Cutoff Wall Research Articles

Influence of short-term binder-bentonite interactions on workability of sand-bentonite-binder mixture for seepage cut-off wall

Bentonite is commonly used as sealing material in seepage cut-off walls to minimize the permeability of sandy soil. To further reduce permeability and stabilize the sand-bentonite mixture, binders such as ordinary Portland cement (OPC) can be added. However, OPC can interact with bentonite and cause cation exchange, resulting in increased flowability and bleeding of soil mixture. Hence, this study proposes alternative bentonites and binders to reduce the influence of the short-term binder-bentonite interactions on the workability. The performance of three bentonite types, including sodium, calcium and salt-resistant bentonites, was evaluated. The use of ground granulated blastfurnace slag (GGBS) with activators of reactive magnesia (MgO) and quicklime (CaO) as alternative binders was also studied. When compared to OPC, MgO-GGBS had flowability of 1.7%–21.7% and bleeding of 0.6%–1.4% less, whereas CaO-GGBS had flowability of 1.9%–3.7% and bleeding of 0.1%–1.6% more. Mg2+ ions from MgO-GGBS had a lower affinity to participate in cation exchange with bentonite than Ca2+ ions from OPC and CaO-GGBS. The flowability and bleeding decreased further by 0.7%–5.4% and 6.2%–30.3%, respectively, when MgO reactivity and content increased. Besides, the binder-bentonite interactions had the least influence on salt-resistant bentonite, followed by calcium bentonite and then sodium bentonite, due to its low exchangeable sodium content. The XRD analysis also revealed that the short-term interactions of binder with bentonite tended to convert Na-based montmorillonite to Ca-based montmorillonite. In summary, the use of MgO-GGBS and salt-resistant bentonite can minimize the binder-induced cation exchange effect on the workability of sand-bentonite-binder mixture in cut-off wall construction.

Stress–strain behavior and constitutive relation of rubberized plastic concrete under uniaxial and triaxial compression

AbstractPlastic concrete is a kind of flexible concrete cutoff wall material, which is used to solve the problem of poor coordinated deformation ability between ordinary concrete cutoff wall and surrounding soil. It has the characteristics of small elastic modulus and large limit strain, which greatly improves the operation condition of cutoff wall. In order to save bentonite resources and recycle waste automobile tires, this paper crushed waste automobile tires into rubber particles and mixed them into plastic concrete, through the uniaxial compression test and triaxial compression test, obtained the corresponding stress–strain curve, rubberized plastic concrete strength and deformation properties are studied and the rubber particles and the relationship between dosage. The test results show that the compressive strength of rubberized plastic concrete decreases with the increase of rubber particles content, but the deformation capacity increases with the increase of rubber particles content. In this paper, the regression analysis of rubberized plastic concrete constitutive relation and failure criterion including the influence factor of rubber particles are helpful to promote the engineering application of rubberized plastic concrete and reduce the exploitation of natural resources and environmental pollution.

Electrochemical Accelerating Leaching Behavior of Plastic Concrete for Cut-Off Walls

Plastic concrete is a ductile material with a low elastic modulus (1000–3000 MPa), good flexibility, a and strong ability to adapt to the surrounding soil deformation. Hydraulic concrete mainly serves in a watery environment, so the leaching behavior of plastic concrete is crucial and cannot be neglected. Meanwhile, improving the crack resistance and effect of anti-seepage is also a primary task for cut-off walls. In this paper, in order to investigate the mechanical performance and leaching behavior of plastic concrete, a uniaxial compressive strength test was performed on plastic concrete specimens of a specific age (28 days) and different percentages of replacement cement by single bentonite (40%, 50%, and 60%) and bentonite (30%) together with clay (10%, 20%, and 30%), and the compressive strength, elastic modulus, pH value of the leaching solution, ultrasonic transmit time, electrical resistivity, and calcium ion dissolution concentration of plastic concrete have been evaluated. Moreover, the quantitative relationship between pH value and calcium ion concentration change was built through the electrochemical accelerating leaching method. According to the results, adding 40–60% soil materials can entirely meet the compressive strength (2–7 MPa), elastic modulus (below 3000 MPa), and relative permeability coefficient (below 1 × 10−7 cm/s) of plastic concrete used for cut-off walls while the compressive strength and elastic modulus of plastic concrete with 30% replacement cement by bentonite would be higher than 7 MPa and 3000 MPa, respectively. The leaching resistance of plastic concrete can be improved by more than 30% by adding bentonite coupled with clay, and three representative zones were observed through SEM and energy spectrum analysis, and Ca/Si molar ratio decreased by 30% after leaching.

Effect of the cutoff wall on the fate of nitrate in coastal unconfined aquifers under tidal action

This paper investigates the effects of the cutoff wall on the fate of nitrate (NO3-), the NO3- removal rate, and the salinity distribution in a coastal aquifer under tidal action. A numerical study was performed based on a coupled model with variable-saturation and variable-density flow and a convection-diffusion-reaction equation for solute transport in a coastal unconfined aquifer. The results showed that the cutoff wall led to a larger upper salinity plume (USP) and that the saltwater wedge (SW) further retreated seaward. The recirculation pathways of saltwater and groundwater were largely modified by the wall. The cutoff wall within the tidal range could increase the NO3- mass of denitrification and the NO3- removal efficiency and decrease the length of the SW and the freshwater flux. This modification of the saltwater and groundwater recirculation pathways was enhanced with increasing wall depth. A deeper cutoff wall led to a further retreated SW, lower freshwater flux, and greater improvements in the NO3- mass of denitrification and the NO3- removal efficiency. In addition, the cutoff wall significantly decreased the terrestrial dissolved organic carbon (TDOC) discharge into the sea. Dissolved organic carbon source (SDOC) promoted a higher NO3- removal efficiency. This study provides us with a better understanding of coastal physical-biogeochemical processes and dynamic mechanisms, as well as a guide for designing engineering measures to mitigate NO3- contamination and thus enhance groundwater quality management.

Influence of Combination of the Cutoff Wall and Horizontal Drainage on Seepage Characterization in Earth Dam Foundations

Influence of Combination of the Cutoff Wall and Horizontal Drainage on Seepage Characterization in Earth Dam Foundations

Vertical and Horizontal Water Penetration Velocity Modeling in Nonhomogenous Soil Using Fast Multi-Output Relevance Vector Regression.

A joint determination of horizontal and vertical movement of water through porous medium is addressed in this study through fast multi-output relevance vector regression (FMRVR). To do this, an experimental data set conducted in a sand box with 300 × 300 × 150 mm dimensions made of Plexiglas is used. A random mixture of sand having size of 0.5-1 mm is used to simulate the porous medium. Within the experiments, 2, 3, 7, and 12 cm walls are used together with different injection locations as 130.7, 91.3, and 51.8 mm measured from the cutoff wall at the upstream. Then, the Cartesian coordinated of the tracer, time interval, length of the wall in each setup, and two dummy variables for determination of the initial point are considered as independent variables for joint estimation of horizontal and vertical velocity of water movement in the porous medium. Alternatively, the multi-linear regression, random forest, and the support vector regression approaches are used to alternate the results obtained by the FMRVR method. It was concluded that the FMRVR outperforms the other models, while the uncertainty in estimation of horizontal penetration is larger than the vertical one.

Numerical simulation using the finite element method to investigate the effect of internal cutoff walls on seepage and hydraulic gradients in homogeneous earth dams

Numerical simulation using the finite element method to investigate the effect of internal cutoff walls on seepage and hydraulic gradients in homogeneous earth dams

Self-healing soil mix cut-off wall materials incorporating reactive magnesium oxide pellets

Recent decades have witnessed the increasing use of soil mixing technology to construct cut-off walls for groundwater control or underground pollutant containment. The strength and permeability of soil mix columns deteriorate considerably when subjected to chemical, mechanical and environmental stresses, leading to the need for repair of crack-originated damage. The development of self-healing grouts could provide more resilient and sustainable soil mix cut-off walls. In this study, for the first time, self-healing soil mix columns incorporating reactive magnesium oxide pellets were developed. The magnesium oxide-based cementitious grout was mixed with a sand soil using a laboratory-scale auger, and the embedded magnesium oxide pellets were ruptured upon cracking, producing expansive healing products due to hydration and carbonation, which can significantly fill the crack volume. The effects of magnesium oxide on the grout and soil mix samples were investigated in terms of both fresh and hardened properties. In addition, the self-healing performance was evaluated by examining permeability recovery and crack closure, and the microstructure and morphology of the magnesium oxide pellets and healing products were analysed. The results demonstrate the great potential of reactive magnesium oxide pellets as a self-healing agent for soil mix cut-off wall materials.

Effective use of farmland soil samples for N and O isotopic source fingerprinting of groundwater nitrate contamination in the subsurface dammed limestone aquifer, Southern Okinawa Island, Japan

Nitrate contamination of groundwater due to improper management of fertilizer application and controls during agricultural activities is an escalating worldwide issue. Numerous studies have reported similar contamination in the Ryukyu limestone aquifer in the southern part of Okinawa Island. The study area has a unique setting where the subsurface dams hold the groundwater behind their cut-off walls, probably holding contaminated waters. This study provides updated insights on the source of contamination using stable N and O isotope ratios of dissolved nitrate (ẟ15NNO3 and ẟ18ONO3) in the aquifer waters. To propose a better reference in the source fingerprinting assessment, a special emphasis was placed on the isotopic comparison between the fertilizers and farmland soils where the respective fertilizers were applied. A total of 176 groundwater samples were collected from several spatial and monthly sampling campaigns for the purpose of source determination. Our isotopic comparison detected the enrichment of 15N of nitrate-nitrogen in the soil samples after the application of both chemical fertilizers and manure due to nitrification and volatilization. Unlike results from previous studies, the ẟ15NNO3 compositions of most groundwater samples did not fall into the range of soils mainly controlled by chemical fertilizers (from +3‰ to +5‰), but were comparable to the soils affected by mixed fertilizers including both chemical fertilizers and manure (from +7‰ to +9‰). The soils beneath untreated manure storage sites characterized by the highest ẟ15NNO3 values (from +18‰ to +20‰) affected some groundwater, which shows an even higher value (> +9‰). The contaminated waters tend to be gathered close to the subsurface dams and this tendency did not differ between two subsurface dam settings, Komesu and Giza. However, due to more impact from manure storage sites, the groundwater in the Komesu area tends to show slightly higher δ15NNO3 values than those in the Giza area. Our study demonstrated the usefulness of soil samples for precise isotopic source identification in farmland areas.

Numerical Simulation Study on the Influence of Construction Load on the Cutoff Wall in Reservoir Engineering

Relying on the Beijing-Shijiazhuang Expressway widening project near the impervious wall of a reservoir, this paper uses FLAC3D two-dimensional and three-dimensional numerical simulation methods to establish the whole process model of the impervious wall of the reservoir affected by the construction load of the high-way reconstruction section. The stress and strain state of the cut-off wall in the high-way reconstruction section and the nearby reservoir is simulated in detail, the overall deformation of the cut-off wall in the reservoir is directly reflected, and the interaction and differential deformation between the wall structures are reflected. The safety and stability of the cutoff wall of the reservoir affected by the construction load are evaluated so that various advanced mechanical behaviors of the cutoff wall can be predicted. Research results show that the horizontal displacement value of the wall gradually increases from bottom to top, and the maximum value appears at the top of the wall. The horizontal displacement value of the 1–3 walls is relatively large, with the maximum value of 22.368 mm, and the horizontal displacement value of the 4–10 walls shows little difference. This is on account of the gravity of the backfill, the strata in the whole project area having settled, and the settlement at the bottom of the cut-off wall being 2.542 mm. At the root of the rigid cut-off wall, the compressive stress concentration occurs, with the maximum value between 1.75 MPa and 2.15 MPa. Due to the size of the structure, the maximum tensile stress of 0.237 MPa appears in the local area near the guide wall of the rigid cut-off wall, which will not endanger the rigid cut-off wall because of its small value. The maximum stress in the rigid impervious wall and the plastic impervious wall are 1.90–2.15 MPa and 1.00–1.12 MPa, respectively. Apart from the small tensile stress at the connecting guide wall between the rigid cut-off wall and the plastic concrete cut-off wall, the cut-off wall is under pressure, especially the plastic cut-off wall. Combined with the analysis of the stress state of the wall, it can be determined that the anti-seepage wall (rigid cut-off wall and plastic concrete cut-off wall) is stable and safe during the construction period.

Repulsion driven by groundwater level difference around cutoff walls on seawater intrusion in unconfined aquifers

Cutoff walls have been widely used to prevent seawater intrusion (SWI) in coastal regions. Previous studies generally concluded that the ability of cutoff walls to prevent seawater intrusion depends on the higher flow velocity at the wall opening, which we have shown is not the most critical mechanism. In this work, we implemented numerical simulations to explore the driving force of cutoff walls on the repulsion of SWI in both homogeneous and stratified unconfined aquifers. The results delineated that the inland groundwater level was raised by cutoff walls, which generated a significant groundwater level difference beside two sides of the wall and thus provided a large hydraulic gradient to repel SWI. We further concluded that by increasing inland freshwater influx, the construction of cutoff wall could result in a high inland freshwater hydraulic head and fast freshwater velocity. The high inland freshwater hydraulic head posed a large hydraulic pressure to push the saltwater wedge seawards. Meanwhile, the fast freshwater flow could rapidly carry the salt from the mixing zone to the ocean and induce a narrow mixing zone. This conclusion explained the reason that the cutoff wall can improve the efficiency of SWI prevention through recharging freshwater upstream. With a defined freshwater influx, the mixing zone width and saltwater pollution area mitigated with the increase of the ratio between high and low hydraulic conductivity values (KH/KL) of the two layers. This was because the increase of KH/KL caused a higher freshwater hydraulic head, a faster freshwater velocity in the high-permeability layer, and the prominent change of flow direction at the interface between the two layers. According to the above findings, we deduced that any way to increase the inland hydraulic head upstream of the wall would improve the efficiency of cutoff walls, such as the freshwater recharge, the air injection, and the subsurface dam.

Engineering performance and microscale structure of activated high titanium slag-soil-cement-bentonite (HTS-SCB) slurry backfill

An innovative soil-cement-bentonite (SCB) slurry composed of cement, clay soil, bentonite and reactive high titanium slag (HTS) was synthesized to block the migration of contaminants in groundwater at tailings ponds. Mechanical activation was applied to activate the HTS, and partial cement was replaced by the reactive HTS. The engineering performance indices of the HTS-SCB slurry, such as the slump, hydraulic conductivity, compressive strength, porosity and leaching risks of heavy metals, were evaluated in this study. The hydration products were identified through a series of analytical tests. The three-dimensional microscale structure of the sample was obtained from the CT experiment, and the microscale geometrical properties were analysed using pore-network models and Avizo software. The lattice Boltzmann method (LBM) was applied to simulate the fluid flow in the HTS-SCB slurry, and the simulated hydraulic conductivities and three-dimensional streamlines were obtained. The results showed that the HTS-SCB slurry had an optimal anti-seepage performance, appropriate mechanical performance and low leaching risks of heavy metals when the cement was replaced by 50 % reactive HTS. The hydration reaction between calcium hydroxide and HTS generates large amounts of fresh calcium silicate hydrates containing Fe and Ti, which fill the pores and contribute to low hydraulic conductivity, micropores (diameter ≤ 2.8 μm) and microflow paths (diameter ≤ 2.5 μm). The LBM results show that the calculated hydraulic conductivities match well with the measured values, and a high tortuosity is obtained from the three-dimensional velocity fields. The HTS-SCB slurry can satisfy the requirement of cut-off walls in land remediation applications and realize solid waste recycling, energy conservation and emission reduction and sustainable development.

Saltwater intrusion management at different coastal aquifers bed slopes considering sea level rise and reduction in fresh groundwater storage

Coastal fresh groundwater management is a challenging research topic due to the relevance of these resources and the huge suffered risks due to global change and overpopulation. The geometrical features of coastal aquifers play a control role in saltwater intrusion (SWI). Seawater level rise and the reduction in aquifer fresh groundwater storage are promoting SWI. All these key factors are considered with two different numerical approaches defining schematic management criteria bottom using the numerical code SEAWAT. The former approach adopts the well-known Henry’s problem; the latter is based on the real study case of the Gaza aquifer (Palestine). Different aquifer bed slopes (ABS), and hydraulic and physical methods for SWI management are considered together with SLR, recharge reduction, and over pumping. The results showed that the land side ABS cases show more SWI than sea side and horizontal ABS. Cut-off walls and check dams are effective to manage SWI in horizontal ABS more than in other slopes, also the subsurface dams, earth fill and recharge of freshwater are good methods in land side ABS while the abstraction of brackish water and combination of recharge with abstraction are better to mitigate of SWI in sea side ABS. Useful comparing tables and considerations are defined with the purpose to guide the preliminary selections of new management solutions for reducing the effect of the global change on groundwater resources for different slopes aquifers around the coastal world.Graphical abstract

Concurrent multiscale analysis of anti-seepage structures in embankment dam based on the nonlinear Arlequin method

The scales of embankment dam and local anti-seepage structures (core wall or cut-off wall) differ heavily, which poses challenges to analyze both the global and local responses simultaneously. In this study, the nonlinear Arlequin method is introduced to evaluate the behavior of anti-seepage structures in embankment dam within a concurrent multiscale framework. Based on the energy blending approach, the governing equations are deduced, in which the material nonlinearity effect and the Arlequin coupling effect can be separated from each other. Then, a list of subroutines of ABAQUS is used to implement the proposed framework jointly. Multiscale analysis of elasto-plastic cantilever beam demonstrates the effectiveness of the proposed method. The effects of coupling operators are discussed, and constant weighting functions are recommended for nonlinear coupling analysis. The analyses of embankment dams indicate that the uneven settlement pattern and the resulted arching effect between the core and the shell as well as the complicated bending deformation state of cut-off wall can be well captured by the proposed method. It is affirmed that the nonlinear Arlequin method facilitates the increase of computational efficiency without losing much accuracy, showing great merits of modeling local nonlinearity and global response simultaneously under a concurrent multiscale framework.

Physical and mechanical characterization of deep soil mixing (DSM) materials: Laboratory vs construction site

Soil mix material is a soil–cement material mixed in place. The process results in the formation of “concrete” in which the soil is being used as aggregate. The objective of this paper is to compare laboratory-proportioning tests with results obtained on three different construction sites (CS_1, CS_2, CS_3) using deep soil mixing method, establishing relationships between physico-mechanical properties of laboratory and construction site materials. In CS_1, Deep Soil Mixing was used to make a temporary retaining wall. Two soil-mixing columns of 80 cm diameter and 3 m depth were realized. After curing, 230 cylindrical specimens of Φ 10 × 20 cm and 130 samples of Φ 5 × 10 cm were drilled. In CS_2 and CS_3, Trench mix method was used for cut-off walls construction. In these sites, 48 specimens and 22 specimens of Φ 11 × 22 cm of fresh mixed-soil were collected, respectively. Two types of soils, from 1 and 3 m depth, were collected from CS_1 to prepare soil–cement laboratory specimens. Soil-cement mixtures contain different cement contents (100, 150, 200, 250 and 300 kg/m3), and 50 % water content. Compressive strength, dynamic modulus, porosity and hydraulic permeability are investigated. The influence of cement dosage and soil type is discussed. There is a close fit between the laboratory results and the site results for soils with low clay content. The difficulty of uniformly mixing the clay results in a lower performance of the mixed in place material compared to the laboratory material.

Enhanced Organic Contaminant Retardation by CTMAB-Modified Bentonite Backfill in Cut-Off Walls: Laboratory Test and Numerical Investigation.

Adding organically modified bentonite into impervious wall materials may improve the adsorption of organic pollutants. In this study, cetyltrimethylammonium bromide organically modified bentonite (CTMAB bentonite) was mixed with sodium bentonite and kaolin to obtain two materials, which were then used as cut-off walls for typical pollutants. Soil column consolidation tests, diffusion tests, and breakdown tests were conducted to study migration of organic pollutants in soil columns. The parameter sensitivity of pollutant transport in the cut-off wall was analysed by numerical simulation. The sodium bentonite mixed with 10% CTMAB bentonite and kaolin-CTMAB bentonite showed the greatest impermeability: with a consolidation pressure of 200 kPa, the permeability coefficients were 1.03 × 10-8 m/s and 3.49 × 10-9 m/s, respectively. The quantity of phenol adsorbed on sodium bentonite-CTMAB bentonite increased with increasing water head height. The kaolin-CTMAB bentonite column showed the best rhodamine B adsorption performance, and the adsorption rate reached 98.9% on day 67. The numerical results showed that the permeability coefficient was positively correlated with the diffusion of pollutants in the soil column. The quantity adsorbed on the soil column was positively correlated with the retardation factor, and the extent of pollutant diffusion was negatively correlated with the retardation factor. This study provides a technical means for the optimal design of organic pollutant cut-off walls.

Experimental study on barrier performance and durability under dry-wet cycles of fly ash based geopolymer cutoff wall backfill

Vertical cutoff wall is one of the most effective technologies to restrict the migration of contaminants. In this study, a geopolymer cutoff wall backfill (GCWB) consisting of reactive sodium silicate, fly ash, sand, and water was developed, which does not need Ordinary Portland Cement (OPC) and bentonite. The basic macroscopic properties of GCWB were tested, in terms of workability, unconfined compressive strength, hydraulic conductivity, and chemical compatibility, then the optimal material ratio was determined. In order to evaluate the barrier performance of GCWB under dry-wet cycles, the hydration products and microstructural characteristics were measured by X-ray Diffraction (XRD), Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). According to the orthogonal analysis, the optimal material ratio with the highest unconfined compressive strength was 30FA1.2M12S and that with the lowest hydraulic conductivity was 40FA1.2M12S. The unconfined compressive strength and hydraulic conductivity of GCWB can achieve 6.62 MPa and 4.83 × 10−11 m/s after cured for 28 days, respectively. The chemical compatibility of GCWB was overall favorable in Na2SO4 and CaCl2 solutions. Under the effect of dry-wet cycles, the variation of hydraulic conductivity, mass and volume in contaminant solutions were basically the same as that in water. The hydraulic conductivity increased sharply by two orders of magnitude after the 1st dry-wet cycle and tended to be stable (slightly larger than 1 × 10−8 m/s) in the subsequent dry-wet cycles. Therefore, the barrier performance and durability of GCWB was desirable in water and contaminant solutions, indicating that it is a promising antifouling cutoff wall material.

Closed-form analytical solution for infinite-depth seepage below diversion dams considering the width of cutoff wall

ABSTRACT The problem of steady-state seepage flow underneath diversion dams on permeable soil of infinite depth with a cutoff wall at the downstream end with a certain width has not been previously solved. This paper uses conformal mapping and the Darcy equation-based approach to investigate the exact solution. Closed-form analytical equations are given for changes in hydraulic gradient, seepage discharge, and uplift pressure. The results are easily used to create design diagrams. These diagrams show that along the specified length of the structure, with increasing cutoff wall width, hydraulic gradient and seepage discharge decrease, and the pressure difference between the two endpoints of the cutoff wall increases. Also, increasing cutoff wall width has a more negligible effect on reducing hydraulic gradient and seepage discharge at large-length structures. In addition, at greater distances from the downstream end of the structure, the effect of the width on hydraulic gradient reduction and seepage discharge is minimal.

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.

Phreatic seepage around a rectilinear cutoff wall: The Zhukovsky-Vedernikov-Polubarinova-Kochina dispute revisited

The problem of steady 2-D potential Darcian flow from a flat-bottomed reservoir of a coffer or other type of dam, separated from an empty tailwater by an impermeable vertical sheet piling, is one of the pillars in groundwater hydrology and soil mechanics. Analytical solutions to this problem are presented in the books by Forchheimer, Zhukovsky, Vedernikov, Polubarinova-Kochina, Harr, among others. For the case of seepage with a phreatic line, descending from the downstream face of the cutoff, the formulae presented in classical textbooks - despite the simplicity of conformal mappings of the complex potential quarter-plane onto the Zhukovsky half-strip - have lacunae and flaws. We discuss and clean the mismatch in the abovementioned formulae. The classical analytical solutions in the textbooks deal with the regime of no backwater, for which the magnitude of the specific discharge vector at infinity coincides with hydraulic conductivity of the soil. We obtain a general solution for most general flows with backwater, for which the specific discharge at infinity is zero and the free surface has an inflection point in the physical plane. The capillary fringe in the hodograph domain is imaged by a circular cut. The pressure head infinitely deep under the cutoff-wall is infinite. A circular tetragon in the hodograph domain is conformally mapped onto the quadrant in the complex potential plane via a reference plane. In the case with backwater, one extra condition is needed to fix a mathematical solution. For this purpose, the location of a point on the free surface is fixed. Mathematically, two affixes in the reference plane are found by solving a system of two nonlinear equations (in the no backwater case, only one affix is found and one equation is solved). We also examine flow with a phreatic line in a low-permeable layer (e.g. concrete apron of a dam) with an Λ-shaped topology of streamlines, bounded from the left by a cutoff. Seepage from a dam reservoir through a subjacent highly-permeable alluvium induces the pore water motion in the upper layer. A Hilbert boundary-value problem is solved for the complex potential in this cell. In the Kirkham-Brock type boundary condition along the interface between the two layers, the horizontal component of the specific discharge is constant. Phreatic surfaces rising on the downstream face of the cutoff caps Λ−shaped streamlines, which turn around a hinge point on the interface between two layers of contrasting permeability.