Slurry wall

This case serves to point out the importance of maintaining a constant hydraulic gradient on the inside and outside of a slurry wall. This can also become a factor if slurry walls are used with groundwater pumping systems (extraction wells). Then water must be reinjected to maintain a... 

Grouted barriers use a variety of fluids injected into a rock or soil mass, which is set in place to reduce water flow and strengthen the formation. Grouted barriers are seldom used for containing groundwater flow in unconsolidated materials around hazardous waste sites because they cost more and have lower permeability than bentonite slurry walls. Nevertheless, they are suited to sealing voids in rock for waste sites remediation. 

There are other methods for containment such as slurry walls and piling sheets, which are only used as methods for containment but not for treatment. 

In addition, cover systems are also used in the remediation of hazardous waste sites. For example, cover systems may be applied to source areas contaminated at or near the ground surface or at abandoned dumps. In such cases, the cover system may be used alone or in conjunction with other technologies to contain the waste (e.g., slurry walls and groundwater pump and treat systems). 

This relatively new application of subsurface dams has proved useful to focus groundwater flow to a recovery area. A typical application, referred to as a funnel-and-gate system (Figure 7.2), involves construction of a sheet pile, or slurry wall,... 

MITU is capable of treating soils, sludges, sediments, or slurries to depths of up to 30 ft, even through very dense soil, concrete, or rock. They are mobile systems that can be transported and set up quickly and easily. Three different MITU models are available, and each is designed for a different site type, size, and/or treatment rate application. The MITU can also be used for trench installation, slurry wall construction, or horizontal well installation. 

According to Halliburton NUS, this technology is not currently commercially available. Researchers state that Soil Saw barrier technology offers the following potential advantages over conventional slurry wall barrier techniques ... 

Uses commercially available slurry wall technology for barrier installation. 

For ISSZT, a physical barrier, such as a slurry wall or vertical membrane barrier, is installed around the area of contamination to a depth that will be slightly below the future lowered groundwater level to limit the escape of air from the area. Existing clay or silt may serve as a cap for the system, otherwise a man-made cap is installed. Wells are installed into the soil to inject the low-pressure compressed air beneath the cap. As the air pressure increases, the groundwater is lowered. The injected air is prevented from escaping by the cap at the top, the barrier along the sides, and the water table at the bottom. 

Costs likely to be incurred in the design and installation of a standard soil-bentonite wall in soft to medium soil range from 540 to 750/m ( 5 to 7/tf) (1991 dollars). These costs do not include variable costs required for chemical analyses, feasibility, or compatibility testing. Testing costs depend heavily on site-specific factors (D109308, p. 2). The installation cost of a cement-based slurry wall ranges from 10 to 20 per vertical square foot for a 2-ft-wide barrier of less than 100 ft in depth (D18976I, p. 6). 

Factors that have the most significant impact on the final cost of soil-bentonite slurry wall installation include ... 

Generally, there is a substantial cost increase associated with emplacing slurry walls at depths greater than 90 ft (D16334I, p. 2). 

The clay-based grouting technique uses clay slurries as a base for grout solutions. These solutions are injected into bedrock fracture systems to inhibit or eliminate groundwater flow through these pathways. The clay slurries may also be used as a base for slurry wall construction. 

Recent variations include the use of backfilled caissons media-filled, hollow-vibrating beams and emplaced reaction vessels. The funnel typically consists of sheet pilings, slurry walls, or some other material and is preferably keyed into an impermeable layer (clay, bedrock) to prevent contaminant underflow. Particular care is required in designing and constructing the connection between the impermeable funnel section and the permeable gate section in order to avoid bypass of contaminated ground water. 

S.S. Papadopulos Associates, Bethesda, MD 1West Valley Nuclear Services Company LLC, West Valley, NY Key words barriers, sorption, slurry wall, zeolite, ion exchange... 

This chapter considers two particular types of sorbing barriers 1) low-permeability slurry walls amended to promote the sorption of hydrophobic compounds (hydraulic conductivity [AT] 10 7 cm/s), and 2) high-conductivity zeolite treatment walls designed to remove inorganic compounds (K 10" cm/s). These examples are selected, in part, because they represent the systems that have received the most attention from researchers and practitioners, but also because considering them together highlights the importance of conceptual issues common to both types of systems. 

In considering an organic-rich slurry wall additive, the governing sorption mechanism is often considered to be hydrophobic partitioning and the effects of solution chemistry (pH, ionic strength, etc.) are considered secondary. The specification ofthe appropriate sorption model requires several choices ... 

Most studies of contaminant transport in slurry walls have relied on the equilibrium assumption, either for convenience or because of the long residence times associated with diffusion-dominated transport. As an alternative, Rabideau and Khandelwal (1998a) have proposed the simple two-compartment mass transfer model to describe nonequilibrium sorption in soil/bentonite systems ... 

Because diffusion dominates the transport of contaminants in barriers and columns constructed of low-permeability materials, model calibrations and predictions are extremely sensitive to the form of the specified boundary conditions. Two issues are of particular importance 1) treatment of the entrance mixing zone in laboratory columns, and 2) specification of appropriate BCs to represent a slurry wall under field conditions. 

In extrapolating laboratory results to installed slurry wall of thickness L, the commonly applied idealized BCs may not provide an accurate representation of the transitions between an engineered barrier and native aquifer material in the field. To provide conservative predictions for design, Rabideau and Khandelwal (1998b) recommend the combination of a constant concentration entrance (x = 0) BC with a zero-concentration exit (x = L) BC. In particular, several commonly used BCs should be avoided because they distort the nature of the diffusive flux at the boundaries (e.g., the Danckwerts constant-flux entrance BC and the zero-gradient exit BC). These issues are discussed in greater detail by Rabideau and Khandelwal (1998b). 

In a comparative study of sorbing additives for soil/bentonite slurry walls, Adu-Wusu et al. (1997) identified a natural humus material as an attractive candidate in terms of cost and sorption capacity. Subsequently, column experiments were performed by Khandelwal and Rabideau (2000) using an SB mixture containing 5 percent humus. To account for possible... 

Model calculations indicated that the addition of 5 percent humus to a SB slurry wall would delay breakthrough (defined as 5 percent of the steady-state flux) from approximately 5 years to more than 100 years (Figure 3). 

The Kd calibrated from the column studies was approximately 58 percent of the value measured in batch isotherm tests using the same materials. The precise reasons for these differences is a subject of further investigation, but the result suggests that caution should be exercised in using the results of batch sorption tests to extrapolate the performance of amended slurry walls. 

Figure 3. Simulated field performance of 1-meter humus-mofidied slurry wall (column conditions summarized in Table 2). Reprinted with permission from Khandelwal and Rabideau (2000). Copyright 2000, Elsevier Science B. V.

Reliance upon sorbing additives necessitates more refined materials handling and Quality Control testing to ensure a reasonably uniform distribution of the sorbing additive within a slurry wall. This need for precision may complicate the current construction practice for slurry walls (e.g., as summarized by U.S. EPA, 1998) and will likely result in increased construction costs. 

Bierck, B. R., and Chang, W. C. (1994). Contaminant transport through soil-bentonite slurry walls attenuation by activated carbon, Proceedings, Water Environment Specialty Conference on Innovative Solutions for Contaminated Site Management, Miami, FL, March 6-9, 1994, 461-472. 

Khandelwal, A., and Rabideau, A. J. (2000), Amendment of soil/bentonite slurry walls with natural humus, Journal of Contaminant Hydrology, 25, 267-282. 

Park, Jae K., Kim, Jae Y., Madsen, C. D., and Edil, T. B. (1997). Retardation of volatile organic compounds movement by a soil-bentonite slurry wall amended with ground tires, Water Environment Research, 69(5), 1022-1031. 

Rabideau AJ, Shen P, Khandelwal A. Feasibility of amending slurry walls with zero-valent iron. J Geotech Geoenviron Eng 1999 125 330-333. 

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