USE OF TENSIOMETER RESPONSE TIME TO DETERMINE THE HYDRAULIC CONDUCTIVITY OF UNSATURATED SOIL

Abstract

The response time of tensiometers is controlled by the soil hydraulic conductivity when the flow resistance of the porous cup is sufficiently small. Therefore, we can determine the hydraulic conductivity of unsaturated soils from the response of a tensiometer to an artificially-induced perturbation of pressure. We evaluated a practical procedure to determine the conductivity by numerical simulations and by laboratory and field tests. The data can be analyzed by a simple graphical technique based on the linear "slug test" equation because nonlinear dependence of hydraulic conductivity on matric potential head is not significant when the head perturbation is small (<0.5 m). Using readily available equipment, a field tensiometer response test generally takes less than 1 h. Tests can be repeated over time to determine the in situ relationship between hydraulic conductivity and matric potential. The estimated conductivity is sensitive to the disturbance of the soil in close proximity to the porous cup. To improve the reliability of response tests, new methods need to be developed to install tensiometers with minimum soil disturbance. Despite the problem of soil disturbance, the tensiometer response test is an attractive method in clay-rich soils where other field methods are not easily applicable.