2 Experimental Setup

[681.1.6.1] The experimental setup of [15] consists of a vertical cylindrical, 72⁢cm long column, whose side walls are impermeable, filled with a homogeneous, isotropic and incompressible porous medium, an unconsolidated sandy material comprising approximately 97.5, 0.8 and 1.7% sand-, silt-, and clay-sized particles respectively. [681.1.6.2] The two fluids considered here are water and air and water is the wetting fluid. [681.1.6.3] The top of the column is connected to the atmosphere so that only air can enter the top of the column. [681.1.6.4] The bottom of the column is connected to a water tank and only water can enter from the bottom. [681.1.6.5] The pressure in the tank is adjustable. [681.1.6.6] Initially, the porous column is completely water saturated and the pressure PW in the water tank is chosen such that it compensates the water column PW(t=0)=72cmH2O(=7.06kPa) . [681.1.6.7] Hence, the capillary fringe is located at the top of the column. [681.1.6.8] The pressures in this section are given in cm column of water because this translates one to one to the position of the water table in the column. [681.1.6.9] Below in Section 5 SI units are used. [681.1.6.10] Figure 1 gives a conceptual picture of the experiment.

[681.1.6.11] The column is drained by lowering the pressure in the reservoir by Δ⁢P=5⁢cmH2⁢O every Δ⁢t=10⁢min 13 times until the water pressure reaches PW=7⁢c⁢m⁢H2⁢O. [681.1.6.12] After a relaxation period of Δ⁢t=50⁢min, the water pressure is raised again in seven steps by Δ⁢P=5⁢cmH2⁢O every Δ⁢t=10⁢min and the column is imbibed again. [681.1.6.13] After Δ⁢t=50⁢min, the water pressure is again lowered by Δ⁢P=5⁢cmH2⁢O every Δ⁢t=10⁢min for 5 times and the column is drained again. [681.1.6.14] Finally, Δ⁢t=50⁢min later, the water pressure is raised again by Δ⁢P=5⁢cmH2⁢O steps every Δ⁢t=10⁢min until it reaches its initial value PW(t>8h)=72cmH2O. [681.1.6.15] However, the original water content in the column is not recovered because a fraction of air remains trapped in the medium [7].

[681.1.7.1] In the laboratory, water saturation and water pressure have been measured at height x=0.4⁢m, x=0.5⁢m, x=0.6⁢m and x=0.7⁢m from the bottom of the column. [681.1.7.2] It may safely be assumed that the air pressure is essentially hydrostatic and atmospheric for two reasons: Firstly, because of the high viscosity and density contrast (μW=0.001⁢kg⁢m-1⁢sec-1, μO=18×10-6⁢kg⁢m-1⁢sec-1 and ϱW=1000⁢kg⁢m-3, ϱO=1.2⁢kg⁢m-3), and secondly, because the column is short. [681.1.7.3] It is therefore concluded that the measurement of water saturation and water pressure suffices to determine the capillary pressure saturation relationship.