Site investigations to evaluate flow liquefaction slides at Sand Heads, Fraser River delta

Spontaneous flow liquefaction is a recurrent process in the progradation of the Fraser River delta. A portion of the Fraser River delta was investigated by seismic and resistivity cone penetration testing, by the spectral analysis of surface waves method and by soil and vapour sampling in an effort to determine site stratigraphy, in situ state, and degree of saturation to explore the enigma of spontaneous now liquefaction. A pore pressure and till sensor package was installed as part of the investigation, to monitor tidal drawdown effects. Results indicated that the seabed at shallow depth was partially saturated, containing a significant volume of methane gas, which affected the dynamic pore pressure response during tidal drawdown. A reduction in effective stress of up to 16.5 kPa was measured, along with a time lag between tidal drawdown and pore pressure response of 50 min, This suggests that gas-damping effects are a significant factor in initiating spontaneous liquefaction of loose unaged sands where they rest on submarine slopes. The in situ test results showed that newly deposited Fraser River sands possess a high susceptibility to flow liquefaction.