2.1. Test Procedure
 Soil Water Characteristic Curves were experimentally determined using a relatively simple device built at the University of Oklahoma. A test cell was fabricated to fit into a one dimensional consolidation apparatus so that incremental vertical loading could be externally applied while independently controlling the pore air and pore water pressure in the soil sample. As shown schematically in Figure 1, the pore water pressure was digitally controlled using a commercially available high-precision motorized piston pump and transmitted to the soil via a high air entry porous disc. A similar pump having a larger piston volume was used to control the air pressure in the cell. These pumps can resolve pressure and volume changes on the order of 1 kPa and 1 mm3, respectively. Vertical normal stress was applied to the sample through a stainless steel piston acting against the rigid top platen above the sample using the incremental loading features of the oedometer. Vertical deformation of the sample was measured via a linear variable differential transformer (LVDT). The experimental apparatus allowed for independent control/measurement of the pore air pressure, pore water pressure, vertical normal stress and measurement of vertical deformation throughout testing so that a variety of loading sequences with respect to net normal stress and matric suction could be investigated.
Figure 1. (a) Schematic of test cell cross section and measurement control system. (b) Photographs of modified oedometer setup.
Download figure to PowerPoint
 As mentioned previously, it was desired to obtain a significant amount of data in a relatively short period. Timely acquisition of experimental data was achieved in three ways: (1) a porous disc with a relatively low air entry value was used (three bar) to gain maximum efficiency with respect to water transmission into and out of the soil. The air entry value is the theoretical maximum pressure difference between the air pressure and water pressure that can be maintained across the porous disc without air fully penetrating and flowing through the pore channels in the disc. (2) A mixture of two commercially available manufactured soils, Sil-Co-Sil 250 manufactured by U.S. Silica Company and Glass Beads, Size BT-9, manufactured by Zero Products was used as the test soil. The mixture of 75% ground silica and 25% glass beads has a relatively higher hydraulic conductivity, compared with natural soils with similar suction range. The grain size distribution of the mixed test soil is given in Table 1. As shown, the test soil has a grain size distribution similar to that of fine sandy silt having about 48% fine sand (0.075–0.25 mm), 46% silt (0.002–0.075 mm), and 6% clay size material (≤0.002 mm). (3) The height of the soil sample, and hence volume of soil and water required to saturate the soil, was minimized to shorten equilibrium time [Khoury and Miller, 2008]. A series of tests was conducted to determine the minimum height of soil sample that could be used while practically achieving results similar to samples with heights more typical of one dimensional testing. The samples used had a height of 6.4 mm and a diameter of 63.5 mm. Note, the time required to complete testing was reduced by about 50% when the sample height was reduced from 25.4 to 6.35 mm. Using the system developed, the total time for testing, including primary drainage, primary wetting, secondary drainage, and a scanning curve loop was about 48 days.
Table 1. Grain Size Distribution for Test Soil
| ||D (mm)||Percent Passing|
|Sieve number 60||0.250||100.0|
|Sieve number 70||0.212||97.4|
|Sieve number 100||0.150||87.1|
|Sieve number 140||0.106||64.5|
|Sieve number 200||0.075||52.1|
 Prior to conducting the research experiments presented in this paper, numerous tests were conducted to calibrate and evaluate the performance of the testing device with respect to experimental error and repeatability. Results of testing on four different samples are presented in this paper, including data from one retest conducted to investigate repeatability. Each sample was prepared in an identical fashion to achieve nominally the same initial void ratio (0.69) and gravimetric moisture content (17.2%) in the test specimens. Samples were prepared by moist tamping (i.e., volume-based compaction) the soil directly into the test cell on top of the preconditioned high air entry porous disc. The test cell was then placed in the oedometer frame and a seating load (5 kPa) was applied to assure a good contact between the top cap and the soil. The cell was then flooded with water and water was forced under low pressure through the sample by increasing the air pressure above the water in the cell. This process continued until a minimum of three pore volumes of water had flowed through the sample to remove entrapped air. Following saturation, samples were loaded incrementally to the desired vertical normal stress, after which the drying (drainage) and wetting cycles were initiated.
 Suction was controlled during testing by using the axis translation method whereby the air pressure was increased while maintaining a constant water pressure of 0 kPa using the precision pumps. To maintain constant net normal stress during an increase in air pressure the axial normal force exerted by the oedometer was adjusted slightly to compensate for the air pressure acting on the portion of the axial load piston inside the air chamber. Axis translation allows one to increase suction without using water pressure below zero gage pressure, thereby avoiding cavitation.
 Two samples were tested at vertical net normal stresses of 10 and 200 kPa. For each net normal stress the samples were subjected to wetting and drying cycles to obtain the primary drainage, primary wetting, secondary drainage and one or more scanning curves. In addition, a repeat test was conducted at 200 kPa net normal stress to investigate repeatability and one test was conducted at 0 kPa net normal stress but in a separate cell not equipped for measurement of vertical deformation. For the 0 kPa test, only primary drainage and wetting curves were obtained.
2.2. Test Results
 Soil water characteristic curves for tests conducted under net normal stress of 10 and 200 kPa are presented in Figures 2 and 3, where pa and pw are pore air and pore water pressures, respectively, and nw is the volumetric water content (volume of water/total volume). The data points in Figures 2 and 3 represent the increments of suction and corresponding measurements of water volume change at equilibrium. Equilibrium was assumed to occur when negligible water volume and total volume change occurred following application of a new suction increment. In Figure 4 an example of water volume change (w is gravimetric water content) versus time for primary drainage at 200 kPa net normal stress is shown; water volume changed fairly rapidly following application of an increment of suction followed by a more gradual change until equilibrium was observed. On the basis of Figure 4 the time to complete primary drainage was about 18 days, which demonstrates the relatively fast equilibrium times that were achieved using the artificial soil and testing system described above.
 In Figure 5, a comparison of the secondary drainage and primary wetting curves for each test is shown. These portions of the SWCC were chosen for comparison since they represent the bounding curves employed in the bounding surface elastoplastic model presented in the next section. Note that for 0 kPa net normal stress, only the primary drainage and a portion of the primary wetting curve were obtained because the test had to be terminated prematurely; these are shown for comparison to the 10 and 200 kPa net normal stress curves.
 In examining Figures 2, 3, and 5, some interesting observations are made.
 1. For the 10 and 200 kPa net normal stresses, the scanning curves are bounded by the secondary drainage and primary wetting curve.
 2. The initial volumetric water content decreases with increasing net normal stress as expected since the void ratio decreases more during application of higher net normal stress.
 3. Generally, the air entry value increases with increasing normal stress, again as expected, because of the lower void ratio. However, the difference between the air entry value for 0 and 10 kPa net normal stress is negligible, probably because the change in void ratio from 0 to 10 kPa is relatively small compared to the difference between net normal stress of 10 and 200 kPa. The air entry value is the matric suction necessary for air to penetrate the void space when the soil is initially saturated.
 4. Generally, the slope of the SWCC is flatter at lower net normal stress and becomes steeper with increasing net normal stress; it is especially apparent as the curves approach the lower residual saturation moisture contents. This observation is reasonable because the pore channels in soil with lower void ratio would be smaller relative to higher void ratio soil. Thus, the lower void ratio soil would generate higher capillary pressure than a higher void ratio (i.e., matric suction) at the same volumetric water content. Also, the residual moisture contents appear to increase with increasing net normal stresses.
 As mentioned previously, the SWCC test was repeated at 200 kPa net normal stress to investigate experimental variability. Figure 6 shows the comparison of results from the two nominally identical tests. For the repeat test, only the primary drainage and a portion of the primary wetting curve were obtained because the system developed a leak that could not be repaired during testing. Nevertheless, the comparison of the SWCC curves is favorable, as is the volume change behavior represented by specific volume (1 plus void ratio) data presented in Figure 7. While the number of repeat tests was limited, the results for duplicate tests demonstrate that the SWCC is reproducible to reasonable accuracy.
Figure 7. Specific volume versus net normal stress during compression for three similarly prepared SWCC test specimens. Model predictions obtained during calibration shown for comparison.
Download figure to PowerPoint
 For the 10 and 200 kPa net normal stress SWCC tests, vertical displacements were recorded throughout testing beginning with the saturation process and continuing through the saturated compression and SWCC testing. In Figure 7, a comparison of specific volume versus net normal stress curves is presented. These curves represent compression starting from the compacted state, followed by wetting induced compression during saturation at a constant total stress, and subsequent compression under saturated conditions to reach the starting point of the SWCC tests. Although the curves exhibit some slight differences, they are generally similar and express similar soil behavior. Since samples were prepared in nearly identical fashion, similar behavior was expected. Interestingly, the samples showed considerable wetting-induced compression under a very low normal stress; the change in specific volume due to wetting represents a volumetric strain of about 1.5%. This collapse may be partly attributed to the relatively loose initial state of the sample following compaction and the significant angularity of the crushed silica particles. Both of these factors contribute to an open soil structure susceptible to collapse.
 During incremental loading the samples behaved similarly as evidenced by the similar slopes of each curve. Comparison of the specific volumes at 10 kPa and 200 kPa net normal stress indicates that significant compression, about 4–5% volumetric strain, occurred during incremental loading up to 200 kPa. This accounts for the differences in the initial volumetric water content for corresponding SWCCs. During the SWCC testing, the volume change as determined from LVDT measurements of vertical deformation was practically negligible with a maximum volumetric strain (due to suction change beginning from the start of the SWCC) of about 0.75%, for both the 10 and 200 kPa net normal stress specimens. While small, the volume change was included in the computation of volumetric water content.