Use of TDR Probes to Measure Water Content in Pumiceous Soils

Abstract

The time domain reflectometry has become a technique commonly used in geotechnical engineering to determine soil water content of soil. The possibility to determine soil properties depends on the proper understanding of the parameters that affect the propagation of an electromagnetic pulse along the TDR wave guide. Dielectric permittivity measurements in coarse grained soils indicate that the determination of the apparent relative permittivity of this materials may be complicated by a non uniform water content distribution along the rods of the TDR probe. This problem is quite evident when pumiceous soils are of concern due to their water retention properties (very low air entry value and sharp transition from full saturation to residual saturation). In order to conceive an interpretation procedure apt to manage this complexity a specific laboratory testing programme was designed and carried out. During the tests the water content of a reconstituted pumiceous soil sample was changed by means of the negative water column technique along a number of wetting and drying cycles, and simultaneously the waveforms were registered. The soil utilized was collected from a trial field in the neighbourhood of Monteforte Irpino (Avellino, Italy).