USE OF CONTACT MATERIAL IN TENSION INFILTROMETER MEASUREMENTS

A critically important aspect of tension infiltrometer (TI) measuremen ts is that good hydraulic contact be established and maintained betwee n the infiltrometer and the soil. This is routinely achieved by placin g a layer of contact material between the TI membrane and the soil sur face. In this study, the effect of contact material on the pressure he ad and hydraulic head gradient under the TI was investigated using com puter simulations and laboratory measurements. The discrepancy between the pressure head at the soil surface (psi(ss)) and the pressure head on the TI membrane (psi(m)) can be substantial and variable. The disc repancy depends on the thickness, sated hydraulic conductivity (K-cm) and water entry value (psi(cm)) of the contact material, and on the fl ow rate out of the TI. The hydraulic head gradients in the soil under the TI (G) are generally greater than unity, and depend on soil type a nd psi(ss). These results indicate that the often used assumptions, ps i(ss) = psi(m) and G = unity, are generally not correct. A relationshi p based on Darcy's law was developed to account for the discrepancy be tween psi(ss) and psi(m). Use of this relationship in TI equations res ulted in estimates of near-saturated hydraulic conductivity that fell within 10% of the actual values. A highly uniform glass sphere medium with K-cm = 1.1 (+/-0.1) X 10(-4) ms(-1) and psi(cm) = -300 (+/-30) mm was proposed as a good contact material for practical field use. Reco mmendations were also given for how the material could be used in fiel d-based TI measurements.