Geophysical characterization of contaminated urban fills

This study presents the use of geophysical methods as a preliminary step in the characterization of urban fills, namely for the assessment of the spatial variability of their internal structure. The test site is located near downtown Montréal (Québec, Canada) and had a surface area of 1600 m2. A heterogeneous fill was encountered on the entire site and mainly consisted of a matrix of remolded natural soil material mixed with various debris (e.g. concrete, brick, metal), as well as industrial and domestic wastes (e.g. ash, slag, clinker, coal). Old foundation walls were suspected to be still in place in the fill at the location of previous buildings. The results obtained show that a combination of geophysical methods sensitive to different physical properties of the subsurface is necessary to confirm the presence of heterogeneities, their nature, and assess their lateral and vertical extent in urban fills. To validate the nature of the anomalies detected during the geophysical survey, trench excavation and sampling were performed after completion of the geophysical survey. By cross-analyzing the horizontal imaging methods and vertical imaging methods, it was possible to delineate zones of distinctive heterogeneity and establish the contact between the fill and the natural soil material. Thus, as opposed to conventional characterization techniques, which rely on trench excavation or borehole drilling and only provide a localized estimation of fill properties, geophysical methods helped in imaging the external and internal geometry of a fill in a continuous manner.