Search

The present study investigated the effectiveness of two soil-bentonite slurry wall backfills, specifically sand/conventional Na-bentonite (NaB) and sand/hexametaphosphate (SHMP)-amended Ca-...

Hydraulic conductivity of soil-bentonite (SB) slurry-trench cutoff walls is highly influenced by the stress state. A model based on force equilibrium is proposed to predict the stresses in ...

Slurry trenches are often used to impede groundwater flow or to contain soil and groundwater contamination. The stability of the slurry trench during excavation and prior to backfilling is a major concern in design. A horizontal slice method for stability analysis of slurry trenches, which allows consideration of soil stratification, is presented. Vertical force equilibrium for each slice of the sliding mass and horizontal force equilibrium for the whole sliding mass are considered on the basis of limit equilibrium. The factor of safety is obtained via the Newton–Raphson method and the critical slip surface corresponding to the minimum factor of safety is located by the pattern search method. The proposed method is verified via consideration of a previously reported problem. It is then applied to a full-scale field experiment in three-layer soils. The obtained minimum factor of safety and location of the slip surface of the sliding mass are in excellent agreement with the experimental measurements. Analyses of trenches having a weak layer at various depths demonstrate potential variations in slip surface inclination between soil layers.

The slug test is a viable method in estimating the hydraulic conductivity of the slurry trench wall backfill because of its ability to consider a more representative volume of the backfill and to reflect the in situ performance of the construction. A three-dimensional numerical model is developed to simulate the slug test in a slurry trench wall with the presence of bentonite cake on the interface boundary between the wall and the surrounding soil formation. Influential factors such as wall width (i.e., proximity of wall boundary), well deviation, vertical position of the well intake section, and compressibility of the wall backfill are taken into account in the model. The experimentally obtained hydraulic properties of the bentonite cake are also incorporated in a series of slug test simulations. The simulation results are then examined to evaluate the bentonite cake effect in analyzing practical slug test results in the slurry trench wall. The simulation results show that the modified line-fitting method can be used without any reduction factor for the slug test in the slurry trench wall with the presence of bentonite cake. A case study is reanalyzed with the assumption of existing bentonite cake. The results are compared with the previously reported results by the approaches used for the case of no bentonite cake (constant-head boundary) and upper-bound solution (no-flux boundary). The modified line-fitting method and the type curve method produce similar results for slurry walls with bentonite cakes. The case study results demonstrate the importance of the bentonite cake effect in estimating the hydraulic conductivity of the slurry wall backfill.

Several different phenomena are involved in the formation and performance of self-hardening slurry walls, including slurry contamination by soil, penetration of slurry into the ground, confining with the trench, filtration, sedimentation, consolidation, and chemical reactions between the slurry constituents. To evaluate the relative influence of these phenomena, a case study is analyzed—the cutoff wall of Santa Clara-a-Velha Monastery, built in 2001–2002. The performance of this wall is related to the in situ phenomena through data obtained during construction and from a site investigation conducted in 2011. An extensive laboratorial program was performed on samples cored from the wall and on laboratory-made samples. It was found that consolidation was the major contributor to the significant settlement of the wall that occurred after trench excavation, whereas filtration and sedimentation made only a secondary contribution. Contamination by sand only interfered with the physical properties and co...

Slurry trench cutoff walls, constructed using self-hardening slag-cement-bentonite (Slag-CB), are the most common form of in-ground vertical contaminant barrier in the U.K., Europe, and Japan, and are increasingly being used in the United States. This paper presents a case study of the hydraulic conductivity evaluation of an 11-year-old Slag-CB wall material at a sulfate-contaminated site, using different in situ techniques and laboratory tests. The laboratory results suggest that the hydraulic conductivity of the samples, which vary in age from 4 weeks to 11 years, decreases with time for the first 3 years but then remains constant. The results indicate that the long-term performance of these containment walls is influenced by various parameters such as aging, the type/duration of contaminant exposure, mixing of surrounding soil during construction, and wall depth. Piezocone tests, packer tests, and self-boring permeameter tests were carried out in the field to determine the suitability of different in s...

Two soil-bentonite cutoff-wall backfills obtained from construction sites, one in New Jersey and one in Delaware, were tested for the existence of membrane behavior. Both backfills were designed as a mixture of dry bentonite (3–4% by dry weight) and the locally excavated soil blended with bentonite water slurry to provide slumps ranging from 100 to 150 mm (from 4 to 6 in. ). The results of the membrane tests indicate that both backfills exhibit membrane behavior. Further, the magnitude of the membrane behavior increases with decreasing void ratio. However, the magnitude of the increase in membrane behavior in these construction-site backfills was lower than that previously reported for model backfills prepared in the laboratory. The difference in the membrane behavior is attributed, in part, to a lower percentage of clay in the construction-site backfills relative to the model backfills. Nonetheless, based on the measured membrane efficiencies for the two field-constructed backfills, the total liquid flux...

In both the United States and the United Kingdom, slurry walls are used as vertical barriers to control groundwater flow and to contain contaminants as part of waste containment systems. In the United States, slurry walls are commonly constructed using soil-bentonite ~SB! and the barrier typically consists of a mixture of select soil, bentonite, and bentonite-water slurry. Alternatively, in the United Kingdom, the barrier wall comprises a mixture of cement, blast furnace slag, and bentonite-water slurry. After a comparison of the two techniques, this paper presents the results of permeability and unconfined compressive strength tests on twenty-one different mixtures of slag-cement-bentonite ~slag-CB!. The slurry wall materials tested in this study were prepared using sample formulations originating in the United Kingdom and materials originating in the United States. Unconfined compression tests were performed on samples after one month of curing, while permeability tests were performed after one, two, three, six, and twelve months of curing. For the mixtures tested and cured twelve months, two mixtures ~one having 20% cementitious material with 70% slag replacement and another having 15% cementitious material with 80% slag replacement! were found to have the lowest hydraulic conductivity s2 3 10 ˛8 cm/ sd. The data show that 0 to 60% slag replacement had little effect on hydraulic conductivity of the resulting slag-CB mixtures. However, the hydraulic conductivity drastically decreases as the slag replacement increases from 70 to 80%. As expected, the unconfined compressive strength increased as the cementitious material content increased from 10 to 15 to 20%. The slag-CB consolidates rapidly and has compression characteristics similar to other high moisture materials.

Two model soil-bentonite (SB) backfills were evaluated for their ability to behave as semipermeable membranes. The base soils for the model backfills consisted of natural clay with 89% (dry weight) fines and a mixture of sand with 5% dry sodium bentonite. Specimens of both base soils were mixed with a sufficient amount of 5% sodium bentonite.water slurry to correspond to 100 mm slumps in accordance with standard practice for SB vertical cutoff walls. Membrane behavior was evaluated by measuring the chemico-osmotic efficiency coefficient ω, resulting from maintaining a 3.88 mM KCl concentration difference across the specimen. The results indicate that both model backfills act as semipermeable membranes, with ω ranging from 0.018 to 0.024 for the natural clay backfill and from 0.118 to 0.166 for the sand.bentonite backfill. More significant membrane behavior (higher ω) is correlated with higher consolidation stress, lower void ratio, and lower hydraulic conductivity. The benefit of the chemico-osmotic liquid flux due to the existence of membrane behavior in SB vertical cutoff walls is illustrated through an example analysis using the measured results. The results of the study provide the first quantitative evidence that SB vertical cutoff walls can behave as semipermeable membranes.

A method for assessing stability of slurry trenches within an inclined ground surface is presented using the Coulomb-type force equilibrium method. The influence of ground surface inclination on trench stability and angle of critical failure surface is investigated via parametric studies on an example trench. As expected, the factor of safety decreases considerably with an increase in the ground surface inclination. It is shown that it is unconservative to neglect ground surface inclination while analyzing trench stability. According to the example, no significant error in the minimum factor of safety is introduced by using the failure surface inclination angle of 45°+ϕ′/2 for trenches with an inclined ground surface. It is also found that differences between the angle of critical failure surface and 45°+ϕ′/2 may exist; therefore, the angle of the critical failure surface needs to be determined instead of using 45°+ϕ′/2 for the design of remedial measures.

Coulomb-type force equilibrium analyses are presented for general two- and three-dimensional stability of a slurry-supported trench. Analytical solutions are derived for the factor of safety and critical failure plane angle in each case for drained effective stress and undrained total stress conditions. The solutions can accommodate variable trench depth, trench length, slurry depth, groundwater table elevation, surcharge loading, tension crack depth, and level of fluid in the tension cracks. Drained analyses can account for c82f8 soil strength and the effect of soil suctions above the groundwater table using the total cohesion method. Undrained analyses can account for undrained shear strength that varies linearly with depth. The solutions reduce to previously published expressions for simplified cases. An example problem is provided to illustrate variations of the factor of safety and critical failure plane angle with the length of a three- dimensional slurry trench. Finally, the method shows excellent agreement with the results of a full-scale field experiment of the failure of a diaphragm wall slurry trench constructed in silty sand.

Some of the deepest structural slurry walls in the world were designed for permanent training walls at the Dam No. 2 Hydropower Project on the Arkansas River. Each of the 152 wall panels had to be optimized for embedment, length, section type, and restraining system. Compatibility for deflection and load transfer of adjacent panels was an integral part in designing individual structural slurry wall panels to behave as a system of panels. Design of the structural slurry walls utilized the computer program RIDO to analyze the deflection, shear, and moment under construction and operation conditions. Structural requirements for the design based on the RIDO analyses were checked using STAAD-III. A third program (SOILSTRUCT) was used to verify the 2D global deformation of the proposed structural slurry wall system. Analyses and design had to be completed in a short 9-month time frame. This paper presents the design methodology, approach, and procedures for optimization of the structural slurry wall system required to make this project a reality.

Rapid degradation of aqueous trichloroethylene (TCE) was observed in batch experiments conducted with soil/bentonite slurry wall materials amended with the addition of zero-valent iron. The first-order TCE decay constants for soil/bentonite/iron mixtures, when normalized to the available iron surface area, were approximately 1–2 orders of magnitude higher than observed in batch experiments with pure iron systems. Permeability tests indicated an increase in SB hydraulic conductivity roughly proportional to the amount of iron added. Based on the observed reaction rates and the assumption of sustained long-term performance, significantly less than one percent added iron would be required to reduce the diffusive flux of TCE across an installed slurry wall by over 10 orders of magnitude. However, the release of hydrogen gas was noted as a potential problem for low permeability systems containing zero-valent iron.

This paper presents a method to analyze the problem of lateral extrusion of sandwiched weak soil in a slurry trench. This problem is modeled as weak material compressed between rough plates. Theoretical solutions are derived on the basis of the limiting plasticity theory. The arching effect due to the finite length of the trench is taken into account in the computation of vertical pressure being exerted upon the weak soil. The derived theoretical solutions are reduced to simple forms that can be used in practice. The stability of sandwiched weak soil is evaluated using a factor of stability that represents a measurement of the effectiveness of slurry pressure on preventing weak soil from lateral extrusion. This factor is defined as a ratio of slurry pressure to lateral extrusion pressure at the level of weak soil. Validity of the proposed method is examined using a field case study.

This paper presents a case study concerning the design and construction of structural and nonstructural slurry walls at the Dam Number 2 Hydropower Project (project) on the Arkansas River. One-hundred four 2.8 m (9 ft, 2 in.) long by four 4.0 m (13 ft) long T-shaped structural slurry wall panels, which included some of the heaviest reinforcing cages ever lifted into place as a single unit, were used along with 24 2.8 m (9 ft, 2 in.) long straight structural slurry wall panels to construct the upstream and downstream channel training walls. Channel training walls guide water to and from the powerhouse at this project. This project is the first known use of structural slurry walls as channel training walls at a hydropower project in the United States. The project also included eight 6.3 m (20 ft, 8 in.) long and eight 2.8 m (9 ft, 2 in.) long straight structural slurry wall panels, and four 4.0 m (13 ft) long L-shaped structural slurry wall panels used to enable powerhouse construction. These panels will ev...