Estimating van Genuchten Parameters α and n for Clean Sands from Particle Size Distribution Data

The effect of median particle size and breadth of particle sizes in clean sands on the SWCC and the van Genuchten parameters α and n is described. For uniform sands, the air entry suction increases (α decreases) and the steepness diminishes slightly (n decreases) for the drying SWCC as the particles become finer. Increasing the breadth of particle sizes also increases the air entry suction (lower α) and reduces the steepness (lower n) of the SWCC. Breadth of the particle size distribution has a larger effect on steepness and n of the SWCC, and median particle size has a larger effect on the air entry suction and α. Similar effects occur for the wetting SWCC, although the impact on the water entry suction is less than the impact on the air entry suction. The systematic effects of median particle size and breadth of particle sizes on the SWCC are captured in a pedotransfer function (PTF) to estimate α and n for clean sand based on d 60 and C u . Comparison with other SWCC data shows that α estimated with the PTF typically is within a factor of 2 of the actual α and n estimated with the PTF is within ± 2 of the actual n. These findings are applicable to α for sands with nonplastic fines provided the fines content is less than 20%, but apply to n only for sands with negligible fines content. A procedure to convert between van Genuchten and Brooks-Corey parameters for sands is also presented.