Raffaele Papa

An Experimental Technique for Determining the Hydraulic Properties of Unsaturated Pyroclastic Soils

The experimental technique proposed in the paper was designed to rapidly and reliably determine both the water retention curve and the permeability function of undisturbed natural pyroclastic soil samples. The technique was developed in the framework of an experimental research project on mudflows in pyroclastic soils in the southern Italian region of Campania (Italy). Grain-size distribution and void ratio of the particular pyroclastic soils suggested investigation of the hydraulic behavior for matric suction values in the range between 0 and 100 kPa. Our experimental technique consisted of a sequence of testing phases to be conducted on a single undisturbed soil sample: A constant head permeation test, a forced evaporation test, and finally a drying test in a pressure plate apparatus. Interpretation of the experimental data was based on inverse modelling and allowed both water retention curves and permeability functions to be determined. The six parameters of the Mualem-van Genuchten model were adopted to describe the hydraulic behavior of the tested soils. Each testing phase supplied experimental data to be used in the interpretation procedure: The permeation phase provided a matching data point for the permeability function near saturation; the forced evaporation test was related to the hydraulic behavior for matric suction values ranging from a few kPa to less than 80-90 kPa (i.e., mini-tensiometer functioning range); and the drying steps in the pressure plate apparatus provided information about the retention properties for matric suction values up to 1 MPa. The results shown in the paper clearly confirm the goodness of the experimental design and the interpretation procedure.

Performances of Two High Capacity Tensiometers

Two High Capacity Tensiometers (HCT) have been developed at the University of Napoli Federico II (UNINA) using a design layout similar to that of Ridley and Burland (1993). One of the HCT includes a O-ring sealing and a high air-entry value disk (HAEV) to protect a water-saturated measurement chamber from desaturation. The second one eliminates the O-ring in the attempt to enhance the time-response and improve the mechanical design for the calibration in the “negative” pressure range. Two different pre-conditioning devices have been developed to water-saturate the HAEV and the measurement chamber. The HCT may also be calibrated in the same chamber by applying positive values of water pressure to the first prototype, and positive value of air pressure, to the second prototype, for ‘negative’ calibration. A number of free evaporation tests and matric suction measurements against preconditioned soil samples are presented in the paper and discussed in order to highlight the performances of the two HCTs.

Use of TDR Probes to Measure Water Content in Pumiceous Soils

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).