United States Department of Transportation
i
Quality assessment of geogrids used for subgrade treatment.
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2012-12-01
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-HIGHWAY/ROAD TRANSPORTATION;NTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and Performance;
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Abstract:Geogrid reinforcements have been used by the Indiana Department of Transportation (INDOT) to construct stable subgrade foundations
and to provide a working platform for construction over weak and soft soils. Use of geogrid reinforcement in a pavement system
ensures a long‐lasting pavement structure by reducing excessive deformation and cracking. The main objectives of this research were to
evaluate the mechanical interaction between a subgrade soil and an aggregate base layer with and without a geogrid in place at the
interface. A series of large‐scale direct shear tests were performed to investigate the effects of geogrid properties, such as geogrid
aperture area, junction strength, and tensile strength, on the interface shear strength of soil‐geogrid‐aggregate systems. The test results
showed that the aperture size and junction strength of the geogrids were relatively important factors affecting the overall interface
shear strength the most. The average values for the peak interface shear strength coefficient for the three normal stresses (50 kPa, 100
kPa and 200 kPa) considered in this study ranged from 0.96 to 1.48. In addition, the test results showed that the average peak interface
shear strength coefficient increases with increases in the junction strength of the geogrid. The optimum aperture area of the geogrid
was found to be equal to 825 mm2 (1.4 in2) for the subgrade soil and aggregate considered in this study. There was no significant
correlation between the geogrid tensile strength at 2% strain and the average peak interface shear strength coefficient. The effect of the
moisture content of the subgrade soil on the peak interface shear strength coefficient was also investigated. The peak interface shear
strength coefficient for the subgrade soil sample prepared at the optimum moisture content and compacted to relative compaction
values of 94–96% (Rsoil = 95–96% and Raggregate = 94–95%) and tested under a normal stress of 100 kPa was 20% less than that for the
subgrade soil sample prepared at a moisture content 4% above the optimum moisture content. Based on the results of the tests
performed in this study, an aperture area requirement of 825 mm2 (1.4 in2) and a junction strength requirement of 11.5 kN/m (788 lb/ft)
were suggested as preliminary guidelines for subgrade reinforcement systems. These requirements are only limited to the use of Type IV
geogrid (INDOT specification 207.04) for subgrade reinforcement with aggregate No. 53.
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