Quasi-static Analysis of Geotextile Reinforced Unpaved Road Resting on c-φ Subgrade

This article provides an estimate of the aggregate thickness required for unpaved roads resting on c-φ soil. Conventional practice of design of unpaved roads mostly considers the subgrade layer to be purely cohesive such as in soft marshy lands. However, a huge bulk of Indian sub-urban and rural unpaved roads rest on c-φ soil subgrade soil whose strength characteristics are contributed both by cohesion (c) and angle of internal friction (φ). It is imperative that if cohesion is considered as the sole strength criterion, it will provide lower bearing resistance, and hence, will overestimate the aggregate thickness required, which will eventually lead to undesirable increase in the overall project cost. In this regard, this article reports the result of an attempt made to identify the diminution in the required aggregate thickness of the unpaved road when both the strength characteristics of the subgrade soil are taken into account. Utility of a single geotextile layer beneath the aggregate has also been investigated towards further reduction of the required aggregate thickness. Incorporating bearing capacity estimation of the c-φ soil, necessary expressions have been developed for estimating the required aggregate thickness as a function of the axle load, tire inflation pressure, cohesion and angle of internal friction of subgrade soil, angle of internal friction and load distribution angle of the aggregate. Extended ranges of the aforementioned parameters, as suitable in Indian context, have been considered and the effect of the same, in the absence and presence of geotextiles, has been reported. Efficacy of the geotextiles has been elucidated in terms of the degree of improvement represented as reduction of aggregate thickness. Encouraging improvement up to the level of 70% has been observed in many instances.