Estimating the California Bearing Ratio (CBR) for some Auckland soils using geotechnical index and physical properties.

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.