IS 17371 : 2020 Geosynthetics - Geogrids for Flexible Pavements - Specification
New Standard from Last Update.
1. SCOPE
1.1 This standard specifies material properties and specifications that are required for geogrids to be used as reinforcement /stabilization in base, subbase and subgrade in design of flexible pavement structures.
1.2 This standard is a specification aid for material purchase and shall not be used as design or construction guideline. Design of geogrid reinforced flexible pavements shall be carried out as per standard practices recommended by Indian Roads Congress using the mechanistic empirical method, or global practices including Structural Number based on the AASHTO Method. The input parameters shall include parameters relating to interaction between the geogrids and the surrounding pavement component material.
1.3 It shall be noted that there are other types of geosynthetics other than geogrids, that can be used as reinforcement /stabilization for flexible pavements which cannot be excluded from being considered as alternate solutions to geogrids.
1.4 Figure 1 explains the common broad terminology for pavement components considered in this standard and the area of the geogrid reinforcement/stabilization or stabilization application is explained in Fig. 2 to Fig. 7.
NOTE - Above figures are general representation of the placement of geogrids in the various granular component of pavement layers. Placement of geogrid shall be as per the designer’s discretion.
1.5 The function of the geogrid reinforcement /stabilization in the application of flexible pavements includes:
a) Tensile reinforcement /stabilization within the structural components of the flexible pavement section. This may include the granular subbase and base courses, between the base course.
b) Tensile reinforcement /stabilization placed above the dressed subgrade and below the pavement subbase. The application is to strengthen the subgrade with a California Bearing Ratio (CBR) =5 percent. It must be noted that neither flexible nor rigid geogrid serves the purpose of separation nor filtration. For these functions, appropriate woven or nonwoven geotextiles shall be considered. The designer is required to exercise caution when geotextiles are used in conjunction with geogrids since the subgrade soil-geotextile-geogrid-GSB interaction need to be considered. Hence, it shall be judiciously evaluated.
1.6 Geogrid reinforcement /stabilization may be classified according to the function within the pavement structure as follows:
1.6.1 Geogrids placed within or directly beneath the base course are classified as base/sub-base reinforcement geogrid. There are two significant objectives of base /sub-base reinforcement geogrids:
a) increase the service life of the road and / or
b) demonstrate an equivalent performance but with reduced thickness(es) of the component(s) of the pavement section.
1.6.2 Geogrids placed at the subgrade/subbase interface are classified as Subgrade reinforcement/stabilization geogrids. The basic objective of Subgrade reinforcement /stabilization geogrids is to strengthen/stabilize the subgrade of the pavement, which may have very low CBR (CBR = 5 percent). Such geogrids would reduce vertical deformations to prevent rutting at the pavement surface.
1.7 Geogrids are classified as flexible or rigid.
1.7.1 In case of flexible geogrids, the yarns are formed into a dimensionally stable grid structure with uniform aperture. Flexible geogrids are bonded or knitted or woven ribs of polyester (PET) or polyvinyl alcohol (PVA) yarn. The geogrid is then given a tough and durable coating to enhance dimensional stability, resistance to installation damage and durability. The resulting grid structure possesses large openings called apertures. Flexible geogrids interact with the surrounding materials essentially through friction.
1.7.2 Rigid geogrids are generally manufactured from polypropylene or polyethylene. The manufacturing process consists of extrusion of a flat sheet of plastic, either high density polyethylene or polypropylene, followed by punching a controlled pattern of holes, and then stretching the sheet in either one (cross direction) or both directions. The resulting grid structure has large openings or apertures. These apertures induce interlocking between the rigid geogrid and the coarse granular (gravel size and above) that initiates interaction between the rigid geogrid and the pavement aggregate component.
1.7.3 By the above processes, both flexible and rigid geogrids develop tensile strength in both machine and cross machine directions, as well as enhance dimensional stability and durability, and develop resistance to installation damage.
2. REFERENCES
The standards listed in Annex A contain provisions which through reference in this text, constitute provision of this standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the standards.