Formatted Title:

Evaluation of Soil Volumetric Expansion during ISCO-ISS: Reduced Soil Swelling and Increased UCS with a Combined Approach

Background/Objectives:

In situ stabilization (ISS) and in situ chemical oxidation (ISCO) are two well-established remediation technologies that have each been used to treat contaminated sites for several decades. These two technologies are used both for the application of upland soils and for the treatment of sediments. In recent years, the combination of these two technologies has increased in popularity after showing a cost-effective combined remedy that both treats and immobilizes contamination in a single application. ISCO and ISS reagents are applied together using a soil mixing strategy where both technologies act in synergy to reduce environmental risk from the contaminants, often by reducing leachate or pore gas impacts. This is caused by ISCO chemically transforming a significant portion of the contaminants and ISS immobilizing and solidifying any residuals in a low permeable higher strength monolith.

A significant cost component of implementing an ISS remedy by itself is that the addition of binders to the soil and sediments can cause the matrix to swell (increase in volume), which often requires treatment or disposal of that excess material. This study evaluated the volumetric expansion (swell) and other key parameters of ISS only and the impacts of adding different dosages of ISCO to ISS.

Approach/Activities:

ISS only and ISCO-ISS has been evaluated in a series of batch tests where varying dose combinations of sodium persulfate and binders to assess the effect on soil unconfined compressive strength (UCS), hydraulic conductivity and volumetric expansion (swell). Increasing dosages of ISS binders without the addition of sodium persulfate were evaluated to determine the swell caused by the addition of the reagents. Sodium persulfate was added with the ISS binders to assess the impact of ISCO reagents on the system. The tests were performed on a composited sandy clay. Volume and UCS of the monoliths were measured after 7, 28, 56, and 90 days. Hydraulic conductivity was evaluated at 28 days.

Results/Lessons Learned:

The study showed that the addition of increasing dosages of ISS reagents alone resulted in a proportional increase in volume of the test monoliths. The percent volume increase was observed to be similar to the dosage of the binder, namely 4% binder resulted in a roughly 4.5% soil volume expansion, a dosage of 8% binder resulted in a roughly 10% expansion and so on. The addition of sodium persulfate did not result in additional soil swelling, but rather in reactors with the same binder dose, increasing dosages of sodium persulfate contributed to a reduction of up to 40% of the volumetric expansion (swell) compared to ISS alone. Furthermore, smaller additions of sodium persulfate (1 to 2% w/w by soil mass) increased the UCS by up to 59% compared to ISS alone for the same ISS binder dosage, potentially allowing for reducing the amount of binder to meet a site’s UCS goals, commonly set at 50 psi. Previous studies have also demonstrated that the addition of sodium persulfate resulted in lower leachate concentrations and lower hydraulic conductivity compared to soils treated with cement only at a similar dose of cement.

The results of this study indicate that the addition of sodium persulfate to an ISS application can help reduce soil swelling in two mechanisms; (1) by decreasing the volumetric expansion and (2) by reducing the amount of binder required to achieve the same UCS. This in turn reduces the mass of soil that is displaced and the need for further handling and disposal of excess soil masses resulting in a potentially significant cost and energy savings to the project. This presentation will review the bench test results and use project examples to illustrate potential cost savings and other benefits of a combined ISCO-ISS approach.

Brianna Desjardins

Affiliation

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Evaluation of Soil Volumetric Expansion during ISCO-ISS: Reduced Soil Swelling and Increased UCS with a Combined Approach