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Hangseok Choi - One of the best experts on this subject based on the ideXlab platform.

  • consideration of bentonite cake existing on vertical cutoff wall in Slug Test analysis
    Journal of the Korean Geotechnical Society, 2013
    Co-Authors: Jeehee Lim, The-bao Nguyen, Dongseop Lee, Jaeyoon Ahn, Hangseok Choi
    Abstract:

    Slug Tests can be adopted to estimate hydraulic conductivity of the slurry trench wall backfill for its abilities to reflect the in-situ performance of the construction. A comprehensive three-dimensional numerical model is developed to simulate the Slug Test in a slurry trench wall considering the presence of bentonite cake on the interface boundaries between the wall and the surrounding soil formation. Influential factors such as wall width (i.e., proximity of wall boundary), well deviation, vertical position of well intake section, compressibility of wall backfill, etc. are taken into account in the model. A series of simulation results are examined to evaluate the bentonite cake effect in analyzing practical Slug Test results in the slurry trench wall. The results show that the modified line-fitting method can be used without any correction factor for the Slug Test in the slurry trench wall with the presence of bentonite cake. A case study is reanalyzed with the assumption of existing bentonite cake. The results are compared with the previously reported results by the approaches assuming no bentonite cake (constant-head boundary) or upper-bound solution (no-flux boundary). The case study demonstrates the bentonite cake effect and the validity of the modified line-fitting method in the estimation of the hydraulic conductivity of the slurry wall backfill.

  • Analytical Interpretation of Slug Test in a Vertical Cutoff Wall
    Ground Water, 2013
    Co-Authors: Jeehee Lim, Dongseop Lee, Vitaly A. Zlotnik, Hangseok Choi
    Abstract:

    An analysis method for Slug Tests performed in a partially penetrating well within a vertical cutoff wall is presented. A steady-state shape factor for evaluating hydraulic conductivity of the material within the wall was derived by applying the method of images to the previously developed analytical solution of Zlotnik et al. (2010) for an infinite aquifer. Two distinct boundary conditions were considered: constant-head boundary for the case of direct contact between the wall and the aquifer, and no-flux boundary representing an impermeable filter cake on the sides of the wall. The constant-head and no-flux boundary conditions yield significantly higher and lower shape factors, respectively, than those for the infinite aquifer. Consequently the conventional line-fitting method for Slug Test analysis would yield an inaccurate estimate of the hydraulic conductivity of a vertical cutoff wall.

  • Effect of Real Bentonite Cake on Slug Test Analysis for Slurry Trench Wall
    Journal of Geotechnical and Geoenvironmental Engineering, 2013
    Co-Authors: The-bao Nguyen, Jeehee Lim, Dongseop Lee, Hangseok Choi
    Abstract:

    The Slug Test is a viable method in estimating the hydraulic conductivity of the slurry trench wall backfill because of its ability to consider a more representative volume of the backfill and to reflect the in situ performance of the construction. A three-dimensional numerical model is developed to simulate the Slug Test in a slurry trench wall with the presence of bentonite cake on the interface boundary between the wall and the surrounding soil formation. Influential factors such as wall width (i.e., proximity of wall boundary), well deviation, vertical position of the well intake section, and compressibility of the wall backfill are taken into account in the model. The experimentally obtained hydraulic properties of the bentonite cake are also incorporated in a series of Slug Test simulations. The simulation results are then examined to evaluate the bentonite cake effect in analyzing practical Slug Test results in the slurry trench wall. The simulation results show that the modified line-fitting method can be used without any reduction factor for the Slug Test in the slurry trench wall with the presence of bentonite cake. A case study is reanalyzed with the assumption of existing bentonite cake. The results are compared with the previously reported results by the approaches used for the case of no bentonite cake (constant-head boundary) and upper-bound solution (no-flux boundary). The modified line-fitting method and the type curve method produce similar results for slurry walls with bentonite cakes. The case study results demonstrate the importance of the bentonite cake effect in estimating the hydraulic conductivity of the slurry wall backfill.

  • analytical solution for transient groundwater flow in vertical cutoff walls application of Slug Test and evaluation of hydraulic conductivity
    Journal of the Korean Geotechnical Society, 2012
    Co-Authors: Jeehee Lim, Dongseop Lee, Hangseok Choi
    Abstract:

    No analytical solution exists for evaluating in-situ hydraulic conductivity of vertical cutoff walls by analyzing Slug Test results with consideration of transient flow. There is an analytical solution proposed to interpret a Slug Test performed in a partially penetrated well within an aquifer. However, this analytical solution cannot be directly applied to the cutoff wall because the solution has been developed exclusively for an infinite aquifer instead of a narrow cutoff wall. To consider the cutoff wall boundary conditions (i.e, constant head boundary and no flux boundary condition), the analytical solution has been modified in this study to take into account the narrow boundaries by introducing the imaginary well theory. Type curves are constructed from the currently derived analytical solution and compared with those of a partially penetrated well within an aquifer. The constant head boundary condition provides faster hydraulic head recovery curve than the aquifer case. On the other hand, no flux boundary condition leads to slower hydraulic head recovery. The bigger the shape factor and deviation of the well and the smaller the width of the vertical cutoff wall are, the more effect of boundary condition was observed. The type curves obtained from the analytical solution for a cutoff wall are similar to those made by the numerical method in the literature.

  • general steady state shape factors in analyzing Slug Test results to evaluate in situ hydraulic conductivity of vertical cutoff wall
    Journal of the Korean Geotechnical Society, 2011
    Co-Authors: Jeehee Lim, The-bao Nguyen, Dongseop Lee, Hangseok Choi
    Abstract:

    No analytical solution exists for evaluating in-situ hydraulic conductivity of vertical cutoff walls by analyzing Slug Test results. Recently, an analytical solution to interpret Slug Tests has been proposed for a partially penetrated well in an aquifer. However, this analytical solution cannot be directly applied to the cutoff wall because the solution has been developed exclusively for an infinite aquifer instead of a narrow cutoff wall. To consider the cutoff wall boundary conditions, the analytical solution has been modified in this study to take into account the narrow boundaries by introducing the imaginary well theory. Two boundary conditions are considered according to the existence of filter cakes: constant head boundary and no flux boundary. Generalized steady-state shape factors are presented for each geometric condition, which can be used for evaluating the in-situ hydraulic conductivity of cutoff walls. The constant head boundary condition provides higher shape factors and no flux boundary condition provides lower shape factors than the infinite aquifer, which enables to adjust the in-situ hydraulic conductivity of the cutoff wall. The hydraulic conductivities calculated from the analytical solution in this paper give about 1.2~1.7 times higher than those from the Bouwer and Rice method, one of the semi-empirical formulas. Considering the compressibility of the backfill material, the analytical solution developed in this study was proved to correspond to the case of incompressible backfill materials.

Hund-der Yeh - One of the best experts on this subject based on the ideXlab platform.

  • Parameter Identification for a Slug Test in a Well with Finite-Thickness Skin Using Extended Kalman Filter
    Water Resources Management, 2012
    Co-Authors: By Yen-chen Huang, Hund-der Yeh
    Abstract:

    Yeh and Chen (J Hydro 342(3–4):283-294, 2007 ) integrated a Slug Test solution for a well having a finite-thickness skin with the simulated annealing (SA) to determine the hydraulic parameters of the skin zone and formation zone. Some results obtained in positive-skin scenarios are however not accurate if compared with the target values of the parameters. This study first employs the sensitivity and correlation analyses to quantify the relationship between two normalized sensitivities and analyze the resulting errors in parameter estimates. It is found that the inaccuracy in parameter estimates can be attributed to following two problems: (1) the normalized sensitivities of the skin thickness and hydraulic conductivity are highly correlated and (2) the SA algorithm is very sensitive to round-off error in well-water-level (WWL) data. A parameter identification approach is thus developed based on the extended Kalman filter (EKF) coupled with the solution used by Yeh and Chen (J Hydro 342(3–4):283-294, 2007 ) to determine the parameters in six positive-skin scenarios where the parameters were not accurately determined before. We show that previous two problems can be overcome by the proposed approach because it is designed to account for uncertainties of measurements. Moreover, the EKF can save 99.8% and 99.9% computing time when compared with the results using the SA in analyzing 20 WWL data and 47 WWL data, respectively.

  • semi analytical solution for a Slug Test in partially penetrating wells including the effect of finite thickness skin
    Hydrological Processes, 2008
    Co-Authors: Hund-der Yeh, Yen Ju Chen, Shaw Yang Yang
    Abstract:

    This paper presents a new semi-analytical solution for a Slug Test in a well partially penetrating a confined aquifer, accounting for the skin effect. This solution is developed based on the solution for a constant-flux pumping Test and a formula given by Peres and co-workers in 1989. The solution agrees with that of Cooper and co-workers and the KGS model when the well is fully penetrating. The present solution can be applied to simulate the temporal and spatial head distributions in both the skin and formation zones. It can also be used to demonstrate the influences of skin type or skin thickness on the well water level and to estimate the hydraulic parameters of the skin and formation zones using a least-squares approach. The results of this study indicate that the determination of hydraulic conductivity using a conventional Slug-Test data analysis that neglects the presence of a skin zone will give an incorrect result if the aquifer has a skin zone.

  • determination of skin and aquifer parameters for a Slug Test with wellbore skin effect
    Journal of Hydrology, 2007
    Co-Authors: Hund-der Yeh, Yen Ju Chen
    Abstract:

    Slug Test is considered to reflect the hydraulic parameters in the vicinity of the Test well. The aquifer parameters are usually identified by fitting an appropriate mathematical solution or graphical type curves with Slug Test data. In this paper, we developed an approach by combining [Moench, A.F., Hsieh, P.A., 1985. Analysis of Slug Test data in a well with finite-thickness skin. In: Memoirs of the 17th International Congress on the Hydrogeology of Rocks of Low Permeability, U.S.A. Members of the International Association of Hydrologists, Tucson, AZ, vol. 17, pp. 17-29] and simulated annealing (SA) approach to estimate five parameters, i.e., three skin parameters and two aquifer parameters. The three skin parameters are hydraulic conductivity, specific storage, and thickness of the wellbore-skin zone, while the two aquifer parameters are hydraulic conductivity and specific storage of the formation zone. It is worthy to note although the thickness of the wellbore-skin zone is usually taken as an input data for the data-analyzed software, it is actually an unknown parameter that cannot be measured directly. This paper proposes a methodology for estimating the thickness of the wellbore-skin zone with other hydraulic parameters at the same time. Eight sets of well water-level (WWL) data of aquifers with both positive and negative skins are generated by Moench and Hsieh [Moench and Hsieh, 1985] and four sets of standard normally distributed noise are then added to each set of WWL data. The results indicate that the negative-skin cases generally give a better estimated result than that of the positive-skin cases. Sensitivity analysis is also employed to demonstrate the physical behavior when Slug Test was performed under positive-skin effect. For the case of an aquifer with a positive-skin, the use of a longer series of WWL data for analysis is strongly recommended for better estimation of aquifer hydraulic conductivity. Analyzing the WWL data of the Test and observation wells simultaneously could significantly improve the estimations on specific storages. Impetuously presuming an arbitrary value for the thickness of the wellbore-skin zone may lead to poor estimation for the other four parameters.

  • A novel analytical solution for a Slug Test conducted in a well with a finite-thickness skin
    Advances in Water Resources, 2006
    Co-Authors: Hund-der Yeh, Shaw Yang Yang
    Abstract:

    An aquifer containing a skin zone is considered as a two-zone system. A mathematical model describing the head distribution is presented for a Slug Test performed in a two-zone confined aquifer system. A closed-form solution for the model is derived by Laplace transforms and Bromwich integral. This new solution is used to investigate the effects of skin type, skin thickness, and the contrast of skin transmissivity to formation transmissivity on the distributions of dimensionless hydraulic head. The results indicate that the effect of skin type is marked if the Slug-Test data is obtained from a radial two-zone aquifer system. The dimensionless well water level increases with the dimensionless positive skin thickness and decreases as the dimensionless negative skin thickness increases. In addition, the distribution of dimensionless well water level due to the Slug Test depends on the hydraulic properties of both the wellbore skin and formation zones.

  • A simple approach using Bouwer and Rice's method for Slug Test data analysis.
    Ground water, 2004
    Co-Authors: Shaw Yang Yang, Hund-der Yeh
    Abstract:

    Slug Test data obtained from Tests performed in an unconfined aquifer are commonly analyzed by graphical or numerical approaches to determine the aquifer parameters. This paper derives three fourth-degree polynomials to represent the relationship between Bouwer and Rice's coefficients and the ratio of the screen length to the radius of the gravel envelope. A numerical approach using the nonlinear least squares and Newton's method is used to determine hydraulic conductivity from the best fit of the Slug Test data. The method of nonlinear least squares minimizes the sum of the squares of the differences between the predicted and observed water levels inside the well. With the polynomials, the hydraulic conductivity can be obtained by simply solving the nonlinear least squares equation by Newton's method. A computer code, SlugBR, was developed from the derived polynomials using the proposed numerical approach. The results of analyzing two Slug Test datasets show that SlugBR can determine hydraulic conductivity with very good accuracy.

Shaw Yang Yang - One of the best experts on this subject based on the ideXlab platform.

  • semi analytical solution for a Slug Test in partially penetrating wells including the effect of finite thickness skin
    Hydrological Processes, 2008
    Co-Authors: Hund-der Yeh, Yen Ju Chen, Shaw Yang Yang
    Abstract:

    This paper presents a new semi-analytical solution for a Slug Test in a well partially penetrating a confined aquifer, accounting for the skin effect. This solution is developed based on the solution for a constant-flux pumping Test and a formula given by Peres and co-workers in 1989. The solution agrees with that of Cooper and co-workers and the KGS model when the well is fully penetrating. The present solution can be applied to simulate the temporal and spatial head distributions in both the skin and formation zones. It can also be used to demonstrate the influences of skin type or skin thickness on the well water level and to estimate the hydraulic parameters of the skin and formation zones using a least-squares approach. The results of this study indicate that the determination of hydraulic conductivity using a conventional Slug-Test data analysis that neglects the presence of a skin zone will give an incorrect result if the aquifer has a skin zone.

  • A novel analytical solution for a Slug Test conducted in a well with a finite-thickness skin
    Advances in Water Resources, 2006
    Co-Authors: Hund-der Yeh, Shaw Yang Yang
    Abstract:

    An aquifer containing a skin zone is considered as a two-zone system. A mathematical model describing the head distribution is presented for a Slug Test performed in a two-zone confined aquifer system. A closed-form solution for the model is derived by Laplace transforms and Bromwich integral. This new solution is used to investigate the effects of skin type, skin thickness, and the contrast of skin transmissivity to formation transmissivity on the distributions of dimensionless hydraulic head. The results indicate that the effect of skin type is marked if the Slug-Test data is obtained from a radial two-zone aquifer system. The dimensionless well water level increases with the dimensionless positive skin thickness and decreases as the dimensionless negative skin thickness increases. In addition, the distribution of dimensionless well water level due to the Slug Test depends on the hydraulic properties of both the wellbore skin and formation zones.

  • A simple approach using Bouwer and Rice's method for Slug Test data analysis.
    Ground water, 2004
    Co-Authors: Shaw Yang Yang, Hund-der Yeh
    Abstract:

    Slug Test data obtained from Tests performed in an unconfined aquifer are commonly analyzed by graphical or numerical approaches to determine the aquifer parameters. This paper derives three fourth-degree polynomials to represent the relationship between Bouwer and Rice's coefficients and the ratio of the screen length to the radius of the gravel envelope. A numerical approach using the nonlinear least squares and Newton's method is used to determine hydraulic conductivity from the best fit of the Slug Test data. The method of nonlinear least squares minimizes the sum of the squares of the differences between the predicted and observed water levels inside the well. With the polynomials, the hydraulic conductivity can be obtained by simply solving the nonlinear least squares equation by Newton's method. A computer code, SlugBR, was developed from the derived polynomials using the proposed numerical approach. The results of analyzing two Slug Test datasets show that SlugBR can determine hydraulic conductivity with very good accuracy.

Jacques Bodin - One of the best experts on this subject based on the ideXlab platform.

  • Cross-borehole Slug Test analysis in a fractured limestone aquifer.
    Journal of Hydrology, 2008
    Co-Authors: Olivier Audouin, Jacques Bodin
    Abstract:

    This work proposes new semi-analytical solutions for the interpretation of cross-borehole Slug Tests in fractured media. Our model is an extension of a previous work by Barker (1988) and Butler and Zhan (2004). It includes inertial effects at both Test and observation wells and a fractional flow dimension in the aquifer. The model has 5 fitting parameters: flow dimension n, hydraulic conductivity K, specific storage coefficient Ss, and effective lengths of Test well Le and of observation well Leo. The results of a sensitivity analysis show that the most sensitive parameter is the flow dimension n. The model sensitivity to other parameters may be ranked as follows: K > Le ~ Leo > Ss. The sensitivity to aquifer storage remains one or two orders of magnitude lower than that to other parameters. The model has been coupled to an automatic inversion algorithm for facilitating the interpretation of real field data. This inversion algorithm is based on a Gauss-Newton optimization procedure conditioned by re-scaled sensitivities. It has been used to interpret successfully cross-borehole Slug Test data from the Hydrogeological Experimental Site (HES) of Poitiers, France, consisting of fractured and karstic limestones. HES data provide flow dimension values ranging between 1.6 and 2.5, and hydraulic conductivity values ranging between 4.4x10-5 and 7.7x10-4 m.s-1. These values are consistent with previous interpretations of single-well Slug Tests. The results of the sensitivity analysis are confirmed by calculations of relative errors on parameter estimates, which show that accuracy on n and K is below 20% and that on Ss is about one order of magnitude. The K-values interpreted from cross-borehole Slug Tests are one order of magnitude higher than those previously interpreted from interference pumping Tests. These findings suggest that cross-borehole Slug Tests focus on preferential flowpath networks made by fractures and karstic channels, i.e. the head perturbation induced by a Slug Test propagates only through those flowpaths with the lowest hydraulic resistance. As a result, cross-borehole Slug Tests are expected to identify the hydrodynamic properties of karstic channels and fracture flowpaths, and may be considered as complementary to pumping Tests which more likely provide bulk properties of the whole fracture/karstic-channel/matrix system.

  • Cross-borehole Slug Test analysis in a fractured limestone aquifer
    Journal of Hydrology, 2008
    Co-Authors: Olivier Audouin, Jacques Bodin
    Abstract:

    International audienceThis work proposes new semi-analytical solutions for the interpretation of cross-borehole Slug Tests in fractured media. Our model is an extension of a previous work by Barker (1988) and Butler and Zhan (2004). It includes inertial effects at both Test and observation wells and a fractional flow dimension in the aquifer. The model has 5 fitting parameters: flow dimension n, hydraulic conductivity K, specific storage coefficient Ss, and effective lengths of Test well Le and of observation well Leo. The results of a sensitivity analysis show that the most sensitive parameter is the flow dimension n. The model sensitivity to other parameters may be ranked as follows: K > Le ~ Leo > Ss. The sensitivity to aquifer storage remains one or two orders of magnitude lower than that to other parameters. The model has been coupled to an automatic inversion algorithm for facilitating the interpretation of real field data. This inversion algorithm is based on a Gauss-Newton optimization procedure conditioned by re-scaled sensitivities. It has been used to interpret successfully cross-borehole Slug Test data from the Hydrogeological Experimental Site (HES) of Poitiers, France, consisting of fractured and karstic limestones. HES data provide flow dimension values ranging between 1.6 and 2.5, and hydraulic conductivity values ranging between 4.4x10-5 and 7.7x10-4 m.s-1. These values are consistent with previous interpretations of single-well Slug Tests. The results of the sensitivity analysis are confirmed by calculations of relative errors on parameter estimates, which show that accuracy on n and K is below 20% and that on Ss is about one order of magnitude. The K-values interpreted from cross-borehole Slug Tests are one order of magnitude higher than those previously interpreted from interference pumping Tests. These findings suggest that cross-borehole Slug Tests focus on preferential flowpath networks made by fractures and karstic channels, i.e. the head perturbation induced by a Slug Test propagates only through those flowpaths with the lowest hydraulic resistance. As a result, cross-borehole Slug Tests are expected to identify the hydrodynamic properties of karstic channels and fracture flowpaths, and may be considered as complementary to pumping Tests which more likely provide bulk properties of the whole fracture/karstic-channel/matrix system

  • Cross-borehole Slug Test analysis in a fractured limestone aquifer
    Journal of Hydrology, 2007
    Co-Authors: Olivier Audouin, Jacques Bodin
    Abstract:

    Summary This work proposes new semi-analytical solutions for the interpretation of cross-borehole Slug Tests in fractured media. Our model is an extension of a previous work by Barker [Barker, J.A., 1988. A generalized radial flow model for hydraulic Tests in fractured rock. Water Resources Research 24 (10), 1796–1804; Butler Jr., J.J., Zhan X., 2004. Hydraulic Tests in highly permeable aquifers. Water Resources Research 40, W12402. doi:10.1029/2003/WR002998]. It includes inertial effects at both Test and observation wells and a fractional flow dimension in the aquifer. The model has five fitting parameters: flow dimension n, hydraulic conductivity K, specific storage coefficient Ss, and effective lengths of Test well Le and of observation well Leo. The results of a sensitivity analysis show that the most sensitive parameter is the flow dimension n. The model sensitivity to other parameters may be ranked as follows: K > Le ∼ Leo > Ss. The sensitivity to aquifer storage remains one or two orders of magnitude lower than that to other parameters. The model has been coupled to an automatic inversion algorithm for facilitating the interpretation of real field data. This inversion algorithm is based on a Gauss–Newton optimization procedure conditioned by re-scaled sensitivities. It has been used to interpret successfully cross-borehole Slug Test data from the Hydrogeological Experimental Site (HES) of Poitiers, France, consisting of fractured and karstic limestones. HES data provide flow dimension values ranging between 1.6 and 2.5, and hydraulic conductivity values ranging between 4.4 × 10−5 and 7.7 × 10−4 m s−1. These values are consistent with previous interpretations of single-well Slug Tests. The results of the sensitivity analysis are confirmed by calculations of relative errors on parameter estimates, which show that accuracy on n and K is below 20% and that on Ss is about one order of magnitude. The K-values interpreted from cross-borehole Slug Tests are one order of magnitude higher than those previously interpreted from interference pumping Tests. These findings suggest that cross-borehole Slug Tests focus on preferential flowpath networks made by fractures and karstic channels, i.e. the head perturbation induced by a Slug Test propagates only through those flowpaths with the lowest hydraulic resistance. As a result, cross-borehole Slug Tests are expected to identify the hydrodynamic properties of karstic-channels and fracture flowpaths, and may be considered as complementary to pumping Tests which more likely provide bulk properties of the whole fracture/karstic-channel/matrix system.

  • Analysis of Slug-Tests with high-frequency oscillations
    Journal of Hydrology, 2007
    Co-Authors: Olivier Audouin, Jacques Bodin
    Abstract:

    Extensive Slug-Test experiments have been performed at the Hydrogeological Experimental Site (HES) of Poitiers in France, made up of moderately fractured limestones. All data are publicly available through the "H+" database, developed within the scope of the ERO program (French Environmental Research Observatory, http://hplus.ore.fr). Slug-Test responses with high-frequency (> 0.12 Hz) oscillations have been consistently observed in wells equipped with multiple concentric casing. These oscillations are interpreted as the result of inertia-induced fluctuations of the water level in the annular space between the inner and outer casing. In certain cases, these high-frequency oscillations overlap with lower frequency (< 0.05 Hz) oscillations, which leads to complex responses that cannot be interpreted using conventional models. Slug-Test data have been processed in the Fourier-frequency domain, in order to remove the high-frequency component by a signal-filtering method. The corrected signals have been interpreted with the model of (McElwee and Zenner, 1998), which accounts for the inertia of the water-column above the well screen, non-linear head losses in the well, and neglects the aquifer storage (quasi-steady-state approximation). Hydraulic conductivity values interpreted from dual-frequency Slug-Tests compare well to those interpreted from "standard" overdamped or underdamped Slug-Test responses.

  • Analysis of Slug-Tests with high-frequency oscillations
    Journal of Hydrology, 2007
    Co-Authors: Olivier Audouin, Jacques Bodin
    Abstract:

    International audienceExtensive Slug-Test experiments have been performed at the Hydrogeological Experimental Site (HES) of Poitiers in France, made up of moderately fractured limestones. All data are publicly available through the "H+" database, developed within the scope of the ERO program (French Environmental Research Observatory, http://hplus.ore.fr). Slug-Test responses with high-frequency (> 0.12 Hz) oscillations have been consistently observed in wells equipped with multiple concentric casing. These oscillations are interpreted as the result of inertia-induced fluctuations of the water level in the annular space between the inner and outer casing. In certain cases, these high-frequency oscillations overlap with lower frequency (< 0.05 Hz) oscillations, which leads to complex responses that cannot be interpreted using conventional models. Slug-Test data have been processed in the Fourier-frequency domain, in order to remove the high-frequency component by a signal-filtering method. The corrected signals have been interpreted with the model of (McElwee and Zenner, 1998), which accounts for the inertia of the water-column above the well screen, non-linear head losses in the well, and neglects the aquifer storage (quasi-steady-state approximation). Hydraulic conductivity values interpreted from dual-frequency Slug-Tests compare well to those interpreted from "standard" overdamped or underdamped Slug-Test responses

The-bao Nguyen - One of the best experts on this subject based on the ideXlab platform.

  • consideration of bentonite cake existing on vertical cutoff wall in Slug Test analysis
    Journal of the Korean Geotechnical Society, 2013
    Co-Authors: Jeehee Lim, The-bao Nguyen, Dongseop Lee, Jaeyoon Ahn, Hangseok Choi
    Abstract:

    Slug Tests can be adopted to estimate hydraulic conductivity of the slurry trench wall backfill for its abilities to reflect the in-situ performance of the construction. A comprehensive three-dimensional numerical model is developed to simulate the Slug Test in a slurry trench wall considering the presence of bentonite cake on the interface boundaries between the wall and the surrounding soil formation. Influential factors such as wall width (i.e., proximity of wall boundary), well deviation, vertical position of well intake section, compressibility of wall backfill, etc. are taken into account in the model. A series of simulation results are examined to evaluate the bentonite cake effect in analyzing practical Slug Test results in the slurry trench wall. The results show that the modified line-fitting method can be used without any correction factor for the Slug Test in the slurry trench wall with the presence of bentonite cake. A case study is reanalyzed with the assumption of existing bentonite cake. The results are compared with the previously reported results by the approaches assuming no bentonite cake (constant-head boundary) or upper-bound solution (no-flux boundary). The case study demonstrates the bentonite cake effect and the validity of the modified line-fitting method in the estimation of the hydraulic conductivity of the slurry wall backfill.

  • Effect of Real Bentonite Cake on Slug Test Analysis for Slurry Trench Wall
    Journal of Geotechnical and Geoenvironmental Engineering, 2013
    Co-Authors: The-bao Nguyen, Jeehee Lim, Dongseop Lee, Hangseok Choi
    Abstract:

    The Slug Test is a viable method in estimating the hydraulic conductivity of the slurry trench wall backfill because of its ability to consider a more representative volume of the backfill and to reflect the in situ performance of the construction. A three-dimensional numerical model is developed to simulate the Slug Test in a slurry trench wall with the presence of bentonite cake on the interface boundary between the wall and the surrounding soil formation. Influential factors such as wall width (i.e., proximity of wall boundary), well deviation, vertical position of the well intake section, and compressibility of the wall backfill are taken into account in the model. The experimentally obtained hydraulic properties of the bentonite cake are also incorporated in a series of Slug Test simulations. The simulation results are then examined to evaluate the bentonite cake effect in analyzing practical Slug Test results in the slurry trench wall. The simulation results show that the modified line-fitting method can be used without any reduction factor for the Slug Test in the slurry trench wall with the presence of bentonite cake. A case study is reanalyzed with the assumption of existing bentonite cake. The results are compared with the previously reported results by the approaches used for the case of no bentonite cake (constant-head boundary) and upper-bound solution (no-flux boundary). The modified line-fitting method and the type curve method produce similar results for slurry walls with bentonite cakes. The case study results demonstrate the importance of the bentonite cake effect in estimating the hydraulic conductivity of the slurry wall backfill.

  • general steady state shape factors in analyzing Slug Test results to evaluate in situ hydraulic conductivity of vertical cutoff wall
    Journal of the Korean Geotechnical Society, 2011
    Co-Authors: Jeehee Lim, The-bao Nguyen, Dongseop Lee, Hangseok Choi
    Abstract:

    No analytical solution exists for evaluating in-situ hydraulic conductivity of vertical cutoff walls by analyzing Slug Test results. Recently, an analytical solution to interpret Slug Tests has been proposed for a partially penetrated well in an aquifer. However, this analytical solution cannot be directly applied to the cutoff wall because the solution has been developed exclusively for an infinite aquifer instead of a narrow cutoff wall. To consider the cutoff wall boundary conditions, the analytical solution has been modified in this study to take into account the narrow boundaries by introducing the imaginary well theory. Two boundary conditions are considered according to the existence of filter cakes: constant head boundary and no flux boundary. Generalized steady-state shape factors are presented for each geometric condition, which can be used for evaluating the in-situ hydraulic conductivity of cutoff walls. The constant head boundary condition provides higher shape factors and no flux boundary condition provides lower shape factors than the infinite aquifer, which enables to adjust the in-situ hydraulic conductivity of the cutoff wall. The hydraulic conductivities calculated from the analytical solution in this paper give about 1.2~1.7 times higher than those from the Bouwer and Rice method, one of the semi-empirical formulas. Considering the compressibility of the backfill material, the analytical solution developed in this study was proved to correspond to the case of incompressible backfill materials.

  • Slug Test analysis in vertical cutoff walls with consideration of filter cake
    Journal of Geotechnical and Geoenvironmental Engineering, 2011
    Co-Authors: The-bao Nguyen, Hangseok Choi
    Abstract:

    In constructing a vertical cutoff wall, bentonite-water slurry is frequently used to maintain the stability of sidewalls during excavation before backfilling the trench with less permeable materials to complete the cutoff wall construction. This procedure leads to a thin but relatively impermeable layer, called a filter cake, on the excavation surface. The aim of this paper is to examine the effect of a filter cake on evaluating hydraulic conductivity of the cutoff wall backfill through a Slug Test analysis with the aid of the verified numerical program, Slug_3D. As an upper bound solution for evaluation of the hydraulic conductivity of the cutoff wall backfill, no-flux boundary conditions for the boundaries of cutoff walls are imposed to consider the effect of filter cakes. The type-curve method and modified line-fitting method are employed to reanalyze the case of EMCON/OWT, Inc., as an example. The previous analysis, without consideration of a filter cake, is compared with the current results that cons...

  • Hydraulic Conductivity Evaluation of Vertical Cutoff Walls Bearing Filter Cake From Slug Test Analysis
    Advances in Environmental Geotechnics, 2010
    Co-Authors: The-bao Nguyen, Chul-ho Lee, Yonghoon Ahn, Hangseok Choi
    Abstract:

    The hydraulic conductivity of a vertical cutoff wall can be estimated through a Slug Test analysis. A filter cake is a thin and impervious layer formed on the interface between the vertical cutoff wall and the natural soil formation. The conventional line-fitting methods for a Slug Test analysis have never considered the presence of the filter cake. Therefore, results of a Slug Test analysis using the line-fitting methods for the vertical cutoff wall is believed to be inaccurate due to the effect of the filter cake. In this study, the hydraulic conductivity of the filter cake was evaluated using a modified fluid loss Test. The result of the Test indicated that a very low hydraulic conductivity is an important characteristic of the filter cake. The Slug Test analysis with the consideration of the filter cake in the line-fitting method was then employed to estimate the hydraulic conductivity of the cutoff wall in a case study. Result of the case study proves the significance of the filter cake in the estimation of the hydraulic conductivity of a vertical cutoff wall through a Slug Test.