Hydraulic Conductivity Of Compacted Soils Research Articles

Changes in Pore-Size Distribution and Hydraulic Conductivity of Compacted Soils by Grass-Derived Hydrochar

Changes in Pore-Size Distribution and Hydraulic Conductivity of Compacted Soils by Grass-Derived Hydrochar

Influence of biochar particle size on the hydraulic conductivity of two different compacted engineered soils

Biochar is a recalcitrant material obtained after pyrolysis of labile biomass under oxygen-limited condition. The conversion of biomass into biochar is considered to be a sustainable way of generating bioenergy, minimizing the emission of greenhouse gases and managing the waste. Recently, the amendment of biochar in soil gained attention due to its multiple soil ameliorating features. Many studies have investigated the effect of biochar particle size on the physicochemical and hydraulic properties of relatively loose agricultural soil. However, such study is rarely conducted for compacted soil suitable for bioengineered structures. The objective of the current study is to investigate the effect of biochar particle size on the hydraulic conductivity of compacted soil for bioengineered structures. Saturated hydraulic conductivity (Ksat) of different compacted soils amended with biochar of different particle sizes ranging from 0.075 to 4.75 mm has been investigated. The results showed that the amendment of 5% to 15% (w/w) biochar of different particle sizes led to the change in Ksat of the silty sand. When compared with bare soil, a higher Ksat with biochar of particle size larger than 0.425 mm and a lower Ksat with particles of size smaller than 0.425 mm was observed. However, in pure sand, a lower Ksat irrespective of particle size of biochar was observed and the lowest Ksat was noted with biochar of particle size smaller than 150 mm. These changes in the Ksat after biochar amendment were attributed to the alteration of pore structure mainly, inter-pores of the soil, which is controlled by the particle size of the biochar. The findings of the present study could be useful for the field application i.e. in bioengineered structures.

Estimation of the compaction characteristics of soils using the static compaction method

Ground improvement using mechanical stabilization is commonly applied by performing the standard Proctor compaction test, which requires a significant quantity of soil, usually obtained from open pits. A static compaction test is an alternative laboratory compaction test. Although researchers have shown that the results of miniature size static compaction tests are comparable with that of standard Proctor tests in terms of the maximum dry density and the optimum water content, no attempt has been made to compare the two fundamental properties of the compacted soil: undrained shear strength and hydraulic conductivity. The scope of this investigation was to estimate the level of static compaction energy required to (1) obtain a compaction curve similar to that of the standard Proctor test; (2) reconstruct compacted soils using the standard Proctor and static compaction tests at the optimum water content; and (3) compare the undrained shear strength and hydraulic conductivity of compacted soils. The compacted soils at the predetermined energy level were subjected to hydraulic conductivity tests using the rigid-wall falling-head permeability method. Undrained shear strength tests were performed by employing a high-capacity laboratory vane shear apparatus on compacted samples of both the standard Proctor and static compaction tests. The present investigation revealed that the static compaction test, requiring about only 10% of the soil necessary to perform the standard Proctor method, provides comparable results in regard to hydraulic conductivity and undrained shear strength.

Pore-scale model for estimating the bimodal soil–water characteristic curve and hydraulic conductivity of compacted soils with different initial densities

This paper proposed a pore-scale model to simulate the effect of initial soil density on the bimodal soil water characteristic curve (SWCC) of compacted soils. The pore-scale model is developed based on the bimodal pore-size distribution. Soils with bimodal pore-size distribution is assumed to have dual porosity, namely the macropore structure and the micropore structure, and hypothesized that the micropore structure is hard to be compressed under mechanical loading. The water stored in the macropore structure is divided into two parts, the bulk water and the meniscus water, respectively. The deformation of soil (bulk water flow) is obtained by horizontal shifting and vertical scaling of the pore-size distribution (PSD) function. A toroidal model is used to describe the influence of initial density on the meniscus water. The pore structure of compacted coal gangue was qualitatively imaged as an evidence of the pore structure changes, and the SWCC of the compacted coal gangue was quantitatively analyzed. Finally, a numerical method is advised for simulating bimodal permeability function by using the proposed model with Mualem function. Laboratory test results in published literature are employed to validate the proposed model and permeability function, and the model predictions showed good agreement with the experimental data in the literature.

Cracking and water seepage of Xiashu loess used as landfill cover under wetting–drying cycles

This paper intends to investigate the anti-seepage characteristics of Xiashu loess used as landfill cover under wetting–drying cycles. A series of tests were performed to systematically evaluate the surface cracks, pores, the hydraulic conductivity and water seepage of the soil under wetting–drying cycles. The cracking tests indicated that the cracking intensity factor (CIF) increased with the number of wetting–drying cycles. After three wetting–drying cycles, the CIF gradually reached a stable value. The influence of the compactness of the compacted clay was significant to cracking. After four drying–wetting cycles, the CIF of compacted clay with relatively low compactness (ρ = 1.65 g/cm3, W 5 = 25 %) was 0.15, and the CIF of compacted clay with higher compactness (ρ = 1.65 g/cm3, W 3 = 21 %) was only 0.06. With the increase in the number of wetting–drying cycles, the accumulated pore volume of the soil particles increased and the maximum pore size was nearly unchanged. After four drying–wetting cycles, the proportion and average diameter of macro-pore in the soil particles were both increased. However, the proportion and average diameter of the meso-pore slightly decreased. Furthermore, the proportion and average diameter of the micro-pores were unchanged. After four wetting–drying cycles, the hydraulic conductivity of the compacted soil was at 8.3 × 10−7–1.5 × 10−5cm/s, which was increased about three orders of magnitude compared with the compacted soil without cracks. The hydraulic conductivity of compacted soil increased linearly as cracks increased. The hydraulic conductivity of compacted soil also gradually increased to a stable value as the pore volume increased. After 1 month of wetting–drying cycles, the cracks of soil with low compactness had extended to a depth of 15 cm below the soil surface while the cracks of soil with higher compactness only reached a depth of 5 cm.

Hydraulic conductivity of compacted soils controlled by microbial activity

The hydraulic conductivity defines the displacement of liquids inside porous media and affects the fate and transport of contaminants in the environment. In this research the influence of microbial growth and decay inside soil pores on hydraulic conductivity is analysed. Long-term tests performed in silt–bentonite mixtures permeated with distilled water and a nutrients solution demonstrated that hydraulic conductivity of compacted silt–bentonite samples decreases with time of permeation as a bioclogging mechanism develops. The injection of antibiotics and antifungals in the specimens produces a rebound in the hydraulic conductivity associated with the decay of microbial activity. These results show that biomediated reactions can be used to control the flow rate through compacted soil liners.

Prediction of saturated hydraulic conductivity of compacted soils using empirical scaling factors

A new empirical-based scaling method is introduced to predict saturated hydraulic conductivity (K s ) of compacted soils. This method is an improvement of the former non-similar media concept (NSMC) model that is generalized for tilled and untilled conditions. In this method, geometric mean particle size diameter (dg ), geometric standard deviation (σ g ) and saturated soil water content (total porosity) are successfully incorporated in the empirical-based scaling factor of K s . Results showed that the scaled model overestimated K s by ∼18%, whereas the NSMC model underestimated K s by ∼21%. However, the scaled model based on the similar media concept (SMC) failed to predict K s . Because of the complexity and high uncertainty in determining the shape factor parameter in the NSMC model, it is suggested that the new scaled model might be used reliably in practical cases to predict K s in the various layers of compacted soils irrespective of the tillage condition. Further assessment of the new scaling model in other areas, in which new collected data are available, is recommended.

Kozeny-Carman Equation and Hydraulic Conductivity of Compacted Clayey Soils

The saturated hydraulic conductivity of a soil is the main parameter for modeling the water flow through the soil and determination of seepage losses. In addition, hydraulic conductivity of compacted soil layers is critical component for designing liner and cover systems for waste landfills. Hydraulic conductivity can be predicted using empirical relationships, capillary models, statistical models and hydraulic radius theories [1]. In the current research work the reliability of Kozeny-Carman equation for the determination of the hydraulic conductivity of compacted clayey soils, is evaluated. The relationship between the liquid limit and the specific surface of the tested samples is also investigated. The resulting equation gives the ability for quick estimation of specific surface and hydraulic conductivity of the compacted clayey samples. The results presented here show that the Kozeny-Carman equation provides good predictions of the hydraulic conductivity of homogenized clayey soils compacted under given compactive effort, despite the consensus set out in the literature.

Long term hydraulic conductivity of compacted soils permeated with landfill leachate

This study evaluates the relative influence of biological, physical and chemical interactions on the percolation of leachate through compacted soil liners. The long term hydraulic conductivity of compacted silt–bentonite mixtures was measured with distilled water, landfill leachate and a nutrient solution. The soil hydraulic conductivity decreased significantly with time when the permeating liquid contained microorganisms. The decrease of hydraulic conductivity was caused by reduction of the effective porosity due to pore clogging. Pore clogging was analyzed considering physical, chemical and biological processes. The effect of microbes on the hydraulic conductivity of soil liners permeated with leachate prevailed over that produced by physical or chemical interactions. The presence of microbial activity was confirmed by direct observations of the microbial population in the permeating liquid, by microbial exopolysaccharides (EPS) encountered in the soil pores, and from inverse modeling analysis of pore bioclogging. The existence of microbes in the compacting and permeating liquids reduced up to two orders of magnitude the long term hydraulic conductivity. The results were in good agreement with the expected behavior when biofilms develop around soil particles.

Can the diffuse double layer theory describe changes in hydraulic conductivity of compacted clays?

Observations related to modifications of the hydraulic conductivity of compacted soils as a consequence of intentional compositional variation of the percolating fluid are often attributed vaguely to calculated changes in double layer thickness. The same calculated decrease or increase in double layer thickness is interpreted differently, depending on which (unconsciously held) boundary conditions authors take into account. The interpretations of different authors seem to be paradoxical only if the boundary conditions have not been mentioned. On the basis of a literature survey and own previously published test results the different boundary conditions have been grouped and divided into classes. With this division the interpretation of changes in double layer thickness will lead to unique interpretations and predictions of changes in hydraulic conductivity.

Modeling the Relationship between Soil Bulk Density and the Hydraulic Conductivity Function

A model is presented that could quantify and predict the effect of an increase in soil bulk density on the hydraulic conductivity function (HCF). Two approaches are formulated to predict the saturated hydraulic conductivity of compacted soils. The first approach is a general expression based on the Kozeny equation that requires only information on the bulk density. The second approach exploits information contained in the water retention curve (WRC). This approach, which relies on Assouline's model for soil HCFs, also provides a basis for a proposed expression to predict the unsaturated HCF of compacted soils. It is shown that a relationship between η, the power parameter in the expression for the HCF, and ε, the coefficient of variation of the WRC, is applicable to a large number of both compacted and uncompacted soils. The model was verified against measured HCFs for soils at different bulk density values. Both the saturated and unsaturated hydraulic conductivities were well reproduced by the suggested expressions.

Hydraulic Conductivity of Soils from Grain-Size Distribution: New Models

This paper presents new developments of regression-based models to predict the saturated hydraulic conductivity of compacted soils from grain-size distribution. The models incorporate parameter values that adequately represent the distribution of grain sizes. Alternative representations of the grain-size distribution, the fractal dimension and entropy of the distributions, as well as porosity, soil density, and fines content are used in the models to estimate the hydraulic conductivity. These parameters that characterize the textural and hydraulic properties of the soil are combined and used in a multidimensional analysis to estimate the hydraulic conductivity. The predictions of the developed models are compared with those of existing models and laboratory measurements of hydraulic conductivity. The results suggest that the newly developed models outperform the existing models in predicting hydraulic conductivity using information from grain-size distribution. The presented models are suggested as alternatives to, for example, laboratory measurements of the hydraulic conductivity of certain soils that may be difficult to prepare or that may take several days or perhaps weeks to perform. In certain circumstances it may also be used to give first-hand information about the hydraulic properties in a field environment.

Hydraulic Conductivity of Compacted Soil Treated with Biofilm

A laboratory study was conducted to evaluate the feasibility of creating low-permeability waste containment barriers using soil treated with bacteria to produce a plugging biofilm. The effects of exopolysaccharide-producing bacteria on the hydraulic conductivity (k) of a compacted silty sand were determined. Values of k for this soil without bacterial treatment ranged from 10−5 and 10−6 cm/s, depending on molding moisture content. Soil specimens were molded with a bacterial and nutrient solution, compacted at optimum moisture content, permeated with nutrient solution, and tested for k using a flexible-wall permeameter. Significant reductions in k were observed, and most specimens reached a stable final k of 10−8 cm/s. The durability of the biofilm was tested by permeation with saline, acidic, and basic solutions, and by subjecting specimens to wet-dry conditions. In most cases these chemical and physical challenges had little or no effect on the reduced k. Results of these tests demonstrate that biofilm treatment may be a feasible technology for creating waste containment barriers in soil.

Electrical‐Resistivity Measurements For Evaluating Compacted‐Soil Liners

Theoretical relationships suggest that the hydraulic conductivity of compacted soils can be evaluated with electrical resistivity measurements. On the material scale, high resistivity may be indicative of low molding water content, high air‐filled porosity, or high sand or gravel content. On the liner scale, resistivity measurements may be useful for identifying desiccation cracking or poor interlift bonding. Methods exist for measuring the resistivity of compacted soils in the laboratory and on field liners. Further evaluation is warranted to fully assess the utility of resistivity measurements for evaluating soil liners.

Depth variations in hydraulic conductivity within a single lift of compacted clay

Compacted clay liners are commonly used as components of the lower portion of composite double liner systems for hazardous waste containment. Because the overlying leachate collection and removal systems and the FMLs are not perfect leachate still comes into contact with the lower liner and thus makes it critical that the clay liner component be constructed to achieve the lowest possible hydraulic conductivity. This research was conducted to evaluate the effects of clod size on the hydraulic conductivity of compacted soils and the uniformity of conductivity with depth within a lift of compacted soil. Two subsoils, one from the Beaumont series (smectitic) and one from the Kosse series (kaolinitic), were evaluated in the laboratory and then compacted in large fixed wall permeameters using maximum clod sizes of <2.5, <5.0, and <7.5 cm to a compacted lift thickness of 23 cm. Measurements were made of the hydraulic conductivity of the entire lift, the lower two thirds of the lift, and the lower one third of the lift. The results show that the conductivity of the lower one third of the lift can be as great as 8.7 times that measured for the entire lift and indicates that liners need to be constructed using thin lifts to achieve more uniform low conductivity throughout the liner. The data also indicated that under the carefully controlled conditions of this study and with the clod sizes used, the clod size did not have a significant effect on the hydraulic conductivity of the soils tested. Soil bulk density was poorly correlated with hydraulic conductivity and indicates that measuring the bulk density of a compacted soil is an inadequate method for assuring low hydraulic conductivity. Measurements of the time to the first appearance of leachate indicated that 8 to 17 d are required for water to penetrate a 23 cm thick compacted liner with an average conductivity of 1 × 10−7 cm s−1.

A Mechanism by which Organic Liquids Increase the Hydraulic Conductivity of Compacted Clay Materials

AbstractThe ability of concentrated organic chemicals, with low dielectric constants (E) to increase the hydraulic conductivity of compacted soils has been well documented. This study was undertaken to evaluate a mechanism by which these increases occur. Theory predicts that the spacing between adjacent clay particles diminishes as the E value decreases. Hydraulic conductivity measurements were made in fixed wall permeameters with a range of concentration of organic liquids in water. Marked increases in hydraulic conductivity occurred at concentrations above which the clay flocculated. Smectite d spacings increased when the clay was equilibrated with acetone and ethanol solutions having E values ranging from 65 to 77, but decreased when equilibrated with solutions having E values &lt;35 to 50 as compared to that in water. All three clays tested exhibited reductions in electrophoretic mobility (µ) and the absolute value of the zeta potential|ζ| which would be associated with flocculation at low E values. Relative clay content measurements indicated that all three soils flocculated in solutions with low E values. At E values &gt;49, all clays remained dispersed. When bulk samples were exposed to organic liquids, the volume changes were proportional to the E values of the permeant. The results indicate that when organic liquids or salts are sufficiently concentrated, soil particles will be drawn closer together until the clays flocculate. Thus, changes in the surface electrical properties of the clays cause shrinkage, resulting in cracking and increases in hydraulic conductivity. Flocculation tests and E values may be used as tools to predict the influence of a solution on the hydraulic conductivity of compacted soils.

Coherence of Clay Crystals in Water

Dry aggregates of Na-illite or Na-montmorillonite usually disperse spontaneously when immersed in water. The corresponding Ca-clays show only limited swelling. However, the latter, when mechanically shaken up with water, do form dispersed suspensions. It follows that at some initial spacing of the Ca-clay crystals, which is less than that corresponding to a suspension, the wet clay will just disperse when immersed in water. This spacing must also be greater than the maximum spacing attained by initially dry, unsheared clay in water.The average spacing between crystals has been varied by adding water to dry clay, followed by a uniform remoulding of the clay. The critical water content or spacing at which dispersion begins has been found. For soil illites this water content can be only 5% greater than the maximum water uptake of the dry, unsheared clay. For montmorillonites with a low N2 surface area, the increase required may be 200% by weight.In contrast to the 2: 1 minerals, Ca-kaolin suspensions flocculate in water. The amount of illite required before a wet, remoulded, kaolin-illite mixture disperses in water has been determined. This amount is equal to that calculated to cover the basal surfaces of the kaolin crystals present.When a dry clay aggregate is immersed in water, although the clay crystals may not disperse, the aggregate can still break up rapidly into discrete fragments (slaking). This is due to differential swelling and entrapped air. If the clay crystals are sufficiently thin, then the pressures developed by entrapped air can be relieved by bending of the crystals, without causing macroscopic dislocation of the aggregate. Thus an aggregate of coarse-grained crystals showing very little swelling in water, e.g. a ceramic kaolin, rapidly fragments when immersed dry in water, whereas an aggregate of a highly swelling clay with very thin crystals, e.g. a Ca-montmorillonite, remains intact.The application of these ideas to two soil properties will be discussed. First, the hydraulic conductivity of compacted soils. Second, the stability of soils to sudden wetting with water.