Cutoff Wall Research Articles

Assessing the Efficiency of Seepage Control Measures in Earthfill Dams

Seepage control in earthfill dams is a major concern during different phases of dam construction and operation. More than 30% of earthfill dam failures occur due to uncontrolled flow in the dam body and foundation. Seepage control measures, designed and installed at suspected sites of uncontrolled flow, thus play a vital role in stabilizing earthfill dams. However, the actual efficiency of seepage control measures often falls short of expected performance due to soil heterogeneity and changes over time. Assessing the performance of seepage control measures based on monitoring and modeling is necessary to avoid abrupt failures in earthfill dams. In this study, we developed a novel method for quantifying the efficiency of seepage control measures in earthfill dams based on combined seepage modeling and monitoring data. We tested the method by applying it to assess the efficiency of seepage control components at the Doroudzan dam, Iran. The results revealed that the overall efficiency of the dam’s seepage control measures (depending on water level in the reservoir) was 51–70%, based on the magnitude of discharged flow. The efficiency of three major seepage control devices, the chimney drain, cutoff wall, and grouting diaphragm in the left abutment, was 76–82%, 68–74%, and 16–19%, respectively.

Uplift pressure and hydraulic gradient in Sabalan Dam

The present study investigates the effects of clay core thickness and cutoff wall depth on some of the design parameters in the Sabalan rock fill dam by the finite-element method using the SEEP/W software. Clay cores with various thicknesses and cutoff walls in different situations of dam foundations were tested. The results revealed that (1) the maximum hydraulic gradient in Sabalan dam foundation occurs in the cutoff wall; (2) the minimum hydraulic gradient can be found by changing the location of the cutoff wall, under the center of clay core; and (3) discharge reduction depends on the optimum position at heel of the core. As the permeability decreases and the depth of the cutoff wall increases, the hydraulic gradient in the cutoff wall increases. As the thickness of the clay core increases, hydraulic gradient and the seepage discharge decreases and the uplift pressure increases.

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.

Plastic-damage analysis of concrete cutoff wall for a concrete face rockfill dam

Through in situ measurement records and numerical analysis, this work investigated the behaviour of a concrete cutoff wall for a concrete face rockfill dam built on an alluvium foundation. Monitoring records were obtained from a detailed monitoring system. A three-dimensional numerical model that uses a plastic-damage model for the stress–strain relationship of concrete was developed to predict the behaviour of the concrete cutoff wall. A coupled hydro-mechanical (HM) method was adopted to characterise the HM coupling effect of the dam and the alluvium foundation. The dam construction and the subsequent reservoir-filling processes were simulated in the numerical model. The results were compared with in situ measurements to validate the numerical model. The loading mechanism, stress and deformation behaviour, damage distribution and cracking of the cutoff wall were analysed. The wall behaviour, especially with relevance to the walls for dams with corewalls or inclined corewalls, and the effects of several factors on wall behaviour were investigated.

Insights of variable permeability full-section wall for enhanced control of seawater intrusion and nitrate contamination in unconfined aquifers

Seawater intrusion (SI) is an increased threat to the groundwater resource in coastal regions. The cut-off wall is widely used to arrest the advance of a seawater wedge, but it is likely to cause groundwater contamination in landward aquifers. In order to alleviate the contamination problem, we proposed a new variable-permeability cut-off wall (VPFW) with a semi-permeable section at the bottom of the physical barrier. This study is the first to investigate the spatial distributions and transient behaviors of SI and nitrate pollution at field scale under the joint effects of physical barriers and denitrification. Results show the dissolved oxygen (DO) decreases almost synchronously with the intrusion of SW near the sea boundary, and the denitrification of NO3− and retreat of the dissolved organic carbon (DOC) wedge occurred in the inland aquifer. Notably, the freshwater discharge flux and nitrate discharge flux for the developed VPFW increase by 35% and 20%, respectively, while the enrichment ratio of specific volume of nitrate contamination zone (SVNR) went down by 15% on average compared with the traditional cut-off wall. Four indices were proposed to quantify the impervious barrier: heights (H), relative hydraulic conductivities (K*), and control effectiveness of SI and nitrate pollution. Moreover, we found the control effectiveness for SI and nitrate pollution was continually enhanced with the reduction of K* and increase of H; however, excessive reduction and increase may result in the oppositeimpacts. The optimal K* and H for the VPFW can be determined with the required value of the reduction rate of saltwater wedge length (SWR) and SVNR; but all cut-off wall cases fail to meet the demand. The results show that the VPFW is much more effective in controlling SI and nitrate pollution in comparison with the traditional cut-off wall.

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.

Investigation of the effect of the different configurations of double-cutoff walls beneath hydraulic structures on uplift forces and exit hydraulic gradients

In the design of hydraulic structures, cutoff walls are needed to reduce uplift force (U) and the exit hydraulic gradient (GR). Usually two cutoff walls are required; an upstream cutoff wall is used to reduce the resultant of uplift force and a downstream cutoff wall is used to reduce the exit hydraulic gradient. This study numerically investigates double-cutoff walls beneath hydraulic structures with variations in their location and depth. Governing equations with specified boundary conditions are solved using the finite element method (FEM). Results show that if the downstream cutoff wall is deeper than the upstream cutoff wall, the resultant of uplift force will increase (compared to the no-cutoff wall case). With a constant hydraulic structure width (B), decreasing the distance between the two cutoffs (L), results in a reduction of uplift force. For L/B = 0.7, the change in the slope of the uplift force is large. Increases in the permeable depth (D) and reductions of B will lower the uplift forces. Increases in the downstream cutoff depth (d2) and L result in a reduction in the exit hydraulic gradient (GR). When the two cutoffs are located at the end of the hydraulic structure, GR is lower than when the downstream cutoff wall is located at a more downstream location. Comparisons between the available analytical solutions and computational results for two equal cutoffs show that the FEM can predict U and GR with a maximum of 5% error.

A numerical modeling study on the seepage under embankment dams

The present study focuses on the flow discharge under the embankment dams with clay core. Using numerical analysis, the effect of the cutoff wall and horizontal drain on the flow discharge, hydraulic gradient and uplift pressure beneath the dam was examined. The finite element program of SEEP/W was applied for modeling embankment dam. Comparing the results of numerical modeling of the embankment dam with the results of physical modeling demonstrated the acceptable accuracy of the numerical method in the flow evaluation. The effect of the position and depth of the cutoff wall on the seepage under embankment dam was also investigated. The seepage discharge was evaluated in terms of using horizontal drain and a combination of the cutoff wall and horizontal drain. The effect of dam’s layering foundation on the flow was investigated. The results indicated that the placement of the cutoff wall under the clay core would have the most reduction of seepage under the embankment dam. By increasing the cutoff wall depth, uplift pressure under the embankment dam was decreased and the hydraulic gradient was increased. The maximum hydraulic gradient occurred at the location of the cutoff wall. The effect of the cutoff wall on the seepage reduction under the embankment dam was decreased through decreasing permeability of the dam foundation. Increased horizontal drain length resulted in uplift pressure decrease. The results of modeling of embankment dam constructed on layering soil foundation suggested that the amount of hydraulic gradient, uplift pressure and seepage flow would change significantly when the cutoff wall depth ratio equals to the two-layer boundary depth.

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

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

A parametric study on behavior of plastic concrete material in earth dams

In the current research, the mechanical behavior of plastic concrete in cut-off walls of embankment dams has been investigated. To do so, several triaxial compression tests have been carried out on the specimens with various mix designs under different confining pressures. It has been observed that the compressive strength of such a material increases dramatically with the confining pressures applied on the samples. Moreover, the material behavior is substantially dependent on the confining pressure values; so that, as the confining pressure increases, the material behavior intends to be more ductile. Furthermore, a parametric study has been done to investigate the influence of mix design on these materials. The results showed that the strength and elasticity modulus of plastic concrete increase with the increase of cement factor and decrease of bentonite content. However, the rate of increase is apparently dependent on the mix design changes.

Laboratory Study on the Cyclic and Postcyclic Behavior of Plastic Concrete Used in Cutoff Walls of Embankment Dams

AbstractIn this paper, the cyclic response of plastic concrete material used in cutoff walls of embankment dams has been studied. To investigate the dynamic behavior of such a material, an extensiv...

Effect of open-framework gravel on suffusion in sandy gravel alluvium

Some accidents of dams are attributed to suffusion around open-framework gravel (OFG) in sandy gravel alluvium. However, whether OFG can be really an internal seepage exit of fine particle migration in the field or not? How much the effect of OFG on suffusion is? These problems have not been investigated yet. This paper presented a list of flume-scale suffusion tests to investigate suffusion at the tip of a cutoff wall in sandy gravel alluvium with OFG. The results indicate that suffusion first initiates at the downstream side of the tip of the cutoff wall in the test without OFG, while in the tests with OFG, it initiates at the upstream side of OFG, and then both mainly progress backward to the upstream side. OFG significantly decreases the hydraulic gradients at the initiation of suffusion and at blowout. In addition, the experimental results also confirm that OFG is likely to be an internal seepage exit of fine particle migration in the field. A large number of fine particles and some coarse particles in the sandy gravel are eroded into OFGs in the tests, and OFG significantly increases the settlement induced by blowout; meanwhile, it makes the settlement unstable after blowout.

Evolution analysis and environmental management of intruded aquifers of the Dagu River Basin of China

The seawater intrusion in the Dagu River Basin, China, has attracted intensive attention from the government and scholars. Increasing data have become available with the development of data acquisition technology. This situation brings unprecedented opportunities and challenges to the hydrochemical evolution analysis and improvement of seawater intrusion. The hydrochemical evolution process of groundwater is studied on the basis of our collected data in the Dagu River Basin by using mathematical statistics, end-element mixing, Durov, and Gibbs. The negative influencing factors of the groundwater environment are determined. Results show that the groundwater on the north side of the cutoff wall is mainly affected by residual saltwater, sulfuric acid leakage, and NO− 3 pollution. The groundwater on the south side of the cutoff wall is seriously affected by seawater invasion, followed by NO− 3 pollution. Meanwhile, the groundwater on the west bank of the Dagu River Basin is mainly affected by NO− 3 pollution. The groundwater on the north side of the cutoff wall must be discharged on a large scale for its improvement. Meanwhile, unpolluted water is recharged to renew the groundwater resources. Groundwater exploitation should be reduced, and the exploitation area should be dispersed to mitigate seawater intrusion and increase the recharge of the groundwater resources. NO− 3 pollution mainly comes from the sowing of chemical fertilizers and pesticides and domestic waste. Therefore, we should reduce the use of pesticides and fertilizers to control the generation, storage, and treatment of domestic pollutants strictly.

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.

Performance of a deep excavation in Lanzhou strong permeable sandy gravel strata

A large air shaft engineered between the Olympic Sports Center Station and Century Avenue Station of the Lanzhou Metro Line one, China, occurred in strong permeable sandy gravel complex strata. The short distance between the Yellow River and the pit (shaft) resulted in many challenges during construction. These challenges mainly included the quick recovery of the water level and the difficulty of plugging the water. Some measures for monitoring the structural safety and response of the surrounding environment were taken. As the data showed, the design combined a bottom-up method with a top-down method using a peripheral cut-off wall for anti-seepage treatment, the underground diaphragm wall and the concrete bracing had a large rigidity, good impermeability and little influence on the surrounding environment in the process of dewatering and excavation, the external plastic concrete cut-off wall could contribute to the groundwater drawdown inside the pit and reduce the soil pressure on the retaining structure of the pit.

Theoretical Basis for Designing Seepage-Control Walls in Dam Foundations

The structures arranged using the method of «slurry trench cutoff wall» or using secant piles are frequently used as seepage-control elements in dam foundations. However, they are designed based on empirical approaches without theoretical validation. With the aid of theoretical modeling the author revealed main specific features of seepage-control walls stress-strain state. They work not only for cross bending but also are subject to bending in their own plane, as well as they perceive considerable vertical forces. These forces appear due to friction between the wall and soil. The author obtained empirical formulae which at preliminary design stages permit predicting maximum values of compressive stresses in the wall at the moment when vertical forces have maximum values. The study made it possible to verify the recommendations for selection of material for arrangement of seepage-control walls. As a rule, deformation modulus of the wall material should not exceed the deformation modulus of the weakest soil by more than two times. Clay-cement concrete or plastic concrete is recommended to be used for the wall material. Its compressive strength is provided due to lateral compression of the wall by lateral compression of soil and groundwater pressure. For not deep «hanging» walls it is possible to use concrete. The assessment was given of foundation deformation impact on the stress-strain state of seepage-control walls. Occurrence of weak interlayers in the foundation causes in the wall appearance of zones concentrating compressive stresses and additional bending deformations. In high dam foundations there is a danger of sharp decrease of the wall compression pressure due to deformations of the foundation lateral expansion. It is not recommended to use the cantilever pattern of the wall conjugation with the dam body, but arrange a concrete gallery above the wall head. Conjugation by a cantilever is possible only for the walls arranged at the edge of the dam profile.

Durability of engineered cementitious composite exposed to acid mine drainage

Acid mine drainage (AMD) is detrimental to concrete cut-off walls due to acid and sulfate attacks. As a recently developed fiber reinforced cement-based composite with high tensile ductility and intrinsically tight crack width, Engineered Cementitious Composites (ECC, also known as Strain-Hardening Cementitious Composites, or SHCC) has potential advantages in resisting transport of water and contaminant if applied to the mine cutoff walls. The durability of ECC exposed to AMD, however, has not been previously studied, and is the main objective of this research. Mechanical properties of ECC exposed to AMD wet-dry cycles were investigated. In addition, MgO was added in ECC to improve self-healing performance. It was found that AMD wet-dry cycles coarsened pore structure and led to ECC's mass loss and compressive strength reduction. The AMD exposure also reduced ECC's first cracking strength, ultimate tensile strength and tensile strain capacity by 23.7%, 17.8% and 36.5%, respectively. Addition of MgO was found effective in enhancing ECC's resistance to AMD attack, by moderating the reductions in compressive strength, tensile ductility and mass loss. Further, the addition of MgO densified ECC's microstructure and improved self-healing behavior. We conclude that ECC containing 6% MgO retains the desirable tensile ductility and possesses acceptable durability against AMD for cutoff wall applications at mining sites.

Permeability Characteristics of Model Clay-Based Cutoff Wall Backfills

Clay-based cutoff walls have been widely applied in the landfills or site remediations. The factors that affect the hydraulic conductivity of cutoff walls formed by mixing formation with clays are still unclear. Aiming at the issue, the Fujian standard sand was used to simulate the formation and three kinds of clays were taken as the additive and slurry material for mixing and pouring. Then the hydraulic conductivity and pore size distributions were measured. The results indicated when the Clay C formed the cutoff walls with the model formation, there was a critical clay additive amount Ccr corresponding to the minimum porosity, and there would be great change in the descent velocity of the hydraulic conductivity k before and after critical clay additive amount Ccr. In regard to Clay A and Clay B, the critical clay additive amount Ccr of sand-clay specimens didn’t appear as the additive amount of Clay A and Clay B reached 25%. For Clay A and Clay B, the pore size distributions of sand-clay specimens were relatively identical, and their hydraulic conductivity k declined with decreasing porosity n. However, the pore size distributions of the mixture corresponding to Clay C gradually widened with the increasing additive amount, and they would develop towards the smaller pore size. Meanwhile, P50 also declined correspondingly, and there was poor correlation between porosity n and hydraulic conductivity k. Comparatively speaking, the microscopic pore structure of the model formation improved by Clay C was the most obvious, and Clay C raising the sealing effectiveness of model formation was also best.

Modelling evaporation processes of cement-bentonite mixtures

Upon drying, matter and energy are exchanged between the atmosphere and porous media through evaporation, which is a coupled process that involves the simultaneous transport of liquid water, water vapour and heat. At shallow depths, evaporation controls the water content and suction of both natural soils and earthworks, affecting their hydraulic response. This impact is particularly relevant when the earthworks are aimed at the containment of aqueous or non-aqueous pollutants, as in the case of cement bentonite cut-off walls. A coupled model for the transport of liquid water, water vapour and heat through cement bentonite mixtures upon evaporation was formulated. The model considers flow of water driven by both total suction and temperature gradients. Model predictions were compared to experimental results obtained in the laboratory on samples having different sizes and imposed boundary conditions. A good agreement between predicted and measured volumetric water contents was obtained, once defined a suitable dependency of the relative permeability of the mixture on degree of saturation. The results suggest that the proposed formulation correctly accounts for the underlying physical processes, and that it might be used to model the real scale behaviour of cut-off walls.

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

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