Effect of gassy sand lenses on a deep excavation in a clayey soil

Gas venting (mainly methane) was encountered during a deep excavation in a 40 m thick clayey deposit (with silty sand lenses) overlying a shale layer believed to be the source of biogenic gas. The upward diffusion of methane and chloride from the bedrock aquifer through the clay till is modeled and the potential for chloride migration contributing to the exsolution of methane due to a reduction in methane solubility is discussed. Two approaches to modeling the lenses are examined where gas exsolution either occurs prior to the excavation, or during the excavation. Both approaches lead to similar conclusions although sand lenses were more prone to liquefaction if the methane was present in its gaseous state prior to unloading. The results suggest that while liquefaction of a sand lens near the base of the excavation may have contributed to the formation of the seeps and vents, the gassy response of sand lenses due to the exsolution of gas during unloading cannot explain the level of venting that was observed in the field.