In-Situ Stabilization at Former Manufactured Gas Plant Site

This former manufactured gas plant was located on a high-visibility, environmentally sensitive waterfront site in California. Our client’s objectives were to address coal tar-impacted soil containing dense non-aqueous phase liquid (DNAPL) from the old gas plant operations. The project’s goals included stopping DNAPL from migrating to bay sediments, preventing exposure to contaminated soil and soil gas, and reducing the risk of chemicals in the soil leaching into groundwater. 

The pilot phase of this project evaluated the feasibility and effectiveness of in situ stabilization (ISS) via Deep Soil Mixing (DSM) for treating soils impacted by MGP by-products in the site subgrade. The objectives were to verify that the selected performance criteria could be met in the field using full-scale mixing equipment and to determine if mix design modifications were needed for actual field conditions. Additional goals included assessing soil homogenization, production rates, swell volumes, potential for vapor and odors, and the adequacy of the QA/QC program. 

The pilot demonstration provided crucial treatability data and operational knowledge, guiding the development and implementation of the final Remedial Action Work Plan. The pilot phase involved drilling 13 ISS columns in two treatment areas, using mixing augers 4 to 8 feet in diameter to test their effectiveness in a debris-obstructed subgrade. ISS treatment was applied to the fill to depths of up to 57 feet, including about 3 feet into the underlying bay mud. The high rock content of this fill soil had the potential to impact the type and size of Deep Soil Mixing equipment and productivity rates. 

Work included mobilization of all personnel, tools, equipment, and incidentals required to perform the work including on-site set-up and calibration of batch plant and ISS drilling/mixing equipment and all support equipment to ensure safe, continuous and efficient production during ISS mixing operations. Equipment included all storage silos, mixers, pumps, regulating and measuring devices required to produce and transfer reagent grout.  Forgen mobilized an ABI 20/25 vertical auger mixing rig and custom batch plant to the project along with all ancillary support equipment. 

The second phase (remedial action) included drilling ISS columns in two distinct treatment areas encompassing approximately 20,000 sf of the site. More than 26,000 cy of ISS was completed using mixing augers ranging from 4 to 8 ft in diameter to treat 100 percent of the DNAPL-impacted soils. Work was performed over a six-month period utilizing two ISS drill rigs and batch plants to complete treatment on time, and to prevent any impacts and delays of subsequent phases of this remediation. 

ISS was completed in the northern and southern continuous DNAPL areas to depths ranging from 20 to 50 ft and included penetration of approximately 3 ft into the underlying bay mud.  

The scope of work also included handling of the ISS swell material within the treatment zones and management of obstructions encountered during drilling. Throughout the project, a number of obstructions such as brick, rocks, concrete, miscellaneous debris, as well as historic timber piles were encountered. For timber piles and wood debris, an auger configuration with aggressive cutting teeth was utilized to allow advancement of the auger to full treatment depths through the debris while incorporating the material within the mixed column. All other obstructions were managed by utilizing an excavator to remove the material up to 20 ft below ground surface. 

The primary performance criteria for the treatment zones were a minimum unconfined compressive strength of 30 psi, maximum hydraulic conductivity of 1 x 10-6 cm/sec, and visual observation of soil cores to ensure there was no unmixed soil or observed DNAPL. Additionally, ISS within the bulkhead area of the northern zone was completed through the fill zone and approximately 15 to 20 ft below the underlying bay mud, with a minimum unconfined compressive strength of 100 psi, in order to stabilize the soil mass behind the bulkhead. 

Our extensive experience with in-situ stabilization (ISS) allowed us to optimize grout proportions, the water-to-reagent ratio, and reagent injection percentages. This expertise was critical in meeting the requirements of both our client and regulatory agencies, resulting in a stabilized soil mass that met all project objectives for industrial site remediation.