Assessment of the effect of Geogrid and Geotextile inclusion on flexible pavement performance

Abstract

The performance of flexible road pavements greatly depends on the quality of the subgrade foundation on which it the pavement structure is supported, as well as the quality of the pavement materials. However, highway pavements are usually constructed on subgrade composed of weak native soil deposits and in some cases, on water sensitive natural soil subgrade (i.e. silt and clay) that need to be replaced with better quality materials (e.g. CRR and gravel) to provide a relatively stable foundation for the overlying pavement. However, these better-quality materials may be very hard to find nowadays, and are also very costly, due to the bulk quantities required, relatively thick pavement structure, as well as compensation, excavation and transportation costs involved. Field observations indicate that the inclusion of geosynthetics such as geogrids and geotextiles within flexible road pavements can significantly improve the performance of such pavements. In this research project, the effect of geotextile and geogrid inclusion in the subgrade and subbase of flexible pavements was investigated. The test samples were obtained from the Nansana-Busunju Road in Wakiso District which was under rehabilitation then. The appropriate geogrid and geotextile types to be used in the study were purchased from Baruga Technical Services located in Namugongo. The subsequent laboratory tests were carried out in the Soil Mechanics Laboratory at the College of Engineering Design, Art and Technology, Makerere University. 7 test section models (each 1m x 1m) were be prepared outside the Soil Mechanics Laboratory and Lightweight Deflectometer Tests were performed on each of the sections to determine their resilient modulus. Reinforcing the subbase using the geotextile and resulted into a reduction in the thickness of the overlying base course layer of 37mm and 68mm respectively. The initial cost of constructing the geotextile and geogrid-reinforced pavement section was greater by 82.91% and 113.86% respectively than the initial costs of construction of the unreinforced pavement section. However, the geotextile and geogrid-reinforced pavement designs are more cost effective over the unreinforced pavement design by 49.85% and 41.36% respectively over the service life of the pavements i.e. 25 years. Geogrids were found to be better reinforcing materials than geotextiles. This was evident from both the laboratory CBR tests and the field pavement models. However, the selection criteria for the geosynthetics to be used in reinforced flexible pavements are not well established yet.