Geo-Con, Inc.

Insitu mixing of dry and slurried reagents in soil and sludge using shallow soil mixing

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Courtesy of Geo-Con, Inc.

Introduction
Recent implementation for the Federal Hazardous Waste Land Disposal Restrictions (LDR) to RCRA and USEPA's willingness to approve on-site treatment and/or containment has allowed in situ treatment methods to become viable options in site remediation. Shallow Soil Mixing (SSM) is one of several in situ mixing techniques now commercially available for use in on-site treatment of hazardous wastes.

The SSM mixing process consists of three units: the materials handling unit, SSM unit and environmental control unit. Figure 1 presents a schematic diagram of the overall SSM process. The materials handling unit is the area in which reagents are stored in bulk as either dry powder or a liquid. This unit is typically located outside the exclusion zone on hazardous waste sites. Given the type of material being mixed the reagents can be introduced either pneumatically as a dry powder or as a pumpable grout slurry.

The second segment of the SSM process is the SSM unit. The SSM unit consists of a single auger tool 6 to 12 feet in diameter, that is attached to a kelly bar which is suspended from a crane. The auger tool is turned by a drill platform capable of developing greater than 300,000 ft lbs. of torque. The auger tool itself is specifically designed to break up the soil and/or sludge and mix it with the reagent without removing the material from the area. The maximum depth to date the auger tool can penetrate is approximately 35 feet.

The third unit in the process is the environmental control unit (ECU). This portion is designed to control dust and potential airborne contaminants. The components of ECU include a hood which enclosed the auger tool, a low pressure blower or vacuum pump which exerts a negative pressure inside the hood during operation an different types of control equipment such as a dust collector, fume incinerator or an activated carbon scrubber. Typically, the ECU is designed to control site specific conditions and contaminants.

In addition to the process equipment, there are two other items which are necessary to successfully apply the SSM process. The first item is the development of the proper design mix. The design mix is the specific amount of reagent (s) introduced in a particular sequence to achieve the performance and/or clean up criteria. The design mix is developed by conducting a bench-scale treatability study that simulates the full-scale operation. The goal of the treatability study is to identify the least amount of reagent (s) that will achieve the performance criteria. Typical performance criteria include: unconfined compressive strength (UCS), permeability, cement content, Ph, TCLP, density and volume increase.

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