Dynamic Water‐Entry Pressure for Initially Dry Glass Beads and Sea Sand

Dynamic water‐entry pressure has been considered an important factor in the mechanism of fingered flow formation. We measured and compared dynamic water‐entry pressures of air‐dried glass beads of 0.4‐mm mean particle size and for 0.30‐ to 0.60‐mm (30–50 mesh) sea sand. Two measurements were conducted. First, breakthrough pressures were measured by increasing the supply water pressure incrementally from a low water pressure at which water could not enter the porous medium. Second, the relationship between the supply water pressure and the initial infiltration rate was measured by keeping the supply water pressure constant at several values. Measured values for the 0.4‐mm glass beads and the 30‐ to 50‐mesh sea sand showed some differences. While the breakthrough pressure was similar to the advancing capillary pressure for both materials, only the 30‐ to 50‐mesh sea sand showed almost the same value as the water pressure at the inflection point on the initial wetting curve. For the 0.4‐mm glass beads, the breakthrough pressure was higher (less negative) than that of the inflection point. Furthermore, the 0.4‐mm glass beads showed a sharp jump in the initial infiltration rate when the supply water pressure exceeded the breakthrough pressure, whereas the 30‐ to 50‐mesh sea sand showed an almost linear increase in the initial infiltration rate. These differences are discussed in terms of oscillatory nature of the water‐entry pressure. An expression for the dynamic water‐entry pressure was fitted to the measured data. The expression was used for estimating the width of fingers formed in the materials during infiltration into layered conditions and was found to be useful for studying fingered flow formation, at least for the glass beads.