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D. Toll - One of the best experts on this subject based on the ideXlab platform.

  • On the Development of an Ultra-High-Capacity Tensiometer Capable of Measuring Water Tensions to 7 MPa
    Géotechnique, 2018
    Co-Authors: J. Mendes, Alessandro Tarantino, Domenico Gallipoli, D. Toll
    Abstract:

    Tensiometers are increasingly used in geotechnical engineering to monitor pore-water tension in the field and to study the hydro-mechanical behaviour of unsaturated soils in the laboratory. Early Tensiometers exhibited a relatively small measuring range, typically limited to a tension of 0·1\,MPa, due to the breakdown of water tension inside the sensing unit at absolute negative pressures. This limitation was subsequently overcome by the design of high-capacity Tensiometers (HCTs), which enabled the measurement of considerably larger pore-water tensions. According to the literature, the highest value of water tension ever recorded by an HCT is 2·6\,MPa. In the present work, this value is almost tripled by designing a novel ultra-high-capacity Tensiometer (UHCT) capable of recording water tensions up to 7·3\,MPa. This is achieved by replacing the traditional ceramic interface with a nanoporous glass (typically employed by physicists for the study of confined liquids), which has never been used before in the manufacture of Tensiometers. The maximum attainable tension has been determined using tests where the UHCT measurement was progressively increased by vaporising water from the glass interface until the occurrence of tension breakdown (often referred to as `heterogeneous cavitation' or `Tensiometer cavitation'). The increased measuring range and the potentially larger measuring stability of the proposed UHCT will contribute to enhance laboratory testing of soils at high suctions and long-term monitoring of earth structures.

  • Tensiometer techniques for determining soil water retention curves
    Unsaturated Soil Mechanics - from Theory to Practice, 2015
    Co-Authors: D. Toll, S.d.n. Lourenço, Joao Mendes, Jd Asquith, A Fraser, Asem Ahmed Hassan, G Liu, T Noguchi, Piotr Osiński, Ross Stirling
    Abstract:

    The water retention behaviour of soils is an essential aspect of understanding the unsaturated behaviour of soil materials. This paper looks at a relatively new method of measurement for soil water retention curves, using high capacity Tensiometers. New equipment for carrying out continuous measurements of water content, suction and volume change is described. The results are compared with traditional pressure plate measurements, as well as filter paper and chilled mirror hygrometer tests. It has been suggested that Tensiometer measurements and pressure plate data give different results. This is confirmed by the larger data set provided here. It is found that the differences between the techniques can be explained by different volumetric responses.

  • Soil suction monitoring for landslides and slopes
    Quarterly Journal of Engineering Geology and Hydrogeology, 2011
    Co-Authors: D. Toll, J. Mendes, Domenico Gallipoli, Charles E. Augarde, S.d.n. Lourenço, F.d. Evans, Yu-jun Cui, Anh Minh Tang, J.c. Rojas, L. Pagano
    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.

  • Advances in Tensiometer-based suction control systems.
    Unsaturated Soils, 2010
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Charles E. Augarde, Fred Evans
    Abstract:

    Cunningham (2000) and Jotisankasa (2005) pioneered the development of Tensiometer-based suction control systems. In these systems, wetting and drying of the soil are achieved by water injection and circulation of air in contact with the specimen while suction is monitored by sample-mounted high suction Tensiometers. Unlike the axis translation technique, these systems avoid using elevated air pressures and better reproduce the drying and wetting conditions occurring in the field. Building upon these earlier works, this pa-per describes an automated Tensiometer-based suction control system that enables direct measurement of water content changes inside the sample. A diaphragm pump forces air to flow inside a closed loop that runs across the sample while a moisture trap ensures that the relative humidity of the circulating air is kept low. As the circulating air dries the soil, the amount of abstracted water is measured by continuous weighing of the desiccant inside the moisture trap. Wetting of the sample is instead achieved by controlled injection of water through a solenoid valve connected to a pressurized volume gauge. The changes of soil water content are given by the difference between the amounts of water injected by the volume gauge and that retained by the desiccant. The system is used to impose cycles of drying and wetting on compacted clayey specimens and results from preliminary tests are presented.

  • on the measurement of water pressure in soils with high suction Tensiometers
    Geotechnical Testing Journal, 2009
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Fred Evans
    Abstract:

    Past studies on the use of high suction Tensiometers for measuring negative pore water pressure have focused on three different aspects, namely, initial saturation of the probe, calibration over both positive and negative pressure ranges, and measurement procedures. Among these three areas, the one focusing on measurement procedures has undeservedly received less attention. Aspects related to measurement conditions during laboratory or field testing are as important as the initial pre-conditioning or calibration of the probe. According to the particular type of measurement, different aspects of the testing procedure have to be considered in order to obtain accurate readings of pore water pressure. This note presents preliminary data to highlight the importance of factors, such as measurement time, soil-probe contact, and material type, when measuring suction by means of high suction Tensiometer.

Alessandro Tarantino - One of the best experts on this subject based on the ideXlab platform.

  • On the Development of an Ultra-High-Capacity Tensiometer Capable of Measuring Water Tensions to 7 MPa
    Géotechnique, 2018
    Co-Authors: J. Mendes, Alessandro Tarantino, Domenico Gallipoli, D. Toll
    Abstract:

    Tensiometers are increasingly used in geotechnical engineering to monitor pore-water tension in the field and to study the hydro-mechanical behaviour of unsaturated soils in the laboratory. Early Tensiometers exhibited a relatively small measuring range, typically limited to a tension of 0·1\,MPa, due to the breakdown of water tension inside the sensing unit at absolute negative pressures. This limitation was subsequently overcome by the design of high-capacity Tensiometers (HCTs), which enabled the measurement of considerably larger pore-water tensions. According to the literature, the highest value of water tension ever recorded by an HCT is 2·6\,MPa. In the present work, this value is almost tripled by designing a novel ultra-high-capacity Tensiometer (UHCT) capable of recording water tensions up to 7·3\,MPa. This is achieved by replacing the traditional ceramic interface with a nanoporous glass (typically employed by physicists for the study of confined liquids), which has never been used before in the manufacture of Tensiometers. The maximum attainable tension has been determined using tests where the UHCT measurement was progressively increased by vaporising water from the glass interface until the occurrence of tension breakdown (often referred to as `heterogeneous cavitation' or `Tensiometer cavitation'). The increased measuring range and the potentially larger measuring stability of the proposed UHCT will contribute to enhance laboratory testing of soils at high suctions and long-term monitoring of earth structures.

  • Benchmark of experimental techniques for measuring and controlling suction
    Géotechnique, 2011
    Co-Authors: Alessandro Tarantino, Domenico Gallipoli, Charles E. Augarde, Vincenzo De Gennaro, Rodrigo Gomez, Lyesse Laloui, Claudio Mancuso, G. El Mountassir, Juan J. Muñoz, Jean-michel Pereira
    Abstract:

    The paper presents a benchmarking exercise carried out within the MUSE network aimed at comparing different techniques for measurement and control of suction. Techniques tested by the seven MUSE research teams include axis-translation (pressure plate and suction-controlled oedometer), high-capacity Tensiometer, and osmotic technique. The soil used in the exercise was a mixture of well graded sand, sodium bentonite (active clay), and kaolinite (non-active clay), which were all commercially available. Samples were prepared by one team and distributed to all other teams. They were normally consolidated from slurry under one-dimensional conditions (consolidometer) to a given vertical stress. The water retention characteristics of the initially saturated samples were investigated along the main drying path. Samples were de-saturated by applying suction through the liquid phase when using axis translation technique or osmotic method and de-saturated by air-drying when suction was measured using high-capacity Tensiometers. In general, the same technique was tested by at least two teams. The water retention curves obtained using the different techniques are compared and discrepancies are discussed in the paper.

  • Field Measurement of Suction, Water Content, and Water Permeability
    Geotechnical and Geological Engineering, 2008
    Co-Authors: Alessandro Tarantino, Andrew M. Ridley, D. Toll
    Abstract:

    This paper presents a review of techniques for field measurement of suction, water content, and water hydraulic conductivity (permeability). Main problems in the use of field Tensiometers are addressed and hints on how to improve Tensiometer performance are given. Advantages and limitations of instruments for indirect measurement of suction including electrical conductivity sensors, thermal conductivity sensors, dielectric permittivity sensors, filter paper, and psychrometer are discussed. Techniques for water content measurement based on dielectric methods are then presented. These include time and amplitude domain reflectometry and capacitance. Finally, a brief overview of methods for measurement of water permeability in the field is presented.

  • Laboratory measurement of matric suction in pyroclastic soil using vacuum and high-suction Tensiometers
    Springer Proceedings in Physics, 2005
    Co-Authors: M. V. Nicotera, Alessandro Tarantino
    Abstract:

    The paper presents suction measurements on pyroclastic soils using vacuum Tensiometer and high-suction Tensiometers. Laboratory measurements with the vacuum Tensiometer aimed at assessing the performance of this instrument, which had to be used for field measurement of matric suction. These measurements were also used to develop and validate a numerical model for predicting air cavity growth in the vacuum Tensiometer. The model was intended to be a tool for detecting possible malfunctioning of the Tensiometers in the field. Measurement with high-suction Tensiometers aimed at corroborating the results of vacuum Tensiometer measurements. When using high-suction Tensiometers, contact can be better controlled and suctions approaching the limit value of the vacuum Tensiometers (about 80 kPa) can be measured with no risk of cavitation.

  • Experimental procedures and cavitation mechanisms in Tensiometer measurements
    Geotechnical and Geological Engineering, 2001
    Co-Authors: Alessandro Tarantino, L. Mongiovì
    Abstract:

    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.

Kamini Singha - One of the best experts on this subject based on the ideXlab platform.

  • Transpiration‐ and precipitation‐induced subsurface water flow observed using the self‐potential method
    Hydrological Processes, 2019
    Co-Authors: Emily Voytek, Holly Barnard, Damien Jougnot, Kamini Singha
    Abstract:

    Movement of soil moisture associated with tree root-water uptake is ecologically important, but technically challenging to measure. Here, the self-potential (SP) method, a passive electrical geophysical method, is used to characterize water flow in situ. Unlike Tensiometers, which use a measurement of state (i.e. matric pressure) at two locations to infer fluid flow, the SP method directly measures signals generated by water movement. We collected SP measurements in a two-dimensional array at the base of a Douglas-fir tree (Pseudotsuga menziesii) in the H.J. Andrews Experimental Forest in western Oregon over five months to provide insight on the propagation of transpiration signals into the subsurface under variable soil moisture. During dry conditions, SP data appear to show downward unsaturated flow, while nearby Tensiometer data appear to suggest upward flow during this period. After the trees enter dormancy in the fall, precipitation-induced vertical flow dominates in the SP and Tensiometer data. Diel variations in SP data correspond to periods of tree transpiration. Changes in volumetric water content occurring from soil moisture movement during transpiration are not large enough to appear in volumetric water content data. Fluid flow and electrokinetic coupling (i.e. electrical potential distribution) were simulated using COMSOL Multiphysics to explore the system controls on field data. The coupled model, which included a root-water uptake term, reproduced components of both the long-term and diel variations in SP measurements, thus indicating that SP has potential to provide spatially and temporally dense measurements of transpiration induced changes in water flow. This manuscript presents the first SP measurements focusing on the movement of soil moisture in response to tree transpiration.

Domenico Gallipoli - One of the best experts on this subject based on the ideXlab platform.

  • On the Development of an Ultra-High-Capacity Tensiometer Capable of Measuring Water Tensions to 7 MPa
    Géotechnique, 2018
    Co-Authors: J. Mendes, Alessandro Tarantino, Domenico Gallipoli, D. Toll
    Abstract:

    Tensiometers are increasingly used in geotechnical engineering to monitor pore-water tension in the field and to study the hydro-mechanical behaviour of unsaturated soils in the laboratory. Early Tensiometers exhibited a relatively small measuring range, typically limited to a tension of 0·1\,MPa, due to the breakdown of water tension inside the sensing unit at absolute negative pressures. This limitation was subsequently overcome by the design of high-capacity Tensiometers (HCTs), which enabled the measurement of considerably larger pore-water tensions. According to the literature, the highest value of water tension ever recorded by an HCT is 2·6\,MPa. In the present work, this value is almost tripled by designing a novel ultra-high-capacity Tensiometer (UHCT) capable of recording water tensions up to 7·3\,MPa. This is achieved by replacing the traditional ceramic interface with a nanoporous glass (typically employed by physicists for the study of confined liquids), which has never been used before in the manufacture of Tensiometers. The maximum attainable tension has been determined using tests where the UHCT measurement was progressively increased by vaporising water from the glass interface until the occurrence of tension breakdown (often referred to as `heterogeneous cavitation' or `Tensiometer cavitation'). The increased measuring range and the potentially larger measuring stability of the proposed UHCT will contribute to enhance laboratory testing of soils at high suctions and long-term monitoring of earth structures.

  • Soil suction monitoring for landslides and slopes
    Quarterly Journal of Engineering Geology and Hydrogeology, 2011
    Co-Authors: D. Toll, J. Mendes, Domenico Gallipoli, Charles E. Augarde, S.d.n. Lourenço, F.d. Evans, Yu-jun Cui, Anh Minh Tang, J.c. Rojas, L. Pagano
    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.

  • Benchmark of experimental techniques for measuring and controlling suction
    Géotechnique, 2011
    Co-Authors: Alessandro Tarantino, Domenico Gallipoli, Charles E. Augarde, Vincenzo De Gennaro, Rodrigo Gomez, Lyesse Laloui, Claudio Mancuso, G. El Mountassir, Juan J. Muñoz, Jean-michel Pereira
    Abstract:

    The paper presents a benchmarking exercise carried out within the MUSE network aimed at comparing different techniques for measurement and control of suction. Techniques tested by the seven MUSE research teams include axis-translation (pressure plate and suction-controlled oedometer), high-capacity Tensiometer, and osmotic technique. The soil used in the exercise was a mixture of well graded sand, sodium bentonite (active clay), and kaolinite (non-active clay), which were all commercially available. Samples were prepared by one team and distributed to all other teams. They were normally consolidated from slurry under one-dimensional conditions (consolidometer) to a given vertical stress. The water retention characteristics of the initially saturated samples were investigated along the main drying path. Samples were de-saturated by applying suction through the liquid phase when using axis translation technique or osmotic method and de-saturated by air-drying when suction was measured using high-capacity Tensiometers. In general, the same technique was tested by at least two teams. The water retention curves obtained using the different techniques are compared and discrepancies are discussed in the paper.

  • Advances in Tensiometer-based suction control systems.
    Unsaturated Soils, 2010
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Charles E. Augarde, Fred Evans
    Abstract:

    Cunningham (2000) and Jotisankasa (2005) pioneered the development of Tensiometer-based suction control systems. In these systems, wetting and drying of the soil are achieved by water injection and circulation of air in contact with the specimen while suction is monitored by sample-mounted high suction Tensiometers. Unlike the axis translation technique, these systems avoid using elevated air pressures and better reproduce the drying and wetting conditions occurring in the field. Building upon these earlier works, this pa-per describes an automated Tensiometer-based suction control system that enables direct measurement of water content changes inside the sample. A diaphragm pump forces air to flow inside a closed loop that runs across the sample while a moisture trap ensures that the relative humidity of the circulating air is kept low. As the circulating air dries the soil, the amount of abstracted water is measured by continuous weighing of the desiccant inside the moisture trap. Wetting of the sample is instead achieved by controlled injection of water through a solenoid valve connected to a pressurized volume gauge. The changes of soil water content are given by the difference between the amounts of water injected by the volume gauge and that retained by the desiccant. The system is used to impose cycles of drying and wetting on compacted clayey specimens and results from preliminary tests are presented.

  • on the measurement of water pressure in soils with high suction Tensiometers
    Geotechnical Testing Journal, 2009
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Fred Evans
    Abstract:

    Past studies on the use of high suction Tensiometers for measuring negative pore water pressure have focused on three different aspects, namely, initial saturation of the probe, calibration over both positive and negative pressure ranges, and measurement procedures. Among these three areas, the one focusing on measurement procedures has undeservedly received less attention. Aspects related to measurement conditions during laboratory or field testing are as important as the initial pre-conditioning or calibration of the probe. According to the particular type of measurement, different aspects of the testing procedure have to be considered in order to obtain accurate readings of pore water pressure. This note presents preliminary data to highlight the importance of factors, such as measurement time, soil-probe contact, and material type, when measuring suction by means of high suction Tensiometer.

Fred Evans - One of the best experts on this subject based on the ideXlab platform.

  • Advances in Tensiometer-based suction control systems.
    Unsaturated Soils, 2010
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Charles E. Augarde, Fred Evans
    Abstract:

    Cunningham (2000) and Jotisankasa (2005) pioneered the development of Tensiometer-based suction control systems. In these systems, wetting and drying of the soil are achieved by water injection and circulation of air in contact with the specimen while suction is monitored by sample-mounted high suction Tensiometers. Unlike the axis translation technique, these systems avoid using elevated air pressures and better reproduce the drying and wetting conditions occurring in the field. Building upon these earlier works, this pa-per describes an automated Tensiometer-based suction control system that enables direct measurement of water content changes inside the sample. A diaphragm pump forces air to flow inside a closed loop that runs across the sample while a moisture trap ensures that the relative humidity of the circulating air is kept low. As the circulating air dries the soil, the amount of abstracted water is measured by continuous weighing of the desiccant inside the moisture trap. Wetting of the sample is instead achieved by controlled injection of water through a solenoid valve connected to a pressurized volume gauge. The changes of soil water content are given by the difference between the amounts of water injected by the volume gauge and that retained by the desiccant. The system is used to impose cycles of drying and wetting on compacted clayey specimens and results from preliminary tests are presented.

  • on the measurement of water pressure in soils with high suction Tensiometers
    Geotechnical Testing Journal, 2009
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Fred Evans
    Abstract:

    Past studies on the use of high suction Tensiometers for measuring negative pore water pressure have focused on three different aspects, namely, initial saturation of the probe, calibration over both positive and negative pressure ranges, and measurement procedures. Among these three areas, the one focusing on measurement procedures has undeservedly received less attention. Aspects related to measurement conditions during laboratory or field testing are as important as the initial pre-conditioning or calibration of the probe. According to the particular type of measurement, different aspects of the testing procedure have to be considered in order to obtain accurate readings of pore water pressure. This note presents preliminary data to highlight the importance of factors, such as measurement time, soil-probe contact, and material type, when measuring suction by means of high suction Tensiometer.

  • Calibrations of a high-suction Tensiometer
    Geotechnique, 2008
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Charles E. Augarde, Fred Evans, Gabriela M. Medero
    Abstract:

    High-suction Tensiometers are able to measure suctions up to 2 MPa. Direct calibration at such high suctions requires the imposition of negative water pressures, which are difficult to achieve using facilities commonly available in soil mechanics laboratories. For this reason, Tensiometers are usually calibrated in the positive pressure range, and such calibration is subsequently extrapolated to negative pressures. This paper examines different experimental techniques to assess the accuracy of such extrapolation. Any error in the calibration process would be directly reflected in the measured values of suction, and might be particularly significant (in relative terms) for the measurement of low suctions. In addition, the results of this study show that calibration in the positive range is affected both by the physical configuration of the Tensiometer during calibration and by aspects of its design. The paper concludes that linear extrapolation of the calibration from the positive to the negative range is sufficiently accurate provided that calibration is done under conditions that closely match the conditions in which the Tensiometer will be used. Owing to structural differences between Tensiometers, and also to suction-induced 'calibration hysteresis', at least one check on the accuracy of the extrapolated calibration equation over a range of negative pressure should be performed, even if at low values of suction.

  • Evaluation of suction measurement by the Tensiometer and the axis translation technique
    Unsaturated Soils. Advances in Geo-Engineering, 2008
    Co-Authors: D. Toll, Domenico Gallipoli, Sérgio D. N. Lourenço, Charles E. Augarde, Gabriela M. Medero, Fred Evans
    Abstract:

    The axis translation technique is a well-established method for imposing values of suction in unsaturated soil samples. High-suction Tensiometers are more recently developed devices used for measuring pore water pressures in soils, including negative pore water pressures (i.e. suctions) below absolute zero. Both these techniques are comparable in terms of the suction range in which they operate. In this work a Tensiometer has been used to measure suction values imposed by the axis translation technique in kaolin samples inside a pressure plate and a triaxial cell. The Tensiometer has been kept in contact with the soil sample to track pore water pressure variations throughout the duration of the tests. The suctions measured by the Tensiometer have been compared to those imposed by the axis translation technique and it was found that the suction measured by the Tensiometer was always smaller than that imposed. Two scenarios are proposed to explain this. The first scenario considers the presence of water inside and below the high air entry value ceramic plate whereas the second one hypothesises the lack of equilibrium in terms of soil water content when suction is measured. The latter scenario seems to be supported by the evidence in the literature of equilibration times for pressure plate tests that are significantly longer than those reported for the present testing programme. Implications of both scenarios for laboratory testing are discussed.

  • Development of a Commercial Tensiometer for Triaxial Testing of Unsaturated Soils
    Unsaturated Soils 2006, 2006
    Co-Authors: Sérgio D. N. Lourenço, Domenico Gallipoli, D. Toll, Fred Evans
    Abstract:

    A new suction probe, the WF-DU Tensiometer, has been developed by Wykeham Farrance in collaboration with Durham University. The performance of the new suction probe is assessed in terms of the minimum negative value of pressure that can be measured by the probe before cavitation. A series of tests performed on four different units of the same design of Tensiometer is presented and the results are discussed in relation to the procedure adopted for the saturation of the probe prior to the tests. Other aspects of the Tensiometer response are interpreted by making reference to similar experiments published in the literature. All Tensiometers used in this work are calibrated in the positive range of pressure. The extrapolation of such calibration to the domain of negative pressures is validated by using the Tensiometer to measure known values of suction. Copyright ASCE 2006.link_to_subscribed_fulltex