Cutoff Wall Research Articles

Numerical investigation of mixed physical barriers for saltwater removal in coastal heterogeneous aquifers

Saltwater intrusion is a prevalent global environmental issue that detrimentally impacts coastal groundwater aquifers. This problem is exacerbated by climate change and increased groundwater abstraction. Employing physical barriers proves effective in mitigating saline water intrusion. In this study, a validated numerical simulation model is utilized to assess the impact of aquifer stratification on the effectiveness of mixed physical barriers (MPBs) and their response to structural variations. Additionally, the performance of MPBs was compared with that of single physical barriers in a laboratory-scale aquifer. Three different configurations were replicated, comprising two stratified aquifers (HLH and LHL) and a homogenous reference aquifer (H). The results demonstrate that MPBs are efficient in decreasing the saltwater penetration length in the investigated cases. The reductions in penetration length were up to 65% in all cases. The removal efficacy of residual saline water for MPBs exceeded that of the subsurface dam by 2.1–3.3 times for H, 2.1–3.6 times for HLH, and 8.3 times for LHL conditions, while outperforming the cutoff wall by 38–100% for H, 39–44% for HLH, and 2.7–75% for LHL. These findings are of importance for decision-makers in choosing the most appropriate technique for mitigating saline water intrusion in heterogeneous coastal aquifers.

Durability of geopolymer cutoff wall backfill incorporating reactive MgO particles under dry-wet cycles

Vertical cutoff wall has been employed for decades to control groundwater flow and subsurface contaminant transport. The barrier performance of cutoff wall material may significantly deteriorate under dry-wet cycles due to precipitation, drought, and groundwater fluctuation. This study adopted active magnesium oxide (MgO) particles to enhance the durability of geopolymer cutoff wall backfill (GCWB) under dry-wet cycles. The influence of particle size and proportion of MgO on the unconfined compressive strength (UCS), hydraulic conductivity, self-healing capacity and durability of GCWB were comprehensively examined. The results indicate that the incorporation of active MgO particles hindered the geopolymerization process of GCWB to a certain extent, leading to an increase in hydraulic conductivity and a decrease in UCS. But the overall performance of GCWB was still favorable. The hydraulic conductivity of samples with MgO gradually increased to more than 1 × 10−8 m/s after at least two dry-wet cycles. In the subsequent cycles, the samples with MgO exhibited a pattern of first gradually declining then ascending in hydraulic conductivity. This is due to hydration and carbonization of the remained MgO in matrix during the dry-wet cycles, which converts into oxygen-rich carbonates, forming well-ramified networks and reducing the proportion of macropores. Thus, the durability of GCWB with MgO is significantly more favorable than that devoid of MgO. In particular, large particle size MgO can exert more persistent self-healing effect, endowing the samples with superior durability under the prolonged dry-wet cycles.

Effects of Cutoff Wall Shape and Size of Dam Gates on Vortex Formation Upstream of Dams

Effects of Cutoff Wall Shape and Size of Dam Gates on Vortex Formation Upstream of Dams

Analysis of the Water/Cement/Bentonite Ratio Used for Construction of Cut-Off Walls

In recent years, because of the continuous expansion of urban areas, an increased necessity to isolate historically polluted sites by means of artificial, flexible, low-permeability barriers has emerged. Moreover, due to cost and efficiency considerations, various combinations of materials that fulfill the previously stated requirements have been proposed. On the basis of a literature review, this paper analyses the relationships between water, cement, and bentonite, and the physical and mechanical properties of the resulting material created in combination with standard sand introduced in the mixture using a ratio of 2:1 with respect to the solid part of the mixture (cement and bentonite). The quantity of standard sand was established following previous research conducted by the authors. The relation between water, cement, and bentonite is analyzed through properties such as viscosity, permeability, and undrained cohesion, and the representation of mixtures and their corresponding parameters was carried out using a ternary diagram. This paper provides a graphical approach to finding the optimum water/bentonite/cement mixture required for barrier design, taking into account permeability, undrained cohesion, and mixture viscosity.

Analysis of Steady Groundwater Flow in Confined Aquifer Due to Long-Strip Pit Dewatering with Suspended Cut-Off Wall

This study investigates the steady flow resulting from dewatering by a partially penetrating well in a confined aquifer with a cut-off barrier. By considering flow in both horizontal and vertical directions and incorporating the barrier and pumping well as flow boundary conditions, separate mathematical models are established for the inside and outside of the cut-off barrier. The interaction between these zones is ensured through continuous conditions along the opening of the two zones. A semi-analytical solution is derived for the problem using the finite Fourier cosine transform and boundary transformation methods. The effectiveness of the method is verified by comparing it with the finite element numerical results and pumping test data respectively. Based on the proposed solutions, we proceed to analyze the influence of some relevant factors: the extent to which the cut-off wall is embedded within the confined aquifer, the depth of the partially penetrating well, and the distance to the lateral head boundary. Results indicate that a greater depth of the cut-off wall leads to a reduced pumping rate requirement for achieving a desired drawdown of the confined water level within the excavation. According to the presented solution, placing pumping wells near the top of the confined aquifer in excavation dewatering projects can facilitate a faster reduction of the confined water head at the excavation bottom. Additionally, proximity of lateral head boundary could significantly impact dewatering, with closer boundaries reducing dewatering effectiveness due to improved aquifer recharge. Finally, the use of the Fourier method showcases impressive convergence properties in the approach presented in this study. The computed results maintain a high level of approximation quality, even with extremely coarse discretization.

Modeling and Analysis of Cutoff Wall Performance Beneath Water Structures by Feed-Forward Neural Network (FFNN)

Cutoff walls are widely used to limit seepage, piping, and the uplift under hydraulic structures. Therefore, this study focused on a numerical investigation of the hydraulic performance of cutoff walls beneath hydraulic structures during both static and dynamic conditions, considering location and inclination angle influences. The results confirmed that placing the cutoff wall at the upstream heel was more effective in reducing uplift pressure compared to other placements during static conditions. The inclination angles for the different placements of the cutoff wall had a significant impact on the total uplift pressure, exit hydraulic gradient, and seepage discharge during both static and dynamic states. The earthquakes had a noticeable effect on uplift pressure, seepage discharge, and exit hydraulic gradient. During static conditions, the inclination angle of 90° was the most effective angle for decreasing seepage discharge, irrespective of the cutoff wall position. During an earthquake, the seepage discharge values were high regardless of the inclination angle. In the case of placing a cutoff wall at the upstream heel, the maximum seepage discharge value occurred at an inclination angle of 45°. This study provided insights into the behavior of cutoff walls under different conditions and can inform the design and construction of such structures for effective seepage control. The experimental feed-forward neural network (FFNN) was also successfully built. According to the following criteria (uplift pressure, seepage, and exit hydraulic gradient), the hydraulic performance of cutoff walls beneath hydraulic structures under static conditions can be examined. The FFNN can make predictions with root mean square errors (RMSE) of 0.0697, 0.0021, and 0.0059, respectively, and R2 values of 1.00, 0.9994, and 0.9997.

Study on barrier mechanism and application of silico-alkaline sol-modified bentonite cutoff wall materials to lead pollution in lead-zinc tailings pond

In this paper, the barrier mechanism of silico-alkaline sol-modified bentonite as cutoff wall materials for lead ions in lead‑zinc tailings ponds was investigated. Mechanical property tests, adsorption capacity tests, and permeability tests were conducted to assess the performance of the materials. The results indicated that the addition of silico-alkaline sol at a proportion of 20% by weight of bentonite improved the mechanical strength, anti-seepage ability, and adsorption capacity of the materials towards lead ions. The modification process of bentonite using silico-alkaline sol was confirmed through XED analysis. It was observed that silico-alkaline sol particles adsorbed onto the end face of montmorillonite crystal layer, altering its charge properties. This modification enhanced the adsorption capacity of bentonite towards metal cations. The interaction between the cutoff walls and lead ions was primarily governed by ion exchange. Through the utilization of CT and the finite element method (FEM), demonstrated the exiguity of connected flow lines in the vertical direction within the cutoff walls. Furthermore, column tests revealed that lead ions permeating through the cutoff walls gradually transformed into residuals and were immobilized within the wall. Visual Modflow analysis confirmed the effective application of the cutoff wall in remediating contaminated sites and the potential for practical implementation.

Study on the anti-corrosion and barrier ability of modified sulfoaluminate cement mortar cutoff wall against sulfate

Calcined layered double hydroxide (CLDH) was used to modify sulfoaluminate cement (SAC) mortar to develop a new sulfate-resistant barrier material. Through investigations into mechanical properties, permeability, microstructure, and migration characteristics, the results demonstrated that the addition of CLDH improved the mechanical properties and pore structure of SAC mortar and inhibited the diffusion of SO42-. Based on its anti-corrosion and barrier ability, the optimal CLDH dosage in SAC mortar was determined to be 2 % of the cement mass. Service life calculations and Visual MODFLOW simulations demonstrated that the SAC-CLDH cutoff wall has significant durability and barrier effect on sulfate. Compared to the site without the SAC-CLDH cutoff wall, the effective dispersion distance and area of sulfate decreased by 88.79 % and 93.43 % during 4000 days. This finding provides a new reference for the containment and management of high-concentration sulfate-contaminated sites.

FIELD AND MODEL RESEARCH OF THE GROUNDWATER FLOWS IN THE AREA OF THE PAKS-2 NPP CONSTRUCTION, HUNGARY

The construction of the nuclear power plant (NPP) requires conducting a large number of engineering and hydrogeological surveys, as well as assessment of the design decisions’ safety. A deep excavation pit at the Paks II NPP construction site requires execution of the cut-off wall due to extremely high groundwater saturation of the alluvial deposits. However, lithological anisotropy and the presence of dislocation zones did not allow identifying the appropriate depth for the cut-off wall construction. Unfortunately, engineering geological boreholes with a 20-meter distance between them and surface and borehole geophysical surveys could not identify the hydrogeological units. Thus, to conceptualize the hydrogeological settings, an analysis of the groundwater head distribution and the large-scale pumping tests were conducted. The interpretation of the geological data and the distinguishing of the hydrogeological units were carried out iteratively using the hydrogeological numerical model. The flow model could represent the hydraulic head distribution, the response of the lithologically heterogeneous layers to the water fluctuations in the Danube river, and the pumping tests carried out at the different depths. The results of the hydrogeological modeling revealed the aquitard to be continuous throughout the territory; however, its top’s depth changes from 30–35 to 90 m within the construction site of the Paks II NPP. This complex geometry of the aquitard is controlled by the plicated dislocation zone, which cuts the construction site in half and is revealed as the right wall of the graben.Correct hydrogeological stratification enabled us to ensure waterproof activities such as the cut-off wall construction using the hydrogeological model when excavating a deep pit for the Paks II NPP. This also minimizes the hydrodynamic impact on the closely located NPP Paks in operation.

Study on the hydraulic conductivity of plastic concrete vertical cutoff walls

With the rapid development of global economy and the accelerating process of urbanization, the prevention and control of groundwater pollution has received great challenges. In the remediation of contaminated sites, the traditional method is to use isolation barriers for in-situ remediation. However, due to the influence of various factors, the permeability of barriers will be changed to different degrees. This paper takes plastic concrete as the research object, through the permeability, water content and SEM test, discusses the water-binder ratio, no pollution and the presence of Ba2+ conditions on the permeability. The results show that in the case of no pollution, the decrease in the water-binder ratio and the increase of bentonite content will decrease the permeability, which decreases with the increase of age and satisfies the exponential relationship. Both Ba2+ and pH had adverse effects on the permeability. Ba2+ inhibits the formation of hydration products and the bonding between aggregate and bonding of cementitious materials, resulting in the decrease of compactness and the increase of porosity, which adversely affects the permeability. The addition of Ba2+ and the increase of pH will have adverse effects on the permeability, and these influencing factors need to be considered in the actual project.

Analytical model for organic contaminant transport in a cut-off wall and aquifer dual-domain system considering barrier arrangements

This paper presents an analytical study of organic contaminants transport in a cut-off wall and aquifer dual-domain system, considering the effects of the inlet boundary conditions and cut-off structural arrangements. The comprehensive sensitivity analysis of parameters focusing on the breakthrough time, attenuation time and cumulative concentration are presented using the Monte Carlo simulation and Sobol global method. The simplified constant inlet boundary condition can lead to an excessively conservative prediction of the contaminant breakthrough compared to the ‘finite mass’ and ‘decaying source’ boundary conditions. The cut-off wall hydraulic performance can be enhanced by reducing the contaminant's head loss, shape factor, half-life and cut-off wall hydraulic conductivity while increasing the retardation factor. The contaminant's half-life can largely influence the maximum contaminant concentration, attenuation time and breakthrough time. For example, the maximum contaminant concentrations for T1/2 = 1.4 years and T1/2 = 100 years are 13 and 123 times greater than that for T1/2 = 0.1 year, respectively. The influence of the variation of shape factor on the breakthrough curve should be taken into consideration. Altering the structural arrangement of the cut-off wall to accommodate a smaller shape factor increases the contaminant breakthrough time. The proposed solution allows the analysis of a cut-off wall and aquifer system with different inlet boundary conditions and structural arrangements of the cut-off wall.

Experimental Study on Piping of Dike Foundation with Suspended Plastic Cutoff Wall Under Vertical Load

Experimental Study on Piping of Dike Foundation with Suspended Plastic Cutoff Wall Under Vertical Load

Study on the performance of sealing slurry at the bottom of geomembrane composite vertical cut off walls

At the bottom of the geomembrane cut-off walls, pollutants may seep out through the bottom gap between the geosynthetic membrane and the low-permeability bedrock, contaminating the groundwater and the surrounding environment. Therefore, to address the occurrence of bottom gap seepage, this study adopts a mixture of attapulgite and cement in a certain proportion, mixed and stirred to form a sealing material. This sealing material is used to connect the bedrock and the geosynthetic membrane, thereby creating an efficient composite vertical barrier with special containment capacity. A series of laboratory experiments were conducted to investigate engineering properties of cement-attapulgite sealing slurry, including flow value tests, unconfined compressive strength (UCS) tests, improved flexible wall permeability tests, and bedrock and sealing slurry adhesion tests. The results show that the flow value of the slurry decreases with the increase of attapulgite content, and it increases with the increase of water-cement ratio. The UCS strength increases with the increase of attapulgite content, and it decreases with the increase of water-cement ratio, but the range of change is different. The hydraulic conductivity measured with hexavalent chromium solution as the penetrant is less than 50 % higher than that measured with water as the penetrant, and hexavalent chromium solution has little effect on the hydraulic conductivity of sealing slurry. The bonding performance between sealing slurry and bedrock is satisfactory, and the hydraulic conductivity of bedrock-sealing slurry is less than 1 × 10−7 cm/s (28d curing age), it is concluded that use of cement-attapulgite sealing slurry as bonding material between bedrock and geomembrane meets the engineering design requirements.

Numerical simulation of different pollutant control measures around an old landfill contaminated site: A field scale study

The remediation of contaminated soils is a great challenge for global environmental sciences and engineering. The landfill was a kind of infrastructure to deal with waste from different sources while it would also cause the threat to groundwater. Cut-off walls and pumping wells were usually applied in the landfill to prevent the spread of pollutants to wider areas. However, the combination of using both of methods was rarely analyzed, especially using field data for calibrating and fitting groundwater flow and pollutant transport. 7 monitoring wells were arranged in the study area to survey the subsurface seepage. The pollution monitoring was carried out for a period of 50 days, covering 31 types of inorganic and organic pollutants. The concentration of 2,4,6-trichlorophenol (TCP) was 556.7 times greater than the standard concentration. A coupled numerical model of groundwater flow and pollutant transport was developed to assess the effectiveness of various control methods. Three options were tested, including the implementation of a single cut-off wall as well as a combination of a cut-off wall and a pumping well, for preventing the discharge of pollutants from landfills. The combination of a cut-off wall and a pumping well is the best strategy for removal of TCP. The combination approaches lead to a reduction of pollution plumes by a factor of 11 compared to the case without pollution control measures. The research findings may provide a basis and reference for the application of cutoff walls and pumping well in landfill sites or contaminated groundwater.

Analytical solution for contaminant transport through a GCL-enhanced composite cutoff wall system

The geosynthetic clay liners (GCLs) have been recently shown to be effective in cutoff wall applications. Yet, its influence on contaminant transport through cutoff wall system is rarely reported. This study presents an analytical solution for the transport of organic contaminant through a composite cutoff wall system comprising a soil-bentonite wall, a GCL, and an aquifer. The analytical solution was obtained by variable separation method and verified against an existing analytical solution and a numerical solution. Parametric analysis was conducted to elucidate the effects of GCL, soil-bentonite (SB) wall thickness (Lw), aquifer thickness (La), and contaminant half-life (t1/2) on contaminant transport. The results show that by adding a GCL layer, the contaminant concentration and the maximum flux can be significantly decreased. When the GCL retardation factor increases 1 to 300, the required time to reach the maximum flux is delayed from about 62 years to 1500 years. Larger aquifer thickness sufficiently weakens the contaminant transport process. Under the same breakthrough criteria, the required thickness of SB wall decreases by about 20% when the GCL retardation factor increases from 1 to 50. The proposed analytical solution provides a useful tool for the design of the SB-GCL cutoff wall system.

Experimental Study into the Mechanical Properties of Plastic Concrete: Compressive Strength Development over Time, Tensile Strength and Elastic Modulus

Plastic Concrete is a low-strength (fcm,28d≤ 1.0 MPa), low-stiffness, impervious concrete used for cut-off wall construction in earthen dams. To date, there has been no systematic study on the effect of mix design on the long-term time development of the mechanical properties (compressive strength, tensile strength, and elastic modulus) of Plastic Concrete. Therefore currently no Plastic Concrete specific constitutive law for cut-off wall design exists. The present study closes this gap. Ten Plastic Concrete mixes with two bentonite-cement ratios and three types of sodium bentonite were produced. Fresh concrete workability tests were performed for all mixes. Compressive strength tests were performed at ages 28 d, 56 d, 91 d and four years. Splitting tensile strength and elastic modulus tests were performed at 28 days. The workability results show a good linear correlation between slump and flow table tests. Plastic Concrete’s compressive strength development over time is notably slower than that of ordinary concrete, and a new time development model following fib MC 2020 is established using a fitted sC coefficient. The splitting tensile strength shows an overall good, linear correlation with compressive strength, with an approximate ratio of 0.135. Furthermore, the elastic modulus EC,S according to EN 12390-13 concrete tests show significantly higher elastic modulus than the available data, and a model approximation for the elastic modulus EC,S as a function of compressive strength is also provided. Overall, this study provides the first Plastic Concrete specific mechanical property models, confirming the critical role mix design plays on Plastic Concrete’s short and long-term mechanical properties. Thus, the developed models enable a more realistic Plastic Concrete cut-off wall design and provide an important basis for future research.

Containment of phenol-impacted groundwater by vertical cutoff wall with backfill consisting of sand and bentonite modified with hydrophobic and hydrophilic polymers

A novel soil-bentonite backfill is proposed for use in vertical cutoff walls to contain phenol in groundwater at contaminated sites. The backfill consists of sand and bentonite modified with tetramethylammonium and carboxymethylcellulose, labeled as STCMB backfill. Flexible-wall permeability and double-reservoir diffusion tests were conducted to investigate the impact of phenol solution on hydraulic conductivity (k), effective diffusion coefficient (D*) and partition coefficient (Kp) of the backfill, respectively. The permeability results showed k of the STCMB backfill decreased by 0.91 times when the permeating liquid was changed from tap water to phenol solution. The diffusion testing results showed that D* values for the STCMB and conventional backfill (labeled as SCB backfill) were 4.0 × 10−10 m2/s and 3.0 × 10−10 m2/s, respectively, whereas Kp values for the STCMB and SCB backfills were 2.0 mL/g and 0.75 mL/g, respectively. The octanol-water partition coefficient model is suitable for estimating Kp for nonpolar organics. Furthermore, a series of solute transport simulations using Pollute V7 program was performed to evaluate the performance of vertical cutoff walls comprising STCMB and SCB backfills in containing phenol in lateral flowing groundwater. Overall, the STCMB backfill has demonstrated superior effectiveness in containing phenol in groundwater.

An approach for quantifying the calcium leaching effect on service performance of concrete cutoff wall of embankment dams

Calcium leaching is one of the essential diseases of concrete impermeable structures; therefore, an effective numerical approach to quantifying the effect of calcium leaching on concrete impermeable structures is critical for evaluating the long-term service performance of the structure. To this end, a seepage and dissolution coupling model for simulating the degradation of hydraulic concrete properties is proposed in this paper by introducing performance parameter evolutionary models based on saturated–unsaturated seepage and calcium dissolution and migration theories. The model is developed in COMSOL with a staggered solution scheme, and a detailed numerical implementation method is presented. The effectiveness of the proposed method is examined by laboratory calcium leaching tests. On this basis, the model was applied to evaluate the effect of calcium leaching on the service performance of a concrete cutoff wall (CCW) of an embankment dam in Shaanxi Province, China. The results demonstrate that the proposed method can effectively reproduce the calcium leaching process of concrete, and the variation of leaching depth based on model simulations is highly consistent with the experimental results. Calcium leaching primarily occurs at the CCW’s upstream waterfront surface, bypass seepage area and the partition of adjacent soil layers where the permeability varies greatly. Under seepage dissolution, the amount of leached calcium, average porosity, diffusion coefficient and hydraulic conductivity of the CCW increased exponentially with service duration, and solid-phase calcium leaching was the underlying cause of structural performance degradation. The length of the seepage path in the CCW decreases as the service duration increases, and the hydraulic gradient within it gradually increases. Based on the allowable hydraulic gradient of CCW, the safe service life of the CCW considering the calcium leaching effect was determined to be approximately 52a.

Comparison of MgO-GGBS, CaO-GGBS, and cement for construction of two-phase TRD cut-off walls in sand: workability, strength, and permeability

Comparison of MgO-GGBS, CaO-GGBS, and cement for construction of two-phase TRD cut-off walls in sand: workability, strength, and permeability

Durability, carbon footprint and contaminant immobilization in self-hardening slurries applied to cut-off walls: a review

Durability, carbon footprint and contaminant immobilization in self-hardening slurries applied to cut-off walls: a review