2019 Volume 25 Pages 487-492
A large number of significant sand boils and sink holes repeatedly appeared during flooding events in recent years in the left side hinterland of Kita River at around 13km from the mouth. The volume of many holes was much larger than ejecta from adjacent sand boils. Extensive in-situ investigation including topography survey, penetration tests and trench excavations was conducted aiming at providing better understanding to the mechanism of the sand boils and sink holes at the site. It was confirmed that high water pressure of river water propagated through a thick gravel layer underlying a 3m thick surface silt layer. Sand ejected selectively at either low ground surface or thin cover layer from the gravel layer. Detailed observation in the trenches revealed that the sand was most likely contained originally in the gravel layer, moved upward to create sand chambers in the silt, and sand in the chamber ejected through preferential path ways to the surface. The Shrinkage in the sand chambers due to ejecting sand is considered to have caused subsidence of soil above and resulted in the sink holes.
