Evaluation of Bouwer-Rice Large-Particle Correction Procedure for Soil Water Characteristic Curves

The Bouwer-Rice correction method to account for large particles excluded during laboratory testing to measure the soil water characteristic curve (SWCC) was evaluated on samples of well-graded alluvium. A large-scale hanging column apparatus was used so that tests could be conducted on specimens containing all particle sizes. The analyses show that SWCCs measured on the fraction of alluvium finer than the No. 4 U.S. sieve (4.8mm) can be corrected reliably to represent the SWCC of bulk soil or fractions of bulk soil corresponding to different largeparticle thresholds. The method can also be used reliably to correct SWCCs measured on soils prepared with different large-particle thresholds (e.g., finer than 25mm, 12.5mm, or 4.8mm). Dry density of the finer fraction being tested must be carefully controlled to match the dry density of the finer fraction in the soils containing large particles. An equation is described for computing the dry density of the finer fraction in a bulk soil. A simplified version of the Bouwer-Rice method is also proposed and evaluated. In this method, a SWCC test is conducted on the finer soil fraction with dry density matching that anticipated in the field, the van Genuchten equation is fit to the measured SWCC to define the shape parameters a and n, and then the SWCC for the field application is computed using the fitted a and n from the test on the finer fraction, saturated volumetric water content of the bulk soil in the field, and a residual water content of zero. Analyses show that this modified Bouwer-Rice method is simpler and results in an accurate representation of the SWCC of soil containing large particles.