A Finite Element Framework for Designing Unreinforced Unpaved Roads

Traditional design practice 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). Conventional methods of utilizing analytical expressions for arriving at the thickness of aggregate layer are affected by the semi-empirical assumptions. Further, these methods do not consider any deformation scenario when such unreinforced unpaved roads are subjected to quasi-static vehicular loading. Furthermore, it is idealistically assumed that the individual components (aggregate and subgrade) would not undergo failure under the transfer of stress generated by surface loads. To overcome these constraints, a finite element (FE) based design methodology is proposed to model unreinforced unpaved road resting on c-φ soil subjected to a typical combination of axle loads and tire pressures. Considering the individual failure of the subgrade and aggregate under construction and vehicular load respectively, a step-by-step design guideline is developed to assess the minimum shear strength parameters of the individual components required to prevent failure considering coupled stress-deformation effects.