The redevelopment of brownfield sites across the Greater Toronto Area presents a unique set of geotechnical and environmental challenges, chief among them being the mitigation of subsurface gases. In many industrial corridors or areas situated near former landfill sites, the soil and groundwater can contain elevated concentrations of methane, volatile organic compounds (VOCs), or other hazardous gases. These gases pose a dual threat: they present an immediate health and safety risk to excavation crews and, if not properly managed, can migrate into the future building’s basement or crawlspaces, creating long-term structural and toxicological hazards.
Subsurface gas mitigation begins with a comprehensive Vapor Intrusion (VI) assessment. This involves the installation of soil gas probes to measure the pressure, concentration, and flow rate of gases within the subsurface matrix. In the GTA, the presence of low-permeability soils like Halton Till can lead to ‘trapped’ pockets of gas that, when punctured during excavation, may be released rapidly. Engineering controls must be established before the first bucket of soil is turned. This typically includes the use of real-time gas monitoring arrays equipped with audible alarms to alert the site team of any threshold exceedances of Lower Explosive Limits (LEL) or toxic concentrations.
Active ventilation is a primary engineering strategy for managing gases during the excavation phase. High-capacity, explosion-proof fans are used to circulate air within the pit, ensuring that any released gases are diluted and exhausted away from the work zone. In cases of concentrated methane or VOC plumes, ‘trenching curtains’ or specialized vapor barriers may be used to isolate the source from the rest of the site. For the permanent structure, a robust gas management system is engineered into the foundation. This often consists of a thick, spray-applied or sheet-form gas-impermeable membrane, such as a reinforced polyethylene or ethylene vinyl alcohol (EVOH) barrier, installed beneath the floor slab.
Beneath this barrier, a sub-slab venting system is typically installed. This system consists of a network of perforated pipes embedded in a high-void gravel layer, designed to collect gases and vent them safely through vertical risers to the building’s roofline. Depending on the gas concentrations found during the initial site characterization, these systems may be passive (relying on natural pressure gradients) or active (using blowers to create a continuous vacuum beneath the slab). The integrity of these systems is critical; every pipe penetration through the membrane must be meticulously sealed with specialized boots and adhesives to ensure the vapor barrier remains airtight.
Aden Earthworks excels in the complex coordination required for brownfield excavation and gas mitigation. We emphasize the integration of safety protocols with technical earth-moving performance, ensuring that gas risks are neutralized before they can impact the project timeline or the safety of the community. By combining advanced vapor barrier installation with precision trenching and active ventilation, we transform contaminated GTA sites into safe, viable foundations for the city’s future growth.
