Fitting particle size distribution models to data from Burundian soils for the BEST procedure and other purposes

Testing the Beerkan Estimation of Soil Transfer (BEST) soil particle size distribution (PSD) model is necessary to evaluate the applicability of the BEST procedure for characterising soil hydraulics. In this investigation, the fitting performance of the BEST PSD model was tested using a database of 114 Burundian soils with 14 measured particle size fractions for each soil sample, and also by considering a reduced number of measured particle size fractions for a sample. The fitting performance of the model developed by Fredlund et al. (2000) (FR model) was also considered for comparative purposes. On average, the BEST model yielded satisfactory results (i.e., mean relative error, Er¯=3.9%). However, low values of Er (<5%) were always obtained in soils with relatively high clay content (≥48%) and Er increased up to 14.3% with the sand content of the soil. In fine textured soils, using 11–12 measured particle size fractions, or 1–3 h of suspension density measurement, did not affect substantially the ability of the BEST model to reproduce the complete PSD. An Er¯=2.4% was obtained with the FR model. It was concluded that, on average, the BEST PSD model is expected to give a reliable description of PSD of Burundian soils. In fine textured soils, reduced experimental information is enough to reproduce the complete PSD. If the objective of fitting a PSD to measured data is theoretical, and not the application of the BEST procedure, then the FR model is preferred.