Unsaturated Conditions Research Articles

A critical state constitutive model for unsaturated structured soils

This paper presents an advanced approach for interpreting the hydro-mechanical response of unsaturated structured soil, including the effect of bond strength degradation and the role of the degree of saturation in unsaturated conditions. The model relies on the definition of a “structure degradation law,” which accounts for the progressive breakage of inter-particle bonding during loading. The soil compression index is assumed to be a function of the degree of structure and the effective degree of saturation. A new volume change equation is proposed to better explain the nonlinear change of compressibility of structured soils under saturated and unsaturated conditions. The proposed volume change equation is generalised to three-dimensional stress states by incorporating the modified Cam-clay model. The basic properties and performance of the proposed constitutive model are then illustrated through numerical examples with various stress paths, degrees of structure, and degrees of saturation. The proposed model is verified using the experimental data available in the literature for both structured soils and reconstituted soils under saturated and unsaturated conditions. The results indicate that the proposed model simulates the mechanical response of structured and reconstituted soils reasonably well in saturated and unsaturated conditions.

Integrating 3D geological modeling and kinetic modeling to alleviate acid mine drainage through upstream mine waste classification

Mine waste classification preceding mining constitutes a proactive solution to classify and segregate mine waste into geo-environmental domains based upon the magnitude of their environmental risks. However, upstream classification requires multi-disciplinary and integrated approaches. This study integrates geological modeling and kinetic modeling to inform upstream mine waste classification based on the pH generated from the main acid-generating and acid-neutralizing reactions once the mine solid waste is stored in oxidizing conditions. Geological models were used to depict the ante-mining spatial distribution of the main reactive minerals: pyrite, albite and calcite. Subsequently, the corresponding block models were created. The dimension of the elementary voxels for each block model was set at 40х40х40 m for this study. The kinetic modeling approach was performed using PHREEQC and VS2DRTI to consider unsaturated conditions. The kinetic modeling simulated a 1D column for each voxel. The column simulates the excavated state of the hosting rock involving kinetic reactions and unsaturated flow under highly oxidizing conditions. Subsequently, the resulting pH for different intervals of time was assigned to its respective voxel. The outcome consists of a spatio-temporal visualization of the pH defining ante-mining geo-environmental domains, thereby providing the opportunity for formulating proactive management measures regarding the hazardous geo-environmental domains.

Coupled effect of porous network and water content on the natural attenuation of diesel in unsaturated soils.

The natural attenuation potential of a vadose zone against diesel is critical for optimizing remedial actions and determining groundwater vulnerability to contamination. Here, diesel attenuation in unsaturated soils was systematically examined to develop a qualitative relationship between physical soil properties and the natural attenuation capacity of a vadose zone against diesel. The uniformity coefficient (Cu) and water saturation (Sw, %) were considered as the proxies reflecting the degree of effects by porous network and water content in different soils, respectively. These, in turn, are related to the primary diesel attenuation mechanisms of volatilization and biodegradation. The volatilization of diesel was inversely proportional to Cu and Sw, which could be attributed to effective pore channels facilitating gas transport. Conversely, biodegradation was highly proportional to Cu under unsaturated conditions (Sw=35-71%), owing to nutrients typically associated with fine soil particles. The microbial community in unsaturated soils was affected by Sw rather than Cu. The overall diesel attenuation including volatilization and biodegradation was optimized at Sw=35% for all tested soils.

Hardening characteristics of Granulated Blast Furnace Slag under saturated and unsaturated conditions

Hardening characteristics of Granulated Blast Furnace Slag under saturated and unsaturated conditions

Soil slope analysis to develop useful correlations in saturated and unsaturated conditions

Shear strength (τ), shear stress (σ s) and slope angle (β) play a very important role in the stability of any soil slope. This research investigates the slope stability of a soil slope to determine correlations between shear strength and slope angle and shear stress. The assessment and calculation of the soil slope are carried out using slope stability analysis methods that assume the failure surface as a circular slip surface. A limit equilibrium software program – namely, Slide – and a numerical modelling software program – namely, Statistical Package for the Social Sciences – are used for the analysis. Total 200 number of analyses were performed out of which 100 analyses in saturated and 100 in unsaturated condition. Correlations are developed between τ, β and σ s, which show a very close relationship among all these three parameters. The applicability of the equations is above 97%. The correlations can be used in any slope stability project to know about the shear strength of the slope with variation in the values of the slope angle and shear stress.

Unsaturated Transport Modeling: Random‐Walk Particle‐Tracking as a Numerical‐Dispersion Free and Efficient Alternative to Eulerian Methods

AbstractLagrangian methods, such as the random‐walk particle‐tracking (RWPT), are often qualified as a potentially valuable alternative to error‐prone Eulerian methods for simulating solute transport in unsaturated porous media. Yet, the RWPT method has not yet been validated against ‐ and compared to ‐ currently used Eulerian solutions for simulating solute transport under a range of typical unsaturated conditions. This paper presents a new implementation of the RWPT approach for advective ‐ dispersive transport problems under variably saturated conditions. We first show that, as previously demonstrated for a heterogeneous dispersion tensor, using an interpolation scheme in the RWPT algorithm performs well for problems with abrupt changes in the water content. The new model is then compared against a simple 1D uniform transport problem, for which an analytical solution exist, and against a variety of 1D and 3D numerical solutions using the different Eulerian schemes implemented in Hydrus software suite. Results show that, while the Eulerian solutions significantly suffer from numerical dispersion in case of a coarse spatial discretization of the simulation domain, the new Lagrangian model provides accurate solutions for all problems. Furthermore, RWPT reproduces accurately solute transport for typical unsaturated flow conditions (infiltration, evaporation). Moreover, the Lagrangian model appears to be orders of magnitude faster than its Eulerian alternative to solve a 3D heterogeneous problem. Thus, RWPT should be seen as an attractive, stable and efficient alternative for simulating solute transport in the vadose zone, especially in case of complex and large problems.

Parametric Analysis of Rainfall-Induced Loess Soil Slope Due to the Rainwater Infiltration

Hydraulic properties (such as soil–water characteristic curves (SWCC) and hydraulic conductivity function (HCF)) play an important role in evaluating the stability of unsaturated soil slopes. Loess soils are widely distributed in Gansu Province in China, and most of them are in unsaturated conditions due to the deep groundwater table (G.W.T). In this study, twenty-eight sets of data published in the literature were analyzed to develop the upper and lower bounds of the SWCC for loess soil in Gansu. The variation of HCF for the loess soil was estimated from the upper and lower bounds curve developed in this study. Subsequently, numerical analyses incorporating scenarios considering different SWCCs, HCFs, and rainfall conditions were conducted for investigating the effects of those factors on the rainfall-induced slope stability. The results of analyses indicate that the infiltration plays an important role in the rainfall-induced slope stability. Higher permeable soil leads to a larger infiltration amount, which, in turn, results in a lower safety factor. In addition, the effect of the hydraulic property on the rainfall-induced slope stability decreases with the increase in slope angle.

Pore‐scale analytic model to calculate the suction stress in partially saturated soils

The expression of effective stress in unsaturated conditions has remained controversial ever since the concept was first proposed in the 1950s by Bishop. In this paper, based on the first law of thermodynamics, a novel approach is proposed to calculate the effective stress in unsaturated soil systems by applying the improved bundled cylindrical capillary model and the probability density function of soil particles. The proposed model provided an expression for Bishop's effective stress model and Bishop's coefficient χ. The basic assumptions of the model are (a) the water in the unsaturated pores can be divided into capillary water and adsorbed water, and (b) the soil pore network is considered as a rigid body, and the particle structure is unchanged with saturation variation. Published experimental data are compared with Lu's effective stress model and our model results. The findings show that the proposed model performs well over a wide range of saturation conditions for nonexpansive soils, such as sand, and the capillary stage for expansive soils, such as silt, and clay materials.

Use of centrifugal systems for investigating water flow processes in unsaturated soils

Centrifugal modelling, both physical and numerical, has been used for studying groundwater flow and transport processes in the past. However, there was disagreement in previous studies whether numerical models can be used in simulating centrifugal systems under unsaturated flow condition. In the present study, a numerical model based on Richards’ equation was developed to predict one-dimensional unsaturated flow in centrifugal systems. The validity of the model was tested using data from physical models in four published benchmark problems. The ability of the numerical model to close mass balance was also tested. It was shown that the newly developed numerical model was able to recreate the four benchmark problems quite successfully, indicating that using such a model under unsaturated flow condition is feasible. The mass conservation result shows that the model is more sensitive to spatial grid resolution than to specified temporal step. Therefore, fine spatial discretization is suggested to ensure the simulation quality. Additionally, adaptive temporal time stepping method can be used to improve the computational efficiency. It was found that the dimensionless factors used for scaling physical dimensions by 1/N, seepage velocity by N, and temporal dimension by 1/N2 were useful parameters for scaling centrifugal systems.

Saturated and unsaturated seepage analysis of earth-fill dams using fractal hydraulic conductivity function and its verification

One of the most important factors concerning the design of earth-fill dams is the accurate seepage estimation, which is highly correlated to the permeability and hydraulic conductivity of the core material. With the geographic coordinates of 37°31′35.9,“ North and 46°07′55.6″ East, Gale chay dam is located in the northwest of Iran in East Azerbaijan province. The dam is highly important regarding the provided economic benefits for the region. In this research, the steady-state seepage analysis of the Gale Chay dam was conducted via the finite element model (FEM) using a novel saturated fractal hydraulic conductivity model, and the results were compared with those obtained from the conventional and fractal models in the unsaturated condition. For estimating the saturated hydraulic conductivity of the core material, the fractal theory was employed using the image processing technique, the Scanning Electron Microscope (SEM) images, and a new fractal permeability model. The estimated saturated hydraulic conductivities were in good accordance with the measured values. Furthermore, since the fractal theory considers the distribution of the pores, the estimated upper and lower bounds of the saturated hydraulic conductivity resulted in better seepage estimations. Using the estimated maximum, minimum, and average fractal hydraulic conductivities, the results of the saturated seepage analysis indicated relative errors of 15.6%, 8%, and 10.8% compared to the observed seepage. The results were acceptable in terms of safety and economic aspects. Conducting the unsaturated seepage analysis, it was observed that the estimated seepage was highly influenced by the saturated hydraulic conductivity value. Hence, the estimated average fractal hydraulic conductivity by the image processing technique affected the unsaturated fractal and conventional models in a way that the unsaturated seepage analysis was improved by an average of 80%. Moreover, the unsaturated fractal model revealed reliable estimations compared to the conventional models and was considered a feasible alternative for estimating the unsaturated hydraulic conductivity.

LANDSLIDE SUSCEPTIBILITY ASSESSMENT: SOIL MOISTURE MONITORING DATA PROCESSED BY AN AUTOMATIC PROCEDURE IN GIS FOR 3D DESCRIPTION OF THE SOIL SHEAR STRENGTH

Abstract. Slope stability is strongly influenced by soil hydraulic conditions. Considering rain-triggered shallow landslides, the stability can be markedly influenced by the propagation of the saturation front inside the unsaturated zone. Soil shear strength varies in the vadose zone depending on the type of soil and the variations of soil moisture. Monitoring of the unsaturated zone can be done by measuring volumetric water content using low-cost instrumentation, such as capacitive sensors that are easy to manage and provide data in near-real time. For a proper soil moisture assessment a laboratory soil-specific calibration of the sensors is recommended. Knowing the soil water content, the suction parameter can be estimated by a Water Retention Curve (WRC), and consequently the soil shear strength in unsaturated conditions is evaluated. Several models are already proposed for shallow landslide susceptibility evaluation, also in FOSS GIS environment. However, these models do not usually consider the soil shear strength in unsaturated conditions, even if it is crucial, especially in the case of shallow landslides. A procedure that allows the estimate of the soil shear strength starting from soil moisture monitoring data (from sensor networks or satellite-derived map) is here presented. Moreover, preliminary results relative to a case study (i.e. the landslide of Ceriana-Mainardo in Italy) are shown. The proposed procedure could be integrated into existing models for landslide susceptibility assessment and also for the emergency management.

Agro waste as immobilization carrier for in situ remediation of 2,4,6-trinitrotoluene contaminated soil

In light of global challenge of environmental sustainability, it has become imperative to devise techniques for eco-friendly remediation of hazardous compounds. 2,4,6-Trintro toluene (TNT) is one such hazardous explosive compound finding application in defence and civilian sectors. In this study, we explore the efficiency of Janibacter cremeus an indigenous bacteria, immobilized on biowaste from agriculture sector, viz, cocopeat and waste egg shells for degrading TNT in soil. Preliminary evaluation on tolerance of the bacteria to TNT in aqueous phase revealed maximum degradation at 60 mg/l. Agrowaste used as carrier was found to enhance the bioavailability of explosive to microbe and enhanced the remediation of the explosive compound. Remediation of TNT was carried out under unsaturated as well as saturated conditions. It was observed that, TNT was degraded efficiently in both conditions (98 and 99 % respectively). Saturated conditions prove to be better due to the disappearance of the intermediates formed by the end of the study (35 days). Both the conditions exhibited release of nitrite in initial stages, followed by its uptake. Also, the microbial activity in both the conditions were monitored in terms of live cell count and bulk enzyme activity. Saturated treatment sets showed higher live cell count as well as enzyme activity in comparison to the unsaturated treatment sets. The maximum microbial growth was observed at day 21 followed by its decrease later. The biowaste based bioformulation was observed to be a sustainable solution for remediation of TNT contaminated soil. • Janibacter cremeus was evaluated for its TNT degradation potential. • TNT contaminated soil was remediated under saturated and unsaturated conditions. • J. cremeus was immobilized on agrowaste for soil remediation. • HPLC analysis was used to confirm degradation. • Saturated/ anoxic conditions lead to better performance of J. cremeus .

Calibration of a constitutive model for volcanic sands under simple shear conditions

The simple shear response of air-fall volcanic (pyroclastic) soils under both saturated and unsaturated conditions is interpreted through an elastoplastic constitutive model with hydraulic-hardening and porosity-dependent critical state. Extensive experimental data collected under various testing protocols (i.e., simple shear, direct shear, and triaxial tests) enable the theoretical identification of the variability of the fundamental physical properties (i.e., frictional resistance, dilatancy, and water retention behavior) and the identification of a band of admissible values for prescribed confidence levels. In this paper, a constitutive model specifically developed to account for the simple shear loading is adopted and the calibration of the model parameters is performed accounting for such data scatter. The calibration procedure led to a single set of constitutive parameters through 3 main steps. The identification of range of variation for each parameter is carried out using the data derived from laboratory tests. Then, sensitivity analyses are performed on the main physical and constitutive parameters. Mathematical indicators quantifying the difference between the measurements and the predictions are proposed to investigate the role of the material properties and evaluate the model performance. Furthermore, an optimization algorithm is adopted to identify the optimal set of model parameters which best fits all the considered tests (i.e., simple shear tests under saturated and unsaturated conditions and wetting tests). The results imply that the variability of the hydro-mechanical properties must be considered to satisfactorily simulate the constitutive behaviors of the volcanic soils under a variety of simple shear testing regimes.

Hydrobiological Mechanism Controlling the Synergistic Effects of Unsaturated Flow and Soil Organic Matter on the Degradation of Emerging Organic Contaminants in Soils.

Hydrology is a key factor influencing microbial degradation of emerging organic contaminants (EOCs) in soils, but the underlying mechanisms are not clear. In this study, biotic and abiotic column experiments were performed to investigate the removal and degradation of five EOCs in soils with different soil organic matter (SOM) contents under saturated and unsaturated flow conditions. In biotic experiments, 54-90% of bisphenol A (BPA) and 9-22% of ibuprofen (IBU) were removed from the aqueous phase of saturated columns due to adsorption and biodegradation. The biodegradation removed 26-65% of BPA and 1-22% of IBU. Decreasing soil pore water saturation from 100 to 80% increased BPA removal to 97-100% and IBU removal to 42-43% due to increased biodegradation (67-81% for BPA and 36-39% for IBU). No significant removal of BPA and IBU was observed in SOM-removed soils under saturated and unsaturated flow conditions. The desaturation did not influence sorptive losses of BPA (<27%) and IBU (<7%), suggesting their negligible adsorption at air-water interfaces but increased biodegradation of BPA and IBU sorbed at SOM-water interfaces. The study shows that soil drying and SOM can synergistically degrade BPA and IBU but have no effect on recalcitrant carbamazepine, tetracycline, and ciprofloxacin.

Variation of the hydraulic properties in sandy soils induced by the addition of graphene and classical soil improvers

In this study, for the first time, the changes in relevant hydraulic parameters (e.g., hydraulic conductivity, effective porosity, and dispersivity) induced by the introduction of graphene in a calcareous sandy soil and a siliciclastic riverine soil were monitored and modelled via leaching column experiments. Column experiments were also run with traditional soil improvers (compost, biochar, and zeolite) to compare the changes induced by graphene versus well-studied soil improvers. Constant pressure head tests were used to calculate the hydraulic conductivity of each column, while leaching experiments were run to estimate porosity and specific retention, and for each treatment three replicates were done. Columns were then run in saturated conditions via a low flow peristaltic pump and monitored for electrical conductivity, temperature, and chloride. CXTFIT 2.0 was employed to inversely model the column experiments and retrieve parameters like effective porosity, longitudinal dispersivity, bulk thermal diffusivity, and thermal retardation factor. Results highlighted small changes of hydraulic conductivity, porosity, and effective porosity induced by graphene addition (as well as by the other soil improvers) for both soils. A marked increase (nearly 20 %) of specific retention values was instead recorded in the amended columns with respect to control ones. Chloride breakthrough curves modelling showed that graphene doubled dispersivity in the calcareous sandy soil (5.82 ± 1.4 cm) compared to the control (2.6 ± 0.29 cm), while it halved dispersivity in the siliciclastic riverine soil (0.31 ± 0.05 cm) with respect to the control (0.65 ± 0.06 cm). Thermal retardation factors were decreased by graphene by approximately 20 % for both soils. The model fitting via TDS (derived from the electrical conductivity monitoring) produced unreliable dispersivity values in most of the experiments due to the nonconservative nature of this parameter compared to chloride. The results highlight that graphene affected dispersivity but did not significantly alter other physical parameters relevant for solutes transport in sandy soils in comparison to classical improvers, thus future studies should focus on the graphene’s effects on nutrients and agrochemicals leaching in unsaturated flow conditions.

Seismic Factor of Safety for 3D Reinforced Soil Slope with Piles under Unsaturated Condition

This study investigates the seismic stability of reinforced slope under a three-dimensional (3D) rotational horn-like failure mechanism, using the pseudo-dynamic approach and provides a new procedure to estimate factor of safety (FS) for slope. Such a procedure is able to figure out the influences of the seismic force, the soil property and the reinforcement by piles. Pseudo-dynamic approach is used to describe the seismic force in the time-to-spatial fields in unsaturated soils where matric suction works. Compared to the pseudo-static approach, the current approach considers the effects of time and spatial through the horizontal and vertical shear waves. For the pile driving, different insertion depths can yield various results, whether two work conditions of considering the pile resistance or not are discussed. FS of this study undergoes a decrement as the increase of the pile resistance and seismic force, indicating that the insertion depth affects FS where FS decreases in the early period of pile driving, after that increases and keeps constant in the late of pile driving, showing evident unimodal characteristics during the pile driving. The new expressions for slope reinforced by piles are presented for practical use in unsaturated soil engineering.

Insight into excess pore pressure generation leading to liquefaction of sand with stress history under saturated and unsaturated conditions

Assessments of the liquefaction resistance of clean sand still involve considerable uncertainties, which are a current research topic in the field of soil liquefaction. The factors considered and discussed in this study include the loading history, degree of saturation, and partial drainage. The effects of each of these factors on pore pressure generation and liquefaction resistance have been studied for decades in the laboratory, and empirical relationships have been derived. In this paper, an attempt is made to explain these effects using the unique index of volumetric strain. A pore pressure generation model is developed which is similar to that of Martin et al. (1975), but based on stress-controlled triaxial tests. The model is verified through comparisons of its results with those of laboratory tests. It is confirmed that the plastic volumetric strain that has accumulated in sand, either by drained or undrained cyclic loading, dominates the increase in the liquefaction resistance of the sand. However, the plastic volumetric strain caused by overconsolidation is less effective in reducing the volumetric strain potential for subsequent cyclic shearing, thus enhancing its resistance to liquefaction. The model provides a better understanding of the physical processes leading to the liquefaction of saturated and unsaturated sand with and without stress history.

Enhancing solute transport by pressure-wave driven flow in unsaturated porous media

Solutes and their limited mobilization in heterogeneous porous media may lead to subsurface salinization and contamination evident in various environmental settings. Previous studies showed that flow and transport driven by pressure waves in porous media under saturated conditions significantly enhance solute displacement compared to drag-dominant (Darcy) flow and transport, resulting in more thorough remediation. However, experimental evidence lacks demonstration of such transport under unsaturated conditions. We investigate solute leaching in a flow cell, experimentally modeling solute displacement from an unsaturated, saline, fine-textured soil region to the surrounding coarse-textured soil. The extent of mobilization efficiency is evaluated by comparing solute breakthrough measurements between two leaching treatments, with drag-dominant continuous flux (CF) and pressure-wave pulse flux (PPF). The results indicate that the PPF treatment provided an additional 75% solute mass that leached from the fine-textured region per unit volume of water compared to the CF. The effect is partly due to the impact of the pressure waves on water imbibition into the medium. The PPF induced relatively high pressure gradients at the wetting front compared to percolation during the CF, thus wetting the media in a more spatially uniform pattern. Consequently, the PPF treatment led to higher water content at and behind the front, which increased the continuity of the water phase across the fine-textured and coarse-textured soils and enhanced transport. The study is a proof of concept for pressure-wave driven flow to enhance local wetting and solute transport in unsaturated soil.

ERRATUM: Contaminant transport model in transient and unsaturated conditions applied to laboratory column test with tailings

ERRATUM: Contaminant transport model in transient and unsaturated conditions applied to laboratory column test with tailings

Soil-water characteristic curve of polypropylene fibrereinforced sandy soil

Fibre reinforcement is considered a good alternative for improving the geotechnical properties of soil. However, studies that investigate its behaviour, accounting for the unsaturated condition, and the hydraulic behaviour of soil mixtures with fibre, are limited. Therefore, the current study evaluates the impact of the inclusion of polypropylene fibres on the hydraulic behaviour of soil through geotechnical characterisation, scanning electron microscopy (SEM), macroporosity and microporosity tests, and filter paper tests. The soil-water characteristic curve (SWCC) of different mixtures of fibre-reinforced soil was adjusted by the models enshrined in the literature, using polypropylene fibres of length 6 mm, diameter 18 μm, and fibre contents 0.25% (SF025), 0.75% (SF075), 1.0% (SF100), and 1.25% (SF125) relative to the dry weight of the soil. The results indicated a transition from unimodal to bimodal shape in the SWCC for the polypropylene fibre-reinforcement, suggesting that their inclusion altered the soil structure. The same bimodal behaviour of SWCC was observed in all reinforced samples that produced similar values of air-entry suction and residual volumetric water content, but with increased water retention for the same level of suction for higher fibre content. The results of the tension table test indicated an increase in the volume of macropores with an increase in fibre content and a decrease in micropore volumes. These results agree with the compaction tests, which showed a decrease in the dry maximum density with an increased fibre content, whereas the optimum water content increased.