Slurry Cutoff Walls Research Articles

Effect of polyanionic cellulose modification on hydraulic and consolidation performance of sand-calcium bentonite slurry trench cutoff walls

Sand-bentonite slurry trench cutoff wall is one of the most common vertical barriers to retard the migration of contaminants. This study aimed to explore the hydraulic and consolidation performance of cutoff-wall materials containing polyanionic cellulose (PAC)-modified calcium bentonite (CaB) through a comprehensive laboratory investigation. First, physicochemical properties tests were conducted to demonstrate the superiority of PAC-CaB over CaB. Subsequently, modified fluid loss tests were conducted to investigate the effects of PAC modification and CaCl2 solution concentration on the chemical compatibility of CaB cake formed during slurry trench protection. Finally, slump, Atterberg limit, oedometer, and rigid/flexible-wall hydraulic conductivity tests were performed to evaluate the effect of PAC modification on the hydraulic conductivity (k) and compressibility of sand-CaB backfill. The results indicate that PAC can significantly enhance the impermeability of CaB cake when permeated with salt solutions. The 4 %PAC-CaB cake in 40 mM CaCl2 solution had the lowest k value (9.3 × 10−10 m/s), and was therefore recommended as the optimum proportion for trench protection. The addition of PAC considerably reduced the consolidation coefficient of CaB backfill and increased its compression index and swell index. Moreover, with the increase of PAC dosage from 0% to 0.2%, 0.4%, and 0.8%, the k values of CaB backfill decreased by 0.75, 0.96, and 1.29 orders of magnitude, respectively. The k values of PAC-CaB materials were lower than those of sodium bentonite materials, indicating that PAC is a promising modifier to achieve a low k value of CaB cake and sand-CaB backfill for slurry cutoff walls.

Analytical solution for transport of degradable contaminant in cut-off wall and aquifer

An analytical solution for contaminant transport through a vertical cut-off wall and aquifer system is developed, which can consider the advection, dispersion and degradation processes. The uniqueness of the solution is that it includes hyperbolic eigenfunctions, considers the degradation process and is suitable for an aquifer with finite thickness due to the existence of a river or lake. The solution is first verified against an existing analytical solution and a numerical model using the Comsol software program. The three important aspects are then investigated and some important conclusions are obtained. In some scenarios, the hyperbolic eigenfunctions must be considered; otherwise, the obtained results are unreasonable. When the aquifer thickness is more than a critical value (approximately ten times the cut-off wall thickness in this study), it is reasonable to regard the aquifer as semi-infinite. Contaminant degradation has significant influence on contaminant transport. A higher degradation intensity leads to a lower contaminant concentration but a larger mass flux in the cut-off wall. Therefore, the present solution is suitable for an aquifer with arbitrary thickness and has better applicability than the existing analytical solutions. It is promising to be used as an applicable tool for the design of vertical slurry cut-off walls.

Effects of natural zeolite and sulfate environment on mechanical properties and permeability of cement–bentonite cutoff wall

Cement–bentonite (CB) slurry cutoff walls are extensively utilized to restrict the horizontal flow of fluids. In this study, cement was replaced by four various contents of 0%, 10%, 20%, and 30% by natural zeolite to evaluate its effects on the unconfined compressive strength, flexural strength, tensile strength, and the coefficient of permeability of cement–bentonite–zeolite (CBZ) specimens. Since underground cutoff walls are always in danger of sulfate attack, the CBZ specimens were immersed in three different containers with three different pH values of 2, 5, and 8 to study the sulfate resistance of this material. Results have indicated that the strength of specimens decreases with an increase in zeolite content at early ages; however, increasing the amount of zeolite up to 20% improves the mechanical properties and permeability of cutoff wall materials at late ages. The bentonite/cement (B/C) ratio equal to 0.2 shows a relatively better response than B/C ratios equal to 0.14 and 0.3. Curing specimens in the sulfate environment increased the mechanical strength of specimens and decreased the coefficient of permeability at first ages. At the late ages, curing in sulfate solutions manifested disadvantageous effects on specimens; however, specimens containing zeolite indicated higher sulfate resistance than specimens without zeolite.

Monitoring drying and wetting of a cement bentonite mixture with Electrical Resistivity Tomography

Cement bentonite slurry cutoff walls are used to encapsulate pollutants within contaminated areas, so avoiding their spreading in the environment. In both temperate and arid climates, at shallow depths, slurry walls are exposed to interaction with the atmosphere and thus to relative humidity values which might induce desaturation and significant shrinkage. This note presents the main results of a study aimed at investigating the impact of drying processes on the integrity and the hydraulic performance of cement bentonite slurry walls. Cement bentonite samples were cured under water for different times (1 months, 2 months and 4 months) and then dried naturally by exposing them to the laboratory environment (T = 21 °C, relative humidity approximately 38%). Once dried, the bottom of the samples was placed in contact with a thin layer of water to induce wetting. The distribution of the electrical conductivity within these samples was evaluated through Electrical Resistivity Tomography measurements, and electrical conductivity maps were converted then into maps of water contents on basis of a phenomenological relationship. The reconstructed water contents compared very well to the measured ones. Drying induced a limited cracking of the samples, which might affect to some extent the hydraulic performance of the barriers.

An investigation into effect of sawdust treatment on permeability and compressibility of soil-bentonite slurry cut-off wall

Landfills containing municipal solid waste (MSW) are increasing with growing population and consumption rate in Australia. However, construction of conventional containment systems such as slurry walls are a cost-effective and timesaving approach to control the leachate diffusion, they are weak barriers when exposed to volatile organic compounds (VOCs) or heavy metals, which are abundant in municipal solid wastes. This study investigates effect of sawdust addition on hydraulic conductivity and compressibility of the Soil-Bentonite (SB) slurry cut-off wall. A series of hydraulic conductivity and consolidation tests performed on SB backfill amended with 2%, 5% and 10% sawdust contents. The results showed that application of sawdust reduced the final hydraulic conductivity (kf) in the range of 2.13 × 10−10 ≤ kf ≤ 3.5 × 10−10 m/s. The computed hydraulic conductivity values using consolidation theory (ktheory) also showed an identical decreasing trend in the range of 5.10 × 10−11 ≤ ktheory ≤ 3.50 × 10−10 m/s under 24 ≤ σ′n ≤ 1280 kPa effective overburden pressure (σ′n). The coefficient of consolidation (cv) was computed using Casagrande and Taylor methods showed a good agreement range value. Additionally, the computed compression index (Ci) and swelling index (Cs) were calculated using void ratio and effective overburden pressure graphs (e - logσ′n) showed that application of sawdust had no significant impact on settlement of the SB backfill.

Evaluation of non-pumped wells with slurry cutoff walls for containing and removing contaminated groundwater

A mass transport model simulated low-energy groundwater remediation alternatives, including linear arrays of non-pumped wells fitted with filter cartridges and non-pumped arrays augmented with slurry cutoff walls. The nearest component of each configuration was located 5 m from the leading tip of a contaminant plume, and arrays were oriented perpendicular to the regional hydraulic gradient. Results indicate that single arrays consisting only of non-pumped wells allow contaminants to pass between wells and potentially migrate offsite. A second backup array offset from the first array more effectively contains the contaminant plume. Alternatively, a short slurry cutoff wall used in place of wells at the center of an array, along with two backup wells on either side of the wall, also facilitates onsite containment. Such low-energy approaches may be an effective means for containing and removing some contaminant plumes encountered in practice.

Reduced-Scale Shake Table Testing of Seismic Behaviors of Slurry Cutoff Walls

AbstractThis paper presents a reduced-scale shake table test on the seismic responses of a section of soil-cement-bentonite (SCB) slurry cutoff wall. The geometric scale of slurry wall width was chosen as 1∶3 (model:prototype). A section of a slurry wall with dimensions of 150 cm long, 20 cm wide, and 160 cm tall was constructed and tested on a one-dimensional shake table. A 187 cm (long)×150 cm (wide)×180 cm (tall) steel-frame box was anchored on the shake table and contained the slurry wall and sandy soil that was compacted on both sides of the wall. Spring-supported wood panels were installed at the bottom and on two sides of the box to create a boundary that has the stiffness of dense sand. The slurry wall and the confining soil were instrumented with accelerometers, LVDT, linear potentiometers, and dynamic soil stress gauges to respectively record the accelerations, vertical and horizontal deformations of the wall, and transient dynamic soil pressures on the wall during the simulated seismic excit...

Engineering behavior and sedimentation behavior of lead contaminated soil-bentonite vertical cutoff wall backfills

Soil-bentonite (SB) vertical slurry cutoff wall is a useful treatment for urban industrial contaminated sites. Due to the clay−heavy metal interaction, significant changes would occur in the engineering behavior of SB cutoff walls. However, previous study is limited to kaolinitic soils or montmorillonitic soils along using solidum chloride and/or calcium chloride as target contaminant. In this work, a series of oedometer tests were conducted to investigate the effects of lead (Pb) on the compressibility and the permeability of kaolin-bentonite (KB) mixtures, a simplified model of in-situ SB cutoff wall backfills. In addition, sedimentation tests were conducted to interpret the mechanism controlling the change of compressibility and permeability from the perspective of soil fabric. The Pb-contaminated KB mixtures for oedometer tests and sedimentation tests were prepared with bentonite contents of 0, 5%, 10%, and 15% by dry mass, and they were mixed with pre-determined volume of lead nitrate solution based on designed Pb concentration and solid-to-solution ratio. The Pb concentration was controlled as 0, 0.1, 0.5, 1.0, 5.0, 10, and 50 mg/g with a solid-to-solution ratio of approximate 0.5. The prepared KB mixtures with bentonite contents of 0, 5%, and 10% were chosen for the sedimentation tests. They were freeze-dried and mixed with DDI with a solid-to-solution ratio of 10 g/100 mL. The results indicate that pH, compressibility, and permeability of KB mixture changed considerably with respect to Pb concentration. It is concluded that the fabric of KB mixture, depending on the particle−particle interaction subjected to different ranges of pH and Pb concentration, governs the sedimentation behavior and permeability. The results of liquid limit (wL) cannot be explained in terms of the sedimentation behavior since it is only ionic-dependent.

Hydraulic Characteristics of Bentonite Cake Fabricated on Cutoff Walls

AbstractBentonite cake is usually formed on the excavated trench surface that is supported by the bentonite slurry during construction of slurry cutoff walls. The lower hydraulic conductivity of bentonite cakes formed during construction of slurry cutoff walls in comparison to backfill materials provides an additional benefit. In the present study, the hydraulic conductivities of bentonite cakes made using three different bentonites were estimated using the modified fluid-loss test under various pressures. Both the hydraulic conductivities of bentonite cakes and cutoff-wall backfill are important in evaluating the in situ hydraulic performance of slurry cutoff-wall construction. Three bentonite slurry concentrations of 4, 6, and 8% were used to fabricate bentonite cakes that represent common field conditions. X-ray diffraction, cation exchange capacity, and swell-index data were collected to characterize the bentonites. Two modified methods for analyzing fluid-loss test results were used to estimate bentonite cake hydraulic conductivities. In addition, the viscosity as a function of time was measured to explain the sealing capacities of the bentonite slurries. The bentonite-cake hydraulic conductivities ranged from 2.15×10−11 m/s to 2.88×10−10 m/s, which were 10 to 500 times lower than the cutoff wall backfill design. Experimental results for 4 and 6% bentonite slurries were relatively similar, but the 8% slurries were noticeably different. Calculated bentonite-cake thickness and stress distribution indicated that the local void ratio and hydraulic conductivity may vary across the cake thickness. The considerably lower bentonite-cake hydraulic conductivities compared to the cutoff wall backfill design show its significance in slurry cutoff-wall construction practices.

Performance assessment of hanging funnel‐and‐gate structures designed by reverse particle tracking for capturing polluted groundwater

AbstractThe objective of this study was to evaluate the capability of partially penetrating (hanging) funnel‐and‐gate structures, designed using reverse flow trajectories, for capturing plumes of contaminated groundwater. Linear capture structures, comprised of two slurry cutoff walls on either side of a permeable gate, were positioned perpendicular to regional groundwater flow in a hypothetical unconfined aquifer. A four‐step approach was used for each of two simulated settings: (1) a numerical mass transport model generated a contaminant plume originating from a source area; (2) a particle‐tracking model projected groundwater flow paths upstream from a treatment gate; (3) the structure was widened and deepened until bounding path lines contained the plume; and (4) mass transport simulation tested the ability of the structure to capture the plume. Results of this study suggest that designing funnel‐and‐gate structures using reverse particle tracking may result in too small a structure to capture a contaminant plume. This practice generally ignores effects of hydrodynamic dispersion, which may enlarge plumes such that contaminants move beneath or around a capture structure. This bypassing effect may be considerable even for low values of dispersivity. Particle‐tracking approaches may also underestimate the amount of time required to reduce contaminant concentrations to acceptable levels. © 2007 Wiley Periodicals, Inc.

Augmenting Groundwater Monitoring Networks near Landfills with Slurry Cutoff Walls

This study investigated the use of slurry cutoff walls in conjunction with monitoring wells to detect contaminant releases from a solid waste landfill. The 50 m wide by 75 m long landfill was oriented oblique to regional groundwater flow in a shallow sand aquifer. Computer models calculated flow fields and the detection capability of six monitoring networks, four including a 1 m wide by 50 m long cutoff wall at various positions along the landfill's downgradient boundaries and upgradient of the landfill. Wells were positioned to take advantage of convergent flow induced downgradient of the cutoff walls. A five-well network with no cutoff wall detected 81% of contaminant plumes originating within the landfill's footprint before they reached a buffer zone boundary located 50 m from the landfill's downgradient corner. By comparison, detection efficiencies of networks augmented with cutoff walls ranged from 81 to 100%. The most efficient network detected 100% of contaminant releases with four wells, with a centrally located, downgradient cutoff wall. In general, cutoff walls increased detection efficiency by delaying transport of contaminant plumes to the buffer zone boundary, thereby allowing them to increase in size, and by inducing convergent flow at downgradient areas, thereby funneling contaminant plumes toward monitoring wells. However, increases in detection efficiency were too small to offset construction costs for cutoff walls. A 100% detection efficiency was also attained by an eight-well network with no cutoff wall, at approximately one-third the cost of the most efficient wall-augmented network.

Vacuum preloading consolidation of Yaoqiang Airport runway

Yaoqiang Airport is an international airport serving the city of Jinan, China. The soil on the site consists of alternate layers of silty sand, silty clay, silt and clay. An under-consolidated soft clay layer approximately 4 m thick is located at between 7·5 m and 11·5 m depth above a silty clay layer. Soil improvement was proposed to consolidate the site prior to the construction of a runway to eliminate excessive settlement under static and dynamic loads on the runway. The results from field pilot tests for both surcharge preloading and vacuum preloading under the same consolidation pressure of 80 kPa are presented. Soil settlement in the range 20–30 cm was achieved in 80–90 days. For the vacuum treatment, in-situ deep mixing slurry cut-off walls were installed to control vacuum loss in a shallow silty sand layer. Data from the successful full-scale vacuum treatment of the site are also presented

Retardation of volatile organic compound movement by a soil‐bentonite slurry cutoff wall amended with ground tires

Laboratory‐scale column tests were conducted to investigate the retardation of volatile organic compound (VOC) movement through a soil–bentonite (SB) slurry cutoff wall material amended with ground tires, which was found to sorb a significant amount of VOCs. The hydraulic conductivity of the SB slurry cutoff wall backfill material was not affected by the addition of ground tires. The hydraulic conductivity of the ground‐tire‐amended SB backfill specimen increased 1.1 to 1.2 times after the spiking with VOCs; however, that of the conventional SB backfill specimen increased 1.7 to 1.8 times. The effective porosity of the backfill specimens tested was in the range of 70 to 100%. The partition coefficient of trichloroethylene (TCE) increased from 0.96 to 13.41 L/kg when 15% by weight of ground tires was added. As a result, the time for TCE to break through to 10% of the influent concentration became approximately 10 times longer by the addition of 15% ground tires by weight. Ground tire addition to the SB slurry cutoff wall appears to provide a significant amount of VOC retardation capacity without deteriorating the performance of the slurry cutoff wall.

Ground Rubber: Sorption Media for Ground Water Containing Benzene and O-Xylene

The purpose of the current study is to examine the ability of ground rubber to sorb benzene and O-xylene from water contained with aromatic hydrocarbons. The study consisted of running both batch and packed bed column tests to determine the sorption capacity, the required sorption equilibration time, and the flow through utilization efficiency of ground rubber under various contact times when exposed to water contaminated with various amounts of benzene or O-xylene. Initial batch test results indicate that ground rubber can attain equilibrium sorption capacities up to 1.3 or 8.2 mg of benzene or O-xylene, respectively, per gram of tire rubber at solution equilibrium concentrations of 10 mg/L. Packed bed column tests indicate that ground tire rubber has on the average a 40% utilization rate when a hydraulic residence time of 15 min is used. Possible future uses of ground rubber as a sorption media could include, but are not limited to, the use of ground rubber as an aggregate in slurry cutoff walls that are in contact with petroleum products. Ground rubber could also be used as a sorption media in “pump-and-treat” methodologies or as a sorption media in in-situ reactive permeable barriers.

Optimum Hydraulic Conductivity to Limit Contaminant Flux Through Cutoff Walls

AbstractCutoff walls are becoming increasingly attractive options for the control of solute migration from long‐term sources of contamination. The main advantage of low permeability enclosures is that they restrict advective transport of solutes away from the source. However, with high concentration source zones surrounded by cutoff walls, there exists the potential for notable mass fluxes outward due to diffusive transport. This paper shows, through the use of the steady‐state flux equations, that there is an optimal range of hydraulic conductivities for barrier materials which permit the outward diffusive flux to be counter balanced by an inward advective and dispersive flux. This concept of designing optimum contaminant containment using an inward advective flux to counter the outward diffusive flux is valid for sealable joint sheet pile walls, bentonite‐slurry walls and clay liners, but not synthetic membrane materials with extremely low hydraulic conductivities. The effective diffusion coefficient for the common chlorinated organic solvents such as TCE in water‐saturated clayey materials is approximately 1 × 10−6 cm2/sec, resulting in an optimum hydraulic conductivity ranging from 1 × 10−6 to 1 × 10−8 cm/sec. This range in hydraulic conductivity is within the range of common barrier materials but not the lowest achievable. The steady‐state concentration profile in a slurry cutoff wall can result in a substantial amount of contaminant mass stored within the wall which will need to be considered over the long term or dealt with during site remediation. Large inward advective fluxes reduce the total chemical mass stored within the low permeability barrier material.

The Role of Bentonite in Soil Sealing Applications

The use of bentonite as a natural soil sealant has experienced a phe­nomenal increase over the past 15 years. Swelling bentonites, which occur mainly in Wyoming, are exported all over the world for various uses, in­cluding the sealing of ponds, lagoons, and landfills and as a component of the slurry in slurry trenching. The natural swelling and adsorption capacity of bentonites are of tremendous value in the sealing off and containment of industrial waste sites. Landfills and lagoons are often constructed in areas where the natural soils have a permeability that is not consistent with safety standards. Admixing 5 to 15 percent bentonite into those soils will quickly reduce the natural permeability to the desired levels. All landfills require a cap once they are filled, to prevent the formation of a “bathtub” full of water and commensurate generation of excessive leachate. A cap consists of a 4 to 6 in. layer of soil-bentonite and two to three ft of natural soil, to allow for vegetation. Over the years, many hazardous waste sites have contaminated the ground water to such a degree that it is not fit for drinking. These sites can be encapsulated by constructing a slurry cut-off wall. These trenches are filled with a bentonite or cement-bentonite slurry during excavation and then backfilled with natural soil or a soil-bentonite mixture. The most important aspects of using bentonites as waste site sealants are the proper installation of the sealing systems and a knowledge of the limitations of bentonites regarding resistance to contamination. Types of bentonite, installation practices, and testing methods are discussed in this paper.