High-capacity Tensiometer Research Articles

Evaluating innovative direct and indirect soil suction and volumetric measurement techniques for the determination of soil water retention curves following drying and wetting paths

Soil Water Retention Curves (SWRC) are a main characterising component in constitutive models developed for describing unsaturated soils behaviour. Their determination requires establishing a combination of accurate soil matric suction measurements and volumetric measurements. While direct suction methods are always preferred, current methods, such as high capacity tensiometers, are limited to a measuring range of only 1.5 MPa. In this work, the novel Northumbria High Capacity Tensiometer, with an extended measuring range of up to 3.5 MPa, is used in a simplified continuous-drying setup, with proposed modifications to enhance the equalisation times and volumetric measurements. The results are then compared and validated with direct and indirect methods in both drying and wetting paths. At the same time, different volumetric measurement techniques were assessed in order to establish an understanding of Soil Shrinkage Curves (SSCs) that describe the moisture-voids relationship during drying and wetting and has the potential to be used for indirect determination of volumetric dimensions of soil specimens. Hysteresis was observed in the water retention behaviour but not in the volumetric behaviour. An established SSC model is proposed in this work and is fitted to experimental data in order to indirectly estimate volumetric measurements from the water content of small-scale specimens. The proposed novel approach standardises volumetric measurements determination, leading to uniform and complete-range SWRCs.

Calibration of a granular matrix sensor for suction measurements in partially saturated pyroclastic soil

Field monitoring of soil moisture and matrix suction is a useful tool for the implementation of a reliable early warning system against rainfall-induced landslide occurrence. Several test fields have been set up in Campania region (southern Italy), frequently affected by flow-like landslides involving pyroclastic soil cover. In particular, at the Mount Faito test site (Lattari Mountains, southeast of Naples), field matric suctions were measured over two years by conventional jet-fill tensiometers and granular matrix sensors (Watermark, Irrometer®) at different depths. Granular matrix sensor is a resistive device that is more and more spread in agriculture applications and that may also be used for geotechnical purposes thanks to a suitable calibration. In order to gain the calibration curve of the Watermark sensor, two small tip tensiometers (STT) and one High Capacity Tensiometer (HCT) were installed at the same depth of the Watermark sensor in the partially saturated pyroclastic soil sampled at the topsoil of the Mount Faito test site. Tests were carried out in the laboratory by performing drying and wetting phases on undisturbed soil sample. By coupling resistance measurements by Watermark and matrix suction provided by the reference tensiometers, it was possible to derive the non-linear relationship between these two quantities. The soil retention curve was also determined thanks to the installation in the soil sample of a decagon probe previously calibrated in the same pyroclastic soil.

Stress-strain behaviour of unsaturated compacted coal rejects and tailings

The paper presents the results of an experimental study aimed at evaluating the stress-strain response of unsaturated tailings (Mixed Plant Reject and Dewatered Tailings) from a mine in Queensland, Australia, which can be applied to further optimise current tailings disposal strategies of the mine. Triaxialtests at constant gravimetric water content were performed on specimens prepared at different compaction states using dynamic and static methods, to determine their shear strength. The dynamically compactedspecimens display a higher strength than that statically compacted ones, which highlights the significance of stress history for tailings strength. As-compacted and post-testing suction measurements, performed using a high-capacity tensiometer, showed a reduction in matric suction irrespective of the material type, which is caused by mechanical wetting. The strength envelope was found to be non-linear, partly because of suction changes during testing. Post-testing suction measurement showed spatial variability within each specimen, with the central part of the samples experiencing the maximum suction reduction. The paper concludes with a discussion on the interpretation of such results.

Evaluating the performance of three sensor types for long-term measurement of suctions in gold tailings

A key aspect of the stability of a tailings dam is the pore pressure regime within the dam. Although gold tailings can exist in an unsaturated state in the field, the measurement of negative pore pressures (matric suctions) in active gold tailings dams is not common. One of the reasons for this may be the uncertainty regarding the ideal sensor type that should be used due to the shortcomings of various types of sensors. To investigate this, a long-term Drying Box experiment was conducted in the geotechnical laboratory at the University of Pretoria. Three sensor types were assessed: a commercially available water potential sensor and two versions of the TUKS tensiometer – a high capacity tensiometer designed and constructed at the University of Pretoria. Gold tailings was deposited in a slurry form and allowed to desiccate. The response from the three sensor types as a result of the change in suctions due to drying and subsequent re-wetting were observed. It was found that the TUKS tensiometer with the steel casing provided rapid, reliable, long-term suction readings. This sensor type is therefore recommended for long-term measurement of suction in gold tailings.

Effect of Soil Moisture Evaporation Rate on Dynamic Measurement of Water Retention Curve with High-Capacity Tensiometer

Effect of Soil Moisture Evaporation Rate on Dynamic Measurement of Water Retention Curve with High-Capacity Tensiometer

Cross-validation of the high-capacity tensiometer and thermocouple psychrometer for continuous monitoring of xylem water potential in saplings.

The pressure chamber, the most popular method used to measure xylem water potential, is a discontinuous and destructive technique and is therefore not suitable for automated monitoring. Continuous non-destructive monitoring could until very recently be achieved only by use of the thermocouple psychrometer (TP). Here we present the high-capacity tensiometer (HCT) as an alternative method for continuous non-destructive monitoring. This provided us with a unique chance to cross-validate the two instruments by installing them simultaneously on the same sapling stem. The HCT and the TP showed excellent agreement for xylem water potential less than -0.5 MPa. Response to day/night cycles and watering was remarkably in phase, indicating excellent response time of both instruments despite substantially different working principles. For xylem water potential greater than -0.5 MPa, the discrepancies sometimes observed between the HCT and TP were mainly attributed to the kaolin paste used to establish contact between the xylem and the HCT, which becomes hydraulically poorly conductive in this range of water potential once dried beyond its air-entry value and subsequently re-wetted. Notwithstanding this limitation, which can be overcome by selecting a clay paste with higher air-entry value, the HCT has been shown to represent a valid alternative to the TP.

Evaluation of instruments for monitoring the soil–plant continuum

The response of the shallow portion of the ground (vadose zone) and of earth structures is affected by the interaction with the atmosphere. Very frequently, the ground surface is covered by vegetation and, as a result, transpiration plays a major role in ground–atmosphere​ interaction. The soil and the plant form a continuous hydraulic system that needs to be characterised to model the ‘boundary condition’ of the geotechnical water flow problem. Water flow in soil and plant takes place because of gradients in hydraulic head triggered by the water tension (negative water pressure) generated in the leaf stomata. To study the response of the soil–plant continuum, water tension needs to be measured not only in the soil but also in the plant (in addition to the water content in the soil). This paper first evaluates three instruments that can be used to measure xylem water tension, i.e. the High-Capacity Tensiometer (HCT) and the Thermocouple Psychrometer (TP) for continuous non-destructive measurement on the stem, and the Pressure Chamber (PC) for discontinuous destructive measurement on the leaves. Experimental procedures are presented and critically discussed, including data quality control and instrument calibration, accuracy, and precision. The performance of these three instruments is evaluated in terms of measurement precision and measurement accuracy via cross-validation. The paper then addresses the problem of monitoring soil suction (pore-water tension) and water content using a second generation profile probe (fully encapsulated) and the use of Electrical Resistivity Tomography (ERT) for coarse characterisation of water content spatial distribution to support the design of spatial configuration of suction and water content sensors.

A comparative study of high capacity tensiometer designs

The High Capacity Tensiometer (HCT) is a soil sensor that can measure large negative pore-water pressures, i.e. in excess of −1 MPa, and is a key instrument for monitoring ground-atmosphere interactions. The design of HCTs has not changed since the first prototype was proposed in the 1990s and still includes three main components, namely a high air-entry value porous filter, a small water reservoir and a pressure transducer, which are all encased inside a protective sheath. Despite many successful examples of measurement of pore-water tension in the laboratory and the field, there are essentially no commercially available HCTs for industrial applications. The rare utilization of HCTs in engineering practice has always relied on bespoke devices that have been purposely designed by research laboratories or suppliers of soil sensors. The dissemination of HCTs into engineering practice has been further hindered by the relatively poor understanding of sensor design and its impact on the reliability of measurements. To overcome such gap of knowledge, this paper explores the influence of distinct design variables (i.e. the porous filter, the pressure transducer, the water reservoir size and the protective casing) on the measuring range of HCTs. Seven prototypes were manufactured with different combinations of the above design variables showing that, if HCTs are properly saturated, the air entry value of the filter has the strongest influence on the measuring range while the effects of reservoir size, pressure transducer and protective casing are relatively modest. It is expected that the results from this work will guide future design of HCTs, thus contributing to the development of resilient sensors for geotechnical applications.

Measurement of plant xylem water pressure using the high-capacity tensiometer and implications for the modelling of soil–atmosphere interaction

The response of shallow geotechnical structures is affected by interaction with the atmosphere. Since the ground surface is very often vegetated, plant transpiration plays a major role in such an interaction. Transpiration in geotechnical applications is generally modelled by way of a transpiration reduction function (e.g. the Feddes function). However, its parameters are generally borrowed from the agricultural literature, where the focus is on crop species and often loosely compacted organic agricultural soils. For the non-crop species in denser soils typically encountered in geotechnical applications, monitoring of the flow taking place in the soil through the xylem up to the leaves can potentially be exploited to characterise the transpiration reduction function. The main challenge is the measurement of the water pressure in the xylem. Techniques currently used include the pressure chamber and thermocouple psychrometer. The pressure chamber is destructive and thus not suitable for continuous monitoring and/or where a relatively small number of leaves is available (as often occurs in laboratory experiments). The thermocouple psychrometer is not accurate at low water tension, is affected by the presence of solutes in the xylem water and is significantly sensitive to temperature. This paper explores a novel application of the high-capacity tensiometer (HCT), initially developed for pore-water pressure measurement in soils. The HCT was installed on the stem or branch of different trees and its measurement validated against pressure chamber measurements over a range of xylem water pressure down to −1300 kPa. In addition, its measurement was used to investigate the response of the soil–plant continuum. Results show that the HCT is a viable and convenient instrument to use for xylem water pressure measurement and can provide field-based data for the modelling of plant transpiration. Installing HCTs on stems and branches is quite straightforward and this will help achieve a step change in testing and modelling the effect of plant transpiration on the soil water regime in the vadose zone.

The influence of rates of drying and wetting on measurements of soil water retention curves

The water retention curve is fundamental for a comprehensive description of the hydro-mechanical behaviour of unsaturated soils. The water retention testing system developed at Durham University allows direct and continuous measurement of suction using a high capacity tensiometer, water content determined from mass readings of a digital balance and measurements of volume change. The system was modified to accommodate an additional tensiometer to measure suction at the top besides the existing one at the bottom of the soil specimen. Soil specimens were subjected to drying and wetting following two procedures: discrete measurements carried out in stages to ensure equalisation and continuous measurement at different rates. All suctions measured during continuous and discrete measurements were very close at high saturation degrees. At lower saturation degrees, the suction values from the top and bottom of the specimen deviated from suctions observed in discrete measurements. This deviation in suction values was more evident in accelerated drying and wetting patterns. This can be explained by the fact that water permeability reduces with the decrease in saturation levels.

Measurement of xylem water pressure using High-Capacity Tensiometer and benchmarking against Pressure Chamber and Thermocouple Psychrometer

The response of the shallow portion of the ground (vadose zone) and of earth structures is affected by the interaction with the atmosphere. Rainwater infiltration and evapotranspiration affect the stability of man-made and natural slopes and cause shallow foundations and embankments to settle and heave. Very frequently, the ground surface is covered by vegetation and, as a result, transpiration plays a major role in ground-atmosphere interaction. The soil, the plant, and the atmosphere form a continuous hydraulic system, which is referred to as Soil-Plant-Atmosphere Continuum (SPAC). The SPAC actually represents the ‘boundary condition’ of the geotechnical water flow problem. Water flow in soil and plant takes place because of gradients in hydraulic head triggered by the negative water pressure (water tension) generated in the leaf stomata. To study the response of the SPAC, (negative) water pressure needs to be measured not only in the soil but also in the plant. The paper presents a novel technique to measure the xylem water pressure based on the use of the High-Capacity Tensiometer (HCT), which is benchmarked against conventional techniques for xylem water pressure measurements, i.e. the Pressure Chamber (PC) and the Thermocouple Psychrometer (TP).

Comparison of high capacity tensiometer designs for long-term suction measurements

This paper investigates the long-term measurement of negative (tensile) pore-water pressures in soils by using high capacity tensiometers (HCTs). Seven different HCT prototypes were designed and manufactured by using different porous filters, pressure transducers, water reservoirs and protective casings. The ability of these prototypes to record negative pore-water pressures over long times was initially assessed by a series of measurements on small clay samples equalised at different suction levels. These tests were followed by two larger scale experiments in which four HCT prototypes were simultaneously installed inside a lysimeter filled with a sandy soil alongside two standard dielectric permittivity sensors measuring suction and water content, respectively. In one experiment, the soil was left to dry until all four HCTs cavitated while, in another test, the soil was allowed to dry up to an intermediate level of suction before triggering a rainfall, after which the soil was left to dry again until all four HCTs cavitated. In each lysimeter experiment, the readings of the HCTs were reasonably consistent, which suggests that sensor design has little effect on the accuracy of measurements. These experiments also indicated that HCTs are more accurate and exhibit a faster response than standard dielectric permittivity sensors. Moreover, the HCTs incorporating a small water reservoir showed a greater ability to sustain suction over long period of time without cavitating.

Assessment of Skempton's pore water pressure parametersBandAusing a high-capacity tensiometer

Saturation of soils is a prerequisite in many laboratory tests involving consolidation, permeability and stress–strain behaviour. The saturation process is usually time consuming, particularly in clay-rich soils, and this can incur substantial cost and potential delays in reporting findings. The saturation of samples is assessed using the well-established Skempton's pore water pressure parameter B. In a situation where the soil is fully saturated, the B-value is approximately one. It is often the case that fine soil samples extracted from the ground, particularly those from below the water table, remain saturated. However, current testing protocols require evidence to verify a complete saturation prior to subsequent laboratory investigations. This paper reports experimental results exploring the hypothesis that, if the sample is ‘perceived’ to be saturated, then further saturation procedures may not be necessary to obtain reliable geotechnical parameters. Laboratory investigations were conducted on three different clays (kaolin clay, Belfast clay and Oxford clay) in a testing chamber instrumented with a high-capacity tensiometer. The confining pressures were applied in a ramped fashion under undrained conditions. The response of the tensiometer confirmed that the samples were saturated from the very beginning of the loading process, as implied by the B-value being close to one. Further supplementary investigations were carried out to assess the Skempton's pore water pressure parameter A and the stress–strain behaviour of the soils. The combined finding provides further evidence to suggest that the saturation process as suggested in standards may not be necessary for fine-grained soils to establish reliable geotechnical design parameters.

Evaluating a new method for simultaneous measurement of soil water retention and shrinkage curves

Water retention and shrinkage curves are two important characteristics of unsaturated soils. Conventionally, two curves are separately measured using multiple carefully prepared “identical” soil specimens. However, it is difficult or impossible to fabricate soil specimens with exactly identical stress history. The measurement of two curves was very time-consuming (each up to several weeks). Recently, a new method has been proposed to simultaneously measure two curves through shrinkage tests. The soil suction, water content, and volume during testing are monitored during testing for the construction of the water retention and shrinkage curves. Preliminary shrinkage test results prove that the new method is efficient in the measurement of two curves. In this study, a series of shrinkage tests on two types of soils were performed to evaluate this new method using a new device in which the soil suction, water content, and volume are, respectively, measured using a high-capacity tensiometer, digital balance, and a non-contact photogrammetric method. Test results indicate that the new method significantly reduces the time required for the measurement of two curves from weeks to 1 day. The new method allows simultaneous determination of the two curves using one single soil specimen, which also eliminates the influence of different stress histories on the test results. The axial strain distribution during drying can also be identified based on the reconstructed three-dimensional model of the soil, which is beneficial for examining the soil deformation non-uniformity and stress history. Finally, some discussions on the measurement of two curves using the new method are presented.

Performance of high-capacity tensiometer in constant water content oedometer test

Matric suction is one of the stress-state variables which governs the behaviour of unsaturated soils. In order to evaluate the effect of matric suction on the behaviour of unsaturated soil under constant water content condition, rapid measurement of change in matric suction during the test is required. High-capacity tensiometer can be used to rapidly measure the change in matric suction of unsaturated soil. In this paper, an oedometer apparatus was modified to incorporate a high-capacity tensiometer to investigate the compression behaviour of unsaturated soil under constant water content condition. The design of the oedometer apparatus was described. Two soils which are kaolin FP grade (commercial kaolin which has a fine grain size) and residual soil from Bukit Timah Granite were used to evaluate the performance of the high-capacity tensiometer. The results showed that kaolin paste is helpful in improving the contact condition between the soil specimen and high-capacity tensiometer. The equilibrium time of the high-capacity tensiometer was about 10 min. Evaporation effect reported by other researchers was not observed with the high-capacity tensiometer. The change in matric suction due to loading and unloading was accurately captured by the high-capacity tensiometer.

Mechanical Behaviour of a Compacted Residual Soil of Gneiss from Brazil under Constant Water Content Condition

Compacted fine-grained soils are widely used in the construction of geotechnical infrastructures such as the embankments, retaining walls and pavements. These infrastructures are typically constructed at a relatively fast rate such that the pore-water phase is not allowed to drain, or under constant water content (CW) condition. The mechanical behaviour of compacted fine-grained soils under CW condition should be well understood in addition to various other loading conditions for rational design of geotechnical infrastructures. In this study, compacted soil specimens of a residual gneiss from Brazil were tested using a conventional triaxial testing equipment under CW condition. A high capacity tensiometer was used to measure the matric suction in the compacted soil specimens. Three groups of soil specimens compacted at different initial water contents representing dry of optimum, optimum and wet of optimum conditions were tested to study the influence of soil structure on the matric suction development in the soil specimens during testing. The test results suggest that the matric suction changes in the soil under CW condition depend mainly on the volume change behavior, which is influenced by the level of confining pressure, the degree of saturation and the soil structure. The simple conventional triaxial testing equipment has been found to be useful for determining the mechanical behavior of compacted fine-grained soils under CW condition with the aid of a high capacity tensiometer.

Building the UPPA high capacity tensiometer

High capacity tensiometers (HCTs) are sensors capable of directly measuring tensile pore water pressure (suction) in soils. HCTs are typically composed of a casing that encapsulates a high air entry value ceramic filter, a water reservoir and a pressure sensing element. Since the creation of the first HCT by Ridley and Burland in 1993 at Imperial College London, HCTs have been almost exclusively built and used in academic research. The limited use in industrial applications can be explained by a lack of unsaturated soil mechanics knowledge among engineering practitioners but also by the technical difficulties associated to the direct measurement of tensile water pressures beyond the cavitation limit of -100kPa. In this paper, we present the recent design and manufacture of a new HCT at the Universite de Pau et des Pays de l’Adour (UPPA) in France. Different prototypes were tried by changing the main components of the device including the type of ceramic filter, pressure transducer and geometry of the external casing. In particular, two ceramic filters of distinct porosity, three pressure transducers with distinct materials/geometries and four casing designs were tested.

An Integrated High‐Capacity Tensiometer for Measuring Water Retention Curves Continuously

The water retention curve (WRC), which characterizes the relationship between the water content and water pressure head, is an important hydraulic property of soils in the vadose zone. The application of a high‐capacity tensiometer (HCT) to WRC measurement under a continuous drying process could significantly reduce the measurement duration compared with other conventional testing methods. However, the electric cable connecting the HCT to the data logger generally introduces errors in the soil mass measurement and, in turn, the water content results. In this study, an integrated HCT was designed, and a novel method was proposed to improve the measurement accuracy by using only one soil specimen. The integrated HCT comprised a digital millivoltmeter and a battery supplying constant voltage. The mass of this integrated HCT along with a soil specimen was recorded by a high‐resolution electronic balance during continuous drying. Meanwhile, the water pressure head was also measured by the integrated HCT. Hence, the errors induced by the HCT cable and the separate soil specimens were avoided. It was demonstrated that the WRC of a compacted kaolin obtained using the integrated HCT was similar to that obtained using the recently proposed method in which two identical specimens are required. Because only one specimen was required, the newly developed HCT is a promising transducer for the WRC measurement of intact specimens under continuous drying.

Experimental study on water evaporation from sand using environmental chamber

Large-scale evaporation experiments were conducted on bare sand using an environmental chamber. Four different atmospheric conditions and various drying durations were imposed on the soil sample. Atmospheric parameters (air flow rate, relative humidity, and temperature) and response of the soil (volumetric water content, temperature, and soil suction) were both monitored simultaneously. Notably, the temperature and matric suction at the soil surface were monitored using an infrared thermometer and high-capacity tensiometer, respectively. The results show that the air and soil temperatures depend on the evaporation process and atmospheric conditions. In addition, volumetric water content in the near-surface zone is strongly affected by the evaporation process and changes linearly over depth. The evaporation rate is strongly dependent on the air conditions.

New insight into cavitation mechanisms in high-capacity tensiometers based on high-speed photography

The high-capacity tensiometer developed by Ridley and Burland in 1993 is a milestone in experimental unsaturated soil mechanics. This device, which relies on development of tension in an enclosed water body, permits direct measurement of negative water potential. Many tensiometers have been built since 1993, all being based on the same principle although the design may differ slightly from the original. In particular, the common characteristic is a very small water reservoir that is believed to be the location of bubble nucleation, a phenomenon referred to as cavitation that impedes the sensor from functioning properly. Many have studied cavitation and different explanations or cavitation mechanisms have been proposed. However, all the considerations put forward were derived without being able to capture what happened inside the water reservoir at cavitation. This is now achieved with the new tensiometer specifically designed to “see” inside the water reservoir during suction measurement. For the first time, cavitation has been captured via high-speed photography and the mechanisms of cavitation can be explained using physical evidence. The first outcome is that it is possible for a high-capacity tensiometer to function to its full range with a large water reservoir. Then, the analysis of high-speed photographs reveals that the bubbles triggering cavitation are nucleated in the ceramic and make their way to the water reservoir. Cavitation occurs only when the air phase reaches the water reservoir.