Alessandro Tarantino

Experimental procedures and cavitation mechanisms in tensiometer measurements

In recent years tensiometers for direct measurement of matric suction have been developed at Imperial College and later on at the University of Saskatchewan. The major drawback of these instruments is water cavitation which may occur before pressure equalisation. A better understanding of the mechanisms that control cavitation inside the tensiometer may therefore help optimise their design and define adequate experimental procedures. This paper presents some of the experiences gained over the past four years using three tensiometers manufactured by Imperial College. Some of the anomalous responses recorded by tensiometers are first discussed. These were probably due to inadequate saturation of the porous ceramic rather than malfunctioning of the instruments. The results from a series of tests conducted with two or three tensiometers positioned on the same sample are then presented. These were helpful in defining suitable experimental procedures. Finally, the history of tension breakdown of each tensiometer is examined to identify some of the factors that affect the maximum sustainable tension as well as measurement duration. On the basis of data recorded before and after cavitation, a possible mechanism of cavitation in a tensiometer is described.

Soil suction monitoring for landslides and slopes

Abstract Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high-capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

Laboratory and Field Testing of Unsaturated Soils

The scope of this special issue focuses on recent advances in laboratory and field testing of unsaturated soils. Leading researchers from fourteen countries to represent global research in the area of experimental unsaturated soil mechanics have been invited to contribute to this issue. Twelve reports are presented dealing with measurement and control of suction and water content, mechanical, hydraulic, and geo-environmental testing, microstructure investigation, and applications of unsaturated soil monitoring to engineering behaviour of geo-structures.

Experimental Evidence and Theoretical Approaches in Unsaturated Soils

We happen to be involved in research. But research does not always lead to experimental results and numerical simulations that are in good agreement with each other. Research is also studded with errors, results that are difficult to interpret and theories that go nowhere. Whether working in a laboratory, struggling with apparatus that sometimes makes fun of us, or using a computer that refuses to co-operate and returns nothing but error messages, we understood one important thing. No research can be carried out alone. We need to share ideas, troubles, and doubts. This was the essence of the work-shop that took place in Trento on 10-12 April 2000. In line with the aim of the workshop, sufficient time was allowed for each speaker to give a detailed presentation. Each presentation was then followed by an equal amount of time that was given to the audience for discussion. In this respect, we wish to thank the discussion leaders of the three sections of the workshop, who led the long debate fol-lowing each presentation. As pointed out above, research needs to be fed by continuous discussion. Thus we also wish to thank the more experienced researchers, often co-authors of the papers presented in this book, for giving us the opportunity to think further. The works published in this book have been revised following comments, discussion and suggestions from the audience and post conference referee. Authors were invited not to be afraid of presenting works still under-way, highlighting limitations and black-points of their experimental and theoretical research. We hope this will be of help to other researchers. At the same time, we are aware that the reader of this book will expect to find some advances in unsaturated soil mechanics. We are confident her or his expectations will not be disappointed.

An approach for the geophysical assessment of fissuring of estuary and river flood embankments: validation against two case studies in England and Scotland

This paper proposes a two-stage geophysical approach to map the vertical cracking and the structural integrity of flood embankments made up of clay geomaterials susceptible to fissuring. The first stage is based on a ‘coarse-resolution’ investigation using conventional electrical resistivity tomography (ERT) equipment to identify the fissured zones in the embankment. This step is complemented by an additional geophysical technique, electromagnetic, to verify the ERT measurements. The second stage is based on a ‘high-resolution’ investigation using a miniature ERT system previously developed at the laboratory scale for detailed mapping of the fissure patterns. The ‘coarse-resolution’ stage is the major focus of this paper and was validated against two case studies in England and Scotland. Longitudinal ERT survey provided a tomographic picture of the upper desiccated zones of the embankments and fissured areas in 2-D, validating the range of resistivity results obtained previously on a fissured clay model in the laboratory. A transversal embankment resistivity tomography was also completed to show the positions of fissured zones in detail in the field. The electromagnetic technique as a fast screening tool allowed cross checking the ERT results and was also efficient in detecting high and low conductivity zones, indicating areas of potential weakness during flash floods and heavy rain. The southern embankment in England showed more fluctuations in the conductivity and resistivity than the north embankment in Scotland, likely to be due to the differences in climate, vegetation and location characteristics between the two sites. Conclusions were also drawn on the potential weaknesses for both embankments and the effect of vegetation on conductivity measurements.

Measurement of osmotic suction using the squeezing technique

The paper presents an experimental determination of the osmotic component of suction by means of the squeezing technique. Soil was mixed with distilled water at different water contents and squeezed at different pressures. Osmotic suction was estimated from electrical conductivity of water extracted from the soil and two different empirical relationships were used to estimate osmotic suction. Experimental results have shown that osmotic suction depends upon the extraction pressure and the initial water content. The osmotic suction measured using the squeezing technique was then compared with the difference between total suction (measured with transistor psychrometer) and matric suction (measured with axis translation technique).

A mechanism for bentonite buffer erosion in a fracture with a naturally varying aperture

Abstract In the deep geological disposal of nuclear waste in crystalline rock, erosion of the bentonite buffer may occur during periods of glaciation. Previous researchers have examined the mechanism and rates of extrusion and erosion for purified montmorillonite samples in smooth planar fractures. In this paper, we investigate the influence of using MX-80 material (as delivered, i.e.including accessory minerals) and a naturally varying aperture on bentonite erosion. A bespoke fracture flow cell was constructed for this purpose and flow through conducted with deionized water. Throughout the experiment, gravimetric analysis was undertaken on the effluent and the swelling pressure of the bentonite monitored. Quantitative image analysis of the extrusion process was also undertaken. When the swelling pressure data were analysed, alongside both the oscillations in erosion rate and the area of the accessory-mineral ring, a two-stage mechanism governing the erosion process became apparent. Once an accessory-mineral ring had formed at the edge of the extruded material, further increases in swelling pressure resulted in a breach in the accessory-mineral ring, triggering an erosive period during which, the mineral ring was supplemented with additional minerals. The cycle repeated until the ring was sufficiently strong that it remained intact. This observed process results in erosion rates one order of magnitude less than those currently used in long-term safetycase calculations.

Suction induced by static compaction

The paper aims at investigating suction induced by static compaction in clay specimens. To this end, kaolin powders prepared at target water contents were statically compacted in a shear box apparatus. Trento (TNT) high-suction tensiometers (0-1800 kPa) were in stalled through the loading pad to monitor suction changes during the loading-unloading paths. Specimens were compacted at water contents ranging from 0.22 to 0.30 and at vertical stresses of 300,600, and 1200 kPa. This made it possible to explore abroad spectrum of compaction conditions. Experimental results are presented and discussed in the paper. The most striking aspect of the results is that suction of unloaded specimens increased as degree of saturation increased. This can be explained by the dependency of the main wetting curve upon void ratio.

MEMS sensor-based monitoring system for engineered geological disposal facilities

Abstract The design of a novel MEMS (Micro-Electro-Mechanical System) sensor-based monitoring system is presented in this article for the in situ monitoring of the conditions (temperature, relative humidity) of an engineered bentonite barrier for the underground disposal of radioactive waste in a geological disposal facility (GDF). A first prototype of the monitoring system incorporating multiple state-of-the-art MEMS sensors has been developed on a PCB-based (Printed Circuit Board) structure, in order to measure the variation of temperature and relative humidity inside a cylindrical bentonite block during the hydration process. The monitoring system comprises separate sensor boards, the microcontroller-equipped interface board and the software user interface in the Labview environment. One of the main design priorities is to reduce the size of the embedded sensors in order to minimize their influence on the hydro-mechanical response of the bentonite block. The sensor boards are encapsulated in different manners to protect them from moisture, chemical corrosion and mechanical shocks. The sensor system has been tested and calibrated in the temperature range between -20°C and 120°C, and at different relative humidity levels implemented by saturated salt solutions in enclosed containers. Test results demonstrate that the sensors have shown good functionality and robustness in harsh test environments such as high temperature and high humidity. Both temperature and relative humidity sensors have shown satisfactory precision level and temporal stability, which are in good accordance with the design specification of these devices.

Effect of Partial Saturation on the Stability of Retaining Diaphragms above the Water Table

Granular ‘cohesionless’ soils above the water table are partially saturated but are commonly assumed to be dry in geotechnical practice. Accordingly, ‘drained’ shear strength is calculated by replacing the ‘saturated’ effective stress with the total stress. The ‘dry soil’ assumption neglects the effect t of suction on shear strength and, as a result, geo-structures are over designed. To investigate the implications of this assumption, this paper presents an approach to design retaining diaphragms in unsaturated soils above the water table. This approach is based on the lower bound theorem of plasticity. As an example, the embedment depth of retaining diaphragms in ‘cohesionless’ granular soils under unsaturated conditions is calculated and compared with the solution obtained from the classical ‘dry’ approach.