Dewatering of Tunneling Slurry Waste Using Electrokinetic Geosynthetics

Laboratory experiments are described that investigate the potential for using electrokinetic geosynthetics (EKGs) [materials that permit the combined exploitation of geosynthetics with electrokinetics (EKs)] to dewater slurry waste from a tunneling operation. The results demonstrate that the EK is reproducible for different slurries and that the process can significantly dewater tunneling slurry wastes. Higher electrode element surface area, increased potential gradient, and longer processing time increase water removal from a slurry waste. Higher potential gradients and current densities were found to consume more energy, with thicker samples (lower voltage gradients) and close element spacing using less power to achieve a particular dewatering efficiency than other configurations tested. The resultant pH of the treated slurry and the effluent water were found not to be sufficiently altered by the EK process to prevent their safe disposal or reuse. The potential of three different forms of EKG to treat tunneling slurry are discussed and a conceptual scheme for an EK enhanced belt press is proposed. While further investigation would be required to optimize their operating parameters, preliminary designs, and cost estimates can be based on the results presented herein.     

Investigation of Load-Transfer Mechanisms in Geotechnical Earth Structures with Thin Fill Platforms Reinforced by Rigid Inclusions

The reinforcement of soft soils by rigid inclusions is a practical and economical technique for wide-span buildings and the foundations of embankments. This method consists of placing a granular layer at the top of the network of piles to reduce vertical load on the supporting soil and vertical settlement of the upper structure. The study focuses on the modeling of load-transfer mechanisms occurring in the reinforced structure located over the network of piles with a coupling between the finite-element method (geosynthetic sheets) and discrete element method (granular layer; concrete slab in some cases). The importance of granular layer thickness to increase load-transfer intensity and to reduce vertical settlement was observed. However, without a basal geosynthetic sheet, the compressibility of soft soil has a great influence on the mechanisms. A method predicting the intensity of load transfers was proposed, based on Carlsson’s solution. The main parameters concerned are the geometry of the work and the peak and residual friction angles of the granular layer.     

Soil–Nail Pullout Interaction in Loose Fill Materials

A laboratory study was conducted to investigate the behavior of soil nails embedded in loosely compacted sandy fills. By varying the overburden pressure, the peak pullout force and the load–displacement behavior were determined by carrying out pullout tests in a displacement-rate controlled manner. The test results were compared to other published ones. The present results show that the pullout resistance can be interpreted with conventional soil parameters. The effect of retrained dilatancy, which is considered to be the reason for high pullout resistance in dense materials, is negligible in loose fill materials except under very low stress level. Furthermore, pullout resistance increases with overburden pressure opposed to some field test results reported in the literature which show no systematic trend in pullout resistance with overburden pressure. A numerical model was developed to simulate the mobilization of pullout force in soil nails. It has been shown that a simple one-dimensional spring model can be used to simulate the pullout load–displacement relationship.     

Development and Validation of a Two-Phase Model for Reinforced Soil by Considering Nonlinear Behavior of Matrix

The paper presents the formulation of a two-phase system applied for reinforced soil media, which accounts for nonlinear behavior of matrix phase. In a two-phase material, the soil and inclusion are treated as two individual continuous media called matrix and reinforcement phases, respectively. The proposed algorithm is aimed to analyze the behavior of reinforced soil structures under operational condition focusing on geosynthetics-reinforced-soil (GRS) walls. The global behavior of such deformable structures is highly dependent to the soil behavior. By accounting for mechanical characteristics of the soil in GRS walls, a relatively simple soil model is introduced. The soil model is formulated in bounding surface plasticity framework. The inclusion is regarded as a tensile two-dimensional element, which owns a linear elastic-perfectly plastic behavior. Perfect bonding between phases is assumed in the algorithm. For validation of the proposed model, the behavior of several single element reinforced soil samples, containing horizontal and inclined inclusions, is simulated and the results are compared with experiment. It is shown that the model is accurately capable of predicting the behavior especially before peak shear strength. The proposed algorithm is then implemented in a numerical code and the behavior of a full-scale reinforced soil wall is simulated. The results of analysis are also reasonably well compared with those of experiment.     

Enhanced Electrokinetic Remediation of Heavy Metals in Glacial Till Soils Using Different Electrolyte Solutions

Previous electrokinetic remediation studies involving the geochemical characterization of heavy metals in high acid buffering soils, such as glacial till soil, revealed significant hexavalent chromium migration towards the anode. The migration of cationic contaminants, such as nickel and cadmium, towards the cathode was insignificant due to their precipitation under the high pH conditions that result when the soil has a high acid buffering capacity. Therefore the present laboratory study was undertaken to investigate the performance of different electrolyte (or purging) solutions, which were introduced to either dissolve the metal precipitates and/or form soluble metal complexes. Tests were conducted on a glacial till soil that was spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, under the application of a 1.0 VDC/cm voltage gradient. The electrolyte solutions tested were 0.1M EDTA (ethylenediaminetetraacetic acid), 1.0M acetic acid, 1.0M citric acid, 0.1M NaCl/0.1M EDTA, and 0.05M sulfuric acid/0.5M sulfuric acid. The results showed that 46–82% of the Cr(VI) was removed from the soil, depending on the purging solution used. The highest removal of Ni(II) and Cd(II) was 48 and 26%, respectively, and this removal was achieved using 1.0M acetic acid. Although cationic contaminant removal was low, the use of 0.1M NaCl as an anode purging solution and 0.1M EDTA as a cathode purging solution resulted in significant contaminant migration towards the soil regions adjacent to the electrodes. Compared to low buffering capacity soils, such as kaolin, the removal of heavy metals from the glacial till soil was low, and this was attributed to the more complex composition of glacial till. Overall, this study showed that the selection of the purging solutions for the enhanced removal of heavy metals from soils should be primarily based upon the contaminant characteristics and the soil composition.     

Required Unfactored Strength of Geosynthetic in Reinforced Earth Structures

Current reinforced earth structure designs arbitrarily distinguish between reinforced walls and slopes, that is, the batter of walls is 20° or less while in slopes it is larger than 20°. This has led to disjointed design methodologies where walls employ a lateral earth pressure approach and slopes utilize limit equilibrium analyses. The earth pressure approach used is either simplified (e.g., ignoring facing effects), approximated (e.g., considering facing effects only partially), or purely empirical. It results in selection of a geosynthetic with a long-term strength that is potentially overly conservative or, by virtue of ignoring statics, potentially unconservative. The limit equilibrium approach used in slopes deals explicitly with global equilibrium only; it is ambiguous about the load in individual layers. Presented is a simple limit equilibrium methodology to determine the unfactored global geosynthetic strength required to ensure sufficient internal stability in reinforced earth structures. This approach allows for seamless integration of the design methodologies for reinforced earth walls and slopes. The methodology that is developed accounts for the sliding resistance of the facing. The results are displayed in the form of dimensionless stability charts. Given the slope angle, the design frictional strength of the soil, and the toe resistance, the required global unfactored strength of the reinforcement can be determined using these charts. The global strength is then distributed among individual layers using three different assumed distribution functions. It is observed that, generally, the assumed distribution functions have secondary effects on the trace of the critical slip surface. The impact of the distribution function on the required global strength of reinforcement is minor and exists only when there is no toe resistance, when the slope tends to be vertical, or when the soil has low strength. Conversely, the impact of the distribution function on the maximum unfactored load in individual layers, a value which is typically used to select the geosynthetics, can result in doubling its required long-term strength.     

Numerical Study of Reinforced Soil Segmental Walls Using Three Different Constitutive Soil Models

A numerical finite-difference method (FLAC) model was used to investigate the influence of constitutive soil model on predicted response of two full-scale reinforced soil walls during construction and surcharge loading. One wall was reinforced with a relatively extensible polymeric geogrid and the other with a relatively stiff welded wire mesh. The backfill sand was modeled using three different constitutive soil models varying as follows with respect to increasing complexity: linear elastic-plastic Mohr-Coulomb, modified Duncan-Chang hyperbolic model, and Lade’s single hardening model. Calculated results were compared against toe footing loads, foundation pressures, facing displacements, connection loads, and reinforcement strains. In general, predictions were within measurement accuracy for the end-of-construction and surcharge load levels corresponding to working stress conditions. However, the modified Duncan-Chang model which explicitly considers plane strain boundary conditions is a good compromise between prediction accuracy and availability of parameters from conventional triaxial compression testing. The results of this investigation give confidence that numerical FLAC models using this simple soil constitutive model are adequate to predict the performance of reinforced soil walls under typical operational conditions provided that the soil reinforcement, interfaces, boundaries, construction sequence, and soil compaction are modeled correctly. Further improvement of predictions using more sophisticated soil models is not guaranteed.     

Finite-Element Simulations of Full-Scale Modular-Block Reinforced Soil Retaining Walls under Earthquake Loading

A finite-element procedure was used to simulate the dynamic behavior of four full-scale reinforced soil retaining walls subjected to earthquake loading. The experiments were conducted at a maximum horizontal acceleration of over 0.8 g, with two walls subjected to only horizontal accelerations and two other walls under simultaneous horizontal and vertical accelerations. The analyzes were conducted using advanced soil and geosynthetic models that were capable of simulating behavior under both monotonic and cyclic loadings. The soil behavior was modeled using a unified general plasticity model, which was developed based on the critical state concept and that considered the stress level effects over a wide range of densities using a single set of parameters. The geosynthetic model was based on the bounding surface concept and it considered the S-shape load-strain behavior of polymeric geogrids. In this paper, the calibrations of the models and details of finite-element analysis are presented. The time response of horizontal and vertical accelerations obtained from the analyses, as well as wall deformations and tensile force in geogrids, were compared with the experimental results. The comparisons showed that the finite-element results rendered satisfactory agreement with the shake table test results.     

Generalized Model for Geosynthetic-Reinforced Granular Fill-Soft Soil with Stone Columns

This paper pertains to the development of a mechanical model to predict the behavior of a geosynthetic-reinforced granular fill over soft soil improved with stone columns. The saturated soft soil has been idealized by Kelvin–Voight model to represent its consolidation behavior. The stone columns are idealized by stiffer springs. Pasternak shear layer and rough elastic membrane represent the granular fill and geosynthetic reinforcement layer, respectively. The nonlinear behavior of the granular fill and the soft soil is considered. Effect of consolidation of the soft soil due to inclusion of the stone columns has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution, and results are presented in nondimensional form. Comparison between the results from the present study and the analytical solution using theory of elasticity shows reasonable agreement. The advantage of using geosynthetic reinforcement is highlighted. Results indicate that inclusion of the geosynthetic layer effectively reduces the settlement. Nonlinearity in the behavior of the soft soil and the granular fill is reduced due to the use of geosynthetic reinforcement layer.     

Effect of Soil Type on Electrokinetic Removal of Phenanthrene Using Surfactants and Cosolvents

Numerous sites have been contaminated with polycyclic aromatic hydrocarbons (PAHs), and these sites pose a significant risk to public health and the environment because PAHs are often toxic, mutagenic, and/or carcinogenic. Furthermore, these sites are often difficult or costly to remediate because PAHs are hydrophobic and highly resistant to degradation. The in situ flushing process using surfactants and/or cosolvents has shown great promise for sites possessing uniform and high-permeability soils, but it is generally ineffective for sites containing heterogeneous and/or low-permeability soils. Thus, for difficult soil conditions, electrokinetics can be integrated with the in situ flushing process to improve soil-solution-contaminant interaction. This investigation was conducted to evaluate the effects of two different low-permeability soils, kaolin and glacial till, on electrokinetically enhanced flushing. Each soil type was used in three bench-scale electrokinetic experiments, where each test employed a different flushing solution, deionized water, a surfactant, or a cosolvent. The results indicated that the contaminant was more strongly bound to the glacial till than the kaolin, and this was attributed to its higher-organic content. The glacial till also generated a greater electrical current and electro-osmotic flow, and this was probably a result of its higher-carbonate content and more diverse mineralogy. Based on the contaminant mass remaining in the soil, it was apparent that the surfactant or cosolvent solution caused contaminant desorption, solubilization, and/or migration in both soils, but additional research is required to improve PAH removal efficiency.     

Loose Fill Slope Stabilization with Soil Nails: Full-Scale Test

Soil nailing is commonly used for stabilizing cut slopes and retaining structures. The technique is, however, seldom used in stabilizing old loose fill slopes that were not constructed to the current standards. There is a concern that soil-nailed loose fill slopes may not render safety during heavy rainstorms. Little work has been carried out to investigate the behavior of soil-nailed loose fill slopes. This paper presents a comprehensive field test on a loose fill slope that was constructed by end tipping without any compaction—in the same way old fill slopes were formed. The slope was 4.75?m high, 9?m wide, and 33?deg to the horizontal. Two rows of five grouted nails were installed at a grid of 1.5?m×1.5?m at an inclination of 20?deg from the horizontal. A surface grillage was used to connect the six nails in the middle of the grid. Performance of the nailed slope was monitored with various instruments for about six months until the slope was tested to fail by surcharging and wetting. The overall results show that soil nailing with a surface grillage is a potentially effective way to enhance the stability of old fill slopes.     

Iodide-Enhanced Electrokinetic Remediation of Mercury-Contaminated Soils

This study investigates using an iodide-enhanced solution at the cathode during electrokinetic treatment to optimize the removal of mercury from soils. The experimental program consisted of testing two types of clayey soils, kaolin, and glacial till, that were initially spiked with 500 mg/kg of Hg(II). Experiments were conducted on each soil type at two voltage gradients (1.0 or 1.5 VDC/cm) to evaluate the effect of the voltage gradient when employing a 0.1 M KI solution. Additional experiments were performed on each soil type to assess the effect of using a higher iodide concentration (0.5 M KI) when using a 1.5 VDC/cm voltage gradient. The tests conducted on the kaolin soil showed that when the 0.1 M KI concentration was employed with the 1.0 VDC/cm voltage gradient, approximately 97% of the mercury was removed, leaving a residual concentration of 16 mg/kg in the soil after treatment. The tests conducted on glacial till indicated that it was beneficial to use the higher (0.5 M KI) iodide concentration and the higher (1.5 VDC/cm) voltage gradient to enhance mercury removal, because, under these conditions, a maximum of 77% of the mercury was removed from the glacial till, leaving a residual concentration of 116 mg/kg in soil after electrokinetic treatment. Compared to kaolin, the lower mercury removal from the glacial till soil is attributed to the more complicated soil composition, such as the presence of carbonates and organic matter, which caused Hg(II) to adsorb to the soil and/or exist as an immobile chemical species.     

On Global Equilibrium in Design of Geosynthetic Reinforced Walls

Common design of MSE walls is based on a lateral earth pressure approach. A key aspect in design is the determination of the reactive force in each reinforcement layer so as to maintain the system in equilibrium. This force leads to the selection of reinforcement with adequate long term strength. It is also used to calculate the pullout resistive length needed to ensure the capacity of each layer to develop strength. Lateral earth pressures used in design may or may not satisfy basic global equilibrium of the reinforced soil mass. Hence, the present work establishes a benchmark test using a simple statically determinate approach, in order to check if different design procedures satisfy equilibrium. Basic statics indicate that such a test is necessary, but not sufficient, to ascertain the validity of the calculated reactive force. Three existing design methods are examined: AASHTO, National Concrete Masonry Association, and Ko-stiffness. AASHTO, which is the simplest to apply and generally considered conservative, satisfies the benchmark test. However, it may yield very conservative results if one considers the facing to play a major role. NCMA is likely satisfactory if one explicitly accounts for the facing shear resistance in assessing the reaction in the reinforcement. The emerging Ko-stiffness approach, which is empirical, may violate statics potentially leading to underestimation of the reinforcement force.     

Predicted Loads in Steel Reinforced Soil Walls Using the AASHTO Simplified Method

The paper investigates the accuracy of the AASHTO simplified method by using load measurements reported in a large database of full-scale instrumented walls for bar mat, welded wire, and steel strip soil reinforced walls. The accuracy of the AASHTO simplified method is quantified by computing the mean and coefficient of variation of the ratio (bias) of measured loads under operational conditions to predicted loads. The paper shows that for steel strip walls, the AASHTO simplified method is reasonably accurate for granular backfill soils with friction angles less than 45°. For bar mat walls, the method is demonstrated to be slightly conservative. The simplified method underpredicts reinforcement loads at shallow overburden depths for steel strip walls with backfill friction angles greater than 45° due to compaction-related effects. It is concluded that these compaction-induced loads near the wall top do not contribute to internal instability due to pullout.     

Electrokinetic Remediation of Cadmium-Contaminated Clay

Electrokinetic extraction has been demonstrated to be very effective in removing heavy metals from Georgia kaolinite. The relatively high removal efficiency depends on the extremely acidic soil environment generated by the electrokinetic process. However, the efficiency observed in Georgia kaolinite cannot be achieved in soils of high acid/base buffer capacity without enhancement. In this study, the effect of ethylenediaminetetraacetic acid (EDTA) to enhance electrokinetic extraction of cadmium from Milwhite kaolinite was examined. The influence of electro-osmotic flow direction on the migration of cadmium, EDTA, and their complexes were also investigated. It was observed that injection of EDTA from the cathode reservoir by a reverse electro-osmotic flow could mobilize the cadmium in the specimen effectively. A less significant mobilization of cadmium was observed when the electro-osmotic flow was directed toward the cathode. However, accumulation of cadmium near the anode was observed regardless of the electro-osmotic flow direction.     

Sequentially Enhanced Electrokinetic Remediation of Heavy Metals in Low Buffering Clayey Soils

This paper presents the results of an experimental investigation undertaken to evaluate different purging solutions to enhance the removal of multiple heavy metals, particularly chromium, nickel, and cadmium, from a low buffering clay, specifically kaolin, during electrokinetic remediation. Experiments were conducted on kaolin spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, which simulate typical electroplating waste contamination. A total of five different tests were performed to investigate the effect of different electrode purging solutions on the electrokinetic remedial efficiency. A constant DC voltage gradient of 1 V/cm was applied for all the tests. The removal of heavy metals from the soil using tap water as the purging solution was very low. When 1 M acetic acid was used as the purging solution in the cathode, the removal of chromium, nickel, and cadmium was increased to 20, 19, and 13%, respectively. Using 0.1 M ethylene diamine tetraacetic acid as the purging solution in the cathode, 83% of the initial Cr was removed; however, the nickel and cadmium removal was very low. A sequentially enhanced electrokinetic remediation approach involving the use of water as a purging solution at both the anode and cathode initially, followed by the use of acetic acid as the cathode purging solution and a NaOH alkaline solution as the anode purging solution was tested. This sequential approach resulted in a maximum removal of chromium, nickel, and cadmium of 68–71, 71–73, and 87–94%, respectively. This study shows that the sequential use of appropriate electrode purging solutions, rather than a single electrode purging solution, is necessary to remediate multiple heavy metals in soils using electrokinetics.     

Experimental Studies on Heavy Metal Extraction from Contaminated Soil Using Ammonium Citrate as Alkaline Chelate during the Electrokinetic Process

Researchers have performed experimental studies using ammonium citrate (AC) during the electrokinetic (EK) remediation process for the extraction of cadmium (Cd) and copper (Cu) from the contaminated soil. They evaluated the efficiency of ammonium citrate by considering it as a washing solution and a purging solution at the anode electrode compartment. The efficiency of electrokinetic extraction was observed to be significantly influenced by the pH and buffering nature of the soil medium. The experimental studies indicate that the removal of cadmium and copper was 48.9% and 30.0%, respectively, when ammonium citrate was used both washing and purging solution. The solubility of both cadmium (Cd++) and copper (Cu++) in EK-treated soils has also been estimated by sequential extraction studies with deionized (DI) water. The analytical techniques, X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscope (SEM) provide the evidence of migration of cations during treatment of contaminated soil by process of electroosmosis (EO). The SEM images of both cadmium- and copper-contaminated soils show that these soils have a fluffier and more porous structure. This might be caused by the change in surface charges of the clay particles as a result of introduction of heavy metals. The mineralogical compositions of soil are not altered significantly by electrokinetic process.     

New Method for Prediction of Loads in Steel Reinforced Soil Walls

The paper describes a new working stress design methodology introduced by the writers for geosynthetic reinforced soil walls (K-Stiffness Method) that is now extended to steel reinforced soil walls. A large database of full-scale steel reinforced soil walls (a total of 20 fully instrumented wall sections) was used to develop the new design methodology. The effects of global wall stiffness, soil strength, reinforcement layer spacing, and wall height were investigated. Results of simple statistical analyses using the ratio of measured to predicted peak reinforcement loads (i.e., method bias) demonstrate the improved prediction accuracy. The AASHTO Simplified Method results in an average method bias of 1.1 with a coefficient of variation (COV) of 45%, whereas the proposed K-Stiffness Method results in an average bias of 0.95 and a COV of 32%. Soil strength was found to have limited influence on reinforcement loads for steel reinforced soil walls, especially for high shear strength soils, while global wall stiffness and wall height had a major influence on reinforcement loads.     

Experimental Study of the Behavior of a Lumpy Fill of Soft Clay

Land reclamation is a major civil engineering activity in Singapore. Due to depletion of suitable local fills and the cost of imported sand, dredged and excavated clay fills, in spite of their poor engineering properties, are being evaluated as a fill material. To reduce double handling, it is desirable for the clay to be used directly in a lump form, instead of the more conventional slurry fill. While the performance of a slurry fill is relatively well understood, the behavior of lumpy fill is not. This paper reports the results of a laboratory study carried out on lumpy fill made of cubical clay lumps of size ranging from 12.5?to?50?mm. The study showed that the interlump voids are substantially closed at a consolidation pressure much lower than the preconsolidation pressure of the lumps. The study also shows that at a consolidation pressure of about 100?kPa, the permeability of a lumpy fill is reduced to an order similar to that for homogeneous clay. However, the shear strength profile obtained using the cone penetration test indicates that the fill is still highly heterogeneous under a pressure of 100?kPa. When the preconsolidation pressure of the lumps is exceeded, the strength profile becomes uniform. The degree of swelling of the lumps plays a significant role. For fully swollen lumps, the consolidation pressure required to close the interlump voids is considerably less than that if the lumps were not allowed to swell. The coefficient of secondary compression of the lumpy fill is comparable to the homogeneous clay indicating that secondary compression is not a serious issue.     

Complicating Factors of Using Ethylenediamine Tetraacetic Acid to Enhance Electrokinetic Remediation of Multiple Heavy Metals in Clayey Soils

Batch and electrokinetic experiments were conducted to investigate the removal of three different heavy metals, chromium(VI), nickel(II), and cadmium(II), from a clayey soil by using ethylenediamine tetraacetic acid (EDTA) as a complexing agent. The batch experiments revealed that high removal of these heavy metals (62–100%) was possible by using either a 0.1?M or 0.2?M EDTA concentration over a wide range of pH conditions (2–10). However, the results of the electrokinetic experiments using EDTA at the cathode showed low heavy metal removal efficiency. Using EDTA at the cathode along with the pH control at the anode with NaOH increased the pH throughout the soil and achieved high (95%) Cr(VI) removal, but the removal of Ni(II) and Cd(II) was limited due to the precipitation of these metals near the cathode. Apparently, the low mobility of EDTA and its migration direction, which opposed electroosmotic flow, prevented EDTA complexation from occurring. Overall, this study found that many complicating factors affect EDTA-enhanced electrokinetic remediation, and further research is necessary to optimize this process to achieve high contaminant removal efficiency.