Correlation between resilient modulus and CBR

The analytical and some semianalytical pavement design methodologies use a fundamental mechanical property for expressing the bearing capacity or strength of the subgrade soil and not the empirical CBR value. This property is the resilient modulus (Mr) or the stiffness 

After extensive work carried out by researchers on behalf of organisations/laboratories, reliable correlation equations have been established for estimating stiffness from CBR values. The most widely used equations for fine-grained soil material are as follows  

In order to express the unit of MN/m in psi unit, the following conversion factor can be used 1 psi = 0.006895 MN/m.  

Equation 1.1 proposed by Powel et al. (1984) is based on the work carried out by Jones (1958) and is used in the current UK pavement design methodology (Highways Agency 2006). The validity of the equation is restricted to fine soil material with laboratory CBR 

Equations 1.2 and 1.3 are proposed to be used by the Asphalt Institute of the United States (Asphalt Institute MS-1). They are applicable to materials classified as A-7, A-6, A-5, A-4 and finer A-2 soils (AASHTO designation) or CL, CH, ML, SC and SP (USCS), or in general for materials that are estimated to have a resilient modulus of 207 MPa or less. 

---