Mark Fleri, PE WRScompass

Presentation Description
In-Situ Stabilization at MGP Sites; A Cost-Effective and Practical Remedial Alternative
WRScompass has completed in-situ stabilization (ISS) at over 8 MGP sites throughout the US. The primary goals of the remedial actions were to limit the leachability of the chemicals of concern, both organic and inorganic, by controlling both their solubility and movement. The organics stabilized included volatile organics and PAHs.
WRScompass treated 32,000 cubic yards of former MGP waste using in-situ single auger mixing. WRScompass conducted bench-scale treatability testing to define and optimize reagent admixture rates. Three mix designs involving combining various addition levels of Portland cement, ground blast furnace slag, water, water, and a rheology modifier into a grout were developed for use in specific areas of the site.

During full-scale treatment, WRScompass is using a two-bladed mixing/injection tool, mounted on the end of a hollow-stem Kelly bar, to mix the soil and grout to a uniform consistency. A conventional crawler-mounted lift crane suspends the Kelly bar and serves as a platform for the diesel-powered transmission which drives the Kelly bar. A batch plant is used to blend prescribed proportions of reagent, water and or Rheomac® to produce a grout. Grout is then pumped from the batch plant to the mixing tool.

Samples of the treated material were collected and analyzed to verify that the treatment criteria (unconfined compressive strength greater than 50 psi at 28 days, permeability less than 1 x 10-6 cm/sec, and leachability less than 0.1 ppm for naphthalene) are achieved. All of the verification samples have met the performance criteria.
WRScompass also implemented ISS of 55,000 coal tar-impacted soil at an MGP site. Prior to starting treatment, WRScompass collected samples from each discrete area of concern and performed bench-scale testing to define and optimize reagent admixture rates for full-scale ISS implementation. Taking the shallow depth into account and considering the propensity to encounter additional unknown subsurface obstructions while performing ISS work, WRScompass determined that ISS via excavator mixing would be the most effective and dependable method for this site versus auger mixing or use of a specialty mixing devise (e.g., Lang tool).

ISS via excavator mixing was successfully demonstrated through a field pilot test and throughout full-scale project implementation. Samples of the treated material were collected and analyzed to verify that the treatment criteria (unconfined compressive strength between 50 and 200 psi at 28 days, permeability less than 5 x 10-6 cm/sec, less than 10% mass loss via the wet/dry cycle test, no free liquids, and leachability less than 1 ppm for total BTEX) are achieved. All of the verification samples met the performance criteria.

ISS is a regulatory-accepted treatment technology for MGP sites and represents an opportunity for cost-effective treatment at these sites.