• Proceedings of the Korean Geotechical Society Conference
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• 2010.09a
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• pp.1264-1273
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• 2010

Soil-bentonite cutoff walls are widely recognized to be the effective barrier for containment of wastes and groundwater. Bentonite cake is usually found remaining on the trench surface due to the use of bentonite slurry during the excavation for the cutoff wall construction. Defects also inevitably take place due to the inappropriate construction procedures or improperly mixed soil-bentonite backfill. The defects include insufficient keys and windows in the soilbentonite cutoff wall. In this study, the performance of the soil-bentonite cutoff wall is evaluated based on the flow rates through the wall. Three-dimensional numerical models were applied to simulate the groundwater flow through the soil-bentonite cutoff walls of typical geometries with consideration of the defects and bentonite cake. Results of the simulations showed that the bentonite cake has no effect in the insufficient key cases. In the keyed wall cases, the bentonite cake with very low hydraulic conductivity significantly impedes the flow of groundwater through the wall. The presence of the bentonite cake not only compromises the window defect but also renders the wall construction more effective in blocking the groundwater flow. These findings show the significance of the bentonite cake in a soil-bentonite cutoff wall construction.

• Journal of the Korean Geotechnical Society
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• v.35 no.8
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• pp.5-15
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• 2019

This study was conducted to propose a reasonable requirement regulation of cutoff barriers composed of bottom layer and vertical barrier of offshore landfill for the prevention of contaminant leakage. The bottom layer was composed of impermeable clay layer; vertical walls were composed of double walls; outer wall was composed of steel sheet pile which registed against outer force; cutoff vertical barrier took the role of inner wall. Seepage-advection-dispersion numerical analysis was conducted using SEEP/W and CTRAN/W programs under steady and unsteady flows. The results showed that the values calculated under steady flow showed higher migration of pollutant than those of unsteady flow. The values calculated under steady flow are more valid from a design point of view. Under steady flow and the bottom clay layer and when the vertical barrier are homogeneous and completely well installed, respectively, the minimum required cutoff regulations for hydraulic conductivity, thickness, and embedded depth of the bottom clay layer and vertical barrier were suggested.

• Journal of the Korean Geotechnical Society
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• v.25 no.12
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• pp.47-55
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• 2009

As a conventional line-fitting method, the Bouwer and Rice method has been popularly adopted to estimate the hydraulic conductivity of an aquifer through a slug test. Because a vertical cutoff wall is usually very compressible and features a small wall thickness, the Bouwer and Rice method should be carefully applied to analyzing slug test results to estimate the hydraulic conductivity of vertical cutoff walls. In addition, a relatively impermeable layer, called a filter cake, formed at the interface between the cutoff wall and the natural soil formation makes it difficult to use the Bouwer and Rice method directly. In order to overcome such limitations, the original Bouwer and Rice method has been modified by incorporating the concept of the flow net method. In this modification, the geometry condition of cutoff walls including the filter cake is effectively considered in evaluating the hydraulic conductivity of a vertical cutoff wall.

• Proceedings of the Korean Geotechical Society Conference
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• 2009.09a
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• pp.1478-1486
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• 2009

As a conventional line-fitting method, the Bouwer and Rice method has been popularly adopted to estimate the hydraulic conductivity of an aquifer through a slug test. Because a ventical cutoff wall is usually very compressible and features a small wall thickness, the Bouwer and Rice method should be carefully used for the vertical cutoff wall. In addition, a relatively impermeable layer, called a filter cake, formed at the interface between the cutoff wall and the natural soil formation makes it difficult to use the Bouwer and Rice method directly. In order to overcome such limitations, the original Bouwer and Rice method is modified by incorporating the concept of the flow net method. In this modification, the geometry condition of cutoff walls including the filter cake is effectively considered in evaluating the hydraulic conductivity of a vertical cutoff wall.

• Journal of the Korean Geotechnical Society
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• v.27 no.10
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• pp.105-116
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• 2011

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.

• Journal of the Korean Geotechnical Society
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• v.28 no.11
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• pp.17-31
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• 2012

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.

• Journal of the Korean GEO-environmental Society
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• v.22 no.6
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• pp.27-32
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• 2021

Due to heavy rainfall and typhoons caused by climate change, it has become common to witness heavy rain that exceeds the design frequency of agricultural reservoirs. This has brought greater attention to the safety of irrigation facilities including agricultural reservoirs. Out of approximately 17,740 reservoirs available in Korea, 83.87% were built before 1970. To ensure the safety of these old reservoirs, their embankments are being repaired and reinforced using various techniques. Among these techniques, using the cement-bentonite cutoff wall makes it possible to construct diaphragm walls with slurry composed of cement and bentonite, while excavation. The advantages of this technique include that it is simple and fast, and ensures the uniformity of cutoff walls by enabling the immediate application of the replacement method to excavation areas; thus excellent performance is guaranteed. However, despite these advantages, the technique is not commonly used in Korea. Thus, this study investigated the changes in strength and permeability by varying the mix ratio of cement and bentonite. As a major experimental results, when the cement of 200 kg/m³ and the bentonite of 60 to 80 kg/m³ is most suitable for the repair and reinforcement of the reservoir embankments.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.03a
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• pp.220-228
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• 2008

In constructing a slurry trench cutoff wall, a thin but relatively impermeable layer called filter cake can be formed on the excavation surface. The filter cake significantly influences the result of slug test analysis in the cutoff wall. This study is to examine the effect of filter cake on evaluating in situ hydraulic conductivity of the vertical cutoff wall along with slug test analyses. The numerical program Slug_3D was modified to take filter cake into account in the slug test simulation. With consideration of filter cake, the type curve method and the modified line-fitting method were used to reanalyze the case study taken from a landfill site. The previous results achieved by Choi and Daniel (2006b) without consideration of filter cake have been compared with the results in this study. The considerable difference between the two results shows the necessity of considering the filter cake in practice.

• Geomechanics and Engineering
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• v.29 no.3
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• pp.237-247
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• 2022

To investigate the curing temperature effect on the engineering properties of ground granulated blast furnace slag-cement bentonite (GGBS-CB) slurry for cutoff walls, the laboratory experiments including the setting time, unconfined compressive strength, and permeability tests were carried out. The mixing procedure for GGBS-CB slurry was as follows: (1) montmorillonite-based bentonite slurry was first fabricated and hydrated for four hours, and (2) cement or GGBS with cement was added to the bentonite slurry. The dosage range of GGBS was from 0 to 90 % of cement by mass fraction. The GGBS-CB slurry specimens were cured and stored in environmental chamber at temperature of 14±1, 21±1, 28±1℃ and humidity of 95±2% until target days. The highest average temperature of three seasons in South Korea was selected and used for the tests. The experimental results indicated that in early age (less than 28 days) of curing the engineering properties of GGBS-CB slurry were primarily affected by the curing temperature, whereas the replacement ratio of GGBS became a main factor to determine the properties of the slurry as the curing time increased.

• Journal of the Korean Geotechnical Society
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• v.27 no.2
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• pp.53-62
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• 2011

During the construction of vertical cutoff walls, filtration of bentonite slurry into the adjacent soil formation fabricates a layer of bentonite filter cake on the wall surface. The bentonite filter cake possesses much lower hydraulic conductivity compared to that of backfill materials in the cutoff wall. Hydraulic conductivity of bentonite filter cakes formed with three different types of bentonites has been measured by performing the modified fluid loss test under various pressure levels. Three different mixture ratios, 4, 6, and 8%, were selected for fabricating bentonite filter cakes to represent common field conditions. Two analysis methods for interpreting the experimental data from the modified fluid loss tests were employed to estimate hydraulic conductivity of the bentonite cakes. The range of hydraulic conductivities of the three bentonite cakes is between $2.15{\times}10^{-11}\;m/s$ and $2.88{\times}10^{-10}\;m/s$ which is 1 to 2.7 orders of magnitude lower than that of the design cutoff wall backfill. The stress distribution and thickness of the bentonite cakes were also evaluated in this paper.