We designed a method, based on Stokes' law for determining falling velocity of a dispersion front, for determining soil susceptibility to dispersion. Results from our method are compared with horizontal soil water penetration. Dispersion experiments for different soils were carried out in a distance-marked test tube (1.2 cm i.d., and 16 cm length) using 1.5 g of oven dry soil and 15 mL of water. The falling distance of the dispersion front (z) was observed as a function of time (t). The linear relationship between z and t for all soils verified the applicability of Stokes' law in the initial distance (z ≤ 8 cm). Dispersiveness was defined as the inverse of the falling velocity of the dispersion front in water, Vw. The latter was related quantitatively to penetration coefficient λw (λw = 0.513 Vw0.338) and hydraulic conductivity, Ks (Ks = 0.001 Vw0.582). The designed dispersiveness method is simple and practical; preparation of sample, handling of instruments, and measurements of z and t are easy and convenient and require less technological input. Reproducibility is very high (CV < 10%), and the method is highly sensitive because its range of variation in falling velocity over different soils is two orders higher than that of penetration coefficient. The method is intended for field use. Under field situations, volume (1 cm3) may be substituted for weight (1.5 g oven dry) of soil, which keeps the variation within the experimental errors.
Falling velocity (V) and penetration coefficient (λ) increased in a 0.1 N CaCl2 solution compared with distilled water in all soils except sand. Effect of the height of suspension on dispersiveness was found to be positive, whereas the diameter of the sedimentation cylinder had no significant effect.