More and more frequently, In Situ Thermal Remediation (ISTR) is used to treat contamination located below buildings, roadways, and other critical infrastructure. Subsidence (vertical downward movement of the ground) and other potential geotechnical effects on buildings and infrastructure are concerns that should be raised and addressed in the initial design phase of a thermal remedy.
The three ISTR methods are Thermal Conduction Heating (TCH), Electrical Resistance Heating (ERH), and Steam Enhanced Extraction (SEE). While the heating approach and main heating mechanisms differ based on the ISTR technology utilized, heating to 100°C (the boiling point of water) will result in some soil expansion, while at the same time, there will be a change in saturation and a slight dewatering of the treated volume due to boil-off of some of the ground water. Depending on the geology, this can lead to changes in the geotechnical integrity of the soils.
We received the following real-life questions about how to avoid the risk of subsidence when using ISTR methods from attendees at our recent webinar, ISTR and Subsidence Risks. The answers were provided by Steffen Griepke, Vice President of Technology.
What are the key factors we should look at to determine if subsidence is a potential problem at a site?
The configuration of the treatment area relative to infrastructure and the geologic conditions are the two most important factors to consider. If the target remediation area is below buildings, utilities, roadways, rail ways and other sensitive infrastructure, subsidence may be a potential problem. Also, if a developer is planning future development in the treatment area, potential changes to soil geotechnical properties post-thermal treatment should be evaluated.
Certain geologies, such as organic soils, are more prone to subsidence during ISTR where breakdown or dewatering of organic material during heating reduces contact between load-bearing particles. This includes peat, high organic clays, and soils with high organic content. In addition, some clays are known to swell or shrink with varying water contents, including smectite, montmorillonite, and bentonite.
What laboratory studies would you recommend?
There are some useful tests to assess the potential for subsidence during ISTR using standard ASTM methods. ASTM D422, Standard Test Method for Particle-Size Analysis of Soils, is used for determining the particle size distribution of the soil. This can identify soils with high clay contents. We also look at the initial moisture content, the Atterberg (liquid and plastic) limits, the shrinkage limits, and the ash and organic content. And then the specific gravity of the solids fractions also plays into the equation. And last, especially for fill sites or an ex situ IPTD (In Pile Thermal Desorption) site, the Modified Proctor test is useful because it shows the compactability of the soil. If any of these tests shows a potential for shrinkage or subsidence, we would then perform a geotechnical evaluation in the lab simulating heating and moisture conditions.
How are you mitigating subsidence during treatment, if you observe subsidence?
First, we wouldn’t ever initiate a project under a building if we thought it could lead to significant subsidence that would damage the building or infrastructure. When the thermal treatment zone is located outside and away from buildings and other critical infrastructure, we are much less concerned about subsidence and manage it all the time. We have had sites that subsided one to two feet during the remedy. As long as you plan for it, it doesn’t impact thermal treatment effectiveness or safety. The main thing is to have a backup plan for repairing the vapor cover that is installed over the treatment zone, because that will be damaged when the subsurface subsides. Any cracks generated in the vapor cover can create preferential pathways that may allow vapor and steam excursions to ambient air. Obviously, if this occurs, we repair and seal the cover right away.
How do you manage ground movement relative to connected subsurface utilities?
This is something we would address up front in the evaluation and design phase. For example, if we have a PVC water line running through a thermal area, either we protect it, or we reroute it temporarily. It’s all part of studying the potential issues and preparing in advance of treatment operations.
If there are abandoned monitoring wells within the ISTR treatment zone that have been backfilled with bentonite, is there a concern about the integrity of those wells?
If the ISTR method is ERH or TCH, it’s less of a concern because these methods don’t create large amounts of steam under high pressure. With SEE, the issue is that the injected steam can erode the bentonite grout, and create preferential pathways for the steam to short-circuit, migrate up along the monitoring well borehole, and eventually discharge directly to the atmosphere. For example, we did a very deep steam project in Arizona, injecting steam to 250 feet below grade, and we had to overdrill and seal some of the abandoned wells to make sure we didn’t create a pathway from the steam zone, where steam was injected at 80 PSI of pressure and up. Often, we can repurpose the abandoned wells by installing a temperature monitoring point with an appropriate high-temperature grout seal.
If you are conducting ISTR next to a building, do you know how far the effects of the thermal treatment might extend below the structure?
With ERH and TCH, we see the effects of heating extending approximately 10 to 15 feet outside of the perimeter of the well fields. The further you get away from the perimeter, the lower the temperature is. For example, at 15 ft outside of the well field, the soil temperature may only be 15-20°C above ambient, and the change in moisture content is negligible. If the treatment zone extends right up to a building foundation, we might see absolute soil temperatures of 60 to 80°C extending a few feet beneath the building and we then assess the potential for subsidence, heating within the building, and increased vapor intrusion. If present, we design appropriate mitigation measures as required. With SEE, when you inject along the perimeter (the treatment zone in most cases has a surrounding ring of steam injection wells to contain and drive the contaminants to the extraction wells located inside the treatment zone), half of the steam flow migrates outwards and the other half inwards. So, for steam, we typically keep a safety distance of at least 25 to 30 feet from buildings and sensitive infrastructure, depending on the spacing of the wells designed for that particular site.
Have other questions about the risk of subsidence, or want to learn more about thermal remediation?
Steffen Griepke is the Vice President of Technology at TerraTherm. In this role, he helps clients determine if a thermal remedy would work on their project site and, if so, which thermal technology or combination of technologies would be the most cost-effective. He then leads the design of the remedy and guides its implementation, operation,…