The Analysis of a Deep Excavation in a Gassy Soil

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Mabrouk, Ahmed
Hydrofracturing , Numerical forensic analysis , Finite element , Excavation , Gassy soil
The study presents a numerical analysis of series of unanticipated events that took place upon the excavation of a landfill in a deep deposit of clayey soil in southwestern Ontario, Canada. During the excavation of a landfill cell to be used for waste disposal, unexpected lateral slope movements were observed followed by gas and water venting in several locations (while the excavation in low permeability clayey till was about 14m above the underlying aquifer). The clayey till is known to be underlain by permeable, natural gas bearing rock, and gas has been diffusing through the clayey deposit over about the last 13,000-15,000 years. Preliminary 2D and 3D elasto-plastic effective stress analyses using conventional soil mechanics –similar to what was used in design- are used to capture the general behaviour of the excavation. However, the analyses revealed the need for model modification to account for other governing factors (gassy soil and hydrofracturing) to be able to explain the mechanism that might have lead to the evolution of gas vents and upward water flow through the thick shale aquitard. The clayey deposit contains silty sand lenses at different elevations. The upward diffusion of methane and chloride from the bedrock aquifer through the clay till is modelled and the potential for chloride migration contributing to the exsolution of methane due to reduction in methane solubility is discussed. Two approaches to modelling the lenses are examined where gas exsolution either occurs prior to or during the excavation. The FE model is modified to account for hydrofacturing and gassy soil behaviour (for sand lenses). 2D and 3D forensic modelling studies are presented examining the potential causes for the unanticipated movements and the gas and water venting observed during the excavation. The model investigates the role of presence of gassy sand lenses and of the presence of a discontinuous weak sandy clayey silt layer between the bedrock and the low permeability till on the hydrofracturing path and gas venting. Finally, a parametric study is conducted to examine the effect of different parameters on the soil behaviour when excavated. Recommendations regarding further excavations within the same soil deposit are presented.
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