Formatted Title
Geotechnical Considerations when Using Thermal Conduction Heating under Buildings and Sensitive Infrastructure
Background/Objectives
More and more frequently, in situ thermal remediation (ISTR) is being used to treat contamination located below buildings, roadways, and other critical infrastructure. Subsidence and potential geotechnical effects on buildings and infrastructure are concerns often raised in the initial design phase of a thermal remedy. While the heating approach and main heating mechanisms are different for each ISTR technology, heating in general will thermally expand soils during treatment and at the same time, change the water saturation of the treated volume. Depending on the geology, this can lead to changes in the geotechnical properties and integrity.
Approach/Activities
Due to this concern, detailed geotechnical evaluations have been conducted at several thermal conduction heating sites as part of the design phase. Examples from sites in both the United States, Europe and Brazil will be presented, along with the design features incorporated in the full-scale remedies to address the specific geotechnical concerns. The basic factors affecting the geotechnical properties during a thermal remedy will be discussed, along with a general description of soil types where subsidence is more likely to happen. Examples of geotechnical pre-investigations and tests to determine the impact of a structure’s stability will be given along with case studies showing data from monitoring programs focusing on geotechnical parameters collected from thermal source zones remediated using thermal conduction heating (TCH) and steam enhanced extraction (SEE) below structures. The monitoring programs include sites treated with both clayey, sandy and highly organic peaty conditions and will present datasets where more than 80 monitoring points within and outside the treatment zone were monitored up to 11 times during the remedy, both preconstruction, during operation, after the heating system was shut down and more than one year after the heating system were demobilized.
Results/Lessons Learned
Data from five sites, where professional surveys with accuracies better than 0.005 inches (0.1 mm) were conducted will be presented. The datasets reveal substantial differences in soil movements depending on the soil type, geological site history and saturation conditions and shows both downwards and upwards elevation movements at the monitored sites, depending on the dominant geology type and phase of the remedy considered. For some of the sites a net increase in elevation of up to 0.2 inches (6 mm) were initially observed but changed to a net decrease of 0.3 inches (8 mm) in the years after the thermal remedy. The dataset also includes a site where an up to 8 feet (7.5 m) thick peat layer contaminated with chlorinated solvents was thermally treated, and the monitoring program revealed a decrease in elevation of up to 1.45 feet (450 mm). The results stress the importance of selecting appropriate heating technologies, conducting thorough site assessments to prevent infrastructure damage, and implementing robust monitoring systems to understand and limit any geotechnical effects introduced by the thermal system. By incorporating these elements, a proper evaluation can be conducted in the initial design phase to evaluate if a thermal remedy is a safe solution for the site, and if needed what precautions and safeguards should be incorporated to minimize geotechnical effects during the thermal remediation.