Changes In Degree Of Saturation Research Articles

Expected stiffness changes during compaction in laboratory and field

The use of stiffness indexes to assess soil compaction has the potential to revolutionise the earthworks industry. However, conventional Proctor based compaction methodology fails to adequately explain the relationship between soil modulus and density. To address this issue, this paper presents recent laboratory and field trial results and analysis demonstrating the importance of soil degree of saturation when evaluating the effectiveness of using stiffness indexes to monitor and assess compaction.During a recent compaction study of silty sand samples, it has been found that the small-strain shear modulus (G0) and dry density (ρd) can be simply related provided changes in degree of saturation (Sr) are monitored. Multi-staged, one-dimensional compaction tests have been conducted at varying moisture contents, and first arrival shear wave velocity measured via Bender Elements (BE). Results presented in this paper show that G0 increases with increasing ρd at a constant Sr rather than constant water content (w). Further analysis indicates that additional compaction past a critical Sr reduces the stiffness response G0 at a constant w, and that when compacting across the typical range of Sr of 40–70%, G0 appears to be largely independent of ρd and is instead indexed by moisture content.At a construction phase field compaction trial in Australia, field dry density was measured against Intelligent Compaction (IC) unit Compaction Meter Value (CMV). Two outcomes of the field trial are discussed. As in the laboratory G0 trial, CMV, a stiffness related parameter, and field dry density were found to be practically independent. Drawing upon the common results of these two trials, a practical framework is proposed involving a method-based specification with respect to the compacted state and stiffness indexes, one in which adequate compaction and strength could be achieved while reducing the requirement for regular nuclear densometer gauge testing.

Modeling seismic compression of unsaturated soils in the funicular regime

A semi-empirical elasto-plastic constitutive model with a hyperbolic stress-strain curve was developed with the goal of predicting the seismic compression of unsaturated sands in the funicular regime of the soil-water retention curve (SWRC) during undrained cyclic shearing. Using a flow rule derived from energy considerations, the evolution in plastic volumetric strain (seismic compression) was predicted from the plastic shear strains of the hysteretic hyperbolic stress-strain curve. The plastic volumetric strains are used to predict the changes in degree of saturation from phase relationships and changes in pore air pressure from Boyle’s and Henry’s laws. The degree of saturation was used to estimate changes in matric suction from the transient scanning paths of the SWRC. Changes in small-strain shear modulus estimated from changes in mean effective stress computed from the constant total stress and changes in pore air pressure, degree of saturation and matric suction, in turn affect the hyperbolic stress-strain curve’s shape and the evolution in plastic volumetric strain. The developments of the new mechanistic model developed in this study will play a key role in the future development of a holistic model for predicting the seismic compression across all regimes of the SWRC.

Study of the isothermal drying characteristics of normal concrete subjected to low air velocity convection

The drying of capillary moisture from structural concrete used in enclosed areas takes place by free convection characterized by low air velocity. The slow drying results in the persistence of an unsaturated state in concrete rendering it susceptible to deterioration. The study of critical process parameters, such as, mass loss flux, convective transfer coefficients, critical and equilibrium moisture contents and their dependence on the pertinent process variables is fundamental to design of concrete elements for durability. While these parameters have been widely researched for non-cementitious porous building materials, studies related to concrete are not comprehensive and far apart in literature. This study addresses the gap by investigating the drying characteristics of OPC-based normal concrete for varying levels of water-to-cement ratio, curing age and characteristic length of concrete specimens besides the dry bulb temperature and relative humidity of air. Interestingly, the constant rate mass flux is noted to depend on water-to-cement ratio thereby revealing the dependence of the convective transfer coefficients on surface characteristics. It is also observed that the Anomalous diffusion model satisfactorily describes the change in degree of saturation of a specimen with drying time and allows a reliable estimation of the critical degree of saturation. The latter, when plotted as a critical point curve, exhibits a non-uniform increase, as has been reported previously for a wide variety of other porous materials. The modified Oswin model is shown to provide a suitable description of the degree of saturation at equilibrium attained under steady-state temperature and relative humidity conditions.

Impact of non-linear elastic behavior on bentonite density evolution at the FEBEX

It is necessary to apply reliable numerical simulation techniques to assess the mechanical behavior of repositories for the long-term geological disposal of radioactive waste. Having a constitutive model that can describe the mechanical behavior of bentonite is key to such numerical simulations. In this study, the applicability of a linear-elastic constitutive model with a swelling term is examined, focusing on density heterogeneity of the in-situ full-scale engineered barrier experiment (FEBEX) in Grimsel. It was found that the constitutive model underestimates the density change. Therefore, the mechanical behavior was re-simulated with an unsaturated elasto-plastic constitutive model using the changes in degree of saturation as input data. Two-dimensional stress and dry density was reproduced.

Sorption, anomalous water transport and dynamic porosity in cement paste: A spatially localised 1H NMR relaxation study and a proposed mechanism

The link between anomalous water sorption and dynamic porosity in cement pastes is explored using spatially resolved GARField 1H nuclear magnetic resonance (NMR) relaxation analysis. A model is developed in which the effective capillary diffusion coefficient is dependent on the instantaneous pore size distribution. This and earlier data show changes in pore size distribution resultant from changes in saturation that do not occur instantaneously with changes in degree of saturation. Therefore, it is assumed that the pore size distribution is always relaxing exponentially towards a (saturation dependent) equilibrium. It follows that the diffusivity is sample history (i.e. time) dependent as well as saturation dependent. This is sufficient to explain anomalies in rapid capillary water sorption. The same concepts are applied to slow drying. In this case, porosity changes occur on a timescale much shorter than drying so the system is always in dynamic equilibrium and anomalies are therefore not seen.

Electric Conductivity Probes to Study Change in Degree of Saturation - Bench Top Laboratory Tests

Sand characteristics such as liquefaction susceptibility can be affected as a result of change in degree of saturation of sand. New liquefaction mitigation technique by inducing partial saturation in sands is introduced by Yegian et al in 2007[1]. This technique requires to monitor changes in degree of saturation of sand. By nature, changes in degree of saturation of sand can lead in changes in its electric conductivity. Electric conductivity is the property of a material that represents its ability to conduct electric current. Fully saturated sand can conduct electric current better than sand with lower degree of saturation. Therefore, the change in measured electric conductivity can be used to calculate the change in degree of saturation of sand. In 1942, Gus Archie [2] expressed that the electric conductivity of soil is a function of its porosity, degree of saturation, tortuosity and electric conductivity of pore fluid. Using Archie’s law electrical conductivity can be related to the degree of saturation in sands. Typically, electric conductivity probes and meters are instruments which are used to measure electric conductivity. Using electrical conductivity probes, sets of bench top tests were conducted on Ottawa sand to study the relation between degree of saturation and electric conductivity in sand. Partial saturation in sands were created by pouring dry sand into sodium percarbonate solution with a known initial concentration. By nature, sodium percarbonate in water, generates oxygen gas bubbles in time. The changes in electric conductivity in the specimen were measured using electric conductivity meters and probes. In addition, changes in degree of saturation of the specimen were measured using soil phase relations equations. Measured electric conductivity data and calculated degree of saturations were correlated to explore relation between electric conductivity and degree of saturation. This paper presents results of bench top tests, and suggests a relationship between, final degree of saturation of sand and initial concentration of sodium percarbonate solution

Impacts of Unsaturated Conditions on The Ultimate Axial Capacity of Energy Piles

This study uses concepts from unsaturated soil mechanics to explain changes in axial capacity observed in geotechnical centrifuge experiments on semi-floating energy piles in unsaturated silt heated monotonically to different temperatures. Thermally-induced drying of the unsaturated silt surrounding energy piles was observed during heating using temperature-corrected dielectric sensor readings. An effective stress-based equation for estimating the ultimate capacity was calibrated using the load-settlement curves for a pile at room-temperature, which was then used to estimate the ultimate capacities of energy piles under elevated temperatures using measured changes in degree of saturation near the energy pile. The predicted capacity matched well with the capacity from the experimental load-settlement curves, confirming the relevance of the effective stress principle in unsaturated soils in nonisothermal conditions and the importance of considering coupled heat transfer and water flow in unsaturated soils surrounding energy piles.

Effect of Suction on the Drained Seismic Compression of Unsaturated Sand

Backfill soils in many geotechnical applications are compacted and are likely to be in an unsaturated state during operation. In earthquake-prone areas, seismic compression of unsaturated backfill soils should be understood as small settlements may have significant impacts on the performance of overlying infrastructure like bridge decks, roadways, or railways. Accordingly, the goal of this paper is to describe the results from a series of constant suction, drained, cyclic simple shear tests on unsaturated sands subjected to a range of shear strain amplitudes. A new cyclic simple shear apparatus was developed that involves control of the matric suction and monitoring of changes in degree of saturation using the hanging column approach along with monitoring of the matric suction using an embedded tensiometer.

Permeability function for oil sands tailings undergoing volume change during drying

The coefficient of permeability function is an important unsaturated soil property required when modeling seepage and contaminant transport phenomena. Inaccuracies in the estimation of the permeability function can lead to significant errors in numerical modeling results. Changes in void ratio and degree of saturation are factors that influence the permeability function. Presently available methodologies for estimating the unsaturated permeability function make the assumption that there is no volume change as soil suction is changed. As a result, volume changes are interpreted as changes in degree of saturation. The commonly used estimation techniques for the permeability function are reasonable for soils such as sands that experience little volume change as soil suction is changed. On the other hand, inaccurate results are generated when soils undergo volume change as is the case with oil sands tailings. Revisions to previous methodologies are proposed to render the estimation of the permeability function more suitable for simulating the drying process associated with soils that undergo high volume changes. The revised methodology independently analyzes the effect of volume changes (i.e., changes in void ratio) and degree of saturation changes (i.e., changes in S-SWCC (degree of saturation - soil-water characteristic curve)). Laboratory data on thickened oil sands tailings are presented and interpreted within the context of the proposed methodology.

A case study on pavement performance due to extreme moisture intrusion at untreated layers

ABSTRACTPrevious studies reveal that a loss in Modulus of Resilience (Mr) has an adverse effect on pavement performances. However, no study investigated this during extreme moisture intrusion at granular and subgrade layers, which may be compared with a flooding situation. This study has aimed to assess the impact of change in Mr due to moisture increase on pavement performance using the HDM-4 and AASHTO 2008’s roughness prediction models. As a case study, the paper utilises seven selected flexible pavement road groups of Queensland, Australia that cover 50% of the major road network. Both the results match closely when pavement strength data are accurate. The deterioration predictions are verified with actual after flood data and t-tests for two major roads in Logan City, and found consistent. The study has observed some interesting findings on change in degree of saturation, pavements flood resilience and life-cycle performances with flooding scenarios. It is believed that these results would be useful in obtaining accurate ΔIRI change at varying Mr values due to a flood to select appropriate post-flood rehabilitation. A road authority can use pavements flood-resistance and life-cycle performance results for deriving a maintenance strategy. Moreover, pavement flood resilience, relationships between Mr loss and strength and Mr loss and deflection would help in pavement design, estimating existing structural strength and rehabilitation design. Finally, the paper highlights major assumptions and suggests some useful future works.

FE design for the numerical modelling of failure induced by differential straining in meso-scale concrete: Algorithmic implementation based on operator split method

The present paper focuses on the design of FE for the numerical modelling of failure induced by differential straining in concrete at the meso-scale. The differential straining is here considered as a deformation field added to the mechanical strain field and coming from the solution of a mass transport problem for instance. This differential straining can be induced by changes in degree of saturation, drying shrinkage or fluid overpressure for example. The meso-structure is based on a two-phase 3D representation of concrete, where stiff aggregates are embedded into a mortar matrix. In order to explicitly take into account these aggregates without any mesh adaptation, a weak discontinuity is introduced into the strain field. In addition, a strong discontinuity is also added to take into account cracking. The FE implementation is performed within the Enhanced Finite Element Method (E-FEM). An operator split method equipped with a return mapping algorithm accounting for differential straining is implemented for the computation of the discontinuities values, treated as local variables. In regards to the operator split method proposed in this paper, a broad range of problems can be solved such as basic mass transport problems or failure problem induced both by mechanical loading and differential straining. This feature is an advantage for the design of FE for durability issues. Finally 1D simulations are presented to assess the FE implementation and the performance of the numerical method. We also demonstrate the capability of the method for solving and analyzing 3D mesoscale problems.

Plasticity with generalized hardening: constitutive modeling and computational aspects

In this work, an extended theory of plasticity with generalized hardening is proposed to describe the response of geomaterials under both mechanical and environmental processes, which include as special cases several elastoplastic constitutive equations proposed in the literature to model such processes as desaturation or suction hardening, thermal softening, chemo-mechanical coupling effects in fine-grained soils, as well as weathering of soft rocks. In the formulation of the theory, the coupling between mechanical and environmental processes takes place at two levels: first, as an additional direct contribution to the constitutive stress changes, taking place in both elastic and elastoplastic processes; and second, as a result of the evolution of the internal state variables induced by changes in the environmental process variables. This last effect is incorporated through a set of generalized hardening rules. As an example of application, the general formulation is specialized to the particular case of weak calcarenite rocks undergoing degradation processes due to plastic deformations, changes in degree of saturation (short-term debonding) and chemical dissolution of the bond material and the solid grains (long-term debonding). The resulting model is implemented in a FE code by means of an implicit generalized backward Euler algorithm, suitably modified to incorporate the full formalism of plasticity with generalized hardening. Results of numerical simulations carried out at the element level show the accuracy and efficiency properties of the proposed stress-point algorithm. The simulation of a representative initial-boundary value problem demonstrates the practical relevance of environmental degradation effects in practical applications, over periods of time comparable with the life cycle of most geotechnical structures.

Settlements in unsaturated granular soils induced by changes in saturation and suction

In this contribution the hydro-mechanically coupled behaviour of a sand is experimentally investigated with the focus on settlements induced by changes in degree of saturation and suction. This phenomenon, referred to as collapse behaviour, is attributed to rearrangements of the grain skeleton due to changing capillary effects on wetting of the soil. For the experimental investigation of the collapse behaviour of a medium coarse sand cyclic drying-wetting tests are performed under oedometric conditions. In the test set-up a sand specimen, subjected to a constant small vertical stress, is cyclically dried and wetted by volume control of the pore water, whereas matric suction is measured using a tensiometer implemented to the specimen loading plate (topcap tensiometer). The test procedure, originally designed to investigate the hysteretic nature of the soil-water characteristic curve of the sand, allows to measure the one dimensional volume change of the specimen as evoked by the applied hydraulic paths under constant net stress. By varying the specimen void ratio the impact of density on the collapse behaviour can be assessed. The test data is important for the development of a mechanical constitutive model which can take the volume change behaviour due to suction changes into account.

Effect of Saturation and Unsaturation of Fatty Methyl Esters on Biodiesel NOx Emission Characteristics

Biodiesel is a renewable fuel and an attractive alternative to replace fossil diesel without major engine modifications. However, the emissions of oxides of nitrogen (NOx) from biodiesel fuelled engines are reported to be higher compared to diesel engine. The characteristics of biodiesel are known to depend on their fatty acid methyl ester (FAME) contents which vary with the feedstock. Thus the contribution of saturation and unsaturation of pure components of fatty acid methyl esters on NOx formation warrants a systematic investigation. This paper attempts to relate the composition of biodiesel with NOx formation. For this purpose, the NO formation from pure fatty acid methyl esters are predicted using extended Zeldovich reaction scheme. Also, the experiments are conducted for measuring oxides of nitrogen from a compression ignition engine operated using neat palm and karanja methyl esters and their blends providing biodiesel combinations of varying degree of saturation for investigation. The measured NOx concentrations are compared with the corresponding predictions to affirm the influence of fatty acid methyl ester on engine NOx characteristics. The results clearly indicate that the change in degree of saturation influences the NOx formation and an increase in the degree of saturation of biodiesel decreases the engine NOx emission.

A generalized variable-head borehole permeameter analysis for saturated, unsaturated, rigid or deformable porous media

The variable-head borehole permeameter (VHBP) method is a long-standing international standard for in-situ measurement of field-saturated hydraulic conductivity, KFS, in natural and engineered porous media. Its applicability is restricted, however, because traditional VHBP theory does not apply for unsaturated or deformable porous media, and because precise knowledge of the boundary condition on the interface of the borehole outlet (screen) is required for accurate KFS determination. This study extends the traditional VHBP theory to include saturated, unsaturated, rigid and deformable porous media, and also clarifies the boundary condition at the screen interface. Using a recent VHBP analysis developed for rigid, unsaturated porous media, it is shown (via flow conservation and the total increment theorem) that change in porous medium water content, ∆θ, can be extended to include change in porosity (deformation) as well as change in degree of saturation. It is also shown that the appropriate boundary condition on the borehole screen is antecedent pore water pressure head, Ha, for saturated porous media, but effective wetting front pressure head, ψf (or sorptive number, α*), for unsaturated porous media. The KFS, ∆θ, Ha, and ψf (or α*) parameters can be determined using numerical optimization (e.g. “Solver” in the Excel spreadsheet) to curve-fit the extended VHBP analysis directly to borehole head versus time measurements; however, fitting to the velocity graph (borehole head plotted against change in head with time) is generally less problematic. In a cursory assessment of the extended VHBP analysis, KFS was determined with ≤7% error, ∆θ with ≤15% error, Ha with ≤1.4% error, and ψf with ≤0.5% error, which is more than sufficient accuracy for most applications. It was concluded that the VHBP method was successfully extended for application to saturated, unsaturated, rigid and deformable porous media.

Grainsize dependence of clastic yielding in unsaturated granular soils

We use a recent reformulation of the Breakage Mechanics theory explaining comminution in wet granular assemblies. By using a dataset for sands, we quantify the relationbetweenageometricdescriptoroftheassembly(i.e., the mean grainsize) and the model constants that control the suction air-entry value and the stress threshold at the onset of crushing. Such relations are used to define two contrast- ing scenarios for the coupling between degree of saturation and yielding. In the first scenario, the suction air-entry value scales inversely with the mean grainsize, while the energy input for comminution is assumed to be independent of the size of the particles. The outcome of this assumption is that changes in degree of saturation are predicted to play a more intense role in finer gradings. Conversely, if we assume that alsotheenergy input forgrainbreakage scalesinversely with the size of the particles, the effect of the degree of saturation is predicted to be stronger in coarser assemblies. In other words, the deterioration of the yielding stress due to grain-

Soil-water characteristics of Gaomiaozi bentonite by vapour equilibrium technique

Soil-water characteristics of Gaomiaozi (GMZ) Ca-bentonite at high suctions (3–287MPa) are measured by vapour equilibrium technique. The soil-water retention curve (SWRC) of samples with the same initial compaction states is obtained in drying and wetting process. At high suctions, the hysteresis behaviour is not obvious in relationship between water content and suction, while the opposite holds between degree of saturation and suction. The suction variation can change its water retention behaviour and void ratio. Moreover, changes of void ratio can bring about changes in degree of saturation. Therefore, the total change in degree of saturation includes changes caused by suction and that by void ratio. In the space of degree of saturation and suction, the SWRC at constant void ratio shifts to the direction of higher suctions with decreasing void ratio. However, the relationship between water content and suction is less affected by changes of void ratio. The degree of saturation decreases approximately linearly with increasing void ratio at a constant suction. Moreover, the slope of the line decreases with increasing suction and they show an approximately linear relationship in semi-logarithmical scale. From this linear relationship, the variation of degree of saturation caused by the change in void ratio can be obtained. Correspondingly, SWRC at a constant void ratio can be determined from SWRC at different void ratios.

Complex Electrical Measurements on an Undisturbed Soil Core: Evidence for Improved Estimation of Saturation Degree from Imaginary Conductivity

Recent studies have shown that induced polarization (IP) coupled with electrical resistivity surveys can be used for in situ lithological and hydrologic discrimination of the subsurface; yet the driving factors behind the effects of water content dynamics on IP are relatively understudied. We sought to improve the understanding of the relationship of IP on variations in water saturation degree for an undisturbed agricultural soil. Our experiment consisted of collecting IP measurements concurrently with hydraulic data during multistep outflow experiments. We determined the hydraulic properties of the undisturbed soil samples and correlated saturation degree with IP data. Due to an increase in pore fluid conductivity in the column pore water with decreasing saturation degree, we found that imaginary conductivity (σ″) may offer distinct advantages for determining water content over real conductivity (σ′) measurements. Although σ″ exhibits a weaker dependence on saturation degree compared to σ′, the relative insensitivity of σ″ to pore fluid conductivity results in a simpler dependence on saturation change in the presence of varying salinities. As changes in pore fluid conductivity are likely to occur in the field simultaneously with water content variations, we suggest that although IP has mostly been used to discriminate lithology, time lapse IP measurements may additionally provide a robust indicator of changes in saturation degree.

Water retention properties of the Callovo-Oxfordian claystone

A possible option for the disposal of high level long lived radioactive waste is to store the waste in galleries excavated at great depth in clays or claystones. In such conditions, the walls of the galleries will be successively subjected to desaturation induced by ventilation followed by resaturation once the galleries are closed. Partial saturation also affects laboratory samples, due to the added effects of coring, transportation, storage and trimming. In this study, a series of tests was conducted to investigate the water retention properties of the Callovo-Oxfordian claystone (in which the ANDRA Underground research laboratory in France has been excavated) under drying/wetting cycles so as to provide a complete description of the changes in water content, volume and degree of saturation from the as-provided state. The results show that water content changes between 3% (at 150MPa of suction) and 12% at zero suction, with a shrinkage of 1.4% (at 150MPa of suction) and a 7% swelling at zero suction. The data obtained complete existing data by defining the initial state with respect to the main drying and wetting paths and by providing volume changes along the suction cycles. Hysteresis effects are also evidenced. Irreversible volume changes are assumed to be due to micro-crack generation and incomplete recovery. The changes in degree of saturation with respect to water content appeared to be reversible, an important fact to consider when simulating volume changes in COx.

SIMULATION FOR CORROSION CRACKING OF COVER CONCRETE IN REINFORCED CONCRETE STRUCTURES

This study was conducted to propose a simulation model for corrosion cracking of cover concrete in reinforced concrete structures located on land and subjected to airborne chloride. Chloride penetration has been known to have great influences on the design corroded mass due to the fact that it changes the corrosion initiating time and corrosion rate. Accordingly, a chloride penetration model was proposed for reinforced concrete structures located on land. This model was verified with the drawn core of concrete specimens taken from structures that were free from the rain washout effect. In this study, the corrosion rate of reinforcing steel was also proposed by considering the change in degree of saturation in concrete due to environmental conditions. With the changes in the degree of saturation, the effective corrosion rate is changed according to three controlling factors: the conductivity of the concrete, the chloride concentration and the supply flux of oxygen. Therefore, the design corroded mass is the integration of the effective corrosion rate from the corrosion initiating time to the design time. Moreover, a numerical formula for limit corroded mass, which causes cracks in cover concrete, was developed and proposed. The accuracy of the formula was approximately 25% as compared to the experimental results of previous studies. Thus, the cracking time is the time when the design corroded mass is greater than the limit corroded mass. Additionally, the simulations at the time of cracking in cover concrete were compared with three actual reinforced concrete bridges subjected to airborne chloride.