Unsaturated Soil (Engineering) Research Papers (original) (raw)
The Intergovernmental Panel on Climate Change (IPCC) provides convincing evidence of global warming as a result of increased greenhouse gas production. There has been a greater occurrence of extreme climate events in recent decades. We... more
The Intergovernmental Panel on Climate Change (IPCC) provides convincing evidence of global warming as a result of increased greenhouse gas production. There has been a greater occurrence of extreme climate events in recent decades. We need to ensure that our buildings and infrastructure can cope with such events and possibly more extreme events in the future. A good grounding in unsaturated soil mechanics will be necessary to understand future changes involving the drying and desiccation of soils that will occur in dry seasons and the wetting and infiltration processes that prevail during wet seasons. To predict the impacts of climate change will require the use of robust numerical modelling of climate/soil interactions that can be used to model the effects of future climate regimes. To achieve this we need high quality field observations involving climate/soil interaction that can be used to validate the models. This paper reports on a study in the UK to acquire such data.
Spatial variability of material properties is inherent in both natural soil deposits and earth structures, yet it is often ignored during geotechnical design. With the objective of developing novel methods for assessing the effects of... more
Spatial variability of material properties is inherent in both natural soil deposits and earth structures, yet it is often ignored during geotechnical design. With the objective of developing novel methods for assessing the effects of soil variability on groundwater flow, this study presents a stochastic finite element model of seepage through a flood defense embankment with randomly heterogeneous material properties. Stochastic modeling is undertaken by means of a Monte Carlo simulation which involves a large number of finite element analyses, each with randomly varied porosity at element level, which leads to a corresponding random variation of both permeability and water retention properties across the embankment domain. This provides a statistical distribution of responses, such as total flow rate and time to reach steady state, instead of a single deterministic result as in conventional studies of seepage through unsaturated heterogeneous soils. As the degree of heterogeneity increases, water tends to flow along the most permeable paths inside the soil mass, resulting in an irregular shape of the predicted wetting fronts and pore pressure contours. The mean and standard deviation of the computed quantities strongly depend on the statistics of the input porosity field. Simulations are also conducted to compare the statistical variation of flow rate with and without dependency of the water retention curve on porosity. With recent growth in computer speed, stochastic finite element models based on the Monte Carlo approach can become a powerful design tool, especially if a quantitative assessment of geotechnical risks is required. Copyright © 2011 John Wiley & Sons, Ltd.
The existing soils in the nature that is used for construction cannot necessarily bear the loadings on the structure. For example, in granular soils, the natural soil may be very loose and show a lot of elastic settlement. Sometimes,... more
The existing soils in the nature that is used for construction cannot necessarily bear the loadings on the structure. For example, in granular soils, the natural soil may be very loose and show a lot of elastic settlement. Sometimes, there are soft layers, saturated clay and swelling soils at the lower depths, which may cause significant settlement in the structure in terms of foundation load and clay layer thickness. To avoid such settlements, it is necessary to use certain techniques to improve
the soil condition. One of the methods that have recently been widely used to reduce the settlement of soft soils and swelling soils is stone columns or single piles. In this research, first of all, the parameters in need for the analysis will be gained by using the experimental data, and then, the static and dynamic behavior of the confined stone columns is examined with geotextile and without geotextile by a group and single manner as in two-dimensional form using Plaxis numerical method of the finite element and the impact of the following parameters will be investigated in both static and dynamic modes: Column length, column diameter, single and group behavior of columns, and soil cohesion effect on the behavior of the confined stone column in geotextile and reduction of soil settlement during use of stone columns. The results of this research indicate correct understanding of the use of geotextile (Woven Geotextile with a specific elastic normal strength) to prevent the camber and the settlement of the column and increase of the strength and bearing capacity of the column.
... Lourenco, SDN, Gallipoli, D. , Toll, DG, and Evans, F. (2006) Use of Tensiometers for the Measurement of Soil Suction. In: 10th Confresso Nacional de Geotecnia, 22 - 26 May 2006, Lisboa, Portugal. ... Authors: Lourenco, SDN,... more
... Lourenco, SDN, Gallipoli, D. , Toll, DG, and Evans, F. (2006) Use of Tensiometers for the Measurement of Soil Suction. In: 10th Confresso Nacional de Geotecnia, 22 - 26 May 2006, Lisboa, Portugal. ... Authors: Lourenco, SDN, Gallipoli, D. , Toll, DG, and Evans, F. ...
Composite geotextiles with polyester yarn reinforcement have been commonly used in combination with unsaturated soils. Both unsaturated and saturated shear strength of the interfaces were investigated between a composite geotextile and... more
Composite geotextiles with polyester yarn reinforcement have been commonly used in combination with unsaturated soils. Both unsaturated and saturated shear strength of the interfaces were investigated between a composite geotextile and three major types of materials: silty sand (SM), low-plasticity silt (ML) and highplasticity clay (CH) in a direct shear box. The interfaces were formed using two methods (A and B) to reflect the wide range of possible contact conditions in practice. Method A involved statically compacting the soil directly on top of the composite geotextile, while for Method B, the soil was statically compacted in a separate mold and later brought into contact with the composite geotextile. Type B interfaces required a larger displacement to mobilize the shear strength than Type A interfaces. The ultimate failure envelopes of SM and ML soils were similar to those oftheir interfaceshearing. Notably, the failureenvelopes for the clay-geotextile interfaceof both types were higher than that of clay alone. The unsaturated soil-only shearing had a higher peak strength and tended to dilate more than saturated soil-only shearing, while unsaturated soil-interface shearing appeared to be more contractant than saturated interface shearing. The strength variations with suction for all tested soils and interface shearing were clearly non-linear. A new model that takes account of the condition of soil-geotextile contact intimacy is proposed for predicting the variation of interface strength with suction, based on the variation of the soil's apparent cohesion with suction and the geotextile-water retention curve.
The paper gives a brief description of double wall cell triaxial equipment for testing soil samples under unsaturated conditions and presents results of some of the calibrations necessary to achieve accurate measurements of inner cell... more
The paper gives a brief description of double wall cell triaxial equipment for testing soil samples under unsaturated conditions and presents results of some of the calibrations necessary to achieve accurate measurements of inner cell water volume, pore water volume, pressure, load and displacement. The calibration of the measurement devices showed linear relationships between the raw readings and applied values with the regression constants differ from 1 and 0. The result of apparent volume change calibrations showed significant correlation between cell water volume fluctuation and temperature fluctuation inside testing room. A correction to the measured volume was therefore required to reduce the effects of long-term temperature fluctuation on the measured volume. Investigation of the accuracy of volume change measurement with the double wall cell showed excellent matching between the inner cell volume change and pore water volume change under saturated conditions, indicating the high reliability of the double wall system for apparent volume change measurements.
The shear strength of an unsaturated soil is controlled by two variables; net stress and suction. The conventional triaxial machine which is used for obtaining the shear strength of saturated soils cannot be used for unsaturated soil... more
The shear strength of an unsaturated soil is controlled by two variables; net stress and suction. The conventional triaxial machine which is used for obtaining the shear strength of saturated soils cannot be used for unsaturated soil conditions due to weakness in suction control during the consolidation and shearing stages. In the view of this, the standard triaxial apparatus needs to modify and develop for unsaturated conditions. The main aim of this paper is devoted to give a description of current methods and techniques using triaxial apparatus for obtaining data to interpret the accurate shear strength behavior of unsaturated soils. In additional some of the key aspects related to air diffusion and volume change measurement are highlighted.
Mounding often occurs beneath engineering structures designed to infiltrate reuse water. AQTESOLV software and a spreadsheet solution for Hantush, together with soil moisture water balance (SWAGMAN farm model), were used for... more
Mounding often occurs beneath engineering structures designed to infiltrate reuse water. AQTESOLV software and a spreadsheet solution for Hantush, together with soil moisture water balance (SWAGMAN farm model), were used for quantitatively predicting the height and extent of groundwater mounding underground to assess the groundwater-flow simulations of infiltration from a hypothetical irrigation site. Horizontal and vertical permeability, aquifer thickness, specific yield, and basin geometry are among the aquifer and recharge properties inputs. For 2.2 ha sites, the maximum heights of the simulated groundwater mound ranges up to 0.29 m. The maximum areal extent of groundwater mounding measured from the edge of the infiltration basins of 0.24 m ranges from 0 to 75 m. Additionally, the simulated height and extent of the groundwater mounding associated with a hypothetical irrigation infiltration basin for 2.2 ha development may be applicable to sites of different sizes, using the recharge rate estimated from the SWAGMAN farm model. For example, for a 2.2 ha site with a 0.0002 m/day recharge rate, the irrigation infiltration basin design capacity (and associated groundwater mound) would be the same as for a 1.1 ha site with a 0.0004 m/day recharge rate if the physical characteristics of the aquifer are unchanged. The study claimed that the present modelling approach overcomes the complications of solving the Hantush equation for transient flow. The approach utilised in this study can be applied for other purposes such as measuring the feasibility of infiltrating water, attenuation zone, risk mitigation essential for decision-makers and planning regulators in terms of environmental effects and water use efficiency.
The Wady system is an extreme case of a temporary inundated ecosystem in which the duration of flooding is shorter than the dry period (Evenary, 1985). Floods in Saharan Wady systems are scarce; their frequency may be spaced from once... more
The Wady system is an extreme case of a temporary inundated ecosystem in which the duration of flooding is shorter than the dry period (Evenary, 1985). Floods in Saharan Wady systems are scarce; their frequency may be spaced from once every few months to once every decade (Dubief, 1953). A flashflood is a disturbance—a relatively discrete and unpredictable event in time (Resh et al., 1988)—which is capable of disrupting the ecosystem (Pickett & White, 1985), limiting the plant biomass on account of the partial or complete destruction it causes (Grime, 1979).
Such disturbance is followed by successive stages of regeneration (Van der Maarel, 1988). The actual state of the vegetation depends on flood frequency and magnitude (Springel et al., 1990; Springel & Sheded, 1991).
The Eastern Desert lies between latitude 22 and 29 N covering about 22% of the
Egyptian territory and bounded by the Red Sea and Gulf of Suez on the east and the Nile Wady on the west. The Eastern Desert has attracted numerous investments in the last few decades, especially for the tourism and mining ventures. The present water supplies in the Eastern Desert are insufficient, however, to meet the expected increase in water demands for civic, industrial, mining and tourism uses.
The Eastern Desert of Egypt contains a huge diversity of ecosystems. The inland
part of the Eastern Desert lies between the Red Sea coastal mountains in the east
and the Nile Wady in the west, an area of about 223,000km (Abdel Moneim, 2005).
It is a rocky plateau dissected by a number of Wadys. Each Wady has a channel
with numerous tributaries and the Eastern Desert is divided piecemeal into the
catchments areas of these drainage systems. Most of the Wadys drain westward into the Nile.
Ground water is believed to be the main source of water throughout the eastern
desert area in Egypt. Any development projects for this location should be preceded by a pre-assessment of the available quantity of water in the groundwater reservoir,
the groundwater quality and whether this groundwater is renewable or not and how much is the rate of the groundwater replenishment.
The main objectives of this study are to identify the boundaries of Wady Degla
basin within the Eastern Desert area and to estimate the expected surface flow
throughout the Wady system corresponding to different return period. The study also will try to estimate the effective groundwater recharge and it will describe the temporal variation of the Wady plantation and its relation with the precipitation over the Wady.
Another objective is to develop estimates of the different water-balance terms in the given Wady system and to characterize how much of the precipitation leaves the watershed as evaporation or as in the form of other components of stream flow.
Other objectives include the determination of the hot runoff points in the Wady, the places that need to be protected against the probable destructive runoff events and the proposed places of check dams (if any) for any future development.
The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code “Compass” for coupled flow-deformation analysis involving unsaturated soils and application of... more
The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code “Compass” for coupled flow-deformation analysis involving unsaturated soils and application of the improved code to the simulation of pressuremeter tests in unsaturated soils.
On the constitutive side, a unique relationship is proposed between degree of saturation, suction and specific volume by introducing dependency on specific volume in the simplified van Genuchten equation. This is a significant improvement over the common assumption of a state surface expression for degree of saturation. If combined with an elasto-plastic stress-strain model predicting the variation of specific volume, the proposed relationship is capable of reproducing irreversible changes of degree of saturation and changes of degree of saturation experimentally observed during shearing. Predictions show very good agreement with experimental results from tests on compacted Speswhite Kaolin published in the literature.
On the numerical side, a number of changes to the code “Compass” have been performed. The new relationship for degree of saturation is implemented in the code and the implementation is validated against three benchmark problems. Use of the new relationship for degree of saturation results in significantly different predictions to those obtained if a conventional state surface expression for degree of saturation is used (as present in the original code).
Implementation of the water and air continuity equations in “Compass” has been corrected by expressing these equations in terms of flux velocities relative to the soil skeleton. This is the form in which the equations should be expressed if they are to be combined with Darcy’s law for liquid and gas flows. The simulation of a notional laboratory test shows that the incorrect combination of Darcy’s law with absolute flux velocities, as present in the original code, causes significant errors.
The convergency algorithm at constitutive level employed in the code has been corrected by introducing residual flux terms in the two flow equations, analogous to residual forces in the equilibrium equation. These terms must be taken into account if a convergency algorithm for an elasto-plastic stress-strain model is used and the relationship assumed for variation of degree of saturation involves any dependency on net stresses. A numerical study of a notional laboratory test shows that omission of residual flux terms results in substantial errors and may cause failure to converge.
The plane_strain formulation of code “Compass” has been corrected by imposing the condition of nullity only on the out-of-plane component of the total strain rate vector instead of the out-of-plane component of each single contribution to the total strain rate, as was done in the original code. Such inconsistency, due to the history of development of finite element programs, also appears in other examples published in the literature. Numerical simulations of two types of bi-axial tests show that significantly different results are generally predicted by the correct and incorrect formulations, and also provide an explanation why this type of error was difficult to detect in codes implementing traditional models for saturated soils.
The potential of the enhanced version of code “Compass” for analysing boundary value problems is demonstrated by simulations of pressuremeter tests in unsaturated soil. This study also provides some initial insight into the interpretation of pressuremeter tests in unsaturated soil by simulating tests at different loading rates in a normally consolidated soil. The mechanical behaviour of the soil is represented by the elasto-plastic Barcelona Basic Model of Alonso, Gens and Josa (1990) while the variation of degree of saturation is modelled by the new relationship proposed in the thesis. The entire range of loading rates, from undrained to fully drained (with respect to liquid), is simulated. Relatively small changes of suction are predicted even in the fastest test and the computed cavity pressure-cavity strain relationships are all very similar regardless of loading rate. It may therefore be possible to model even rapid pressuremeter tests in unsaturated soils as a drained (constant suction) process. Further work is required to investigate the generality of this conclusion.
The paper presents a bounding surface model that describes the gradual yielding of unsaturated soils subjected to isotropic loads. The model originates from consideration of the capillary bonding between soil grains, which leads to the... more
The paper presents a bounding surface model that describes the gradual yielding of unsaturated soils subjected to isotropic loads. The model originates from consideration of the capillary bonding between soil grains, which leads to the definition of a “unified normal compression line” that is valid in both saturated and unsaturated conditions. This line has the same slope and intercept of the saturated normal compression line but is formulated in terms of a “scaled stress” variable, which takes into account the mechanical effect of capillarity by factoring the average skeleton stress (also known as Bishop’s stress) with a power function of degree of saturation. The normal compression behaviour of unsaturated soils is therefore described by only one additional parameter, which is the exponent of the degree of saturation in the scaled stress expression. For over-consolidated soils, the occurrence of gradual yielding is introduced by assuming that, as the soil state moves towards the unified normal compression line, the slope of the loading curve tends towards the slope of the unified normal compression line according to an expression requiring only one extra parameter. Interestingly, this expression can be integrated in a closed form to provide a general equation for all loading paths in saturated and unsaturated conditions. Different loading curves are simply distinguished by the different values of the integration constants. Unloading paths are also simulated in a similar way. The proposed model requires a total of five parameters, which include the three standard parameters for saturated soils (i.e. the slope and intercept of the saturated normal compression line and the slope of the swelling line) plus one parameter to describe unsaturated behaviour and one parameter to describe the gradual yielding of over-consolidated soils subjected to loading.
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... more
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.
The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code "Compass" for coupled flow-deformation analysis involving unsaturated soils and... more
The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code "Compass" for coupled flow-deformation analysis involving unsaturated soils and application of the improved code to the simulation of pressuremeter tests in unsaturated soils. On the constitutive side, a unique relationship is proposed between degree of saturation, suction and specific volume, by introducing dependency on specific volume in the simplified van Genuchten [48] equation. This is a significant improvement over the common assumption of a state surface expression for degree of saturation. If combined with an elasto-plastic stress-strain model predicting the variation of specific volume, the proposed relationship is capable of reproducing irreversible changes of degree of saturation and changes of degree of saturation experimentally observed during shearing. Predictions show very good agreement with experimental results from...
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... 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 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.
The Discussers read the article by Avunduk et al. (2021) (the Authors), with special interest focused on the fall-cone (FC) penetrometer and torvane shear testing reported for the investigated fine-grained soil–anticlay foaming agent... more
The Discussers read the article by Avunduk et al. (2021) (the Authors), with special interest focused on the fall-cone (FC) penetrometer and torvane shear testing reported for the investigated fine-grained soil–anticlay foaming agent mixtures. These mixtures were prepared at the soil natural water content wn, employing the Ayvali 2, M.-Mkay and Kaolinite soil materials, with constant foam concentration and foam expansion ratios of CF = 3% and FER = 16, respectively. The Discussers would like to make some comments on the data analyses and interpretations by the Authors for the above-mentioned tests, especially regarding the FC data plotted for reducing saturation level (increasing foam injection ratio (FIR)) in their Figures 7, 10b, 11 and 12. But the Discussers would like to begin with some fundamentals of FC penetration testing for shear strength determinations of saturated fine-grained soil samples. Then, from reanalysis of digitized vane shear and FC data reported by the Authors for their conditioned soil samples, the Discussers glean some insightful observations on the variations of the cone factor K for reducing levels of saturation (increasing FIR); an aspect on which there is presently little published research. As such, this discussion article represents an important addition to the research literature.
A lecture given at the Civil and Environmental Engineering Department of the University of Cyprus in Nicosia on 21/1/2015
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... more
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.
The paper presents the analytical solution for the steady-state infiltration from a buried point source into two types of heterogeneous cross-anisotropic unsaturated half-spaces. In the first case, the heterogeneity of the soil is... more
The paper presents the analytical solution for the steady-state infiltration from a buried point source into two types of heterogeneous cross-anisotropic unsaturated half-spaces. In the first case, the heterogeneity of the soil is modelled by an exponential relationship between the hydraulic conductivity and the soil depth. In the second case, the heterogeneous soil is represented by a multilayered half-space where each layer is homogeneous. The hydraulic conductivity varies exponentially with moisture potential and this leads to the linearization of the Richards equation governing unsaturated flow. The analytical solution is obtained by using the Hankel integral transform. For the multilayered case, the combination of a special forward and backward transfer matrix techniques makes the numerical evaluation of the solution very accurate and efficient. The correctness of both formulations is validated by comparison with alternative solutions for two different cases. The results from typical cases are presented to illustrate the influence on the flow field of the cross-anisotropic hydraulic conductivity, the soil heterogeneity and the depth of the source. Copyright © 2004 John Wiley & Sons, Ltd.
An analysis of results from published laboratory tests on Jossigny silt and Barcelona clayey silt is presented to confirm the existence of a unique capillary bonding function linking the quotient between unsaturated and saturated void... more
An analysis of results from published laboratory tests on Jossigny silt and Barcelona clayey silt is presented to confirm the existence of a unique capillary bonding function linking the quotient between unsaturated and saturated void ratio, at the same mean average skeleton stress, to a single capillary bonding scalar variable. The analysis confirms that the same capillary bonding function applies to both normally consolidated and critical stress states.The above two experimental sets with the addition of further published data for Speswhite Kaolin are also used to study the relationship between unsaturated critical shear strength, mean average skeleton stress and capillary bonding variable. The results of such analysis are assessed in the light of a similar modelling framework proposed in the literature.
This paper presents a methodology for the identification of parameter values in the Barcelona Basic Model (BBM) by inverse analysis of the experimental cavity pressure–cavity strain curve from pressuremeter tests in unsaturated soils.... more
This paper presents a methodology for the identification of parameter values in the Barcelona Basic Model (BBM) by inverse analysis of the experimental cavity pressure–cavity strain curve from pressuremeter tests in unsaturated soils. This methodology involves a high-dimensional optimization process which is particularly challenging due to the existence of a large number of local minima caused by the nonlinearity of the BBM. A novel parallel modified particle swarm optimization algorithm is utilized to minimize the difference between measured and computed values on the cavity pressure–cavity strain curve. The computed cavity pressure–cavity strain curve is obtained by using a finite element model of an unsaturated soil whose mechanical behaviour is described by the BBM. An example is presented to validate the proposed methodology making use of artificial experimental results that had been calculated by a finite element simulation of pressuremeter tests. Finally, the application to a real case is presented by showing that the proposed methodology can safely identify the values of at least six BBM parameters via inverse analysis of pressuremeter tests at different suction levels.
The paper presents a collaborative piece of research undertaken by seven research teams from different universities within the ‘Mechanics of Unsaturated Soils for Engineering’ (MUSE) network. The objective is to benchmark different... more
The paper presents a collaborative piece of research undertaken by seven research teams from different universities within the ‘Mechanics of Unsaturated Soils for Engineering’ (MUSE) network. The objective is to benchmark different approaches to constitutive modelling of mechanical and water retention behaviour of unsaturated soils by comparing simulations of suction-controlled and constant water content laboratory tests. A set of 13 triaxial and oedometer laboratory tests, covering the mechanical and water retention behaviour of an unsaturated compacted silty soil under a variety of stress paths, has been provided by one of the seven participating teams. This data set has been used by the other six teams for calibrating a constitutive model of their choice, which has been subsequently employed for predicting strains and degree of saturation in three of the 13 tests used for calibration, as well as in one ‘blind’ test for which experimental results had not been previously disclosed. By comparing predictions from all teams among themselves and against experimental data, the work highlights the capability of some of the current modelling approaches to reproduce specific features of the mechanical and water retention behaviour of unsaturated soils helping to identify potential areas of weakness where future research should focus. It also demonstrates the dispersion of results to be expected when different constitutive models, independently calibrated by different teams of researchers, are used to predict soil behaviour along the same stress path.
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... 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 unsaturated soil mechanics may also be applied to this material.
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... more
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.
Experiments were conducted to investigate the odometric swell behavior of expansive soil specimens mixed independently with two different granular additives: silica sand and granulated tire rubber (GTR). All specimens were prepared with... more
Experiments were conducted to investigate the odometric swell behavior of expansive soil specimens mixed independently with two different granular additives: silica sand and granulated tire rubber (GTR). All specimens were prepared with the same global water content. However, specimens that contained sand swelled less than did analogous specimens that contained GTR, regardless of the surcharge stress imposed before inundation. Phase relationship analyses based on " oversize correction " equations derived for mixtures with three solid phases indicate that the specimens possessed clay portions with different dry density and initial water content. Such differences are attributed mainly to the role of additive grain stiffness during specimen preparation. Results from complementary discrete element method simulations indicate that the stiffness of the additive grains may also play a role in mitigating the swell via contact force alterations upon swell.
This paper presents a new method for selecting parameter values in elasto-plastic models for unsaturated soils under isotropic stress states. The proposed method is an improvement over more traditional calibration approaches as it... more
This paper presents a new method for selecting parameter values in elasto-plastic models for unsaturated soils under isotropic stress states. The proposed method is an improvement over more traditional calibration approaches as it explicitly takes into account experimental yielding behaviour when defining the unsaturated normal compression surface. This is achieved by “enriching” measured yield stresses with corresponding values of specific volume (estimated from the observed elastic and virgin responses), which generates experimental points that can be interpolated together with virgin compression data. The proposed methodology is general and independent of the chosen mathematical forms of normal compression surface, elastic law and constitutive variables. The validity of the method is demonstrated by calibrating a reference model from a set of constant suction isotropic compression tests. Both improved and traditional calibration approaches are employed to select model parameter values which are then used to predict yield curves in the mean net stress–suction plane. Because of the incorporation of yielding data in the interpolation of the normal compression surface, the improved calibration method produces a more accurate prediction of yield stresses at different suctions compared with the standard method. Due to its generality and limited subjectivity, the proposed method provides an unbiased tool to compare strengths and weaknesses of different constitutive models.
The popularity of the Barcelona Basic Model (BBM) has grown steadily since publication in 1990 due to its ability of capturing key aspects of unsaturated soil behaviour. Nevertheless, BBM is still rarely employed by practitioners partly... more
The popularity of the Barcelona Basic Model (BBM) has grown steadily since publication in 1990 due to its ability of capturing key aspects of unsaturated soil behaviour. Nevertheless, BBM is still rarely employed by practitioners partly because of the absence of simple and objective methods for selecting parameter values from laboratory tests. One difficulty is that, in BBM, individual aspects of the isotropic virgin behaviour are controlled by multiple parameters while, at the same time, a single parameter controls more than one aspect of soil behaviour. This has led to iterative procedures where parameter values are adjusted in turn to match experiments, which requires significant experience and can lead to the selection of widely different parameter values depending on the user. The proposed method streamlines parameter selection with a view to increasing the appeal of BBM for practitioners. The method adopts a “sequential” procedure where the five parameters governing isotropic virgin behaviour are matched to degrees of freedom in BBM, which are then fixed one at a time, in a specific order, without any assumption about other parameters. The simplicity and reduced subjectivity of the method in comparison with iterative procedures is demonstrated by selecting parameter values from laboratory tests.
Simulation-based optimization methods have been recently proposed for calibrating geotechnical models from laboratory and field tests. In these methods, geotechnical parameters are identified by matching model predictions to experimental... more
Simulation-based optimization methods have been recently proposed for calibrating geotechnical models from laboratory and field tests. In these methods, geotechnical parameters are identified by matching model predictions to experimental data, i.e. by minimizing an objective function that measures the difference between the two. Expensive computational models, such as finite difference or finite element models are often required to simulate laboratory or field geotechnical tests. In such cases, simulation-based optimization might prove demanding since every evaluation of the objective function requires a new model simulation until the optimum set of parameter values is achieved. This paper introduces a novel simulation-based “hybrid moving boundary particle swarm optimization” (hmPSO) algorithm that enables calibration of geotechnical models from laboratory or field data. The hmPSO has proven effective in searching for model parameter values and, unlike other optimization methods, does not require information about the gradient of the objective function. Serial and parallel implementations of hmPSO have been validated in this work against a number of benchmarks, including numerical tests, and a challenging geotechnical problem consisting of the calibration of a water infiltration model for unsaturated soils. The latter application demonstrates the potential of hmPSO for interpreting laboratory and field tests as well as a tool for general back-analysis of geotechnical case studies.
The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is... more
The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress, which includes the average fluid pressure acting on the soil pores, and an additional scalar constitutive variable, 'csi', related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. The formulation of the model in terms of variables closely related to specific behaviour mechanisms leads to a remarkable unification of experimental results of tests carried out with different suctions. The analysis of experimental isotropic compression data strongly suggests that the quotient between the void ratio, e, of an unsaturated soil and the void ratio es, corresponding to the saturated state at the same average soil skeleton stress, is a unique function of the bonding effect due to water menisci at the inter-particle contacts. The same result is obtained when examining critical states at different suctions. Based on these observations, an elastoplastic constitutive model is developed using a single yield surface the size of which is controlled by volumetric hardening. In spite of this simplicity, it is shown that the model reproduces correctly many important features of unsaturated soil behaviour. It is especially remarkable that, although only one yield surface is used in the formulation of the model, the irreversible behaviour in wetting–drying cycles is well captured. Because of the behaviour normalisation achieved by the model, the resulting constitutive law is economical in terms of the number of tests required for parameter determination.
This study was carried out to investigate the undrained behaviour of weakly bonded soil. Artificial specimens were used and were prepared from a mixture of sand and kaolin. The mixture was fired at 500°C for 5 h to create uniform strength... more
This study was carried out to investigate the undrained behaviour of weakly bonded soil. Artificial specimens were used and were prepared from a mixture of sand and kaolin. The mixture was fired at 500°C for 5 h to create uniform strength of artificial weakly bonding between sand particles. Based on microscopic observation of the specimens revealed that the fired kaolin formed bridges between the sand particles and most of the sand particles were entirely coated by kaolin. It was observed that very few sand grains made a direct grain to grain contact. The occurrences of inter-particle voids either isolated or connected to form larger voids. Fissures found in between the bonding and the grains were due to high temperature firing. A series of consolidated undrained tests (CU) were performed using conventional triaxial compression test. The stress ratio curves for the bonded specimens showed a significant peak at early strain and the values of maximum stress ratios dropped with increas...
In high suction tensiometers, cavitation occurs when the tension in the water inside the device exceeds a certain threshold. The threshold corresponds to the formation of air bubbles either by air entry into or formation within the porous... more
In high suction tensiometers, cavitation occurs when the tension in the water inside the device exceeds a certain threshold. The threshold corresponds to the formation of air bubbles either by air entry into or formation within the porous stone or reservoir. The standard procedure to inhibit air bubble formation in
the device is to flood under vacuum and apply water at high pressure to force any remaining air bubbles to dissolve. A series of factors are, however, likely to influence the cavitation behaviour of high suction tensiometers. This paper demonstrates that: (1) there are no precursory phenomena leading to cavitation, (2) higher pressures are not essential for saturation, (3) cooler temperatures and tensiometer usage decrease the pore water pressure at cavitation, and (4) high suction tensiometers record increasing pressures when plunged dry in free water. The findings provide useful guidance to researchers using these devices for suction measurement.
This study was carried out to investigate the undrained behaviour of weakly bonded soil. Artificial specimens were used and were prepared from a mixture of sand and kaolin. The mixture was fired at 500°C for 5 h to create uniform strength... more
This study was carried out to investigate the undrained behaviour of weakly bonded soil. Artificial specimens were used and were prepared from a mixture of sand and kaolin. The mixture was fired at 500°C for 5 h to create uniform strength of artificial weakly bonding between sand particles. Based on microscopic observation of the specimens revealed that the fired kaolin formed bridges between the sand particles and most of the sand particles were entirely coated by kaolin. It was observed that very few sand grains made a direct grain to grain contact. The occurrences of inter-particle voids either isolated or connected to form larger voids. Fissures found in between the bonding and the grains were due to high temperature firing. A series of consolidated undrained tests (CU) were performed using conventional triaxial compression test. The stress ratio curves for the bonded specimens showed a significant peak at early strain and the values of maximum stress ratios dropped with increasing applied confining stresses. For the destructured (or unbounded) specimens, no apparent peaks were observed and the stress values were slightly lower compared to that of bonded specimens. The bounding surface for the destructured specimens is represented by a straight line equivalent to stress ratio of 1.24. This line is equivalent to strength parameters of internal friction angle, φ’ and cohesion intercept, c’ of 31° and 21 kPa, respectively. Meanwhile, the bounding surface for the bonded specimens is indicated by some curvature at low stress (p′<150 kPa) and high stresses (p′>1100 kPa). The study clearly suggested the role of bonding on the engineering behaviour of residual soils in undrained conditions.