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In-Situ Stabilization


Cambridge, MA: Release Abatement Measure, In-Situ Soil Stabilization

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As a Release Abatement Measure (RAM) under the provisions of Massachusetts Environmental Regulations, Geo-Con performed in-situ Shallow Soil Mixing (SSM) of impacted soil (LNAPL and DNAPL) over an area of approximately 2.2 acres. Approximately 103,000 cy of soils were stabilized in-place to an average depth of 22 feet bls for this project.

The property was the former location of a Manufactured Gas Plant (MGP). Past MGP operations resulted in the release of hazardous chemicals into the soil and groundwater. Hazardous chemicals, including but not limited to volatile and semi-volatile organic compounds, were present in soil and groundwater at concentrations greater than Reportable Concentrations specified in the Regulations for the State of Massachusetts.

The equipment being used to perform the stabilization includes the following; a 4000 series Manitowoc crane, a rotary drill table (Caldwell CH-200 capable of developing 350,000 ft-pounds of torque), a 10-ft diameter mixing tool, a batch plant and a Soil-Vapor Extraction (SVE) system, shroud and carbon filter. The SVE system was used to collect and treat any hazardous gases that may be released during the stabilization process. Cement-Kiln Dust (CKD) and Bentonite are the mixing reagents.

mgp-insitu-stabilization-cambridge-ma

A spacing of 8.67 feet between the center of adjacent columns was implemented based on the effective ten-foot column mixed by the ten-foot diameter-mixing tool. This pattern layout allows for mixing of 100% of the soil mass and includes approximately 35% overlap of each interior column. The SSM operation required approximately 590 columns for treatment of the LNAPL impacted soils and approximately 1620 columns for treatment of the DNAPL impacted soils inside the treatment boundry.

All columns within the LNAPL stabilization area were stabilized to a depth of 12 feet below the existing ground surface. In areas where both LNAPL and DNAPL are present, stabilization occurs to a depth of 2 feet below the clay surface. All columns within the boundry of the DNAPL stabilization area were stabilized to a depth of 2 feet below the clay surface.

Prior to stabilizing the soil, excavation is performed within the limits of the stabilization area to locate and remove rock obstructions and abandoned subsurface structures to a depth of approximately 22 feet below the existing land surface. The subsurface structures consist of gas holders, tar tanks, boiler house and other structures. The obstructions are either removed or broken in place and then removed. The existing ground water elevation is approximately 8-10 feet below land surface. To reduce the odors generated during the excavation of the substructures, the disturbed area was constantly foamed.



 

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