High Capacity Tensiometers, or HCTs, are sensors that can measure negative pore water pressures (... more High Capacity Tensiometers, or HCTs, are sensors that can measure negative pore water pressures (soil suctions) between-1.5 and-2 MPa. To achieve such measured values, HCTs first need to be fully saturated by water. For the first saturation of an initially dry HCT, the most common procedure involves application of vacuum followed by forced flooding by pressurised water. Instead, for the resaturation of a HCT that has cavitated (and is therefore still water flooded), only application of water pressure is necessary. Typically, the procedures for the first saturation and resaturation of HCTs can last days or weeks, which hinders adoption of these devices by the geotechnical industry. In this paper, faster procedures are presented for both first saturation and resaturation of HCTs built with 1.5 MPa air entry value filters. The duration of the first saturation can be reduced to less than 24 hours if high vacuum is first applied to the ceramic filter followed by water pressurisation at about twice the air entry value of the filter. Even more, resaturation of a cavitated HCT can be achieved in less than 10 minutes by simple water pressurisation of the ceramic filter. This is however true only if the HCT is not left to dry out to the atmosphere completely and is submerged in water just after cavitation.
A new suction probe, the WF‐DU tensiometer, has been developed by Wykeham Farrance in collaborati... more 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 ...
Tensiometers are increasingly used in geotechnical engineering to monitor pore-water tension in t... more 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 be...
Geological Society, London, Engineering Geology Special Publications, 2012
Climate change has the potential to have significant effects on the stability and serviceability ... more Climate change has the potential to have significant effects on the stability and serviceability of earthworks slopes, impacting on the performance of transport infrastructures. This paper describes how a unique facility for engineering and biological research was established in NE England through the BIONICS Project (Biological and Engineering Impacts of Climate Change on Slopes). It describes the building and monitoring of a full-scale embankment representative of road/rail embankments in the UK. The paper presents the results of monitoring of pore water pressure carried out between 2007 and 2009. The pore water pressures within the upper 3 m of the embankment have been largely positive, in some case approaching hydrostatic conditions after wetter periods. This is largely due to the wet nature of the summers in both 2007 and 2008 since monitoring began, as well as the application of artificial inundation using a climate control system. Negative pore water pressures (suctions) of t...
The mechanical behaviour of a sandy clay soil has been investigated by a series of constant water... more The mechanical behaviour of a sandy clay soil has been investigated by a series of constant water content triaxial tests on unsaturated samples with suction measurements. The tests were carried out in double cell triaxial cells on compacted samples, and also on samples wetted and dried from the as-compacted conditions. A series of tests on saturated samples was also performed to provide a reference state for the unsaturated tests. Since specimens were at high degrees of saturation (generally greater than 80%), calculations based on effective stress showed a reasonable interpretation of the data for the Critical State parameters M and λ. However, the intercept of the Critical State line in the ν axis (Γ) differed, increasing with an increase in water content (at compaction). The data were also analysed using the Bishop stress (p*) approach and a better fitting was achieved in the stress plane where it was possible to define a unique Critical State line, where M was 0.91. However, in the ν-p* plane, different critical state lines were still obtained for different compaction water contents. The similarity in the critical state parameters M and λ show that subjecting the soil to wetting and drying paths had little influence on the soil at Critical State conditions, when the pore water pressure changes were taken into account. Compaction curve-unsieved Compaction curve-sieved * Each test is identified in the form of Sxx(yy) by the as-compacted water content (xx) and confining pressure applied during the test (yy).
This paper presents the results of a set of 22 triaxial tests on an unsaturated artificially ceme... more This paper presents the results of a set of 22 triaxial tests on an unsaturated artificially cemented sand. The results are used to explore the applicability of a number of unsaturated soil frameworks for interpreting the shear strength. Constant water content triaxial tests were carried out on unsaturated specimens, using the axis translation technique to measure suctions during shearing. The test results on the unsaturated material were referenced against a series of drained and undrained triaxial tests that were carried out on saturated specimens. The results of the unsaturated tests were analysed to investigate the effect of the suction and degree of saturation on the shear strength at the critical state. The results show that the critical state stress ratio in terms of net stress (Ma) was found to be larger than the saturated critical state stress ratio (Ms). It was also found that the stress ratio in terms of suction (Mb) reduced as suction increased (when the degree of satura...
Unsaturated Soils. Advances in Geo-Engineering, 2008
ABSTRACT: Thispaper reportsona set of triaxial test dataonanartificially bondedsand tested inunsa... more ABSTRACT: Thispaper reportsona set of triaxial test dataonanartificially bondedsand tested inunsaturated conditions. Tests were performed using the axis translation technique to measure suctions while shearing under constant water content conditions. The results at the Critical State are interpreted to obtain the variation in the stress ratios: Ma representing the net stress component and Mb representing the suction component. These are also presented as Critical State angles of friction,(φa) c and (φb) c. It is found that Ma is higher ...
Proceedings of the International Conference held in Nottingham, UK, 25-27 August 2008, 2008
... Full text not currently available from Enlighten. Item Type: Conference Proceedings. Status: ... more ... Full text not currently available from Enlighten. Item Type: Conference Proceedings. Status: Published. Refereed: Yes. Glasgow Author(s): Gallipoli, Dr Domenico. Authors: Toll, DG, Mendes, J., Augarde, CE , Karthikeyan, M., Phoon, KK, Gallipoli, D. , and Lin, KQ. ...
An alternative technique for the determination of the soil water retention curve has recently bee... more An alternative technique for the determination of the soil water retention curve has recently been proposed whereby a tensiometer is used to measure soil suction and a balance to record the water content variations. The soil water retention curve is obtained by drying the soil either continuously or by stages (i.e. each drying stage is followed by an equalization period). Initial results from tests on compacted soil suggest that the relatively fast evaporation rate during continuous drying affects the water retention curve whereas the stage drying procedure provides more accurate results. Factors such as sample geometry and tensiometer position (relative to the sample) are also likely to affect the response obtained during continuous drying. These are the object of future investigation.
Quarterly Journal of Engineering Geology and Hydrogeology, 2011
Abstract Rainfall is the most frequent triggering factor for landslides and the development of ea... more 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- ...
Proceedings of the Institution of Civil Engineers - Engineering Sustainability, 2009
A unique facility for engineering and biological research has been established with the aim of im... more A unique facility for engineering and biological research has been established with the aim of improving fundamental understanding of the effects of climate change on slopes. This paper describes the building and monitoring of a full-scale embankment representative of UK infrastructure, the planting and monitoring of representative vegetation, and the construction of a system of sprinklers and covers to control climate. A summary of the results of the first experiments simulating predicted future UK climate and the response of the embankment is also presented. The information that has begun to be gathered is providing data related to the failure modes anticipated as a result of climate change and hence on the sustainability of UK infrastructure slopes.
The paper presents a benchmarking study carried out within the ‘Mechanics of Unsaturated Soils fo... more The paper presents a benchmarking study carried out within the ‘Mechanics of Unsaturated Soils for Engineering' (MUSE) network aimed at comparing different techniques for measurement and control of suction. Techniques tested by the eight ‘Mechanics of Unsaturated Soils for Engineering' 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 uniform 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 specimens were investigated along the main drying path. Specimens were de-saturated by applying suction through the liquid phase when using an ...
Loose particles in moist granular materials, such as unsaturated soils, are held together by capi... more Loose particles in moist granular materials, such as unsaturated soils, are held together by capillary forces acting at the interparticle contacts. The magnitude of these capillary forces depends on the surface tension and on the radius of curvature of the menisci, which in turn depends on the contact angle of the air/water interface against the surface of the solid particles. Menisci are usually assumed to be predominantly concave on the side of the air with negative water pressure (relative to the atmosphere). Evidence for this comes from direct observations at the millimetre scale or from theoretical assumptions. This note presents data from environmental scanning electron microscopy of particles at the micrometre scale that contradict this assumption and show for the first time that, for a given water content, the contact angle between air/water interfaces and grains can give rise to a variety of meniscus shapes, with curvatures not all concave on the side of air. It was found t...
Rammed earth is a manufactured material comprising sand, gravel and clay, which is compacted betw... more Rammed earth is a manufactured material comprising sand, gravel and clay, which is compacted between forms to build walls. Primarily a historic method of construction, it is now receiving considerable interest worldwide owing to its zero reliance on materials such as cement, and its potential for recycling. Despite its longevity, the source of its shear strength is poorly understood. This paper presents initial laboratory test results that point to the main source of strength in rammed earth being suction, and indicating that recent advances in unsatu-rated soil mechanics may also be applied to this material.
High Capacity Tensiometers, or HCTs, are sensors that can measure negative pore water pressures (... more High Capacity Tensiometers, or HCTs, are sensors that can measure negative pore water pressures (soil suctions) between-1.5 and-2 MPa. To achieve such measured values, HCTs first need to be fully saturated by water. For the first saturation of an initially dry HCT, the most common procedure involves application of vacuum followed by forced flooding by pressurised water. Instead, for the resaturation of a HCT that has cavitated (and is therefore still water flooded), only application of water pressure is necessary. Typically, the procedures for the first saturation and resaturation of HCTs can last days or weeks, which hinders adoption of these devices by the geotechnical industry. In this paper, faster procedures are presented for both first saturation and resaturation of HCTs built with 1.5 MPa air entry value filters. The duration of the first saturation can be reduced to less than 24 hours if high vacuum is first applied to the ceramic filter followed by water pressurisation at about twice the air entry value of the filter. Even more, resaturation of a cavitated HCT can be achieved in less than 10 minutes by simple water pressurisation of the ceramic filter. This is however true only if the HCT is not left to dry out to the atmosphere completely and is submerged in water just after cavitation.
A new suction probe, the WF‐DU tensiometer, has been developed by Wykeham Farrance in collaborati... more 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 ...
Tensiometers are increasingly used in geotechnical engineering to monitor pore-water tension in t... more 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 be...
Geological Society, London, Engineering Geology Special Publications, 2012
Climate change has the potential to have significant effects on the stability and serviceability ... more Climate change has the potential to have significant effects on the stability and serviceability of earthworks slopes, impacting on the performance of transport infrastructures. This paper describes how a unique facility for engineering and biological research was established in NE England through the BIONICS Project (Biological and Engineering Impacts of Climate Change on Slopes). It describes the building and monitoring of a full-scale embankment representative of road/rail embankments in the UK. The paper presents the results of monitoring of pore water pressure carried out between 2007 and 2009. The pore water pressures within the upper 3 m of the embankment have been largely positive, in some case approaching hydrostatic conditions after wetter periods. This is largely due to the wet nature of the summers in both 2007 and 2008 since monitoring began, as well as the application of artificial inundation using a climate control system. Negative pore water pressures (suctions) of t...
The mechanical behaviour of a sandy clay soil has been investigated by a series of constant water... more The mechanical behaviour of a sandy clay soil has been investigated by a series of constant water content triaxial tests on unsaturated samples with suction measurements. The tests were carried out in double cell triaxial cells on compacted samples, and also on samples wetted and dried from the as-compacted conditions. A series of tests on saturated samples was also performed to provide a reference state for the unsaturated tests. Since specimens were at high degrees of saturation (generally greater than 80%), calculations based on effective stress showed a reasonable interpretation of the data for the Critical State parameters M and λ. However, the intercept of the Critical State line in the ν axis (Γ) differed, increasing with an increase in water content (at compaction). The data were also analysed using the Bishop stress (p*) approach and a better fitting was achieved in the stress plane where it was possible to define a unique Critical State line, where M was 0.91. However, in the ν-p* plane, different critical state lines were still obtained for different compaction water contents. The similarity in the critical state parameters M and λ show that subjecting the soil to wetting and drying paths had little influence on the soil at Critical State conditions, when the pore water pressure changes were taken into account. Compaction curve-unsieved Compaction curve-sieved * Each test is identified in the form of Sxx(yy) by the as-compacted water content (xx) and confining pressure applied during the test (yy).
This paper presents the results of a set of 22 triaxial tests on an unsaturated artificially ceme... more This paper presents the results of a set of 22 triaxial tests on an unsaturated artificially cemented sand. The results are used to explore the applicability of a number of unsaturated soil frameworks for interpreting the shear strength. Constant water content triaxial tests were carried out on unsaturated specimens, using the axis translation technique to measure suctions during shearing. The test results on the unsaturated material were referenced against a series of drained and undrained triaxial tests that were carried out on saturated specimens. The results of the unsaturated tests were analysed to investigate the effect of the suction and degree of saturation on the shear strength at the critical state. The results show that the critical state stress ratio in terms of net stress (Ma) was found to be larger than the saturated critical state stress ratio (Ms). It was also found that the stress ratio in terms of suction (Mb) reduced as suction increased (when the degree of satura...
Unsaturated Soils. Advances in Geo-Engineering, 2008
ABSTRACT: Thispaper reportsona set of triaxial test dataonanartificially bondedsand tested inunsa... more ABSTRACT: Thispaper reportsona set of triaxial test dataonanartificially bondedsand tested inunsaturated conditions. Tests were performed using the axis translation technique to measure suctions while shearing under constant water content conditions. The results at the Critical State are interpreted to obtain the variation in the stress ratios: Ma representing the net stress component and Mb representing the suction component. These are also presented as Critical State angles of friction,(φa) c and (φb) c. It is found that Ma is higher ...
Proceedings of the International Conference held in Nottingham, UK, 25-27 August 2008, 2008
... Full text not currently available from Enlighten. Item Type: Conference Proceedings. Status: ... more ... Full text not currently available from Enlighten. Item Type: Conference Proceedings. Status: Published. Refereed: Yes. Glasgow Author(s): Gallipoli, Dr Domenico. Authors: Toll, DG, Mendes, J., Augarde, CE , Karthikeyan, M., Phoon, KK, Gallipoli, D. , and Lin, KQ. ...
An alternative technique for the determination of the soil water retention curve has recently bee... more An alternative technique for the determination of the soil water retention curve has recently been proposed whereby a tensiometer is used to measure soil suction and a balance to record the water content variations. The soil water retention curve is obtained by drying the soil either continuously or by stages (i.e. each drying stage is followed by an equalization period). Initial results from tests on compacted soil suggest that the relatively fast evaporation rate during continuous drying affects the water retention curve whereas the stage drying procedure provides more accurate results. Factors such as sample geometry and tensiometer position (relative to the sample) are also likely to affect the response obtained during continuous drying. These are the object of future investigation.
Quarterly Journal of Engineering Geology and Hydrogeology, 2011
Abstract Rainfall is the most frequent triggering factor for landslides and the development of ea... more 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- ...
Proceedings of the Institution of Civil Engineers - Engineering Sustainability, 2009
A unique facility for engineering and biological research has been established with the aim of im... more A unique facility for engineering and biological research has been established with the aim of improving fundamental understanding of the effects of climate change on slopes. This paper describes the building and monitoring of a full-scale embankment representative of UK infrastructure, the planting and monitoring of representative vegetation, and the construction of a system of sprinklers and covers to control climate. A summary of the results of the first experiments simulating predicted future UK climate and the response of the embankment is also presented. The information that has begun to be gathered is providing data related to the failure modes anticipated as a result of climate change and hence on the sustainability of UK infrastructure slopes.
The paper presents a benchmarking study carried out within the ‘Mechanics of Unsaturated Soils fo... more The paper presents a benchmarking study carried out within the ‘Mechanics of Unsaturated Soils for Engineering' (MUSE) network aimed at comparing different techniques for measurement and control of suction. Techniques tested by the eight ‘Mechanics of Unsaturated Soils for Engineering' 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 uniform 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 specimens were investigated along the main drying path. Specimens were de-saturated by applying suction through the liquid phase when using an ...
Loose particles in moist granular materials, such as unsaturated soils, are held together by capi... more Loose particles in moist granular materials, such as unsaturated soils, are held together by capillary forces acting at the interparticle contacts. The magnitude of these capillary forces depends on the surface tension and on the radius of curvature of the menisci, which in turn depends on the contact angle of the air/water interface against the surface of the solid particles. Menisci are usually assumed to be predominantly concave on the side of the air with negative water pressure (relative to the atmosphere). Evidence for this comes from direct observations at the millimetre scale or from theoretical assumptions. This note presents data from environmental scanning electron microscopy of particles at the micrometre scale that contradict this assumption and show for the first time that, for a given water content, the contact angle between air/water interfaces and grains can give rise to a variety of meniscus shapes, with curvatures not all concave on the side of air. It was found t...
Rammed earth is a manufactured material comprising sand, gravel and clay, which is compacted betw... more Rammed earth is a manufactured material comprising sand, gravel and clay, which is compacted between forms to build walls. Primarily a historic method of construction, it is now receiving considerable interest worldwide owing to its zero reliance on materials such as cement, and its potential for recycling. Despite its longevity, the source of its shear strength is poorly understood. This paper presents initial laboratory test results that point to the main source of strength in rammed earth being suction, and indicating that recent advances in unsatu-rated soil mechanics may also be applied to this material.
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Papers by David Toll