Unsaturated Permeability Function Research Articles

Influence of Degree of Compaction on Unsaturated Hydraulic Properties of a Compacted Completely Decomposed Granite

It is crucial to understand hydraulic properties, i.e., soil-water characteristic curve (SWCC) and unsaturated permeability function (UPF), of completely decomposed granite (CDG) for relevant engineering projects in southeastern China. Previous studies mainly focused on SWCCs of CDG, whereas UPFs of CDG have not yet been well understood. In this study, the effects of the degree of compaction (DOC) on SWCCs and UPFs of CDG were investigated based on experiments where suction range was from 0 to 500 kPa. The microstructure of soil specimens was then analyzed by mercury intrusion porosimetry (MIP). Furthermore, the UPFs of CDG under different values of DOC were calculated using four prediction models and compared with experimental data. Results showed that the pore volume of specimens at higher DOC was smaller than that at lower DOC, and there were more macropores observed in specimens at lower DOC. Meanwhile, it was found that increasing compaction effort produced negligible influence on the volume of micropores. When the suction was less than 100 kPa, the permeability was reduced with the increase in DOC, due to the decrease of macropore volume. However, the influence of DOC on SWCCs and UPFs became marginal when the suction exceeded 100 kPa. The Fredlund and Xing model provided the best prediction of UPF among the four models when suction was smaller than air entry value (AEV). It is suggested that these models could be improved to capture UPFs at higher suctions than AEV by considering suction-induced volume contraction.

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.

Rainfall thresholds for landslide early warning system in Nakhon Si Thammarat

Transient seepage in unsaturated soil slope is one of the significant triggering factors in rainfall-induced landslides. Rainfall infiltration leads to the decrease in stabilizing effect because of increased positive pore-water pressures. SEEP/W and SLOPE/W used in this study have been widely employed to describe frameworks for understanding transient seepage in soil slope, and to perform slope stability analyses, respectively. The study area is in Sichon District in Nakhon Si Thammarat Province, southern Thailand. A landslide there was investigated by modeling the process of rainfall infiltration under positive and negative pore-water pressures and their effects on slope stability. GIS (Geographic Information System) and geotechnical laboratory results were used as input parameters. The van Genuchten’s soil water characteristic curve and unsaturated permeability function were used to estimate surface infiltration rates. An average rainfall was derived from 30-year monthly rainfall data between 1981 and 2011 in this area reported by the Thailand Royal Irrigation Department. For transient condition, finite element analysis in SEEP/W was employed to model fluctuations in pore-water pressure during a rainfall, using the computed water infiltration rates as surface boundary conditions. SLOPE/W employing Bishop simplified method was then carried out to compute their factors of safety, and antecedent precipitation indices (API) calculated. Heterogeneous slope at the site became unstable at an average critical API (APIcr) of 380 mm, agreeing well with the actual value of 388 mm.

New approach to improve soil-water characteristic curve to reduce variation in estimation of unsaturated permeability function

The unsaturated permeability function is often estimated from the soil-water characteristic curve (SWCC) of a soil. A complete SWCC measurement can improve the estimation of the unsaturated permeability function. In most laboratories, the SWCC can be measured up to a suction of 100 kPa using a Tempe cell. However, complete measurement of the SWCC is an expensive and time-consuming task. Therefore, this paper presents a new approach to estimate SWCC data points beyond 100 kPa suction to complement the SWCC measured up to a suction of 100 kPa. The new SWCC is then used to estimate the unsaturated permeability function. The proposed approach uses knowledge of the grain-size distribution curve and measured SWCC data at 100 kPa suction to estimate the SWCC data points beyond 100 kPa suction. To verify the proposed procedure, SWCC tests were conducted over a wide range of suctions for coarse kaolin and a triaxial permeameter system was used to directly measure unsaturated permeability of the coarse kaolin. The proposed procedure is found to reduce the variation between unsaturated permeability functions estimated by various estimation models.

Water Permeability Function for Soils that Undergo Volume Change as Suction Changes

The saturated/unsaturated coefficient of permeability is required when modeling engineering problems involving transient seepage and contaminant transport. Inaccuracies in the estimation of the coefficient of permeability function can cause errors in numerical modeling results and as a result, affect subsequent engineering decisions. Void ratio changes and degree of saturation changes influence the coefficient of permeability function of a soil. Existing methodologies for estimating the unsaturated permeability function give consideration to degree of saturation changes and may produce reasonable results for soils such as sands and silts which exhibit little volume change with suction change. However, inaccurate results are generated when the soils involved change volume as soil suction changes (e.g., initially slurry Regina clay). Revisions to previously proposed methodologies are needed for the estimation of a saturated/unsaturated coefficient of permeability function that accommodates both void ratio change and degree of saturation change when simulating the drying process. A modification of the Fredlund–Xing–Huang (Can Geotech J 31(4):533–546, 1994) estimation procedure for the permeability function is presented in this paper. Consideration is given to both volume change and desaturation of the soil as soil suction is increased. A laboratory data set for Regina clay is presented and interpreted using the revised methodology. The results illustrate a procedure that can be used to estimate the saturated/unsaturated coefficient of permeability function for a soil that undergoes volume change and desaturation as soil suction increases.

Examination of the estimation of relative permeability for unsaturated soils

The unsaturated permeability function is an important soil property function used in the numerical modeling of saturated–unsaturated soil systems. The permeability function is generally predicted by integrating along the soil-water characteristic curve (SWCC) starting at saturated soil conditions. The integration is based on a particular integral formula. The Fredlund–Xing–Huang permeability function is a flexible integration technique used for calculating the unsaturated permeability function. The original permeability theory published by Fredlund, Xing, and Huang in 1994 specified that the air-entry value (AEV), ψaev, be used as the lower limit of the integration when calculating the permeability function. However, as there was no analytical procedure available for the calculation of the AEV on the SWCC, it became common practice to start the integration procedure from a value near zero. The assumption was made that the error associated with starting the integration from an arbitrary low value was minimal. While this might be the case in some situations, the error can be quite substantial in other situations. This paper undertakes a study of the effect of the lower limit of integration on the calculation of the permeability function. Comparisons are made between starting the integration from various values below the AEV and starting the integration from the calculated AEV, ψaev. A mathematical algorithm is also proposed for the calculation of the AEV for integration purposes. The results show that the relative coefficient of permeability can be significantly underestimated when the lower limit of integration is smaller than the AEV. The recommendation is that the AEV always be used as the lower limit of integration in the Fredlund–Xing–Huang permeability equation.

Effects of soil–water characteristic curve and relative permeability equations on estimation of unsaturated permeability function

Unsaturated permeability function can be estimated by theoretical models from soil–water characteristic curve (SWCC). To date, there are numerous estimation models that can be used to obtain the unsaturated permeability function from SWCC. However, each model results in a different estimation curve. The reason for this difference is not well understood.In this study, the available SWCC equations and the available relative permeability (kr) equations were combined to form a matrix of unsaturated permeability estimation models. The matrix of unsaturated permeability estimation models was used to study the effect of SWCC equations and relative permeability (kr) equations as controlling factors in the estimation of unsaturated permeability function. The study was conducted using twenty sets of published experimentally measured SWCC and unsaturated permeability data (kw). The effects of the SWCC equations and kr equations on a variation of estimated unsaturated permeability functions are presented in this paper. It was found that the SWCC equation has a more significant effect on the estimation of unsaturated permeability function than that of the kr equation.

Effect of hydraulic properties of soil and fluctuation velocity of reservoir water on landslide stability

Water fluctuation is the main triggering factor of reservoir slope failures, especially in the area of the Three Gorges Reservoir of China. Fluctuation velocity of reservoir water and hydraulic properties of soil as defined by soil–water characteristic curve (SWCC), saturated permeability coefficient and unsaturated permeability function are key potential properties that control reservoir landslide stability. The effect of reservoir water fluctuation velocity and hydraulic properties of soil on landslide stability are investigated through a series of numerical simulations with different parameters. The results of simulations show that fitting parameters in the SWCC [i.e., a, n, m for Fredlund and Xing (Can Geo J, 31(3):521–532. doi: 10.1139/t94-061 , 1994) equations] have significant effect on the stability of landslide for reservoir water’s drawdown or impounding process. The saturated permeability coefficient of soil and velocity of water level fluctuation have comprehensive (defined as Impact Factor, α) and significant influence on the stability of reservoir landslide. A relative equation: m > α > n > a has been drawn for the susceptibility of effect on the stability of reservoir landslide for the parameters “a”, “n”, “m” and “α”.

Effect of range of soil–water characteristic curve measurements on estimation of permeability function

The most commonly used indirect method to determine unsaturated permeability of soil is to estimate the unsaturated permeability function from soil–water characteristic curve and saturated permeability. The suction range that soil–water characteristic curve (SWCC) can be measured in the laboratory depends on the type of equipment used for measurement. As the estimation models use the available measured SWCC data to obtain the unsaturated permeability function, the estimated permeability function can be affected by the number of measured data of SWCC and the suction range over which the SWCC data are measured. Therefore, there is a need to investigate the effect of the range of SWCC measurements on the estimation of unsaturated permeability function. In this study, the effect of the range of SWCC measurements is investigated through different estimation models that are based on four different best-fit soil–water characteristic curve equations and three different relative permeability equations. It was found that the range of SWCC measurements greatly affect the estimated permeability functions. It was also found that the effect of the range of SWCC measurements is more significant than the selected best-fit SWCC equation used.

정수슬러지 혼합토의 함수특성곡선과 불포화 투수 특성

In this paper, in order to solve high water content of water sludge and promote its recycle, sludge mixtures with various mixing ratios were produced. Sludge mixture consisted of water sludge and weathered granite soil. Their physical properties and unsaturated characteristics (soil-water characteristic curve, and unsaturated permeability function) were investigated by laboratory tests. Experimental test results indicated that at a given matric suction volumetric water content of sludge mixture increased as water sludge content increased. Air entry values of sludge mixture increased from 0.9 kPa to 2.4 kPa with an increase in water sludge content or fine content. In addition, unsaturated permeability function, which is an important factor for performing infiltration analysis, was predicted using saturated permeability and soil-water characteristic curve of sludge mixture.

An analytical solution for rainfall infiltration into an unsaturated infinite slope and its application to slope stability analysis

SUMMARYSurficial slope failures in residual soils are common in tropical and subtropical regions as a result of rainfall infiltration. This study develops an analytical solution for simulating rainfall infiltration into an infinite unsaturated soil slope. The analytical solution is based on the general partial differential equation for water flow through unsaturated soils. It can accept soil–water characteristic curve and unsaturated permeability function of the exponential form into account. Numerical simulations are conducted to verify the assumptions of the analytical solution and demonstrate that the proposed analytical solution is acceptable for the coarse soils with low air entry values. The pore‐water pressure (pwp) distributions obtained from the analytical solution can be incorporated into a limit equilibrium method to do infinite slope stability analysis for a rain‐induced shallow slip. The analysis method takes into account the influence of the water content change on unit weight and hence on factor of safety. A series of analytical parametric analyses have been performed using the developed model. The analyses indicate that when the residual soil slope, consisting of a completely decomposed granite layer underlain by a less permeable layer, is subjected to a continuous heavy rainfall, the loss of negative pwp and the reduction in factor of safety were found to be most significant for the shallow soil layer and during the first 12 h. The antecedent and subsequent rainfall intensity, depth of a less permeable layer and slope angle all have a significant influence on the pwp response and hence the slope stability. Copyright © 2012 John Wiley & Sons, Ltd.

A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong

It is well recognised that field measurements of stress-dependent soil–water characteristic curves (SDSWCCs) and the permeability function (coefficient of permeability with respect to the water phase) are vital for assessing transient seepage, pore water pressure changes and the stability of unsaturated soil slopes. In this study, SDSWCCs and the permeability function were measured by using the instantaneous profile method on a saprolitic hillside in Tung Chung, Hong Kong. The ground profile comprises colluvium and completely decomposed tuff (CDT). A 3 m diameter circular plot was instrumented and subjected to two wetting–drying cycles. Throughout a 48-day monitoring period, variations of volumetric water content and matrix suction in the uppermost 3 m of soil were measured continuously using time-domain reflectometry moisture probes and jet-fill tensiometers, respectively. The field observations reveal that there is a clear trend of decreasing sizes of hysteretic loops of SDSWCCs with depth. In a comparison of the field- and laboratory-measured SDSWCCs, general agreement can be seen in the first wetting–drying cycle but not in the second cycle. The field-observed unsaturated permeability function is highly dependent on suction history (wetting–drying cycles). The field-measured permeability ranges from 4 × 10−7 m/s to 3 × 10−4 m/s in the colluvium, whereas the corresponding measured values vary from 4 × 10−7 m/s to 1 × 10−4 m/s in the CDT. Predictions of SWCC and permeability function using existing empirical approaches based on particle size distributions (PSDs) should be treated with caution. There is a clear qualitative and quantitative inconsistency between predictions of permeability function from PSD and field measurements.

Effect of hydraulic properties of soil on rainfall-induced slope failure

Rainfall-induced slope failure is a common occurrence in many parts of the world. Particularly in tropical regions which are covered extensively with residual soils, the temporal frequency of rainfall-induced slope failure is very high. Hydraulic properties of soil as defined by soil–water characteristic curve, saturated coefficient of permeability and unsaturated permeability function are essential properties that affect rainfall-induced slope failures. The effects of hydraulic properties of soil on rainfall-induced slope failure are investigated through a series of parametric studies. The results of the parametric studies show that soil–water characteristic curve fitting parameters affect the stability of poor drainage soil slopes more significantly than the stability of good drainage soil slopes. The stability of good drainage soil slopes is not sensitive to the variation of soil–water characteristic curve fitting parameters. The saturated coefficient of permeability, k s, of soil has a unique effect on the stability of both good and poor drainage soil slopes.

A Drainage Column Test for Determining Unsaturated Properties of Coarse Materials

Abstract A column test was developed to define the water retention curve and the unsaturated permeability function of coarse materials during drainage. The test includes five steps. First, the material is placed in the column at a constant density, using vacuum and deaired water to reach close to 100 % saturation, which is checked using a mass and volume method. Second, the saturated hydraulic conductivity is determined by a constant head test. Third, a gravity drainage test is performed and the volume of drained water is monitored versus time. Fourth, after full drainage, which can take several weeks, the material is removed from the top of the column, to determine the water content versus elevation and thus the water retention curve. Fifth, the water retention data and selected models are used to predict the drainage flow rate and compare the predictions to the experimental data. This helps to define the best hydraulic functions for the material. Examples are provided to illustrate the key elements of the test.

Measurement and estimation of pore shrinkage and pore distribution in a clayey till during soil-water characteristic curve tests

The soil-water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In many applications, such as design of mine waste covers and landfill liners, the unsaturated permeability function, k(h), is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and mono-modal. PSDs of a clayey till compacted at various water contents were measured after compaction, after flexible-wall permeability testing, and during and after SWCC tests. The measurements show that the PSD changes significantly during permeability and SWCC testing. A method is advanced for predicting the observed changes in PSD during SWCC testing. PSDs are determined for soil samples subjected to the highest and lowest suctions applied during the SWCC test. The measured PSDs are transformed to account for pore trapping; the transform assumes that flow occurs through two sets of randomly distributed pores in series. To model pore shrinkage, the pores are idealized as elastic cylinders. PSDs measured after different suction applications in the SWCC tests are compared with predictions of the shrinkage model. The method can also be used to predict the SWCC. Measured and predicted values are compared.Key words: landfill liners, mine waste covers, soil-water characteristic curve, pore-size distribution.