Analysis of Reinforced Unpaved Roads by Modified Structural Number Method

Abstract

By inclusion of geosynthetics in cross-sections of pavements, either traffic capacity of pavements can be enhanced for the same structural section or structural section can be reduced for the same traffic capacity. In the past research works, two methods were mainly adopted for the analysis of reinforced pavements, one is by conducting static plate load tests over unpaved roads on the basis of subgrade bearing capacity failure and secondly by applying dynamic or cyclic loads on the paved sections and determining the traffic benefit on the basis of rutting failure criterion. In the present research study, an effort has been made to evaluate the performance of road sections reinforced with geosynthetics using AASHTO 1993 Method. The structural number (SN), which denotes the strength of the pavements, has been modified using load equivalency factor concept. The unpaved sections were reinforced in a single layer at interface and within the subgrade layer at different positions. The maximum load intensities at a settlement of 12.5 mm in unreinforced and reinforced sections were converted into Equivalent Single Axle Load at the surface of pavement using Botswana guidelines on the basis of axle load surveys (2000). Using Load equivalency factor concept, new traffic capacities were determined followed by calculation of modified SNs for reinforced sections using AASHTO 1993 Design equation. The structural contribution of geosynthetics has been quantified in the terms of Base Course Reduction (BCR) values, which is defined as the percentage reduction in structural section of base course layer for reinforced sections as compared to unreinforced section. Laboratory tests were conducted to determine the grain size distribution and other important physical parameters of soil used for preparing subgrade. It is clear from the results that there is a considerable increase in load carrying capacity of unpaved sections due to reinforcements achieving an average BCR value of about 44%, with bi-axial geogrid (BX2020) and about 30% with CE121 geonet when reinforced within top one-third of subgrade layer.