Marco Valerio Nicotera

A TRIAXIAL AND OEDOMETER APPARATUS FOR TESTING UNSATURATED SOILS

The paper describes a new system for testing unsaturated soils designed at the University of Naples Federico II. The system is modular and can be configured either as an oedometer or a triaxial cell. Triaxial specimens are 68 mm in diameter and 140 mm in height, and oedometer specimens have a diameter of 56 mm and a height of 20 mm. The suction control is based on the axis translation technique. Radial strains are determined by an accurate measurement of the difference in pressures between the water filling an inner cell coaxial to the sample and that filling a reference double-walled burette, submitted to the same pressure. The water content variations are determined by measuring the difference in pressure in two double-walled burettes, one of which being connected to the drainage circuit, the other one operating as a reference. The calibration of all the devices, as well as their precision and accuracy, is widely discussed in the paper. Finally, the results of tests performed on an unsaturated pyroclastic soil are presented in order to highlight the features of both apparatuses.

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.

Hydro-mechanical Analysis of an Unsaturated Pyroclastic Slope Based on Monitoring Data

In recent decades in Campania (Italy) there have been rapid landslides involving unsaturated soil covers resting on steep bedrock. The paper summarises the results of both monitoring at a pilot site in the municipality of Monteforte Irpino (AV) in southern Italy and hydro-mechanical properties of soils from the same site. The first part of the paper concerns the description of the pilot site and the hydro-mechanical characterization of the soils based on wide-ranging laboratory investigation. In the second part of the paper the seasonal features of the pore water regime are described. Measurements are elaborated to estimate the groundwater head spatial distribution and, in turn, head gradients. Finally, a simple stability analysis is presented, schematizing the slope as infinite.

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).