Abstract:
The Auckland urban area is rapidly expanding and consequently the need for engineering assessment of the
subgrade soil is increasing. The California Bearing Ratio (CBR) is a globally recognised and comparable critical
soil strength index test performed to evaluate the shear strength of the subgrade soil for road and pavement
design. The CBR test is however highly time-consuming and tedious placing significant strain on limited soil testing
facilities. Due to these limitations, models have been developed internationally to provide engineers with a quick
CBR estimation tool to predict the behaviour of the soil. No such models have yet been developed within New
Zealand.
Using soil samples collected from four different locations within the Auckland region a database of geotechnical
physical properties of soil including index properties (liquid limit, plastic limit, plasticity index and linear shrinkage),
compaction characteristics (optimum moisture content and maximum dry density), mean grain size and grain size
distribution (the percentage volume of clay, silt and sand) was developed. The use of international models to
estimate the CBR value of Auckland soils was not reliable due to the heterogenous nature of Auckland soils and
therefore region-specific models are essential for estimating CBR. Regardless of the soil origin as the liquid limit,
plastic limit, plasticity index, linear shrinkage and optimum moisture content increase the CBR value tends to
decrease. Whereas, as the maximum dry density increases the CBR value tends to increase. Using the soil data
collected from the four Auckland locations, Auckland-specific CBR estimation models were developed including
single linear regression analysis (SLRA) equations, multiple linear regression analysis (MLRA) equations and
charts. The results of these models showed that SLRA was not effective to estimate the CBR value from other
physical properties of soil whereas the MLRA and charts provided a more reliable estimate of the CBR values.
This suggests than an improved Auckland-specific model using a greater number of soil samples could be used
by engineers as a preliminary estimate but should not replace the results of an actual CBR test for road subgrade
design.