Conventional remediation options requiring excavation, removal and disposal of impacted soils can be cost prohibitive when a large quantity of material is involved. Everything from labor and equipment costs to disposal and restoration costs go up when the volume of material to be addressed increases. In many cases, the need for excavation dewatering and earth support can also substantially increase the cost of remediation as well.
In situ stabilization (ISS), a technique largely associated with geotechnical stabilization of loose soils, can be an ideal alternative for many of these projects. ISS is essentially the mixing and fixating of reactive admixtures into the soil matrix. It can be a containment remediation strategy when using amendments such as cement and slag to fixate or entrain contaminants, as well as a destructive approach when using chemical amendments to oxidize or reduce contaminant concentrations. Since most environmental releases, impacts, and residual sources are within the range of mixing capabilities, ISS can be considered for a large percentage of environmental remediations, including both shallow mixing and deep soil mixing alternatives. From a cost perspective, the benefits of treating in place increases in direct proportion to the volume of impacted soil.
In this blog post, I’ll explain how you can assess site suitability, select the right mixing technology, and administer the correct treatment. I will also be presenting a webinar next week, titled ISS 101: What You Need to Know When Considering In Situ Stabilization. Join me and fellow presenters, Peter Palko, P.E., CHMM, LSRP and Jack Twomey, and we’ll dive deeper into the topic than we can in a single blog post, and take your questions during a Q&A.
In the environmental sector, ISS works by reducing contaminant leachability, mobility and matrix permeability. Contaminants can be chemically or physically altered but generally remain in place unless a destructive oxidation approach is employed. Selection and appropriate concentration of a suitable admixture/binder depend on site-specific factors, such as:
Candidate COCs suitable for fixation or containment ISS treatment include metals, inorganics, non-volatile organics and semi-volatile organic compounds such as PAHs, PCBs, pesticides dioxins and furans. When used as a destructive approach, ISS can be used to treat organic compounds. It should be noted, however, that the presence of organic compounds or presence of product at high levels can have a detrimental effect on cementitious reagents. “Soft sediments” or un-lithified materials comprised of sands, silts or clays are all ideal mediums for treatment.
ISS can be applied using several different methods to mix the binding reagents into the contaminated media. The most common methods are auger mixing (which is often called deep soil mixing [DSM]), which also has many variations in technique and equipment, and bucket mixing which uses an excavator bucket or various forms of excavator mounted mixing implements. Auger mixing allows treatment to achieve greater depths (60 feet or greater), depending on lithology, and can provide consistent mixing energy and repeatability. Excavator mixing is limited to shallow depths (less than 30 feet) and the reach of the equipment, but it can be preferable when foundation remnants or subsurface obstructions are present.
In any ISS program, “bench scale” treatability and pilot testing are essential to selecting and optimizing the appropriate reagent components. Additionally, treatability testing must be utilized to establish performance criteria prior to implementation. Pilot testing can be utilized to plan and assess implementation of treatment in comparison with treatability results prior to full scale treatment. Both treatability and pilot testing are key to assessing the medium volume increase that will occur as a result of soil mixing and reagent addition, commonly referred to as “swell.” The swell will need to be considered, either in final restoration grades or as a disposal item cost.
The efficacy of ISS relies on successful implementation during the construction phase. The formulations developed during the bench scale must be repeated and achieved in the field. Parameters that influence the mixing and homogenization of treatment zone materials—such as reagent/grout delivery rate and dosing, and mixing speed and time—must be consistently monitored and maintained during the construction phase. Finally, samples of treated media are collected during implementation and tested for performance criteria metrics against the treatability study results. Gathering real time data daily allows our team to make adaptive field decisions on mix ratios so that project goals are guaranteed.
ISS can be the perfect technology for meeting remedial goals when site conditions are right, but successful execution requires the right team of professionals, the right equipment, and proper planning and execution.
Learn more about when ISS can and should be used in our webinar next week, ISS 101: What You Need to Know When Considering In Situ Stabilization. Even if you can’t make it, register anyway and a link to the recording will be emailed to you afterward.