Use of Slurry Wall Systems to Support CCR Impoundment Closure and Corrective Action

Dale W. Evans, P.E.1, Kathleen Whysner1

1Remedial Construction Services, L.P., 9977 W. Sam Houston Parkway N., Suite 100, Houston, TX 77064

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KEYWORDS: coal combustion residuals (CCR), slurry wall, soil-bentonite, groundwater containment, dewatering, corrective action ABSTRACT

Since the inception of the Resource Converstaton and Recovery Act (RCRA), soil-bentonite slurry wall systems have received broad acceptance at environmentally-impaired sites for managing and protecting groundwater. The broad-based applicability of these systems to provide site-specific variants for isolation, containment, and diversion of groundwater may serve as a valuable tool for coal combustion residuals (CCR) impoundment closure projects. The technology provides cost-effective support for both near-term impoundment dewatering programs, as well as future containment of impacted groundwater where may be appropriate for corrective action. Accordingly, slurry wall systems may provide a potentially valuable technology to consider for many pending closure programs. This paper provides an overview of the broad applications available for CCR-specific slurry wall programs, including the means of application in the field and the site factors that should be considered to define project-specific efficacy. In particular, three distinct technology applications for CCR impoundments have been highlighted herein.

• Impoundment Dewatering – Impermeable soil-bentonite barrier walls can be used to isolate impoundments and, thus, contain groundwater in contact with CCR materials. This impounded water can then be recovered using permeable infiltration gallery(s) constructed using similar slurry trenching methods. Where site conditions permit, this approach essentially converts the base of unlined impoundments into a subdrain system capable of accelerating the rate of lowering within the phreatic surface to increase in effective stresses, near-surface shear strength, and the ultimate bearing capacity.

• Corrective Action – Where corrective action may eventually be required, slurry wall systems provide an effective and positive means of groundwater containment, which eliminates routes of exposure to potential receptors. The isolation of potentially impacted groundwater within CCR impoundment footprints from external freshwater systems supports active various types of

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active remediation processes inside the containment while facilitating natural attenuation external to the containment barrier. In effect, such isolation interrupts the connection between the CCR source mass and groundwater, which isolates groundwater impacts so they can be addressed using active or passive recovery and treatment methods.

• Comprehensive Combined Groundwater Management System – Use of a soil-bentonite containment system early as part of impoundment closure can integrate immediate needs for dewatering with the longer-term goal of groundwater isolation, containment, and as appropriate, remediation. Such combined applications support a single expenditure to address both issues, which may reduce life-cycle closure costs. Therefore, early integration of such systems into closure plan designs may provide a financially-accretive reduction within overall life-cycle closure costs.

OVERVIEW

As CCR materials were originally deferred from regulation under RCRA, the primary impetus for operation was to employ processes that offered low operating costs for delivery of ash materials from the plant to the containment impoundment. Therefore, many facilities were operated using “wet” operations protocols that employed hydraulic conveyance methods coupled with spigot-based discharges within impoundments. These impoundments were typically constructed using confinement embankments or dam-based valley fill systems. Such operation protocols dramatically lowered operating costs compared to more equipment intensive methods but had the negative effect of creating saturated fly ash deposits. These saturated deposits routinely exhibit limited shear strength while the applied heads present promote migration of entrained water that follow potential subsurface routes of exposure. Therefore, where closure is required, consideration of how best to manage the limited shear strength conditions present as well as the remaining inventory of water contained within the saturated CCR materials will be required.

Managing the unique relationship between retained moisture and the limited shear strength, as well as the longer-term issue associated with porewater chemistry form the basis for the most complex actions required under most impoundment closure programs. Accordingly, defining closure activities for many CCR impoundments will require a means to evaluate dewatering options as the basis for defining an optimally cost-effective process to provide the shear strength and associated bearing capacity required for site regrading and capping.

While traditional dewatering methods (wells, well-points, wick drains, etc.) have merit for consideration at most sites, the application of slurry trench methods also merit consideration when site conditions support installation. In function, an impermeable containment-based barrier system provides isolation of groundwater while an internal recovery system can be used to remove the volume of water necessary to support

regrading and capping efforts, such as is depicted on Figure 1. As shown, a full perimeter system creates dead storage by preventing inflow or outflow which, combined with recovery, converts the entire base of the impoundment into a sub-drainage-based drainage pathway. The flexibility of these systems can be used to support both dewatering in the near-term and as appropriate, for long-term environmental isolation and corrective action.

FIGURE 1: Perimeter Isolation with Internal Recovery System

The following sections of this paper examine the applicability and use of the slurry trench technology and the considerations appropriate to validate the efficacy of application.

SLURRY WALL CONSTRUCTION OVERVIEW

Trenching-Based Sidewall Support

Slurry wall construction is based on excavating a trench that, concurrent with the excavation process, is filled with a specially-formulated fluid capable of providing a lateral hydraulic load to the sidewall of the trench. The fluid selected for a particular application varies based on whether an impermeable barrier is required or whether a permeable flow through wall section is required. Regardless of the type of system desired, the effectiveness of the fluid selected to provide trench stability depends upon the ability to develop an impermeable membrane (via filter cake) along the trench wall such that water cannot migrate into the sidewall of the trench. The result is to provide a hydraulic load to the trench sidewall equivalent to the sum of the trench depth and unit weight of the fluid. These concepts are demonstrated on Figures 2 and 3.

FIGURE 2: Filter Cake Formation Basis

FIGURE 3: Trench Fluid-Enhanced Stability

Depending upon the application, impermeable trenching systems typically use slurries of water and sodium bentonite, which when mixed with a backfill material placed in the trench, can be used to create relatively impermeable barriers (typically 10-7 to 10-9 centimeters/second). The backfill placed within the trench can include a host of options, including:

Soil-Bentonite Impermeable Systems

• Excavated trench spoil blended with bentonite slurry. • Excavated trench spoil blended with added dry bentonite as well as slurry. • Imported low permeability backfill.

Impermeable, Structurally-Improved Systems

• Cement-bentonite systems. • Soil-cement-bentonite systems.

As shown, where structural stability issues may exist, the use of cement to increase shear strength may have merit to meet site-specific needs. The process employed for

these systems parallels that for soil-bentonite walls but often requires adaptation to meet site-specific conditions.

Where a high permeability system is desired, such as for a groundwater infiltration and recovery gallery, the trench fluids employed typically use biopolymer or synthetic polymers. These fluids provide the same basic function of developing an impermeable filter cake for hydraulic support as provided by bentonite in impermeable systems. However, as these systems are typically employed where higher permeability sections are needed to support active fluid recovery, they are typically backfilled with coarse-grained materials such as graded rock. The fluids employed can then be degraded and the interstitial voids flushed to ensure a high degree of internal conductivity through the completed wall section. The combined vertical and horizontal conductivity provided offers substantial efficiency and value compared to the expense and effectiveness of well-based systems.

Slurry Wall Configuration

Slurry wall systems can typically be adapted to a wide array of site conditions. While the basic premise to be satisfied by slurry wall systems is to provide a known level of control over groundwater flow (both impermeable and permeable systems), consideration of each of the geometric dimensions should be carefully planned. Typically, the first key factor for planning is to define the depth required. Impermeable systems are often configured to provide a barrier that extends from the ground surface to a key-in layer with a permeability low enough to ensure a positive barrier is provided to lateral groundwater movement. The optimal key-in layer will be an aquitard that serves as a defined boundary to migration. Where such a layer does not exist, site-wide hydraulic containment may still be possible through keying into a sequenced stratigraphy of relatively low permeability layers or using a hanging wall section that extends deeply enough to balance inward and outward gradients. Each of these configurations is depicted as shown on Figure 4.

FIGURE 4: Slurry Wall Configurations – Full Penetration and Hanging System

Based on the depth required for a given slurry wall system, consideration must then be given to the surface-based elevations and alignment. As trench fluids need to be maintained very close to the ground surface to provide optimal trench sidewall support, the ground surface must be maintained along the alignment at a relatively constant elevation. While slopes can be accommodated incrementally, the maximum degree of fluid lowering allowable must be considered such that excavation and backfill efforts can be properly sequenced.

Through consideration of the depth required and the elevation-based topographic considerations, the alignment of the trench can be defined in combination with the working platform that will be used to support excavation and backfilling. The primary factors influencing trench alignments are typically associated with physical site constraints and the presence of hard boundaries (property, materials being contained, surface features, etc.), equipment needs, and the working geometries associated with equipment and construction. The collective consideration made between depth, elevation, and alignment often requires a somewhat iterative approach to the design

process to ensure that the basic design premise for the slurry wall system can be met while also providing a constructable, cost-effective system.

Slurry Wall Excavation

Slurry wall construction methods can be used to support a wide array of site conditions to support the installation of positive containment systems. In general, the depth of slurry wall excavation depends upon the type of equipment employed, which may include the following:

• Hydraulic excavator – depths from 0 to approximately100-feet. • Clamshell – depths from 0 to greater than 300-feet. • Hydro-mill – depths from bedrock surface to greater than300-feet.

Based on the depth capabilities represented, under the most common conditions, a hydraulic excavator will be used to excavate to the maximum depth possible which may include both soil as well as a key-in into an aquitard. Once these depths have been achieved or refusal criteria have been met, a clamshell can be used to continue the excavation to the final depth. In the event that clamshell penetration cannot be used to extend the trench to the required depth, such as due to hard materials, a hydro-mill can be used to extend the trench vertically-downward to the final elevation. As would be expected, installation depths will often have the most profound effect on the construction resources required, construction productivity, and thus the cost for installation.

Slurry wall excavation using this equipment typically proceeds following the process shown on Figures 5 and 6.

FIGURE 5: Slurry Wall Excavation Details

As shown on Figure 5, the working platform is typically configured to support the footprint of the excavator on one side while leaving sufficient room for equipment access. The dimensions shown may vary based on site constraints but typically, those shown are typically considered minimum values for most projects.

FIGURE 6: Slurry Wall Excavation and Backfilling Process

Slurry Wall Backfill

Using the generalized sequence noted on Figures 5 and 6, excavation can be extended methodically along a sufficient length of the alignment to support the placement of backfill. As noted previously, a host of backfill materials may be used to accommodate site conditions and the intended function of the wall. Therefore, planning efforts should be supported through analysis of the following elements which typically control project-specific decision-making for trench backfill and completion.

• Trench soil conditions • Backfill permeability required • Bentonite addition requirements (impermeable systems) • Gradation (permeable recovery systems) • Blending and placement (impermeable systems) • Internal piping systems and backfill placement (permeable recovery systems) • Construction quality assurance methods (all systems)

For impermeable wall systems, the backfill materials are routinely placed using an initial tremie-based process followed by a displacement-based process to prevent free-falling backfill through the slurry within the trench. Since the coarse-grained materials commonly used for backfill in recovery trenches will not be easily displaced, placement is typically limited to tremie-based processes from the base to the top across the entire length of the trench.

BASIS FOR DEWATERING

The hydraulic balance that often exists between the phreatic surfaces present within impoundments and those within groundwater external to the impoundment often represent a near-equilibrium condition due to the head loss intrinsic to the low permeabilities represented by the fly ash. These low permeabilities may require active dewatering to increase gradients as a stimulus to groundwater recovery. Such conditions may require consideration for active dewatering methods within or external to the limits of the impoundment. In locations where permeable subsoils exist, and where fly ash permeabilities are sufficient to support a reasonable time-rate of drainage, dewatering may be readily achievable using trench-based recovery systems external to the impoundment.

Therefore, the effectiveness of trench-based systems will rely upon the capacity to efficiently and timely support removal of the inventory of groundwater required to support closure using the isolation and recovery system employed. The ability to selectively manage only that inventory of groundwater present within an impoundment

can be supported through construction of an impermeable slurry wall system around the entire perimeter of the impoundment coupled to a recovery trench system, such was shown on Figure 1. Through eliminating the continuity of groundwater flow beneath the impoundment, when groundwater removal can be accomplished, the only means of recharge will be through internal drainage of the impoundment. The basic means to support such drainage is depicted as shown on Figure 6.

FIGURE 6: Combined Containment and Dewatering Systems

The combined containment and recovery process noted supports three key positive attributes, i) comprehensive containment of groundwater, ii) an economical drainage solution and iii) an increase in effective stress above the saturated drainage interface with a corresponding increase in shear strength.

The increases in shear strength are developed through consideration that the saturated ash within an impoundment will behave as an incompressible body. Therefore, the only means to support increases in the effective stresses present will rely on creating a

functional drainage pathway to reduce the intrinsic moisture content. Thus, where conditions exist that provide a high degree of volumetric efficiency for drainage, the use of drainage galleries in combination with confinement is likely to have merit.

IMPOUNDMENT DEWATERING

As noted, the use of a slurry trench system that incorporates both impermeable containment and recovery creates volumetric efficiency by promoting removal of only that inventory of groundwater within the perimeter of the isolation barrier system while removing available migration pathways and routes of exposure such as is shown on Figure 7.

FIGURE 7: Isolation from Groundwater Migration

Where substantial drainage pathways can be created, the process can support somewhat uniform vertical drawdown which enhances dewatering efficiency by removing that inventory of moisture most detrimental to near-surface shear strength.

The effect of such vertical drawdown increases the near-surface effective stresses through creating a negative pore pressure front that enhances strength gains with an attendant increase in shear strength and bearing capacity. The resulting relationship between shear strength and bearing capacity can thus be represented as shown on Figure 8.

FIGURE 8: Dewatering-Based Incremental Shear Strength Increases

CORRECTIVE ACTION

As many CCR impoundments were constructed without liners, the motive forces present due to impounded and atmospheric moisture inputs, the porewater mobility within the environs adjacent to CCR impoundments, and the geochemical instabilities intrinsic to environmental reactions between water and CCR materials may conspire to create groundwater impacts external to the limits of the impoundment. If such porewater was subject to migration, corrective action liabilities may be triggered. The potential time-rate of flux from the impounded CCR materials, the degree of impacts likely from porewater migration, and the degree of naturally-mediated sequestration or attenuation can be generally quantified using data recovered from the Work Area Quantification Process (WAQP) in combination with various analytical tools.

Where Corrective Action liabilities may exist, an impermeable slurry wall system may be appropriate to provide both environmental isolation as well as recovery. The environmental isolation provided by an impermeable slurry wall has the capacity to eliminate the available routes of exposure, a necessary consideration to optimally limit

liabilities. Aside from complete removal, containment of groundwater impacts coincident with the area covered by an impoundment becomes the next best remedy. Such approach can be used to monitor and positively validate that system performance continually meets containment criteria. Where appropriate, such containment can be further enhanced through supplementation with nominal recovery-based systems to support a continual inward gradient.

Where environmental impacts may exist external to the isolation system approach employed, the continuity maintained within groundwater flow will likely lead to gradual attenuation. The rate and effectiveness of such attenuation will depend upon the soil characteristics and geochemistry, porewater geochemistry, and overall groundwater flux represented. Such conditions may support the ability to avoid other active forms of remediation where such impacts may exist.

COST CONSIDERATIONS

As noted, successful applications of dewatering using slurry trenching methods will depend upon the cost-based efficiencies as may be represented on a cost per unit basis, such as cost/acre-foot recovered, or more appropriately for capping, the cost/foot of reduction in the piezometric surface. The ability to provide competitively priced costs for this approach versus more conventional technologies must consider (assuming equivalency in water management costs) the summation of costs for the impermeable barrier system, the cost for the recovery trench system, and the cost for the equipment, materials, and power used for the recovery process.

SUMMARY

Slurry wall systems have proven to be effective for application to conditions where groundwater flow requires diversion or containment or collection. The unique conditions associated with many CCR impoundments can in many instances be accommodated by the inherent flexibility and function of these types of systems.