Soil-bentonite Cutoff Wall Research Articles

Analytical Solution for Contaminant Transport through a Soil–Bentonite (SB)/Geosynthetic Clay Liner (GCL)/Soil–Bentonite (SB) Composite Cutoff Wall and an Aquifer

This study develops a one-dimensional analytical solution for contaminant advection, diffusion and adsorption through a soil–bentonite (SB)/geosynthetic clay liner (GCL)/SB–aquifer composite cutoff wall (CCW) system. The solution agrees well with an existing double-layer model. Adopting toluene as a representative contaminant, using the present solution, the analysis systematically investigates the impact of hydraulic gradient (i) and the hydraulic conductivities of GCL (kgcl) and SB (ksb). The results show the following: (1) Increasing i from 0.1 to 1 reduces the concentration breakthrough time (tcb) from 20 to 11 years and mass flux breakthrough time (tfb) from infinite to 11 years, indicating lower i significantly extend both tcb and tfb, which is crucial for optimizing CCW barrier performance; (2) lowering kgcl from 5.0 × 10−11 m/s to 1 × 10−12 m/s and reducing ksb from 1.0 × 10−9 m/s to 1.0 × 10−11 m/s, would increase the tcb by 36% and 100%, respectively. It demonstrates that reducing kgcl and ksb could enhance barrier performance. (3) To achieve equivalent barrier performance, soil–bentonite cutoff wall (SBCW) requires greater thickness compared to SB/GCL/SB CCW, indicating that GCL reduces the required amount of bentonite; and (4) CCWs can use SB with lower adsorption capacity to achieve equivalent performance, further reducing bentonite requirements. The present solution can aid in the design and optimization of GCL-enhanced CCWs.

Centrifuge Modeling of Backfill Consolidation in Soil-bentonite Cutoff Walls

Soil-bentonite (SB) is commonly adopted for the backfill of vertical cutoff walls to contain contaminated groundwater. The permeability of SB backfill is dependent on its effective stress state, which is affected by the backfill consolidation process. This paper presents a geotechnical centrifuge model test to simulate the primary consolidation of typical SB cutoff walls with the advantages of reduced scale in time and size. The moisture content and earth pressure of the wall were monitored at varied depths. Test results showed that the rapid consolidation stage developed swiftly and various consolidation indicators exhibited significant development. Within the buried depth over the half (approximately two-thirds), the distribution of vertical effective stress was similar to the Bucknell field testing results, namely that it increased nonlinearly and then tended to stabilize. Also, the cumulative effect of lateral frictional resistance increased with depth and reached its peak value. In the deeper buried areas, lateral frictional resistance decreased sharply before increasing again with increasing depth, while vertical effective stress followed a pattern of significant increase followed by a decrease. This centrifuge test may be useful for achieving a comprehensive evaluation of the consolidation behaviour and mechanism of SB cutoff walls.

Hydro-mechanical behavior and microstructure evolution of red clay-bentonite backfills

Soil-bentonite (SB) cutoff walls have been extensively used as vertical engineering barriers to control groundwater flow and subsurface contaminant migration. As the pollution containment capability of the SB backfills is intimately related to the hydro-mechanical behavior, consolidation tests and hydraulic conductivity tests were carried out to investigate the effects of bentonite content on the hydro-mechanical behavior of red clay-bentonite backfills, and the microstructure evolution of backfills under vertical loading was evaluated based on mercury intrusion porosimetry (MIP) tests and field emission scanning electron microscope (FESEM) observations. The results of microstructure analysis also evaluated the influence of bentonite on the clay fabric which changed the relationship between hydraulic conductivity and void ratio. The results indicated that the void ratio and hydraulic conductivity decreased linearly with the increase of the effective consolidation stresses, and the compression index decreased with the increase of bentonite content. Meanwhile, the pore size distribution (PSD) curve of the backfill specimen almost turned into unimodal characteristics with only micropores distributed after consolidation. Additionally, the flow paths became more tortuous under compression which decreased the hydraulic conductivity. The montmorillonite aggregates in red clay-bentonite backfill formed into tortuous network structure which bound the clay aggregates together and effectively increased the tortuosity of flow paths.

The Chemical Compatibility of Sand–Attapulgite Cut-Off Walls for Landfills

Soil–bentonite cut-off walls have been widely used to control landfill pollution but they do not have good chemical compatibility with landfill leachate. Attapulgite can be substituted for bentonite in landfill cut-off walls. However, little is known about the chemical compatibility of attapulgite cut-off walls and leachate. This study experimentally investigated the chemical compatibility of attapulgite cut-off wall specimens with organic and inorganic contaminants and found that a sand–attapulgite cut-off wall has good chemical compatibility with organic contaminants. A CaCl2 solution was used to represent inorganic contaminants, and chemical oxygen demand (COD) was used as an indicator of organic content. The hydraulic conductivity of the cut-off wall initially decreased and then increased to become approximately constant as Ca2+ concentration increased. Changes in COD concentration were divided into a decreasing stage (0–10,000 mg/L) and a constant stage (10,000–40,000 mg/L). The increase or decrease in hydraulic conductivity was by no more than one order of magnitude. The increase in the hydraulic conductivity of the sand–attapulgite cut-off wall is explained in terms of bound water content and pore structure. An increase in Ca2+ concentration decreased the bound water content of the cut-off wall while the CaCl2 solution increased macropore and mesopore volume and decreased small pore volume in the sand–attapulgite cut-off wall. The purpose of this study was to elucidate the chemical compatibility of a sand–attapulgite cut-off wall with organic and inorganic contaminants and to increase the understanding of the interactions between the cut-off wall and the contaminants. The results of this research are informative for improving the application, design, and construction of sand–attapulgite cut-off walls.

Performances of the Soil–Bentonite Cutoff Wall Composited with Geosynthetic Clay Liners: Large-Scale Model Tests and Numerical Simulations

The geosynthetic clay liner (GCL) overlap plays a key role in maintaining hydraulic performance of the barrier systems, e.g., the bottom liner and cover systems. However, its influences on the behavior of the vertical barrier have been rarely investigated. This paper aims to address this issue using the large-scale model test and 3-dimensional finite element (FE) modeling. In the model test, the GCL overlap at the width of 500 mm was tested under a constant hydraulic head of 1 m and confining stress ranging from 10 to 150 kPa using a newly developed large-scale apparatus. Compared with the flexible wall permeameter, this apparatus could guarantee full field-scale GCL overlap to be tested. Results showed that the effective hydraulic conductivity of the GCL overlap decreased from 10−8 to 10−9 cm/s as the confining stress increased from 10 to 150 kPa. The addition of supplemental bentonite paste in between the overlap with a water-to-bentonite ratio of 19:1 contributed to reducing the effective hydraulic conductivity by 60% compared with that for a GCL overlap without bentonite paste. The breakthrough time for the soil-bentonite (SB) cutoff wall composited with GCLs was 64% longer in comparison with that for the single SB wall. Additionally, the breakthrough after 50 years is made for the entire depth of the single SB wall while at the depth no more than 0.9 m for the composite wall with bentonite paste at the GCL overlap. With consideration that the depth of the groundwater table is generally greater than 1 m, the GCL–SB composite cutoff wall will exhibit a good performance in containing groundwater contaminants in the field, especially when applying bentonite paste at the GCL overlap.

Centrifuge modelling of lead retardation in soil–bentonite cut-off walls

Soil–bentonite (SB) walls are commonly used as barriers for in situ containments at landfills. SB walls are designed to control the movement of persistent pollutants such as heavy metals in the long term. This paper presents centrifuge and numerical modelling of 50 years of performance of a loess-amended SB (LSB) wall in comparison with a conventional SB wall. Both walls were 28 m high and subjected to a hydraulic head difference of 2 m in the prototype. A purposely designed apparatus was developed to impose the hydraulic head and a pollutant loading of 200 mg/l lead(II) on two barriers simultaneously. The apparatus was loaded on the ZJU400 geotechnical centrifuge and spun to 100g. Similitudes of dispersion and adsorption of lead(II) in a centrifuge are discussed. Stress redistribution was observed in small-scale walls due to the enhanced soil arching effect. The lead(II) migration distance in the LSB wall after 50 years was 44.3% shorter than that in the SB wall. The predicted breakthrough time for the LSB wall was 149.1% longer than that for the SB wall when the thickness was 0.6 m. Retardation effects could be attributed to the significant ability of loess to absorb lead(II).

Impact of In Situ Soil in Soil-Bentonite Cutoff Wall Backfill on Compressibility and Hydraulic Conductivity

Soil-bentonite cutoff walls, consisting of excavated in situ soil and bentonite as backfills, are used extensively as vertical barriers for groundwater pollution control. Sand mixed with high-quality natural sodium bentonite (NaB) is commonly used as a research object to investigate the hydraulic and compression properties of soil-bentonite backfills. However, pure sand could rarely be found in real conditions, and natural NaB may not be available readily in some countries such as China, India, and Turkey. This paper presents a comprehensive laboratory investigation on the compressibility and hydraulic conductivity (k) of soil-bentonite backfills created by simulated in situ soil and low-quality sodium activated calcium bentonite (SACaB). The simulated in situ soils are prepared using sand-natural clay mixtures with sand to natural clay mass ratios ranging from 0.5 to 6.0, and the bentonite content (BC) in the base mixture ranges from 0 to 15%. The result indicates that BC dominates the compression index (Cc) of the backfill, and a unique relationship between void ratio at effective vertical compression stress of 1 kPa and compression index is proposed for various types of soil-bentonite backfills. An increase in either BC or clay size fraction (CF) in simulated in situ contributes to reducing k, but the impact of CF in simulated in situ soil on k tends to be insignificant for backfill with BC higher than 6%. A new characteristic parameter based on the concept of void ratio of bentonite (eb), named apparent void ratio of clay size fraction (eC), is developed for predicting soil-bentonite backfills created by in situ soils and bentonites with various contents.

Study on the microscale structure and anti-seepage properties of plastic concrete for cut-off walls modified with silica fume: Experiment and modelling

A modified plastic concrete cut-off wall was synthesized using different mass ratios of silica fume (0–21.3%) and constant cement content (8%) based on the conventional soil-bentonite cut-off wall. The slump test, hydraulic conductivity and mechanical performance were analysed to determine the optimal material ratio. The hydraulic conductivity of the optimal sample could meet theanti-seepagerequirement (~10-9 m/s). The amended plastic concrete cut-off wall could obtain the appropriate compressive strength (0.896 MPa) and a low elastic modulus (1244.5 MPa). The characterization of the clayey soil materials and modified plastic concrete cut-off walls were analysed by X-ray diffraction (XRD) methods, which demonstrated that the clayey soil mainly contained quartz and feldspar (anorthite). The pozzolanic reaction between the silica fume and calcium hydroxide and the refinementofthe porestructure could increase the compressive strength and reduce the hydraulic conductivity. Microscale three-dimensional real geometry images of the modified plastic concrete for cut-off walls were obtained from X-ray microcomputed tomography. The internal structural parameters were analysed through pore-scale modelling. The hydraulic conductivity and velocity fields were simulated using the lattice Boltzmann method. The results showed that most of the pores and throats in the modified plastic concrete for cut-off walls were micropores and microflow paths (diameter ≤ 5 μm), and the connected porosity was approximately 25.89%. The simulated hydraulic conductivity was in good agreement with the measured value, and the velocity fields indicated that the microflow paths were tortuous and complex. The silica fume-modified plastic concrete for cut-off walls enabled the realization of the requirements for good controlof the waterseepage in hydraulic engineering construction applications at a low cost.

Nonlinear constitutive model of soil–bentonite based on triaxial tests along different stress paths

To investigate the constitutive behavior of soil–bentonite, which is commonly used as backfill for cutoff walls, a series of triaxial tests was conducted along different stress paths. The tested soil–bentonite comprises 5% Wyoming bentonite and 95% silty clay excavated from a landfill site located in Jiangsu Province, China. Some mechanical properties of the soil–bentonite, including the compression characteristic, shearing characteristic, and coupled deformation of mean and deviatoric stress, were examined based on the test data. Then, a nonlinear constitutive model was developed under the axisymmetric condition based on a modified hypoelastic model framework. All six independent parameters included in the model were calibrated according to the test data. The mechanical behaviors that the triaxial tests revealed can be fully reflected by the nonlinear model; therefore, it can reasonably describe the stress–strain behaviors of the soil–bentonite in triaxial tests of this study and other literature. Compared with the Modified Cam-Clay model, the prediction effect for the shear strain of the nonlinear model is better. According to a large-scale in situ test employing a soil–bentonite cutoff wall, the actual stress paths of soil–bentonite are basically included in the application scope of the nonlinear model, preliminarily indicating its applicability for practical engineering projects.

Comparison of Laboratory and Field Measurements of Backfill Hydraulic Conductivity for a Large-Scale Soil-Bentonite Cutoff Wall

AbstractSoil-bentonite (SB) cutoff walls are commonly employed to control groundwater flow and contaminant migration, and the hydraulic conductivity (k) of the SB backfill is a critical parameter i...

Influence of cement addition on barrier performance of soil-bentonite cut-off wall

Influence of cement addition on barrier performance of soil-bentonite cut-off wall

Application of a Bentonite Slurry Modified by Polyvinyl Alcohol in the Cutoff of a Landfill

Soil‐bentonite cutoff walls are usually used to contain the contaminants of landfills. The pumpability, permeability, and adsorption capability of the slurry are all crucial to the performance of the cutoff wall. In this study, the effect of polyvinyl alcohol (PVA) was used to improve the performance of bentonite slurry. A series of lab tests were conducted to evaluate the pumpability, permeability, and adsorption capacity with different concentrations of PVA treatment. Results show that the addition of PVA can increase the fluidity and pumpable period of slurry, which facilitates the casting and grouting during construction. The addition of PVA also helps to reduce the permeability coefficient of slurry and improve the adsorption capability which enhances the cutoff performance of the walls.

Apparent salt diffusion coefficients for soil–bentonite backfills

Apparent diffusion coefficients, Da, were measured for two soil–bentonite (SB) backfills characteristic of those used in SB vertical cutoff walls for subsurface control of contaminant migration. The base soils for the backfills comprised either a natural lean clay or sand–bentonite mixtures. The base soils were mixed with 5% bentonite–water slurry to obtain a slump of 125 mm, resulting in total bentonite contents ranging from 4.76% to 7.31%. Values of Dafor sodium chloride were measured using a recently developed dialysis-leaching test method. The Davalues for the clay–bentonite and sand–bentonite backfills ranged from 2.5 × 10−10to 5.3 × 10−10m2/s and from 1.4 × 10−10to 8.1 × 10−10m2/s, respectively. Values of Dafor both backfills increased with increasing average salt concentration in the specimen (Cave). Values of Dadecreased by ≤50% with increasing backfill bentonite content. For all Cavevalues, the clay backfills exhibited lower Dathan the sand–bentonite backfills, likely due to additional fines from the lean clay. Results of this study enhance understanding of solute diffusion through SB cutoff walls, as well as support future use of the dialysis-leaching test method to measure diffusion properties of SB backfills.

In situ stress states and lateral deformations of soil–bentonite cutoff walls during consolidation process

A long-term in situ test was performed on two soil–bentonite (SB) cutoff walls with different dimensions. The total stresses and pore pressures in the walls were monitored for 8 months, and the lateral wall deformations were monitored for 15 months. The monitoring results revealed that the primary consolidation of the presented SB cutoff walls took approximately 8 months. In the first 5 days, the stress states of the walls were nearly isotropic. The total stresses were less than the geostatic stress, and the lateral wall deformations were negligible. As the consolidation developed, the total horizontal stresses in the walls decreased and then remained unchanged at most depths after 1 month, while the stresses at the bottoms continued decreasing. The nonlinear profiles of the horizontal effective stresses in the cutoff walls were similar to the active earth pressure distributions behind the retaining walls moving in translation mode. The trench sidewall movements were found in translation mode within a specific depth range. Obvious dependencies of the horizontal effective stress and the horizontal strain on the wall height were observed. Comparison with the literature illustrates the influence of backfill properties on the consolidation behavior of the SB cutoff walls.

Compressibility and hydraulic conductivity of sand-attapulgite cut-off wall backfills

Soil-bentonite cut-off walls have been used widely to control pollution in landfills but their antifouling property (their ability to prevent contaminants in landfills from polluting the surrounding environment) decreases significantly over time due to a variety of factors (e.g. contaminant concentrations). In recent years, attapulgite has been considered as a backfill material for cut-off walls, but relevant studies are lacking. In this study, the compressibility and hydraulic conductivity of sand-attapulgite backfills were investigated using consolidation and hydraulic conductivity tests. In these tests, attapulgite comprised 10%, 20%, 30%, 40%, 60%, 80%, or 100% (dry weight) of the backfills. The results showed that (1) the compression (Cc) and swell (Cs) indexes of the backfills increased linearly with increasing attapulgite content (Ap); (2) both the consolidation coefficient (Cv) calculated by the Casagrande and Taylor methods and the hydraulic conductivity (ktheory) calculated according to Terzaghi consolidation theory decreased with increasing attapulgite content. In the case of an effective consolidation stress σ′<100 kPa, ktheory <10−9 m/s when Ap≥30%, which was supported by the hydraulic conductivity tests. Two methods were developed based on laboratory data, for predicting the hydraulic conductivity of sand-attapulgite backfills. We conclude that the use of sand-attapulgite backfills applied to cut-off walls as substitutes for soil-bentonite backfills is technically feasible.

Studies on the chemical compatibility of soil-bentonite cut-off walls for landfills

Leachate contains composite contaminants, and the chemical compatibility of soil-bentonite cut-off walls is unclear. To better understand the issue, Fujian standard sand is used to represent a sandy soil stratum. Two clays were used as additive to examine the chemical compatibility of the soil-bentonite model backfills under the condition of composite contaminants. The results indicate that there is a representative cation when the backfills are permeated with NaCl, CaCl2, and ZnCl2 solutions and an NaCl-CaCl2-ZnCl2 mixed solution of the same ionic strength. Ca2+ has the highest maximum ionic strength among all cations from leachates. Moreover, the change in hydraulic conductivity, bound water content and effective porosity of sand-bentonite with the Ca2+ concentration or chemical oxygen demand (COD) exhibit a concentration threshold; i.e., when the concentration is smaller than the threshold, the hydraulic conductivity and effective porosity significantly increase, whereas the bound water content rapidly decreases; when the concentration is higher than the threshold, the hydraulic conductivity, bound water content and effective porosity tend to stabilize. In addition, under the condition of composite contaminants, the threshold is observed, while the hydraulic conductivity, bound water content and effective porosity vary with the COD. Thus, both the type and concentration of chemicals can change the effective porosity and affect hydraulic conductivity. Furthermore, there is a power function relationship between permeability and the effective pore.

Compression behavior of reconstituted soils mixed with bentonite for a cutoff wall in a landfill site

Soil-bentonite (SB) backfill is used extensively in cutoff walls at landfill sites; the walls are used as engineered geotechnical barriers for contaminant control. With increasing bentonite content, the coefficient of consolidation and hydraulic conductivity of the SB decrease. However, when the bentonite content is increased beyond a certain percentage, the hydraulic conductivity of the SB decreases very little. One of the aims of this paper is to introduce the concept of optimal bentonite content (OBC) for SB cutoff walls, in which the hydraulic conductivity (kh) is expected to be lower than 1 × 10−9 m/s. Additionally, the paper introduces a new index consolidation stress ratio, cvσ′, which is used to obtain the OBC. For this study, the initial water contents of the SB backfill material are selected to be 0.8, 1.0, and 1.2 times their corresponding liquid limits. The clayey soils are amended with different bentonite contents, 0, 5, 8, and 10% (by dry weight basis) for the oedometer tests. Then, piezocone penetration test (CPTU) is applied in SB cutoff wall at a landfill site in Jingjiang city, China. The results of the laboratory and field studies show that the introduction of a new index, cvσ′, is very useful for calculating the OBC and for evaluating the coefficient of consolidation and hydraulic conductivity of SB backfill. The advantage of SB backfill with OBC is that it can achieve the design requirement of very low hydraulic conductivity and improve the safety reserves.

Modified bentonites for soil-bentonite cutoff wall applications with hard mix water

The influence of mix water hardness and bentonite type on the quality of bentonite slurry and hydraulic performance of soil-bentonite backfill mixtures was investigated through slurry quality testing and flexible-wall hydraulic conductivity (k) testing of soil-bentonite backfill specimens. A conventional sodium bentonite (NG) and two types of modified bentonite, HYPER clay (HC) and multiswellable bentonite (MSB), were used to prepare the slurries and backfills. Slurries were prepared with mix waters containing CaCl2 concentrations (CM) ranging from 0.5 to 25 mM (hardness, HM = 50–2500 mg/L as CaCO3), and backfills with similar total bentonite contents (5.5–5.9%) were prepared by combining dry sand-bentonite mixtures with the slurries in a benchtop mixer. Backfill specimens were permeated using CaCl2 solutions with concentrations, CP, ranging from 0.5 to 50 mM. The results of the slurry testing showed that the impact of the mix water on slurry quality parameters (Marsh viscosity, filtrate loss, and bleed) was dependent upon CM, bentonite type, and bentonite content. For CM ≤ 5 mM (HM ≤ 500 mg/L), slurry containing 5% NG exhibited adequate viscosity, filtrate loss, and bleed. However, NG contents of 7% and 12% were required to obtain adequate viscosity, filtrate loss, and bleed for CM = 10 mM (HM = 1000 mg/L) and 25 mM (HM = 2500 mg/L), respectively, whereas adequate slurry properties were obtained for CM = 10 mM using lower percentages (5–6%) of HC or MSB. In terms of backfill k, the specimens containing HC amended with polyanionic cellulose appeared to be the most resilient against hard mix water. The k of backfill specimens containing NG generally exceeded 10−9 m/s when prepared using slurry with CM = 10 mM and exceeded 10−8 m/s when prepared using slurry with CM = 25 mM. In contrast, HC backfill specimens prepared using slurry with CM = 10 or 25 mM exhibited k ranging from 4 × 10−11 to 5 × 10−10 m/s. Overall, CM had greater influence on backfill k relative to CP for the range of CP considered in this study.

Non-monotonic piezocone dissipation curves of backfills in a soil-bentonite slurry trench cutoff wall

Pore pressure dissipation during piezocone testing provides a unique tool for estimating the hydraulic properties of in-situ backfills in soil-bentonite (SB) slurry trench cutoff walls. Six tests were performed in an SB slurry trench cutoff wall located in Jiangsu Province, China. The pore pressure dissipation curves obtained are non-monotonic, which, as far as the authors are aware, is reported for the first time in SB cutoff walls. The non-monotonic dissipation curves are attributed to the redistribution of excess pore pressures between the base soil clods and the rest of the backfill around the cone. Four existing interpretation methods are adopted to analyze the measured non-monotonic piezocone dissipation curves. The horizontal coefficients of consolidation (ch) of the backfills obtained by three methods are close to each other and in agreement with the results of fixed-ring consolidometer tests, while the other method gives a high overestimate. The hydraulic conductivities (kh) of the backfills are also estimated by four methods, three based on dissipation test results and one based on piezocone penetration data. kh estimated by consolidation theory are close to the results of flexible wall permeameter tests. Two empirical expressions for dissipation tests give relatively low kh, but the method based on penetration gives kh much larger than the laboratory test results.

Influence of recycled tyre amendment on the mechanical behaviour of soil-bentonite cut-off walls

Abstract Generation of the scarp tyre has increased in recent years, and finding innovative reusing methods is of interest to researchers. This study aims to investigate the hydraulic conductivity and one-dimensional consolidation behaviour of soil-bentonite (SB) backfill amended with powdered recycled tyre (PRT) and crumbed recycled tyre (CRT) by performing a series of oedometer consolidation and rigid-wall hydraulic conductivity tests. Three values of PRT and CRT (i.e., 2%, 5% and 10% by dry weight) were used to prepare the specimens. The investigation on vertical strain-time graphs showed that the addition of PRT and CRT caused an increase in settlement characteristics of the SB backfill. The results also showed that the addition of PRT and CRT caused an increase in the compression index (Ci) and the swelling index (Cs) of the SB specimens. The coefficient of consolidation (cv) based on the Casagrande and Taylor methods showed a consistent increasing trend by increasing the PRT and CRT. The hydraulic conductivity was computed based on the Terzaghi consolidation theory (ktheory), and the results showed that increasing the PRT and CRT caused an increase in hydraulic conductivity of the SB backfill. The hydraulic conductivity (kf) measured using a rigid-wall permeability compaction mould showed a similar increasing trend by increasing the PRT and CRT contents of the backfills.