Determination of Compacted Subgrade Thickness on Weak Subgrade Using Odemark’s Method Based on Mechanistic-Empirical Design Approach

Abstract

Subgrade in flexible pavement is considered as the foundation of pavement structure. Improvement in subgrade stiffness in terms of California Bearing Ratio (CBR) is done by placing suitable thickness of compacted subgrade with higher CBR. Such increase in subgrade CBR reduces the total pavement thickness and increases the durability of pavement against rutting failure. Present paper deals with formulation of a methodology for determination of suitable thickness of a compacted subgrade on the top of weak natural subgrade using mechanistic-empirical design approach. In this paper, the allowable vertical stress on top of a compacted subgrade has been determined from the CBR—depth relationship corresponding to the axle load and tyre pressure developed by Yoder and Witczak. Moreover, the vertical compressive strain on the top of the natural subgrade has been determined from the rutting criteria as recommended in IRC-37:2018. The two layered system with compacted subgrade with higher CBR on weak natural subgrade with lower CBR has been transformed in this paper into a homogenous system by Odemark’s method to use the formulations of mechanistic approach. The thickness of compacted subgrade thus has been back calculated so that the layered system can withstand the design load repetitions by limiting the vertical compressive strain on the top of natural subgrade. It has been found that, appropriate thickness of compacted subgrade is necessary to achieve the specified CBR when placed over the weak natural subgrade for specified axle load repetitions. Such layer thickness significantly varies with the ratio of elastic modulus in a two layered system. In this backdrop, the concept of effective CBR as recommended in IRC:37-2018 and the provision of SP:72-2015 specification for adopting a fixed thickness of compacted subgrade in low volume rural road need to be revisited.