and some reinforciment	Ca<2000psf(90kpa)CBR<3	Wet,saturated fine grained soils(ie.silt,clay and organic soils)
Base reinforcement	ReinforcementSsecondary separation	600pfs≤Ca≤5000psf3≤CBR≤8	All subgrade conditions.Reinforcement locatedWithin 6 to12 in. of pavement
Drainage	Transmission and filtrationSecondary;separation	Not applicable	Poorly draining subgrade

4. ROADWAY DESIGN USING GEOGRIDS
Certain design principles are common to all types of roadways, regardless of the design ethod or the type of geosynthetic (i.e., geotextile or geogrid). Basically, the design of any roadway involves a study of each of the components of the system, (surface, aggregate base courses and subgrade) detailing their behaviour under traffic load and their ability to carry that load under various climatic and environmental conditions. All roadway systems, whether permanent or temporary, derive their support from the underlying subgrade soils. Thus, when placed at the subgrade interface, the geosynthetic functions are similar for either temporary or permanent roadway applications. However, due to different performance requirements, design methodologies for temporary roads should not be used to design permanent roads. Temporary roadway design usually allows some rutting to occur over the design life, as ruts will not necessarily impair service. Obviously, ruts are not acceptable in permanent roadways.
4.1 Functions of Geogrids in Roadways and Pavements As indicated in the introduction section, the geogrid improves the pavement system performance through reinforcement, which may be provided through three possible mechanisms.
1. Lateral restraint of the base and subgrade through friction (geotextiles) and interlock (geogrids) between the aggregate, soil and the geosynthetic (Figure 2a).
2. Increase in the system bearing capacity by forcing the potential bearing capacity failure surface to develop along alternate, higher shear strength surfaces (Figure 2b).
3. Membrane support of the wheel loads (Figure 2c).

Figure 2. Possible reinforcement functions provided by geosynthetics in roadways: (a) lateral restraint, (b) bearing capacity increase, and (c) membrane tension support
When an aggregate layer is loaded by a vehicle wheel or dozer track, the aggregate tends to move or shove laterally, as shown in Figure 2a, unless it is restrained by the subgrade or geosynthetic reinforcement. Soft, weak subgrade soils provide very little lateral restraint, so when the aggregate moves laterally, ruts develop on the aggregate surface and also in the subgrade. A geogrid with good interlocking capabilities or geocomposiste with good interlocking and frictional capabilities can provide tensile resistance to lateral aggregate movement. Another possible geosynthetic reinforcement mechanism is illustrated in Figure2b. Using the analogy of a wheel load to a footing, the geosynthetic reinforcement forces the potentialbearing capacity failure surface to follow an alternate higher strength path. This tends toincrease the bearing capacity of the subgrade soil. A third possible geosynthetic reinforcement function is membrane-type support of wheel loads, as shown conceptually in Figure 2c. In this case, the wheel load stresses must be great enough to cause plastic deformation and ruts in the subgrade. If the geosynthetic has a sufficiently high tensile modulus, tensile stresses will develop in the reinforcement, and the vertical component of this membrane stress will help support the applied wheel loads. As tensile stress within the geosynthetic cannot be developed without some elongation, wheel path rutting (in excess of 4 in. {100 mm}) is required to develop membrane-type support. Therefore, this mechanism is generally limited to temporary roads or the first aggregate lift inpermanent roadways. A geosynthetic placed at the interface between the aggregate base course and the subgrade also functions as a separator to prevent two dissimilar materials (subgrade soils and aggregates) from intermixing. Geotextiles perform this function by preventing penetration of the aggregate into the subgrade (localized bearing failures) and prevent intrusion of subgrade soils up into the base course aggregate (Figure 3). Geogrids can also prevent aggregate penetration into the subgrade, depending on the ability of the geogrid to confine and prevent lateral displacement of the base/sub-base. However, the geogrid does not prevent intrusion of subgrade soils up into the base/sub-base course, which must have a gradation that is compatible with the subgrade based on standard geotechnical graded granular filer criteria when using geogrids alone. Subgrade intrusion can also occur under long term dynamic loading due to pumping and migration of fines, especially when open-graded base courses
are used. It only takes a small amount of fines to significantly affect the structural characteristics of select granular aggregate (e.g., see Jornby and Hicks, 1986). Therefore, separation is important to maintain the design thickness and the stability and load-carrying capacity of the base course. Thus, when geogrids are used, the secondary function of separation must also be considered.

Figure 3. Concept of separation in roadways
4.2 Design for Stabilization In stabilization design, the geogrid and aggregate thickness required to stabilize the subgrade and provide an adequate roadbed are evaluated. Recall that this application is primarily for construction expedience. For design of permanent roads, this stabilization lift also provides an improved roadbed (i.e., less subgrade disturbance, a gravel layer that will not be contaminated due to intermixing with the subgrade, and a potential for subgrade improvement of time). The base course thickness required to adequately carry the design traffic loads for the design life of the pavement may be reduced due to the improved roadbed condition, provided an assessment is made of the improvement. Geosynthetics used in this application perform multiple functions of separation, filtration and reinforcement. Separation design requirements were discussed in previous section. Because the subgrade soils are generally wet and saturated in this application, filtration design principles are also applicable with respect to reinforcement requirements, there are two main approaches to stabilization design. The first approach inherently includes the reinforcement function through improved bearing capacity and there is no direct reinforcing contribution (or input) for the strength characteristics of the geosynthetic. When this approach is used for geogrids, a geotextile or graded granular soil separation layer is also required to address these functional requirements. The second approach considers a possible reinforcing effect due to the geosynthetic. It appears that the separation function is more important for roadway sections with relatively small live loads where ruts, approximating 2 in to 4 in. (50 to 100 mm) are anticipated. In these cases, a design which assumes no reinforcing effect is generally conservative. On the other hand, for large live loads on thin roadways where deep ruts (> 4 in. {100 mm}) may occur, and for thicker roadways on softer subgrades, the reinforcing function becomes increasingly more important if stability is to be maintained. It is for these latter cases that reinforcing analyses have been developed and are appropriate. The reinforcing mechanisms mobilized in subgrade stabilization are different between geogrids and geotextiles. Due to the open structure and large apertures, the geogrids interlock with base course aggregate and change the stress and strain conditions in the vicinity of the geogrid. The efficiency of the geogrid-aggregate interlock depends on the relationship between aperture size and aggregate particle size, and the in-plane stiffness of the geogrid ribs and junctions. A design method that recognizes these distinctions is presented in next section along with the empirical method, which was originally developed for geotextiles, and later modified for geogrids. The separation function of geogrids, which is considered secondary in geogrid reinforced stabilization applications, is less obvious mainly because of the geogrids’ open structure. It is recommended that a geotextile be used as a separator beneath a geogrid to prevent migration of fines into the aggregate layers over time. However, it is possible to eliminate the geotextile by designing the gradation of the base or a subbase layer to provide separation based on well-known graded granular filter design principals .Graded granular subbase layers are conventionally used in roadway design. Geogrids provide a stable platform for the base aggregate, which may be sized to adequately filter the subgrade fines to prevent pumping. The movement of fine grained soils into coarse aggregates can be prevented if the pore spaces of the aggregates are small enough to hold the particles in place. When a geogrid is present at the subgrade base course interface, the relative movement of the soil particles is further constrained due to confinement provided by the geogrid-aggregate interlock. As a result, the possibility of soil migration is further reduced.

Figure 4. Filtration at the interface of two dissimilar materials
4.3 Material Properties used in Design
As with any geosynthetic applications, the material properties required for design are based on the properties required to perform the primary and secondary function (s) for the specific application over the life of the system and the properties required to survive installation. Some strength is, of course, required for the reinforcing function, which is based on the requirements in the specific design approach. The separation function is related to opening characteristics and are determined based on the gradation of the adjacent layers (i.e., subgrade, base and/or subbase layers). If the roadway system is designed correctly, then the stress at the top of the subgrade due to the weight of the aggregate and the traffic load should be less than the bearing capacity of the soil plus a safety factor, which is generally a relatively low value compared to the strength of most geosynthetics. However, the stresses applied to the subgrade and the geosynthetic during construction may be much greater than those applied in-service. Therefore, the strength of the geosynthetic in roadway applications is usually governed by the anticipated construction stresses and the required level of performance. This is the concept of geosynthetic survivability — the geosynthetic must survive the construction operations if it is to perform its intended function. Table 2 relates the elements of construction (i.e., equipment, aggregate characteristics, subgrade preparation, and subgrade shear strength) to the severity of the loading imposed on the geosynthetic. If one or more of these items falls within a particular severity category (i.e., moderate or high), then geosynthetics meeting those survivability requirements should be selected. For the high category in Table 2, geosynthetics that can survive the most severe conditions anticipated during construction should be used and are designated as Class 1 geosynthetics in the following geosynthetic property requirements tables. Geosynthetics that can survive normal construction conditions are Class 2 geosynthetics and may be considered for the moderate category. Variable combinations indicating a NOT RECOMMENDED rating suggests that one or more variables should be modified to assure a successful installation. Some judgment is required in using these criteria. Table2 Construction survivability rating

Site Soil CBR atInstallation	<1		1 to 2		>3
Equipment GroundContact Pressure	>50psi	<50 psi	>50psi	<50psi	>50psi	<50psi
Cover thickness(compacted)4in.(100mm)6in.(150mm) 12in.(300mm) 18in(450mm)	NR5NR NR 1	NRNR 1 2	151 2 2	11 2 2	252 2 2	22 2 2

Table 3 lists the survivability requirements for geogrids in stabilization and base reinforcement applications. A national guide of practice has not been established for geogrids. Therefore the recommended requirements were developed specifically for this manual and were based on a review of research on construction survivability a review of state and federal agency specifications on geogrids , and on the properties of geogrids which have performed satisfactorily in these applications . The specific property requirements were conservatively selected with consideration for high reliability required on public sector projects. Field trials or construction survivability tests following the recommendations in note 5 of Table 3 for both the material and junction strength could be used to reduce this conservatism. Table 3 Geogrid Survivability Property Requirement

Property	Testmethord	Units	Requirements
SURVIVABILITY			Geogrid Class
			Class1	Class2	Class3
Ultimate Multi Rib Tensile Strength	ASTMD6637	lb/ft	1230	820	820
JUNCTION STRENGTH	GSI GRIGG2	lb(N)	255	25	8
ULTRAVIOLETSTABILITY	ASTM D4355	%	50%AFTER 500 HOURS OF EXPOSURE

let. In the latter case, the contractor is required to demonstrate that the proposed subgrade condition, equipment, and aggregate placement will not significantly damage the geogrid. If necessary, additional subgrade preparation, increased lift thickness, and/or different construction equipment could be utilized. In rare cases, the contractor may even have to supply a different geosynthetic.
5. DESIGN PROSEDURE OF AGGREGRATE SURFACE REINFORCEMENT STEP 1. Determine soil subgrade strength .Determine the subgrade soil strength in the field using the field CBR, cone penetrometer, vane shear, resilent modulus, or any other appropriate test. The undrained shear strength of the soil, c, can be obtained from the following relationships: • For field CBR, c in psi = 4.3 x CBR (c in kPa = 30 x CBR); • For the WES cone penetrometer, c = cone index divided by 10 or 11, depending on the soil type; and • For the vane shear test, c is directly measured. Other in-situ tests, such as the static cone penetrometer test (CPT) or dilatometer (DMT), may be used, provided local correlations with undrained shear strength exist. Use of the Standard Penetration Test (SPT) is not recommended for soft clays. STEP 2. Determine wheel loading. Determine the maximum single wheel load, maximum dual wheel load, and the maximum dual tandem wheel load anticipated for the roadway during the design period. For example, a 10 yd3 (7.6 m3) dump truck with tandem axles will have a dual wheel load of approximately 8,000 lbf (35 kN). A motor grader has a wheel load of 5,000 to 10,000 lbf (22 to 44 kN) STEP 3. Estimate amount of traffic. Estimate the maximum amount of traffic anticipated for each design vehicle class. STEP 4. Obtain bearing capacity factor(s). Obtain appropriate subgrade stress level in terms of the bearing capacity factors. Values may be obtained for both the conditions with geogrid and without geogrid for estimating the cost effectiveness of using a geogrid.
STEP 5. Determine required aggregate thickness(es). This is obtained from the from Design Curve.The minimum aggregate thickness is 6 inches

Graph 1

Graph 2

Graph 3

6. INSTALLATION PROCEDURES
* Roll Placement
Successful use of geosynthetics in pavements requires proper installation, and Figure 5 shows the proper sequence of construction. Even though the installation techniques appear fairly simple, most geosynthetic problems in roadways occur as the result of improper construction techniques. If the geosynthetic is ripped or punctured during construction activities, it will not likely perform as desired. If a geogrid is placed with a lot of wrinkles or folds, it will not be in tension, and, therefore, cannot provide a reinforcing effect. Other problems occur due to insufficient cover over the geotextiles or geogrids, rutting of the subgrade prior to placing the geosynthetic, and thick lifts that exceed the bearing capacity of the soil. The following step-by-step procedures should be followed, along with careful observations of all construction activities.
1. The site should be cleared, grubbed, and excavated to design grade, stripping all topsoil, soft soils, or any other unsuitable materials (Figure 5). If moderate site conditions exist, i.e., CBR greater than 1, lightweight proofrolling operations should be considered to help locate unsuitable materials. Isolated pockets where additional excavation is required should be backfilled to promote positive drainage. Optionally, geotextile wrapped trench drains could be used to drain isolated areas

a. Prepare the ground by removing stumps,                      b. Unroll the geosynthetic directly over the boulders, etc.; fill in low spot. ground to be stabilized. If more than one roll is . requires overlap rolls. Inspect geosynthetic
PREPARE THE GROUND                                                               UNROLL THE GEOSYNTHETIC

c. Back dump aggregate onto previously place d. Spread the aggregate over the aggregate. Do not drive on the geosynthetic. geosynthetic to the design thickness maintain 6 to 12 inch cover between truck tires and geosynthetic
BACK DUMP AGGREGATE                                                          SPREAD THE AGGREGATE

Figure 5. Construction sequence using geosynthetics
2. During stripping operations, care should be taken not to excessively disturb the subgrade. This may require the use of lightweight dozers or grade-alls for lowstrength,saturated, noncohesive and low-cohesive soils. For extremely soft ground, such as peat bog areas, do not excavate surface materials so you may take advantage of the root mat strength, if it exists. In this case, all vegetation should be cut at the ground surface. Sawdust or sand can be placed over stumps or roots that extend above the ground surface to cushion the geogrid. Remember, the subgrade preparation must correspond to the survivability properties of either the geogrid.
3. Once the subgrade along a particular segment of the road alignment has been prepared, the geogrid should be rolled in line with the placement of the new roadway aggregate (Figure 5.b). Field operations can be expedited if the geogrid is manufactured to design widths in the factory so it can be unrolled in one continuous sheet. Geogrids should be placed directly on top of geotextiles when used together.The geosynthetic should not be dragged across the subgrade. The entire roll should be placed and rolled out as smoothly as possible. Wrinkles and folds in the geogrid should be removed by stretching and staking as required.
4. Parallel rolls of geotextiles or geogrids should be overlapped, sewn, or joined as required.
5. For curves, the geogrid should be cut and overlapped in the direction of the turn.
6. When the geogrid intersects an existing pavement area, the geosynthetic should extend to the edge of the old system. For widening or intersecting existing roads where geotextiles or geogrids have been used, consider anchoring the geogrid at the roadway edge. Ideally, the edge of the roadway should be excavated down to the existing geosynthetic and the existing geosynthetic mechanically connected to the new geosynthetic (i.e., mechanically connected with plastic ties to the geogrid).Overlaps, staples, and pins could also be utilized.
7. Before covering, the condition of the geogrid should be checked for excessive damage (i.e., holes, rips, tears, etc.) by an inspector experienced in the use of these materials. If excessive defects are observed, the section of the geosynthetic containing the defect should be repaired by placing a new layer of geosynthetic over the damaged area. The minimum required overlap required for parallel rolls should extend beyond the defect in all directions. Alternatively, the defective section can be replaced.
8. The base aggregate should be end-dumped on the previously placed aggregate(Figure5.c) For very soft subgrades, pile heights should be limited to prevent possible subgrade failure. The maximum placement lift thickness for such soils should not exceed the design thickness of the road.
9. The first lift of aggregate should be spread and graded to 12 in. (300 mm), or to the design thickness if less than 12 in. (300 mm), prior to compaction (Figure 5.d). At no time should traffic be allowed on a soft roadway with less than 8 in. (200 mm), (or 6 in. {150 mm} for CBR ≥ 3) of aggregate over the geogrid. Equipment can operate on the roadway without aggregate for geocomposite installation under permeable bases, if the subgrade is of sufficient strength. For extremely soft soils, lightweight construction vehicles will likely be required for access on the first lift. Construction vehicles should be limited in size and weight so rutting in the initial lift is limited to 3 in. (75 mm). If rut depths exceed 3 in. (75 mm), it will be necessary to decrease the construction vehicle size and/or weight or to increase the lift thickness. For example, it may be necessary to reduce the size of the dozer required to blade out the fill or to deliver the fill in half-loaded rather than fully loaded trucks.
10. The first lift of base aggregate should be compacted by tracking with the dozer, then compacted with a smooth-drum vibratory roller to obtain a minimum compacted density (Figure 5e). For construction of permeable bases, compaction shall meet specification requirements. For very soft soils, design density should not be anticipated for the first lift and, in this case, compaction requirements should be reduced. One recommendation is to allow compaction of 5% less than the required minimum specification density for the first lift. 11. Construction should be performed parallel to the road alignment. Turning should not be permitted on the first lift of base aggregate. Turn-outs may be constructed at the roadway edge to facilitate construction. 12. On very soft subgrades, if the geogrid is to provide some reinforcing, pretensioning of the geosynthetic should be considered. For pretensioning, the area should be proofrolled by a heavily loaded, rubber-tired vehicle such as a loaded dump truck. The wheel load should be equivalent to the maximum expected for the site. The vehicle should make at least four passes over the first lift in each area of the site. Alternatively, once the design aggregate has been placed, the roadway could be used for a time prior to paving to prestress the geogrid-aggregate system in key areas

7. CONCLUSION
The positive effects of geogrid reinforced base courses can economically and ecologically be utilized to reduce reinforced aggregate thickness. And it can also increase the life of the pavement and can also decrease the overall cost of the pavement construction with an increased lifetime.

 

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USE OF GEOGRIDS IN FLEXIBLE PAVEMENT