Vertical Cutoff Walls Research Articles

Heavy metals containment by vertical cutoff walls backfilled with novel reactive magnesium-activated slag-bentonite-sand: Membrane and diffusion behavior

This study aims to investigate the membrane and diffusion behavior of a novel vertical cutoff wall backfill comprised of reactive magnesium-activated slag, bentonite and sand (referred as MSBS backfill) for the containment of heavy metals in groundwater. The MSBS backfill specimens were subjected to a series of multi-stage chemico-osmotic tests using Pb(NO3)2 and Zn(NO3)2 solutions at different concentrations. Chemico-osmotic efficiency coefficient, effective diffusion coefficient, and retardation factor of the backfills were computed. For computing chemico-osmotic efficiency coefficient, the theoretical chemico-osmotic pressure differences were computed using both van't Hoff equation and water activity method. The results showed that the MSBS backfill exhibited semipermeable membrane behavior in tested solutions. The chemico-osmotic efficiency coefficients of the backfills decreased, whereas effective diffusion coefficients of heavy metal cations remained nearly constant, and retardation factors of heavy metal cations increased slightly with the increase of metal concentration. For the range of concentrations of Zn(NO3)2 in this study, the maximum percent error of theoretical chemico-osmotic pressure difference calculated using van't Hoff equation relative to that calculated using water activity method was 14.4%. A comparative assessment of reported chemico-osmotic efficiency coefficient, effective diffusion coefficient, and retardation factor values for various soil-bentonite backfills and geosynthetic clay liners with those for MSBS backfill was performed. Overall, the results are useful to assess containment performance of MSBS vertical cutoff walls at metal-contaminated groundwater sites.

A customized fragment for seepage beneath a dam with a vertical wall

Based on the method of fragments for the analysis of steady confined seepage, a customized type E fragment is developed, with results presented in the form of charts for the estimation of seepage quantities and exit gradients under embedded water-retaining structures with a vertical cut-off wall. The type E fragment is shown to be an extension of previously derived type A and type D fragments, allowing for both embedment and a cut-off wall. The charts are generated using finite element analysis and cover both isotropic and anisotropic permeability cases. Validation of the fragment is confirmed by comparison with existing method of fragments and full finite element analysis. The charts are shown to predict the flow rate and exit gradient more precisely than existing methods of fragments. The design charts presented in this paper cover a wide range of confined flow problems of practical interest.

Using MgO activated slag and calcium bentonite slurry to produce a novel vertical barrier material: Performances and mechanisms

Slag-cement–bentonite (SCB) slurry has been widely used for the construction of vertical cutoff walls to restrain the movement of groundwater at contaminated sites. However, the conventional cement-based slurry needs a long time to evolve satisfactory performance along with a relatively high environmental load. This paper proposes an innovative material for the self-hardening slurry system, consisting of reactive MgO, slag, bentonite and lots of water. The properties of fresh slurry, unconfined compressive strength (UCS) and permeability of the MgO activated slag and bentonite (MASB) slurry are investigated in comparison with SCB slurry. The results indicate that compared with SCB slurry, MASB slurry with appropriate proportions has comparative fresh properties, but possesses higher UCS and much lower hydraulic conductivity (close to 1.0 × 10−10 m/s) at a later age. A massive formation of expansive hydration products (hydrotalcite phases) could satisfactorily fill in the voids of matrix, leading to a dense microstructure which is responsible for the excellent mechanical and penetrative performance of MASB slurry. The overall findings from this study well demonstrate that the developed MASB slurry holds great potential as a novel, eco-friendly and cost-effective material in the construction of vertical cutoff walls.

Membrane behavior and diffusion properties of sand/SHMP-amended bentonite vertical cutoff wall backfill exposed to lead contamination

This paper presents a laboratory experimental investigation on the membrane behavior and diffusion properties of sand/sodium hexametaphosphate (SHMP)-amended bentonite backfill (referred as SHMP-SB backfill) in vertical cutoff walls for containment of lead-contaminated groundwater. A multi-stage chemico-osmotic test was conducted on the SHMP-SB backfill sample using lead nitrate (Pb(NO 3 ) 2 ) solution with concentrations varying from 0.5 mM to 50 mM. The chemico-osmotic efficiency coefficient ( ω ), effective diffusion coefficient ( D *), and retardation factor ( R d ) of the backfill were calculated. Both water activity method and van't Hoff equation were adopted to compute the maximum chemico-osmotic pressure difference across the backfill used for determination of ω . The results demonstrated that the SHMP-SB backfill acted as a semipermeable membrane material in the Pb(NO 3 ) 2 solutions. The ω of the backfill and R d of lead ions (Pb 2+ ) decreased with increasing concentration of Pb(NO 3 ) 2 solution, whereas D * of Pb 2+ increased. When Pb(NO 3 ) 2 concentration was less than 50 mM, the maximum error in the ω calculated using the van't Hoff equation relative to the water activity method was found to be as high as 22.2%. A comprehensive comparison in ω , D *, and R d was made between the results for the backfill in this study and those for the other types of bentonite-based engineered barrier materials including geosynthetic clay liners (GCLs), a compacted soil-bentonite liner (CSBL), and soil bentonite (SB) backfills exposed to calcium chloride (CaCl 2 ), potassium chloride (KCl) or sodium chloride (NaCl) solutions reported in previous studies. It was found the barrier material type (backfills, GCLs or CSBL), cation concentration, cation valence (di-valent versus mono-valent), and cation hydrated radius had considerable effect on the ω , D *, and R d for the range of studied bentonite-based engineered barrier materials. A linear equation is proposed to quantify the relationship of ω and D * for Pb 2+ , Ca 2+ , K + and Na + on semi-logarithmic scale for all the bentonite-based engineered barriers reported in this study and previous studies. • Effect of lead on ω and D * of SHMP-amended bentonite backfill is investigated for the first time. • Amendment of bentonite with SHMP improves chemico-osmotic efficiency coefficient. • Van't Hoff equation results considerable error in estimating ω in Pb(NO 3 ) 2 solutions. • Factors influencing ω, D * and R d of different bentonite-based barrier materials are assessed.

Diffusion through soil-bentonite backfill from a constructed vertical cutoff wall

Diffusion coefficients were measured for soil-bentonite (SB) backfill sampled from a constructed vertical cutoff wall in Pennsylvania, USA. The backfill consisted of lean clay (the base soil) combined with bentonite-water slurry containing 5–6% sodium bentonite, resulting in a total bentonite content of ~1%. Ten dialysis leaching tests (DLTs) were conducted to measure apparent diffusion coefficients (Da) for chloride (Cl−) diffusion through the lean clay-bentonite backfill. In addition, model sand-bentonite backfills (total bentonite content = 4.7%) were prepared in the laboratory, and eight DLTs were performed for comparison with the field-sampled backfill and the diffusion literature. Both backfill types were tested with monovalent and divalent salt solutions (KCl and CaCl2), over a wide range of concentrations (60–350 mM). Despite the difference in bentonite content, values of Da were similar for the two backfills. Changes in Da with increasing average Cl− concentration were relatively minor, increasing by less than half an order of magnitude over the wide range of salt solutions, likely due to the low percentages of bentonite (<5%) in both backfills. Thus, for SB backfills with bentonite contents <5%, Da appears to be relatively insensitive to backfill composition, salt type and concentration.

Effects of ammonium ion and bentonite content on permeability of bentonite-clay mixture

Sanitary landfill is a common method for the treatment of municipal solid waste. The landfill leachate contains a large amount of NH4+, which will affect the physicochemical properties of bentonite, and subsequently change the anti-seepage function of the bentonite enhanced cutoff wall. The permeability tests were carried out on bentonite-clay mixed soil infiltrated by the ammonium salt solutions using a head pressure control permeameter. The results show that saturated permeability coefficient of the bentonite-clay mixture increases with the decrease of bentonite content and with the increase of NH4+ concentration. Moreover, the reasonable mass content of bentonite is about 10% if the studied bentonite-clay mixture are used in vertical cutoff wall. The scanning electron microscope (SEM) and the nuclear magnetic resonance (NMR) measurements were conducted on the bentonite-clay mixed soil samples with different bentonite contents, and samples infiltrated by the ammonium salt solutions with different concentrations, and afterwards the variations of the micro pore structure and pore-size distribution were obtained. Connecting the Donnan osmotic pressure with saturated permeability of bentonite-clay mixed soils, the effect mechanism of bentonite content and ammonium solution concentration on the permeability of the bentonite-clay mixture was analyzed.

Effect of Polymer Amendment on Hydraulic Conductivity of Bentonite in Calcium Chloride Solutions

Hydraulic conductivity of polyanionic cellulose (PAC)–amended bentonite (PB) in calcium chloride (CaCl2) solutions was investigated to access its chemical compatibility in vertical cutoff walls application. PB was synthesized by mixing conventional bentonite (CB) powder with PAC (2% dry weight). The specific gravity (Gs), liquid limit (wL), pH, swell index (SI), and cation exchange capacity (CEC) of CB and PB were measured, and hydraulic conductivities and microstructures of the PB and CB filter cakes were evaluated by performing modified fluid loss (MFL) tests and scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS) analyses, respectively. The results showed that PB had higher wL, SI, and CEC but lower Gs and pH than CB; in particular, PB possessed higher SI in CaCl2 solution as compared to CB. Increase in the applied overall pressure and decrease in the CaCl2 concentration resulted in a decreased hydraulic conductivity for both PB and CB. However, the hydraulic conductivity of PB was found to be one to two orders of magnitude lower than that of CB when exposed to the same CaCl2 solutions, indicating superior chemical compatibility of PB. SEM-EDS image analyses demonstrated that polymer formed a three-dimensional net structure between bentonite particles, which could clog the intergranular pore space, resulting in a narrow and tortuous flow path for liquid and low hydraulic conductivity.

An analytical model for contaminant transport in cut-off wall and aquifer system

The vertical cut-off wall is a practical means of site contamination control. It is of great importance to predict the service time of a vertical cut-off wall. Previous analytical models on simulation of a cut-off wall neglected the existence of the aquifer, which can lead to simulation errors. An analytical model was developed for contaminant transport in a vertical cut-off wall and an aquifer system. Three cases were considered – that is, advection and diffusion of contaminant towards the same direction, advection and diffusion towards the opposite direction and the pure diffusion case. The analytical solution is obtained by using Laplace transform and binomial theorem. Increasing the scale of the aquifer can extend the breakthrough time. With a fixed hydraulic gradient of 0·5, as the dispersivity of the aquifer increases from 1 to 10 m, the breakthrough time for chloride (Cl−) increased by a factor of 1·7. The breakthrough behaviour of chloride is more sensitive to the scale of the aquifer than the breakthrough behaviour of lead is. The breakthrough time of lead obtained by the single-layer model is always longer than that of the proposed model due to overestimation of retardation.

Experimental Study on the Deformation of a Cut-Off Wall in a Landfill

To effectively solve the leakage problem of landfill leachate, it is necessary to establish a vertical cut-off wall around the landfill site for seepage control. To accurately analyze the deformation of the cut-off wall, a 1/8-scale landfill cut-off wall miniature model is designed and fabricated. The actual force of the cut-off wall is simulated by filling sand on the outer side of the model wall. Meanwhile, a nonlinear finite element model of the wall is established using the ANSYS to simulate the deformation of the seepage wall. The results show that the cut-off wall made from a polyvinyl alcohol-bentonite-fly ash-cement (PBFC) anti-seepage slurry has a high ultimate bearing capacity, and its ultimate strain is approximately 5%, which is much higher than the limit strain of the actual force conditions of the cut-off wall. The horizontal displacement of the cut-off wall is approximately linear and decreases gradually from the top to the bottom, and no reverse displacement is observed. The main stress of the wall is smaller than that of the slurry, and there is no tensile stress. The simulated results are similar to the measured values of the cut-off wall, which indicates the accuracy of the simulation analysis.

The permeability of dredged material-bentonite backfills.

Extensive attention has been paid to the treatment and disposal of dredged material, and there is a need to clarify the feasibility of recycling dredged material by using it as backfill in soil-bentonite vertical cutoff walls. By setting the dredged material in the Baimao storage yard of Meiliang Bay in Taihu Lake and bentonite as the research objects, this paper studied the influences of bentonite content, confining pressure and pore size distribution on the permeability of dredged material-bentonite backfills. According to the test results, from the perspective of medium-term and short-term permeability, it is feasible to recycle dredged material by using it as backfill in a vertical cutoff wall. The permeability of the dredged material-bentonite soil mixture decreases with increasing bentonite content, but the degree of decrease is not significant. At the same time, the higher the confining pressure is, the smaller the variation in hydraulic conductivity with bentonite content. The permeability of the soil mixture decreases with increasing confining pressure, and the range of reduction is within a certain order of magnitude. Moreover, the confining pressure has a similar impact on the decrease in the permeability of the soil mixtures with different bentonite contents. The hydraulic conductivity of the dredged material-bentonite mixture decreases because the addition of bentonite changes the pore size distribution and reduces the porosity and characteristic pore size D50 of the soil mixture.

Hydraulic conductivity and self-healing performance of Engineered Cementitious Composites exposed to Acid Mine Drainage.

Engineered Cementitious Composite (ECC) is proposed as a promising vertical cutoff wall material to contain acid mine drainage (AMD). The study presents comprehensive investigations of hydraulic conductivity of ECC permeated with AMD and self-healing of ECC subjected to wet-dry cycles. The effectiveness of incorporating reactive magnesia (MgO) into ECC for self-healing enhancement is also investigated. The chemical species formed in ECC and MgO-ECC specimens after exposure to AMD are investigated via SEM, FTIR, XRD and TGA analyses. The results show hydraulic conductivity of un-cracked and cracked ECC and MgO-ECC specimens pre-strained up to 1.32% is below commonly accepted limits of 10-8m/s when permeated with AMD. The self-healing capacity of ECC specimens subjected to wet-dry cycles using both tap water and AMD as immersing liquids is improved by MgO addition. MgO addition is also beneficial for reducing hydraulic conductivity of un-cracked and cracked ECC specimens permeated with AMD. MgO addition results formation of new self-healing products including hydromagnesite and brucite when exposed to tap water, and hydrotalcite-like phase (Ht) when exposed to AMD.

Influence of wet-dry cycles on vertical cutoff walls made of reactive magnesia-slag-bentonite-soil mixtures

The strength and hydraulic conductivity of vertical cutoff walls consisting of reactive magnesia-activated ground granulated blast furnace slag (GGBS), bentonite, and soil (MSB) have been investigated in previous studies. However, there has been little comprehensive study of the influence of wet-dry cycles on the mechanical and microstructural properties of MSB backfills. In this paper, the durability of MSB backfills when exposed to wet-dry cycles is investigated. The variations in mass change, dry density, pH value, pore size distribution, and mineralogy are discussed. The results show that the mass change of ordinary Portland cement (OPC)-based and MSB backfills increases with respect to wet-dry cycles. The MSB backfills exhibit up to 8.2% higher mass change than OPC-based ones after 10 wet-dry cycles. The dry density, pH value, and unconfined compressive strength of MSB backfill decrease with the increasing number of wet-dry cycles. Increasing the GGBS-MgO content from 5% to 10% in MSB backfills results in 2.1–2.3 times higher strength, corresponding to a reduction of 2%–12% in cumulative pore volume; while increasing the bentonite content slightly reduces the strength of MSB mixtures, corresponding to an increase of cumulative pore volume by 4.6%–7.9%. The hydrotalcite-like phases and calcium silicate hydrate (C-S-H) are the primary hydration products in MSB backfills. Moreover, the continuous wet-dry cycles result in the precipitation of calcite and nesquehonite.

Comparative analyses of alternative breakthrough curves from cumulative mass column testing of soil–bentonite backfills

The results of eight cumulative mass column tests were analyzed via several different methods to evaluate the dispersion coefficient, D, and the retardation factor, Rd, governing the migration of chloride (Cl−), potassium (K), and zinc (Zn) through soil–bentonite backfills for vertical cutoff walls. Regression of the measured relative (effluent) concentration (RC) breakthrough curves (BTCs) resulted in relatively accurate determinations of Rd, but relatively inaccurate determinations of D for all three solutes. Values of Rd based on dimensionless time, T, corresponding to an RC of 0.5 were underestimated for all three solutes due to the significance of diffusion on solute transport. With a few exceptions, Rd for K and Zn based on analyses of the steady-state portions of measured cumulative mass ratio (CMR) BTCs and T – CMR BTCs were relatively accurate, whereas analysis of measured T – CMR BTCs was more accurate for determining Rd of Cl−. Overall, there is no advantage to analyzing the results of cumulative mass column tests in the form of RC BTCs, whereas the CMR and T – CMR BTCs offer the advantage of determining Rd based on simple linear regressions of the steady-state portions of the BTCs, i.e., provided steady-state solute transport has been established.

Engineering Properties of Vertical Cutoff Walls Consisting of Reactive Magnesia-Activated Slag and Bentonite: Workability, Strength, and Hydraulic Conductivity

AbstractSoil–cement–bentonite (SCB) vertical cutoff walls are commonly used to control the flow of contaminated groundwater at polluted sites. However, conventional backfill consisting of ordinary ...

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.

Mixed metals migration through zeolite-amended backfills for vertical cut-off walls

Column tests were performed with two sand–bentonite backfills comprising 5·8% bentonite with 5% of one of two high-cation-exchange-capacity (CEC) zeolite amendments – clinoptilolite and chabazite upper bed (chabazite-UB). The backfill specimens were permeated with a mixed salt solution of 17·5 mM potassium chloride (KCl) and 10 mM zinc chloride (ZnCl2) to evaluate the extent of attenuation of two metals – potassium (K) and zinc (Zn). The specimens were characterised by low hydraulic conductivity (&lt;1·0 × 10−9 m/s) and delayed breakthrough of potassium and zinc that was consistent with exchange of potassium (K+) and zinc (Zn2+) ions for the sodium (Na+) and/or calcium (Ca2+) ions initially occupying the exchange sites of the bentonite and zeolite components of the backfills. The results were consistent with those previously reported for the same backfills permeated with single-salt solutions of the same metals, indicating that the competition between potassium and zinc ions in the mixed salt solution had little effect on the metals migration. Overall, the metals attenuation was enhanced relative to that for the unamended backfill by a factor ranging from 2·0 for zinc with both zeolite-amended backfills to 3·7 for potassium with the chabazite-UB-amended backfill, further illustrating the benefit resulting from amending a sand–bentonite backfill with a small amount of high-CEC zeolite.

Retention of Pb and Cr(VI) onto slurry trench vertical cutoff wall backfill containing phosphate dispersant amended Ca-bentonite

This study investigated the potential use of sodium hexametaposphate (SHMP) as an amendment in calcium bentonite (Ca-bentonite) and sand/Ca-bentonite backfill to enhance retention of two heavy metals, lead (Pb) and hexavalent chromium (Cr(VI)), commonly encountered in contaminated groundwater by conducting batch equilibrium experiments. The pH of the equilibrated bentonites and backfills was also measured. The results showed that the SHMP amendment and the presence of source chemicals lowered the equilibrium pH of the bentonites and backfills. All the tested bentonites and backfills exhibited nonlinear retention behavior of Pb and Cr(VI) which was well described by Langmuir and Freundlich models. SHMP amendment increased the Pb retention capacity of the bentonite and backfill by a factor of 1.45 and 1.72 respectively, whilst it helped little on improving Cr(VI) retention capacity of the bentonite and backfill. Moreover, the percent retention (PR) and partition coefficient (Kp) were found to increase first and then decrease with the initial metal concentrations, and the peak PS or Kp occurred at initial concentration of 874 mg/L for Pb retention on all tested bentonites and backfills, whilst 1 and 48 mg/L for Cr(VI) retention on the tested bentonites and backfills respectively. Based on the variations of PR and Kp with initial metal concentration, the enhanced retention of Pb on bentonite and backfill with SHMP amendment was pronounced when the initial Pb concentration was higher than the threshold value of 1642 mg/L. However, the bentonite, even though amended with SHMP, is not an ideal barrier material for Cr(VI) retention.

Stochastic analysis of the efficiency of coupled hydraulic-physical barriers to contain solute plumes in highly heterogeneous aquifers

The expected long-term efficiency of vertical cutoff walls coupled to pump-and-treat technologies to contain solute plumes in highly heterogeneous aquifers was analyzed. A well-characterized case study in Italy, with a hydrogeological database of 471 results from hydraulic tests performed on the aquifer and the surrounding 2-km-long cement-bentonite (CB) walls, was used to build a conceptual model and assess a representative remediation site adopting coupled technologies. In the studied area, the aquifer hydraulic conductivity Ka [m/d] is log-normally distributed with mean E(Ya)=0.32, variance σYa2=6.36 (Ya=lnKa) and spatial correlation well described by an exponential isotropic variogram with integral scale less than 1/12 the domain size. The hardened CB wall’s hydraulic conductivity, Kw [m/d], displayed strong scaling effects and a lognormal distribution with mean E(Yw)=-3.43 and σYw2=0.53 (Yw=log10Kw). No spatial correlation of Kw was detected. Using this information, conservative transport was simulated across a CB wall in spatially correlated 1-D random Ya fields within a numerical Monte Carlo framework. Multiple scenarios representing different Kw values were tested. A continuous solute source with known concentration and deterministic drains’ discharge rates were assumed. The efficiency of the confining system was measured by the probability of exceedance of concentration over a threshold (C∗) at a control section 10years after the initial solute release. It was found that the stronger the aquifer heterogeneity, the higher the expected efficiency of the confinement system and the lower the likelihood of aquifer pollution. This behavior can be explained because, for the analyzed aquifer conditions, a lower Ka generates more pronounced drawdown in the water table in the proximity of the drain and consequently a higher advective flux towards the confined area, which counteracts diffusive fluxes across the walls. Thus, a higher σYa2 results in a larger amount of low Ka values in the proximity of the drain, and a higher probability of not exceeding C∗.

Long-Term Column Testing of Zeolite-Amended Backfills. I: Testing Methodology and Chemical Compatibility

AbstractEight column tests were performed with specimens comprising unamended and zeolite-amended sand-bentonite backfills for vertical cutoff walls permeated with solutions of either 35 mM KCl (fi...

Numerical Evaluation of Vertical Cutoff Walls Comprising Zeolite-Amended Backfills for Enhanced Metals Containment

AbstractThe potential for enhanced containment of two metals, potassium (K) and zinc (Zn), by soil-bentonite (SB) vertical cutoff walls comprising zeolite-amended backfills was evaluated on the bas...