Hydro-Mechanical Properties of an Unsaturated Frictional Material

188 CHAPTER 9. NUMERICAL SIMULATION OF COLUMN TEST BY FEM
Table 9.1: Input parameters for the soil-water characteristic
Van Genuchten parameter Swr Snr α n
Transient state test - drainage 0.13 0.005 0.0005 8.0
Transient state test - imbibition 0.13 0.18 0.001 6.5
Steady state test - drainage 0.05 0.01 0.00055 6.0
Steady state test - imbibition 0.02 0.01 0.0015 2.8
In the experiment and also in the numerical simulations air is not allowed to reach the
bottom of the sand column and of the model domain respectively. At the bottom of the
numerical model, a Neumann no-flow boundary condition is assigned to air, that ensures that
no mass of air flows through the bottom. For the water phase, also a Neumann boundary
condition is applied. In this case, the amount of water flux in or out of the model domain
is a predefined function of time as presented in Chapter 5. The fixed flux of water phase
introduced by the Neumann boundary condition implies that at a certain time a certain mass
of water has entered the model domain. For instance, during imbibition the total mass of
water in the domain does not depend on the hysteresis model, or even on whether hysteresis is
taken into account at all, but only on the value of water influx at the bottom boundary. The
same is true for the hydraulic properties of the sand that are used as input in the numerical
simulation. Permeability, porosity and soil-water characteristic curve do not relate to the
amount of water mass in the domain, but strongly influence how this mass distributes in it.
The water mass outflow and inflow is shown in Fig. 9.2.
Two different sets of parameters for the hysteretic soil-water characteristic curve were
identified that are given in Table 9.1. First the hysteretic soil-water characteristic curve is
determined directly from the sand column test results during the transient state test. An
additional hysteretic soil-water characteristic curve is determined based on the results from
classical steady state experiment.
9.4 Comparison of Simulation Results and Experimental Results
Experimental pore-water pressure and volumetric water content versus time results and nu-
merical pore-water pressure and volumetric water content versus time results as well as ex-
perimental and numerical soil-water characteristic curves are compared.
- Comparison using soil-water characteristic curve from transient state test:
In the first numerical simulation, the soil-water characteristic curve parameters are used
as input parameters directly measured by the TDR sensors and tensiometer sensors.
Simulation results are compared to the experimental results in terms of saturation versus