VALGO Environment was commissioned for a project in Sorel-Tracy, Quebec, to implement the environmental remediation of soils and non-compliant materials at a site formerly operated by “Les Aciers Inoxydables Atlas” between 1962 and 2004, during which the site became contaminated.
The site was subsequently acquired by Rio Tinto Fer et Titan (RTFT), which initiated a complete site remediation. A first phase of remediation was carried out in 2019 and 2020; however, due to the presence of a high-pressure natural gas pipeline (2,400 kPa), non-hazardous residual soils and materials not compliant with the remediation plan had to remain on-site. Indeed, excavation of the contaminated soils was limited to a distance of 1 meter from the pipeline, on either side of it.
Type of pollution
The contamination primarily consisted of shallow residual metals (generally < 1m deep), although it extended to depths exceeding 2.5 meters in some areas of the site. There was also residual contamination of HC C10-C50, observed sporadically and often associated with metal contamination in approximately half of the cases.
The non-hazardous residual materials (NHRM) found on the site included steel powder, slag, metal fragments, dolomitic brick, and a hardened crust composed of these various residual materials (RM) listed previously. These materials were found in distinct layers of varying thicknesses, mixed together, or present in small quantities within the soils.
Solutions implemented by VALGO Environment
VALGO Environment opted for the environmental remediation solution involving excavation and off-site disposal of soils and non-compliant materials whose contamination levels exceeded the criteria set by the Guide d’intervention – Protection des sols et réhabilitation des terrains contaminés (Beaulieu, M, 2021) of the Quebec Ministry of Environment and Fight against Climate Change (MELCCFP).
The excavation of non-compliant soils and materials was limited to the soils and materials above and on either side of the gas pipeline that could be excavated without requiring support of the pipeline or compromising its stability or integrity. Soft excavation techniques were required near the pipeline.
Excavation of soils and materials beneath the gas pipeline was not included in the tender.
Infiltration water in the excavations had to be managed properly during the work.
VALGO Environment conducted this decontamination using pneumatic excavation techniques near the gas pipeline.
The advantages of pneumatic excavation over hydraulic excavation are as follows:
- Simplified management of excavated materials: Excavating soils in an unsaturated zone does not generate sludge, making it easier to manage excavated materials and potentially allowing for their reuse to backfill the excavation.
- Precision in excavation: The pneumatic lance produces clean and straight cuts, allowing for precise soil excavation. This enables the segregation of contaminated soils or soils with unsatisfactory geotechnical properties for use as backfill.
- Reduced risks for operators: The pneumatic lance operate at approximately 140 psi, significantly minimizing risks associated with pressure compared to a water lance, which operates at around 3000 psi.
The integrity and stability of the high-pressure pipeline were ensured while maintaining an optimal excavation rate through the following operational procedures:
- Notching of the high-pressure pipeline every 25 meters,
- Excavation using mechanical excavator outside the buffer zone determined by Energir,
- Pneumatic excavation within the buffer zone.
Details and key figures
VALGO Environment’s work involved excavating, transporting, and disposing of:
- 690 tons of non-hazardous residual materials (MRND)
- 24.5 tons of A-B category soils
- 488 tons of C-RESC (inorganic or mixed) soils
- 122 tons of >RESC (organic) soils
- 554.5 tons of >RESC (inorganic or mixed) soils
Additionally, 195,000 liters of excavation water were managed. This water, compliant with municipal regulations, was discharged into the wastewater system.
Project duration: 4 months (from September to December 2023)