Engineers Australia

Spatial and statistical distribution models using the CBR test

Burt Look


The California Bearing Ratio (CBR) is the most common test used in the design of pavements. This simple test has many pitfalls in its application. This is an empirical test, and one must be aware of its many considerations if the design value is to be appropriately applied.

A design value must consider its spatial variation, level of compaction and its relationship with its surrounding layers. The design risk is used to determine the characteristic value for a given project. Characterisation using the spatial and statistical variation of a CBR is used for a project site in Queensland to illustrate the requirement to use an appropriate prediction model. The results of this curve fitting show the normal distribution is inappropriate due to negative values and the lognormal distribution is an appropriate statistical model to characterize the design CBR.