Column test apparatus for the inverse estimation of soil hydraulic parameters under defined stress condition

The experimental determination of soil hydraulic properties (soil water characteristic curve and unsaturated conductivity) over a large range of saturation is of utmost importance for any prediction of soil water dynamics and for evaluation of soil mechanical behaviour of unsaturated soils. However the hydraulic properties are expected to vary under stress, since inter alia they are mainly determined by the pore structure, which itself depends on the density of the soil and thus on the stress condition. A column test apparatus was developed to carry out multi-step outflow and inflow experiments under defined stress conditions on samples 45 cm in height and with a diameter of 19 cm. The simultaneous recording of pressure and changes in water content within the soil column at a reasonable temporal and spatial resolution is of crucial importance for the inverse parameter estimation process. A new measuring technique based on the method of Time-Domain-Reflectometry (TDR) permits the recording of continuos water profiles over the sample at a high temporal resolution. Transient in/outflow rates, water content profiles as well as tensiometer readings allow a detailed inspection of the hydraulic processes and yield a suitable data set for a more unique inverse parameter estimation. In the following paper the test apparatus will be introduced and results of an experiment will be presented.