Permeability Of Unsaturated Soils Research Articles

Measurements of Drying and Wetting Gas Diffusion Coefficients and Gas Permeability of Unsaturated Soils Using a New Flexible-Wall Device

Measurements of Drying and Wetting Gas Diffusion Coefficients and Gas Permeability of Unsaturated Soils Using a New Flexible-Wall Device

Influence of bio-cementation on gas permeability of unsaturated soils in landfill cover system

Influence of bio-cementation on gas permeability of unsaturated soils in landfill cover system

Simple method to determine coefficient of permeability of unsaturated soil

This study introduces a novel transient method for determining the unsaturated coefficient of soil permeability, requiring the measurement of volumetric water content and corresponding pore-water pressures at three distinct points. This distinctive advantage of the method lies in eliminating the need to measure or control the flux at any point within the soil profile. The proposed method was validated through numerical simulations that performed one-dimensional seepage analysis, followed by a comparison of the calculated unsaturated coefficients of permeability with the input data. The strong agreement between the compared results validates the feasibility of the proposed method for determining the unsaturated coefficient of soil permeability. Moreover, a spatial interval of less than 5 cm is recommended to ensure accurate results for the unsaturated coefficient of permeability when employing this method.

Gas permeability and emission in unsaturated vegetated landfill cover with biochar addition

Plant–biochar interaction has been recognized to affect the hydraulic properties of landfill cover soils, while its influence on landfill gas emission is rarely studied. This study investigated the coupled effects of biochar and vegetation on gas permeability and emission in unsaturated landfill cover through an integrated theoretical modelling and laboratory investigation. First, a gas permeability model was developed for vegetated coarse-grained soils with biochar addition. Then, a well-instrumented laboratory column test and two tests from the literature, considering bare, grass, biochar and grass + biochar conditions, were used for model validation. Finally, a numerical parametric study was conducted to investigate the influence of root growth and drought conditions on the gas emission rate. Results showed that the developed model can satisfactorily capture the gas permeability of unsaturated soils at various degrees of saturation. The lowest water retention capacity, the highest gas permeability and gas emission rate after 24 months of growth were observed in the grassed column. However, adding biochar in vegetated soils can maximize the water retention capacity and decrease the gas permeability, resulting in the lowest gas emission rate. The measured gas emission rates for the four cases meet the recommended value by the design guideline. The parametric study showed that the increased root depth from 0.2 m to 0.4 m improved the gas emission rate by 170% in the grass case but decreased by 97% in the grass + biochar case. Under the severe drought condition with soil suction around 500 kPa, the gas emission rate in the grassed case exceeded the design value by 18%, while those in the biochar cases were far below the allowable value. Therefore, peanut shell biochar should be considered to amend the grassed landfill cover using coarse-grained soils as it can significantly improve engineering performance in reducing gas emissions under extreme drought conditions.Graphical abstract

A New Method and Instrument for Measuring In Situ Gas Diffusion Coefficient and Gas Coefficient of Permeability of Unsaturated Soil

A new method and instrument were developed for in situ measurement of the gas diffusion coefficient (Dg) and gas coefficient of permeability (kg) of unsaturated soil. Air containing tracer gas of volumetric concentration of 5%–10% was continuously pumped into a point source at a constant flux rate (q0) until approaching steady state. Then, the pseudo-steady-state tracer gas concentration (Cr0) and gas gauge pressure (ΔPr0) were recorded at a distance (r0) away from the point source. The Dg and kg were determined through the Cr0−q0 and ΔPr0−q0 relationships, respectively. Soil column tests (silty sand) and field tests (sandy lean clay) were conducted to validate the new method. The in situ measured Dg and kg were comparable with those of undisturbed specimens measured by a two-chamber apparatus. The in situ measurement of Dg and kg ranged from 2×10−8 m2 s−1 to 4×10−6 m2 s−1 and 1.9×10−8 ms−1 to 3.9×10−7 ms−1, respectively. The measurement accuracy increased at deeper depth, due to reduced boundary effect at soil surface. The measurement of in situ Dg was more susceptible to the boundary effects than that of kg at shallower depth (i.e., 20 cm deep) in general. The field test results showed that the ratio of in situ Dg (or kg) to that measured by the element tests was generally between 1.2 and 3.3 at or below depth of 0.30 m, due to the in situ heterogeneity of soil.

Prediction of Coefficient of Permeability of Unsaturated Soil

A simple technique is proposed in this paper for estimating the coefficient of permeability of an unsaturated soil based on physical properties of soils that include grain size analysis, degree of saturation or water content, and porosity of the soil. The proposed method requires the soil-water characteristic curve for the prediction of the coefficient of permeability as most of the conventional methods. A procedure is proposed to define the hydraulic conductivity function from the soil water characteristic curve which is measured by the filter paper method. Fitting methods are applied through the program (SoilVision), after indentifying the basic properties of the soil such as Attereberg limits, specific gravity, void ratio, porosity, degree of saturation and wet and dry unit weights.

A New Method for Simultaneous Measurements of Gas Dispersion Coefficient and Gas Coefficient of Permeability of Unsaturated Soils

A New Method for Simultaneous Measurements of Gas Dispersion Coefficient and Gas Coefficient of Permeability of Unsaturated Soils

New simplified transient method for determining the coefficient of permeability of unsaturated soil

The instantaneous profile (IP) method is widely used for determining the coefficient of permeability of unsaturated soils. IP method requires the measurement or control (i.e., zero flux) of flux at any one end of a soil profile. Besides, there are requirements of variations in volumetric water contents (VWCs) and pore-water pressures (PWPs) at various points from the above-mentioned one end of the soil profile to a particular point in which the unsaturated coefficient of permeability is derived. The measurement or control of flux at one end of the soil profile requires high accuracy which causes difficulty while performing tests in the laboratory and field. To overcome the difficulty of the IP method, this study proposes a new simplified transient method for determining the coefficient of permeability of the unsaturated soil. Variations in volumetric water contents and corresponding pore-water pressures at three points are required in the new simplified transient method. The intellectual advantage of this method as compared to the IP method is that there is no need to measure or control the flux at one end of the soil profile. Therefore, this new method is more practical and is expected to be widely used in both laboratory tests and field tests. The validity of the new simplified transient method is verified through numerical simulations considering a one-dimensional seepage for four different types of soils including coarse-grained soil, sand, clay, and silty sand with gravel. The applicability of the new simplified transient method is demonstrated by comparing the unsaturated coefficients of permeability of soils computed by the new simplified transient method and the IP method. Good agreements in comparisons prove that the new simplified transient method is effective for determining the unsaturated coefficient of permeability of soil regardless of soil types and hydraulic functions features.

Evaluating the methods for predicting permeability of unsaturated soils

The unsaturated permeability (kuns) is one of the key parameters in geotechnical and geo-environmental engineering involving unsaturated soils. To measured kuns in a matrix suction range of few Pa to thousands of kPa is time consuming and sometimes not practical. Thus, various methods for predicting the values of kuns were established, and their performances are quite different. The performance of two commonly used methods, i.e. Mualem’s method adopting Van Genuchten’s soil water characteristic curve (SWCC) (designated as MVG), and Fredlund et al.’s method (designated as FXM), have been evaluated comparing the predicted values with the measured data, which are test data from this study and the data from literature. The analysis results indicate that MVG method yielded a better prediction.

Simple Graphical Prediction of Relative Permeability of Unsaturated Soils under Deformations

At present, there are only a few existing models that can be used to predict the relative permeability of unsaturated soil under deformations, and the calculation process is relatively complex. In order to fit the measured value of the relative permeability coefficient of unsaturated soil before deformation, this work employs the simplified unified model of the relative permeability coefficient of unsaturated soil, and it obtains the index λ before deformation of the soil. In addition, the value of index λ remains unchanged before and after deformation. Based on the actual measured value of the soil–water characteristic curve before deformation, the air-entry value prediction model is used to predict the air-entry value of soil with different initial void ratios. The relative permeability coefficient of unsaturated soil is then conveniently predicted using the graphical method in combination with the simplified unified model. The method is validated by using the test data of silt loam, sandy loam, and unconsoildated sand. The results show that the predicted results are consistent with the measured values. The prediction method in this paper is simple and overcomes the limitations associated with the determination of the index λ. It expands the application range of the unsaturated relative permeability coefficient model while improving the accuracy of predictions.

A simple method for predicting the hydraulic properties of unsaturated soils with different void ratios

Reliable estimates of the unsaturated soil hydraulic properties are needed in many research and engineering projects. The void ratio or bulk density is an important factor affecting the unsaturated soil hydraulic properties. The bulk density of one soil can change considerably due to the influences of rain, irrigation, and traffic compaction. It remains a difficult task to accurately describe the hydraulic hysteresis of the unsaturated soil with different void ratios or bulk densities. The aim of this study was to evaluate hydraulic hysteresis behavior and permeability of unsaturated soil with different void ratios or bulk densities based on the normalized soil water retention curves (SWRCs) method. After the normalization, the normalized main drying and wetting SWRCs are independent of the void ratio or bulk density. The hysteresis behavior between the normalized main drying and wetting SWRCs can be described by referring to the existing void ratio independent hysteresis model. Additionally, a simple estimation method to simulate the effects of the void ratio or bulk density on the relative permeability coefficient of unsaturated soils is proposed. The approach proposed here requires no additional parameters. The results show that the proposed hysteresis model can be used to describe the arbitrary main drying, wetting, and scanning curves at arbitrary void ratios over a wide suction range, and the predictions for the relative permeability coefficient with different void ratios by the proposed permeability model can match the experimental data well.

A new method and apparatus for measuring in situ air permeability of unsaturated soil

A new apparatus and an interpretation method simplified from existing solutions of spherical air flow in soil were developed to measure in situ air permeability (ka) of unsaturated soil. By pumping air with a controlled volumetric flux (qv) into a spherical aeration bulb and measuring the corresponding pseudo-steady-state air gauge pressure (Δp0), kawas determined based on the slope of a qv–Δp0relationship. The method was applied to measure kain a full-scale landfill cover. The kameasurements were verified by finite-element modelling of air flow. The measurements show good agreement with the computed results. Sensitivity analyses reveal that using a smaller bulb radius (r0) would increase the measurement accuracy by minimising the boundary effects associated with atmospheric conditions and contrasting kabetween neighbouring soils. Yet, the use of a small bulb would result in more prominent effects of preferential flow and hence substantial overestimation of ka. For a small bulb size (r0= 0.01 m), the measurement accuracy of kacould be within 15% (i.e., 2.4 × 10−13to 6.3 × 10−13m2) of the true values when the ratio of vertical-to-horizontal kais between 0.4 and 1.

A System to Measure Permeability of Unsaturated Soils using Local Pin-Type Sensors in Triaxial Apparatus

A System to Measure Permeability of Unsaturated Soils using Local Pin-Type Sensors in Triaxial Apparatus

Effect of hysteresis on hydraulic properties of soils under multiple drying and wetting cycles

The importance of the hydraulic properties of unsaturated soil for geotechnical designs has been well understood. Many studies on the soil water retention curve (SWRC) and the relative coefficient of permeability under drying and wetting cycles have been performed. However, most of the previous investigations of hydraulic properties are limited to a single drying and wetting cycle or are based on multiple drying–wetting cycles with limited soil experiments. This might be because the measurement of both the SWRC and the coefficient of permeability of unsaturated soil is complex and time consuming. In this study, the hydraulic properties of silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were studied for the same soil specimen under five cycles of drying and wetting in multistep outflow experiments. Significant hysteresis of SWRCs was observed under the first drying and wetting cycle for both soils. However, the hysteresis effect decreased as the number of drying and wetting cycles increased. Furthermore, the coefficient of permeability was estimated using the van Genuchten–Mualem (VGM) model. Results also show that hysteresis plays an important role in the coefficient of permeability for both soils subjected to multiple drying and wetting cycles.

Water permeability of unsaturated soils in the arid zone

The moisture permeability of sandy soils in the unsaturated moisture content regime has been studied. The intensity of moisture conductivity was measured at certain values of the current moisture. It is shown that the expansion of the range of unsaturated soil moisture makes it possible to determine the point of break in the monotony of the curve of simulation of moisture permeability due to the rupture of the capillary connection. The mathematical model of moisture permeability that takes into account the soil porosity is presented.

Numerical modelling of soft ground improvement by vacuum preloading considering the varying coefficient of permeability

Increasing the accuracy of numerical prediction of vacuum preloading methods is very urgent to the geotechnical design engineers. This paper describes the application of the finite element program method incorporating the varying coefficient of permeability with changes in suction (like the variation of permeability in unsaturated soil) to increasing the accuracy of numerical prediction of an axisymmetric single vacuum well point and one case history with prefabricated vertical drains combined with vacuum preloading. The axisymmetric single vacuum well point was modeled using four numerical models: Model 1 with the hydraulic permeability of saturated soil, Model 2 with a thin layer of unsaturated elements activated at the vacuum well boundary, Mode 3 where hydraulic conductivity function was defined as permeability of unsaturated soil, Model 4 where vacuum pressure value remained a constant along the vacuum well drain. Afterward, the case history was modeled using two numerical models: one was created to verify input parameters, and the other was with the application of the varying coefficient of permeability with changes in suction. The results obtained by comparing all the numerical models data with the measured data indicate that the proposed method improved the accuracy of numerical prediction and using the exact distribution of vacuum pressure value or constant vacuum pressure value along the vacuum wellpoint did not make a significant difference in the numerical prediction in the case of shallow vacuum wellpoint.

A New Triaxial Testing System for Unsaturated Soil Characterization

Suction-controlled triaxial tests have been widely used to characterize unsaturated soils. However, this type of test requires sophisticated and therefore expensive equipment, and is very time consuming because of the low permeability of unsaturated soils. Only a few research universities can afford the equipment, which limits the advancement and implementation of unsaturated soil mechanics. This paper proposes a new triaxial testing system for unsaturated soils based upon minor modifications on the conventional triaxial test apparatus for saturated soils. Instead of controlling suction, high-suction tensiometers are adopted to monitor matric suction variations during constant water content triaxial testing. Also, a photogrammetry-based method is used to measure volume changes of unsaturated soil specimens during triaxial testing. To evaluate the capabilities of the proposed testing system, a series of constant water content triaxial tests were performed on unsaturated soils with different moisture content. Matric suction and volume variations during testing were monitored by the high-suction tensiometers and the photogrammetry-based method, respectively. New methods were also proposed to analyze the test results. Analysis results showed that the proposed system is cost effective and efficient for unsaturated soil characterization.

Modification of triaxial apparatus for permeability measurement of unsaturated soils

Both the shear strength and the permeability of unsaturated soils are important engineering properties that are required in numerous geotechnical designs. Many studies on the shear strength of unsaturated soils have been reported; however, only a limited number of studies on the permeability of unsaturated soils have been presented. This might be due to the fact that the time and the costs associated with unsaturated permeability measurements are excessive. The purpose of this paper is to introduce the modification of a triaxial apparatus for the direct measurement of permeability in conjunction with shear strength tests on unsaturated soils under multiple cycles of drying and wetting. Detailed designs and modifications are carried out to allow the unsaturated permeability and the shear strength of a soil to be measured for the same soil specimen. The test results are found to be more consistent as both measurements are conducted on the same specimen. A series of unsaturated permeability tests and then a series of unsaturated consolidated drained (CD) triaxial tests, under multiple cycles of drying and wetting, are conducted on three different soils, and the results are shown to be compatible with the theory reported in literature.

A macroscopic model for predicting the relative hydraulic permeability of unsaturated soils

A macroscopic model for predicting the relative hydraulic permeability of unsaturated soils is proposed. In this model, pores in unsaturated soils are considered to be parallel flow tubes. The water flow in the pores is assumed to take place in the water film on the inside wall of the flow tubes. The viscosity of pore water is considered to be different from the viscosity of pure water and variable with the variation of degree of saturation. The values of tortuosity factor and pore shape factor of unsaturated soils are estimated theoretically. The theoretical model is verified using experimental data for 32 different soils. For application in engineering practice, the value of viscosity of pore water in different soils is proposed.

Testing Study on the Permeability Characteristics of Unsaturated Soils

The coefficient of permeability of unsaturated soil is an important parameter which is used to analyze moisture migration. For complexity and diversity of unsaturated soil behaviors, it is difficult to perform test of unsaturated soil because of many factors. With GDS system, the permeability characteristics of unsaturated soil are tested and studied in this paper which makes it possible to directly measure the coefficient of permeability at various combinations of net normal stresses and matric suction values. The research shows the coefficient of permeability of unsaturated soil is the function of suction and cell pressure.