Subgrade design CBR and surface stiffness modulus

The design CBR of the subgrade is determined by executing CBR tests in the laboratory, over a range of conditions to reproduce, as far as possible, the conditions of moisture content and density, which are likely to be experienced during construction and in the completed pavement. 

Where it is not possible to collect material samples for laboratory assessment of CBR, the design CBR may be estimated from Table 10.1. 

The design CBR, for performance design, must be converted to subgrade surface modulus, E. The subgrade surface modulus is an estimated value of stiffness modulus based on subgrade CBR and is estimated by the following equation, derived from work on certain soils (Powell et al. 1984)  

The validity of the above equation is restricted to fine soil material with laboratory CBR values ranging from 2% to 12%. For coarser materials, the plate bearing test may also be appropriate. 

The subgrade surface modulus used in the design must be the lower value of the long-term and short-term CBR obtained. 

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